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GHG Emissions
A Pioneer's Summary of Causes & Solutions in California

Not only are our problems based in established cultures -- industrial -- but our solutions will also draw on new technological cultures such as biomimicry, and alternative energies from algae and sunlight to build the communities of OUR future.

Find green business solutions
Understanding WHY green and sustainable strategies are valuable to our way of life, and our future is essential for career development and even in making seemingly simple choices such as food or where to live, or how many pets or children to have! We are living in challenging times -- not bad times, just increasingly complex times.

Think like pioneers...

We are pioneers of a new era, just as our American forerunners and settlers were pioneers of a new land-based era. We're pioneers of a new technologically based era. Not only are our problems based in established cultures -- industrial -- but our solutions will also draw on new technological cultures such as biomimicry, and alternative energies from algae and sunlight to build the communities of OUR future.

This summary is understandable by everyone -- to some degree. It is the lighthouse to guide our socked-in journey across open seas. It is an entrepreneurial treasure chest of innovation and market opportunities. It gets into chemistry -- which I don't understand -- but which I struggle to learn just like my grandparents struggled to understand new laws when they emigrated to the US. They came for opportunity. They found it. We face opportunity... and can find it as well.

Greenhouse gases don't JUST affect climate temperature. They also affect the quality of the air we breathe TODAY. They affect which crops can be grown. And the quality of our water. And the cost of our food. These issues matter to us today, and we can ALL take steps today to be more successful pioneers in this new era.

California's GHG Emissions Challenges

We hear about carbon dioxide causing global climate change ... but 18% of the problem is NOT CO2. This is a very powerful 18% of the toxic cocktail. These chemicals are MUCH MORE powerful than CO2 in their impact on our environment. Take a look at these numbers and tease out their actual impact.

The non-CO2 greenhouse gases (NCGGs) emissions in California were 75 MMTCO2-Eq. in 2004, approximately 18% of the total GHG emissions.

Out of this 18% total GHG emissions,

  • 7.6% came from nitrous oxide
  • 6.4% from methane
  • 3.2% from HFCs, PFCs, and SF6
To meet the California GHG targets set in the Executive Order S-3-05, emission reductions from NCGGs are critical.

NOW, THEREFORE, I, ARNOLD SCHWARZENEGGER, Governor of the State of California ... do hereby order effective immediately: That the following greenhouse gas emission reduction targets are hereby established for California: 2010, reduce GHG emissions to 2000 levels; 2020, reduce GHG emissions to 1990 levels; 2050, reduce GHG emissions to 80 percent below 1990 levels

California State University, Fullerton conducted an extensive study of California's GHG emissions challenges in 2008. This editorial "key points" document helps non-scientists survey the findings and pick the solutions that can be applied to their immediate challenges -- be they entrepreneurial innovation, regional governance, or information and outreach.

"Clearinghouse of Technological Options for Reducing Anthropogenic Non-CO2 GHG Emissions from All Sectors"


California Air Resources Board
Research Division
PO Box 2815
Sacramento, CA 95812

Prepared by:
Jeff Kuo, Ph.D., P.E.
Principal Investigator
Dept. of Civil and Environmental Engineering
800 N. State College Blvd.
Fullerton, CA 92834-6870
(714) 278-3995
May 14, 2008
Contract No.: CARB 05-328
NOTE: Anthropogenic means "caused or produced by humans"
We encourage you to download the full report (DOWNLOAD: PDF, 2,530K) and have it handy as you find solutions that apply to your challenge (and related resources are available at CARB's website... because this report is VERY thorough, offering excellent market research information to help pioneers in green tech, clean tech, and sustainable communities move forward with solid information and good resources to connect with.

Causes of Greenhouse Gas Emissions

THIS problem, scale and solutions oriented research project started with a literature search to identify sources of NCGG emissions from various sectors in California. After the emission sources were identified, another comprehensive literature search was conducted to identify available technological options for NCGG emission reductions.

A study was then conducted on gathered information to evaluate identified technological options for their applicability in California. Data and information regarding reduction efficiency, market penetration, technical availability, service lifetime, and costs on many viable technological options were then gathered, evaluated, and presented in a systematic way for easy comparison and use.

With financial resources a struggle for everyone for the next several years, it's probably prudent to look at the size of the problems ... and the mix of benefits that come from each possible solution.

The top contributors for CH4 emissions in California, in the order of magnitude are

  • landfills (30.2%)
  • enteric fermentation (25.9%)
  • manure management (21.6%)
  • wastewater treatment (6.1%)
  • natural gas systems (5.0%)
  • stationary combustion, (4.7%)
  • mobile combustion (2.2%)
  • rice cultivation (2.2%)
  • petroleum system (1.8%)
  • field burning of agricultural residues (0.4%)
As you will notice, three sources stand out -- and contribute a lion's share of the volume of these greenhouse gases: landfills, enteric fermentation (herd animal burps!), and manure management.

What do YOU have to do with these causes? Lots! Urban areas contribute as much as 90% of the trash to landfills. Enteric fermentation is ruminant stomach gas (cattle, sheep, etc). We eat beef and lamb, we contribute to demand for these domesticated animal herds by the millions! They belch a lot. And they poop a lot. And those gaseous releases are the #2 and #3 causes of greenhouse gas problems.

Personal Solutions + Technological Solutions

While the "technological solutions" identified in this research report can be turned into business opportunities on a large scale, consumers and purchasers of goods and services can understand the relative scale of their choices. For example... by eating (and catering) more vegetarian meals, you can reduce the number of animal products that need to be produced. By reducing packaging and waste in landfills --- you can reduce the emissions from landfills. And by using recycled content in your products, or buying products with recycled content in them, you can also foster "urban mining" of landfill waste -- and turn it into valuable resources. That changes the mindset of how we view all materials -- and can transform our economy into a sustainable closed system that reuses everything -- much like Mother Nature.

Technological Solutions

There are many viable technological options for emission reductions for these sectors, especially in oil and natural gas systems, landfills, manure management, enteric fermentation, and wastewater treatment were identified and described.

The contributors for N2O emissions in California, in the order of magnitude, are

  • agricultural soil management (57.5%)
  • mobile combustion (35.3%)
  • human sewage (3.2%)
  • manure management (2.7%)
  • stationary combustion (0.6%)
  • nitric acid production (0.5%)
  • field burning of agricultural residues (0.2%)
  • municipal solid waste combustion (0.1%)

Personal and Purchasing Solutions

The technological options listed below, again, are just the "business scale solution". As consumers and leaders of small groups -- families, colleagues, communities -- we can make specific choices that impact the largest of these problem areas. For example:

Agricultural soil management accounts for 57% of the problem. We can buy organic food and fabrics -- not only because of their health properties, but because organic production reduces wasteful, petroleum-based conventional agricultural land practices. (And don't forget eating more vegetarian foods.) By returning green waste to the soil, we keep erosion in check, we improve water retention of the soil, we support native wildlife populations, and we keep pollution out of our very limited and very precious fresh water supplies.

And Number TWO on the list -- mobile combustion -- is about the vehicles we drive. Practical choices include walking, biking, and carpooling. And taking public transportation. And using the telephone, Internet and webinars when possible. Reducing our travels is the most impactful choices we can contribute to a problem that produces 35% of the N2O emissions in California.

Technological Solutions

Many viable technological options for emission reductions for these sectors, especially in agricultural soil management, manure management, mobile and stationary combustion, nitric acid production, and wastewater treatment were identified and described in this detailed report.

Ozone Depleting Substances

Substitution of ozone-depleting substances (ODS) with hydrofluorocarbons (HFCs) and Perfluorocarbons (PFCs) is the dominant emission source of high-GWP (Global warming potential) gases in California and it represents 88.8% of total high-GWP gases emissions.

Ozone Depleting Substances = 88.8%
of total High-GWP Gas Emissions

Electrical transmission and distribution (7.2%) and semiconductor manufacture (4.0%) are the other two significant emission sources of high-GWP gases in California.

Personal Solutions

And what can we do about electrical transmission and distribution, you might ask. We can use less electricity. And we can learn about, and move into distributed generation of power -- so that less long distance transmission is necessary. You know... solar panels, small wind turbines, solar thermal, passive solar to reduce the need for electrical power... and your finger on the controls -- the most powerful of ALL GHG reduction strategies!

Technological Solutions

Many viable technological options for emission reductions for all these three sectors were identified and described

To more easily identify innovative solutions, emission sources can be categorized into economic sectors:

  • Residential
  • Commercial
  • Industrial
  • Agricultural
  • Transportation
  • Electricity generation

Personal Solutions

We don't often sit down and take stock of how complex our lives are -- how our decisions affect all six of those economic sectors. But our financially-based economy makes us players in all of them. We live in houses and buy furnishings; we shop and work in commercial workplaces; we rely on manufacturing to make and transport goods to us; we eat food and consume resources from agriculture; we travel in cars and public transport systems; and we use electricity. WE have met the polluters -- and they are us.

And we have also met the solution providers -- and they are us!

Within those sectors, there are 6 potential source sectors, as defined by United Nations Intergovernmental Panel on Climate Change (IPCC), including the systems that we depend on every day for the lifestyle to which we've grown accustomed:

  • Energy
  • Industrial processes
  • Solvent use
  • Agriculture
  • Land-use change and forestry
  • Waste

Let's look at these sources of goods and services in detail so that we can identify our own heat map of real value, of over-use, waste and potential for innovative solutions.


Energy-related activities are the primary sources of anthropogenic GHG emissions in the United States.

They accounted for 86 percent of total emissions on a carbon equivalent basis in 2004.

Thirty-nine percent of nation-wide methane emissions are energy-related.

And 15% of nitrous oxide emissions are energy-related.

There are ten sub-sectors considered in the EPA’s inventory report:
1. Stationary combustion
2. Mobile combustion
3. Coal mining
4. Abandoned underground coal mines
5. Petroleum systems
6. Natural gas systems
7. Municipal solid waste combustion
8. Natural gas flaring and ambient air pollutant emissions from oil and gas activities
9. International bunker fuels
10. Wood biomass and ethanol consumption

Industrial Processes

GHG emissions are also produced as a by-product of various non-energy related industrial activities. For example, raw materials can be chemically transformed and this transformation may result in emissions of GHGs including carbon dioxide, methane, and nitrous oxide. The processes addressed in the EPA’s inventory report include the following twenty industries:

1. Iron and steel production
2. Cement manufacture
3. Ammonia manufacture and urea application
4. Lime manufacture
5. Limestone and dolomite use
6. Soda ash manufacture and consumption
7. Titanium oxide production
8. Phosphoric acid production
9. Ferroalloy production
10. Carbon dioxide consumption
11. Petrochemical production
12. Silicon carbide production
13. Nitric acid production
14. Adipic acid production
15. Substitution of ozone depleting substances
16. HCFC production
17. Electrical transmission and distribution
18. Aluminum production
19. Semiconductor manufacture
20. Magnesium production and processing

Solvent and Other Product Use

GHG emissions can be produced as a by-product of various solvents and other product uses. However, in the United States, emission from nitrous oxide product usage is the only source of GHG emissions from this sector, and it accounted for less than 0.1 percent of total U.S. anthropogenic GHG emissions on a carbon equivalent basis in 2004.


A variety of agricultural activities contribute to GHG emissions, with methane and nitrous oxide being the primary ones. The main emission sources in this sector can be grouped into five sub-sectors:

1. Enteric fermentation (CH4)
2. Manure management (CH4 and N2O)
3. Rice cultivation (CH4)
4. Agricultural soil management (N2O)
5. Field burning of agricultural residuals (CH4 and N2O)

Land-use Change and Forestry

Change matters. IPCC’s Good Practice Guidance for Land Use, Land-Use Change, and Forestry recommends reporting fluxes according to changes within and conversions between certain land-use types (forestland, cropland, grassland, settlements, and wetlands). For its inventory report, U.S. EPA estimated greenhouse gas fluxes from the following sub-sectors:

1. Forest land remaining forest land
2. Land converted to forest land
3. Croplands remaining croplands
4. Lands converted to croplands
5. Settlement remaining settlements
6. Lands converted to settlements

Personal Solutions

We don't think about families having an impact on large-scale land changes, but we do. Suburban housing has a huge impact on land use changes. The conversion of agricultural land to urban developments increases not only heat islands from pavement, but increased water usage, increased commuting distances which emit emissions and eat up natural resources -- and increased air pollution.

And when housing and commercial developments encroach into wilderness areas, we not only destroy forests that filter pollutants out of air and replenish fresh water reservoirs, but destroy habitat and native species in both the animal and plant populations.

Living in denser areas makes more sense. California also spends billions of dollars every year fighting forest fires -- which also produce pollutant that affect our air and water resources. It is the responsible thing to do -- NOT to move to the suburbs.


Waste management and treatment activities are sources of GHG emissions, especially for methane and nitrous oxide.

Landfills were the largest source of anthropogenic methane emissions in 2004, accounting for 25% of total methane emissions in the United States. Wastewater treatment accounts for another 7% of the U.S. methane emissions. Discharge of wastewater treatment effluents in receiving waters and the treatment process itself are the sources of nitrous oxide emissions. The following waste and waste management groups were addressed in the inventory report by USEPA:

1. Landfills
2. Wastewater treatment
3. Human sewage (domestic wastewater)

Evaluation of Technological Options / Solutions

Human waste starts with personal decisions about quality and quantity -- and our lifestyle choices. There are many personal decison making values that can affect the community as a while by affecting attitudes and expectations, as well as emotional judgments about what we admire and aspire to. For example, houses have grown in size over the past 100 years from an average of about 1,000 sq. ft to 4,000 sq. ft. At the same time, family size has decreased dramatically!

It's an emotional thing!

Going green isn't always green. Bamboo flooring might be a more ecologically sustainable option, but if you rip out perfectly good hardwood floors and throw them into the not only waste unnecessarily, but contribute to the largest problem for air pollution -- landfill emissions.

Rethinking what we admire is a personal solution. Reuse. Reduction of consumer thrashing about will make a huge difference because our actions are observed by others -- like children, and employees, and neighbors, and even third world residents who want what American have. Frugality and conservation are personal, emotional and very powerful solutions.

Some of the technological options identified from the literature search are already in use, but many of them are still in conceptual, bench-scale studies, or research and development (R&D) stages.

About the Identified Solutions

Not all technology is created equally -- and not every application can use the same technology. You know that. But we sometimes overlook "possibilities" due to regional, educational or industry traditions and entrenched supply chains.

To evaluate the applicability and implementability of a technological option, it is important to have the data on

  • reduction efficiency (RE)
  • market penetration (MP)
  • technical applicability (TA)
  • service lifetime
  • costs (capital and O&M).

A Clearinghouse of Problems and Solutions

To serve as a clearinghouse, this report includes information on as many technological options as possible. Those options with sufficient and definite information on lifetime, RE, MP, TA, and costs are summarized in tables for easier comparison and use.

More detailed data and information on these (and more) technological and behavioral options are provided in the appendices of the full report.



The top five contributors for Methane (CH4) emissions

United States

  • Landfills(25.3%)
  • Natural gas systems (21.3%)
  • Enteric fermentation (20.2%)
  • Coal mining (10.1%)
  • Manure management (7.1%)
  • Wastewater treatment (6.6%)


  • Landfills (30.2%)
  • Enteric fermentation (25.9%)
  • Manure management (21.6%)
  • Wastewater treatment (6.1%)
  • Natural gas systems (5.0%)
California Methane Emissions
Why care about Methane? Because it is so impactful. AND because the emissions are increasing dramatically!

Methane (CH4) is approximately twenty-three times as impactful as carbon dioxide (CO2) in trapping heat in the atmosphere over a 100-year time horizon (IPCC, 2001).
The chemical lifetime of CH4 in the atmosphere is about twelve years. Since 1750, the global-average atmospheric concentrations of CH4 have changed from about 700 to 1,745 parts per million by volume (ppmV), a 150% increase (IPCC, 2001).

This increase is mainly due to anthropogenic emissions, including emissions from landfills, natural gas and petroleum systems, manure management, coal mining, wastewater treatment, stationary and mobile combustions, and some industrial processes (USEPA, 1999; USEPA 2006a).


California accounts for only 1.8% of the nationwide methane emissions from the energy sector. The major contributors for methane emissions in the energy sector in California are natural gas systems (36.8%), stationary combustion (34.2%), mobile combustion (15.8 %), and petroleum systems (13.2%).

Five sub-sectors within the energy sector in California have CH4 emissions:

  • Petroleum systems
  • Natural gas systems
  • Stationary combustion
  • Mobile combustion
  • Abandoned underground coal mines

Methane emissions from petroleum systems are mainly associated with the following activities:

  • Crude oil production field operations
  • Crude oil transportation
  • Refining operation

Technological Solutions

The measures to reduce methane emissions from the petroleum systems as well as natural gas systems mitigation strategies.

Stationery Combustion

Stationary combustion includes all fuel combustion activities from fixed sources (versus mobile combustion). For stationary sources, methane may result from incomplete combustion of fuels. The IPCC Guidelines categorize emissions of NCGGs from stationary combustion-related activities into five sectors (IPCC, 1997):
  • Energy industries (electricity generation, charcoal production, etc.)
  • Manufacturing industries
  • Commercial/institutional sector
  • Residential Sector
  • Stationary agriculture/forestry/fishing sources

Methane is produced in small quantities from fuel combustion due to incomplete combustion of hydrocarbons in fuel. The production of CH4 is a function of the temperature in the boiler/kiln/stove. In large facilities and industrial applications, the combustion is more efficient and the emission rate is very low. On the other hand, emission rates from smaller combustion sources are often higher, particularly when smoldering occurs.

The highest rates of CH4 emissions from fuel combustion occur in residential applications such as small stoves and open burning.

In addition to electricity combustion, industrial stationary sources for methane emissions include:

  • Gasoline and diesel used in various equipment (manufacturing and industrial sectors, food and agricultural processing, off-road equipment, ships and commercial boats, and trains).
  • Commercial stationary sources for methane emissions include diesel and liquefied gas used in asphalt paving and roofing, commercial lawn and garden equipment, and others. They also include natural gas emissions from commercial water and space heating, cooking, and commercial off-road equipment

Technological Solutions

Basically, reducing energy demand and improving combustion efficiency can reduce methane emissions from this sector.

Mobile Combustion

Methane emissions from mobile sources depend on methane content of the motor fuel, the amount of hydrocarbons remained un-burned in the engine exhaust, the engine type, and post-combustion controls.

In vehicles without emission controls, the amount of CH4 emitted is highest at low speeds and when the engine is idle. Poorly tuned engines would have higher CH4 emissions.

Emissions from mobile combustion are often grouped by transport mode (e.g., highway, air, rail), fuel type (e.g., motor gasoline, diesel fuel, jet fuel), and vehicle type (e.g., passenger cars, light-duty trucks).

Road transport accounted for most of the mobile source fuel consumption, and, consequently, the majority of mobile combustion emissions. Mobile combustion was responsible for a very small portion of methane emissions (0.5%) in the United States.

Due to the control technologies employed on highway vehicles that reduce CO, NOx, VOC, and methane emissions, methane emissions from mobile combustion have declined 38% from 1990 to 2.9 MMTCO2-Eq. in 2004.

Technological Solutions

Little information regarding technological options for methane emission reduction in this sector was found from the literature search. Basically, using alternative fuels, reducing travel, and improving vehicle efficiency can reduce methane emissions from this sector.

Abandoned Underground Coal Mines

Active underground coal mines contribute a large share of methane emissions in the United States.

As mines mature and coal seams are mined through, mines will be closed and abandoned. Many abandoned mines were sealed and some were flooded through groundwater intrusion or by surface water. Some abandoned coal mines are vented to the atmosphere to prevent the buildup of methane gas. After an initial decline, abandoned coal mines can liberate methane gas at a steady-state rate for a long period of time.

Although there are no active coal mines in California, there were coal-mining activities in the past.

Technological Solutions

Flaring of the collected off-gases is a viable option to reduce methane emissions from abandoned underground coal mines.

Methane Emissions from Industrial Processes

Out of the twenty industries that may have GHG emissions, petrochemical production is the only industrial process that may have sizable methane emissions in California.

Methane Emissions from Agriculture

Agricultural activities currently generate the largest share of anthropogenic methane emissions both in California and the United States.
Methane emissions from agricultural activities can be grouped into the following categories:
  • Enteric fermentation
  • Manure management
  • Rice cultivation
  • Field burning of agricultural residues

Enteric fermentation and manure management are the two dominant methane emission sources in agriculture sector, followed by rice cultivation. Each is responsible for almost half of the total methane emission from the agricultural sector in California.

Enteric Fermentation Methane is produced as part of the normal digestive processes in animals. During digestion, microorganisms in the digestive system ferment the ingested feed. This enteric fermentation process produces methane as a by-product, which can be eructed by the animal through the mouth or gut. The amount of methane produced and excreted by an individual animal depends mainly on the animal’s digestive system and on the amount and type of food it consumes.

Because of their unique digestive system, ruminant domestic animals (e.g., cattle, sheep, and goats) are the major emitters of methane.

Technological Solutions

In the United States, beef cattle accounts for 71% of total livestock methane emissions in 2004, followed by 24% for dairy cattle, and the remaining 5% was from horses, sheep, swine, and goats (USEPA, 2006a).

US EPA has voluntary Ruminant Livestock Efficiency Program underway as part of the Climate Change Action Plan toward emission reductions. The methane emissions from enteric fermentation in California were 25.9% of the state’s total methane emissions.

Strategies to reduce methane emission enteric fermentation include

  • Reduction of livestock
  • Increase of feed conversion efficiency by adjusting animal diets
  • Increase of animal production through the use of growth hormones
  • Increase of animal production by improved genetic characteristics
  • Improve nutrition through strategic supplementation
  • Improved reproduction

The key reduction options are changing animal diets and use of more productive animal types

Methane Emissions from Manure Management

Livestock manure is another significant source of methane emission as a result of anaerobic decomposition of manure. When livestock or poultry manure is handled as a solid (e.g., in stacks or dry-lots) or deposited on pasture, range, or paddock lands, it tends to decompose aerobically and produce little or no methane gas. However, if manure is stored or treated in systems that promote anaerobic conditions (e.g., as a liquid or slurry in lagoons, ponds, tanks, or pits), methane would be generated.

Technological Solutions

Manure composition also affects the amount of methane production. The composition varies by type of animal’s digestive system and diet. Generally speaking, the greater the feed’s energy content, the greater the methane emission potential. For example, feedlot cattle fed with a high-energy grain diet generate manure with a high methane-producing capacity. Range cattle fed with a low-energy diet of forage material produce manure with approximately 50% of methane-producing potential of feedlot cattle manure. However, some higher energy feeds are more digestible and will result in less manure. Consequently, the quantity and characteristics of the manure depend on both the feed type and the growth rate of the animal (USEPA, 2006a). US EPA has the voluntary AgStar (livestock manure systems) program underway as part of the Climate Change Action Plan toward emission reductions.

Although national dairy animal populations have been decreasing, some states, including California, have seen increases in their dairy populations as the industry becomes more concentrated in specific areas of the country. These areas of concentration tend to utilize more liquid systems to manage (flush or scrape) and store manure. Use of liquid manure management systems has higher methane emission potential than dry systems.

In general, measures to mitigate methane emissions from manure management include livestock reduction, prevention of fermentation during stabling, controlled fermentation of manure, composting, and aerobic digestion. The key reduction option is the capture and use of methane emissions through the use of anaerobic digesters that can be farm scale or centralized for the intensive agricultural zones.

Aerobic digestion – The organics in manure can also be biodegraded under aerobic conditions. In this process, no methane will be generated and, thus, there will be little or no methane emissions.

Methane Emissions from Rice Cultivation

All rice in the United States is grown on flooded fields. When fields are flooded, aerobic decomposition of organic material gradually depletes the oxygen present in soil and floodwater and creates anaerobic conditions in soil. Under the anaerobic environment, methane is produced through decomposition of soil organic matter (mainly plant wastes that remain after harvest) by methanogenic bacteria. In California, there is typically one crop per year.

One of the most important factors affecting methane emissions is the water management system under which rice is grown. Under continuously flooded conditions, rice fields have higher methane emissions than those that are not flooded. Other factors that influence methane emissions from flooded rice fields include fertilization practices (especially the use of organic fertilizers), soil temperature, soil type, rice variety, and cultivation practices.

The methane emissions from rice cultivation in California in 2004, were 2.2% of the state’s total methane emissions.

Various mitigation strategies have been proposed; however, the formulation of such strategies is a very sensitive issue because the emission control measures may exert a negative impact on rice production.

Specific technological options to reduce CH4 emissions from rice cultivation include water management regimes, shallow flooding, upland rice species, soil amendments alternative fertilizers, and off-season straw applications.

Methane Emissions from Field Burning of Agricultural Residues

Large quantities of agricultural crop residue are produced from farming activities. The crop types whose residues are typically burned in the United States are wheat, sugarcane, corn, barley, soybeans, peanuts, and rice. Less than 5% of these residues are burned each year, with the exception of a significantly higher proportion of rice straw residue burned. Crop residue burning is a net source of methane, which is released during combustion (USEPA, 2006a).

The methane emissions from field burning of agricultural residues in California were 0.4% of the state’s total methane emissions,

Land-use Change and Forestry

The only potential source for methane emissions in this sector is forest fires.

Methane Emissions from Landfills

Landfills are the largest anthropogenic source of methane emissions in the United States, 25.3% of total methane emissions. Municipal solid waste (MSW) landfills accounted for 94% of the total landfill emissions, while industrial landfills accounted for the reminder, 6%.

Landfills are also the largest anthropogenic source of methane emissions in California, contributing 30.2% of the state’s total methane emissions.

Although the annual quantity of waste buried in landfills in the United States increased 33% from 1990 to 2004, net annual methane emissions decreased by approximately 18%. The downward trend in overall methane emissions from landfills is due to the increasing amount of landfill gas collected and combusted by landfill operators (USEPA, 2006a).

California started collecting and combusting landfill gas earlier than other states. Thus, the trend in California is that methane emissions from landfills are relatively flat from 1990 onward, since the controls were largely in place before 1990. US EPA has the voluntary Landfill Methane Outreach Program underway as part of the Climate Change Action Plan toward emission reductions

Technological Solutions

Key reduction options for methane emissions from landfills are reduction of the amount of organics deposited into landfills, and energetic use or flaring of landfill gas. Specific technological options to reduce CH4 emissions from landfills include the following:

  • Landfill gas recovery and utilization (direct gas use) – Landfill gas is recovered and used as a medium BTU fuel for boilers or industrial processes. The gas is directly piped to a near-by user and serves as a replacement fuel.
  • Landfill gas recovery and utilization (electricity generation) – Recovered landfill gas is used for electricity generation projects.
  • Landfill gas recovery and utilization (upgrade to natural gas) – Several methods such as membrane separation can separate carbon oxide and other compounds in landfill gas from methane. The treated gas can be injected to a local natural gas distribution grid, converted to compressed natural gas (CNG), liquefied natural gas (LNG), methanol, or ethanol.
  • Anaerobic digestion – Using a reactor vessel to enhance natural decomposition under anaerobic environment and generated methane can be used to produce heat and/or electricity
  • Anaerobic digestion includes source separation of waste prior to disposal in the anaerobic digestion system .
  • Composting – Degradation of organic matter under aerobic conditions and has a market value used to enhance soil in horticulture/landscape and agricultural sites
  • Mechanical biological treatment – the whole waste stream is composted in order to degrade the organic fraction anaerobically and inorganic materials are disposed of in a landfill
  • Increased oxidation – Methane emissions are reduced by top capping and restoration layers of the landfills.
  • Optimize and enhance landfill gas formation with increase of moisture content and movement to accelerate the speed and increase the completeness of conversion of organics to landfill gas.
  • Waste treatment in bioreactors (the sustainable landfill) – in which biological, chemical, and physical processes occur in a controlled way. In this approach waste is deposited in relatively small and shallow compartments with an impermeable bottom liner for one year maximum, to prevent the on-set of methanogenesis before the top liner is installed. After the installation of the top liner, biological process in the waste is accelerated through infiltration and recirculation of leachate Processes include anaerobic, aerobic and hybrid bioreactors
  • Aerobic landfilling or aerobic pretreatment – for reducing methane emissions with compressed air injection into the landfill to collect the product gas mixture at extraction wells.
  • Source reduction – Reducing the amount of degradable waste landfilled will reduce methane emissions.

Methane Emissions from Wastewater Treatment

Wastewater from domestic and industrial sources is treated aerobically and anarobically to remove suspended solids, organics, pathogens, and some chemical constituents. Anarobic removal of organics can form methane as a by-product.

In the United States, methane emissions from domestic wastewater treatment in 2004 were 20.0 MMTCO2-Eq. The emission rates are increasing due to the increase in the U.S. human population and the increase in per capita organic loading to wastewater.

Industrial Wastewater Treatment

The five largest industrial methane emission sources are those that generate wastewater with high biodegradable organic concentrations including):

  • Pulp and paper
  • Meat and poultry packing
  • Vegetables, fruits and juice processing
  • Refineries
  • Petrochemicals

The 2004 methane emissions from wastewater treatment in California were 6.1% of the state’s total methane emissions.

Technological Solutions

Key reduction options for methane emissions from wastewater are addition of more wastewater treatment plants, aerobic wastewater treatment, and recovery of methane from anaerobic wastewater treatment processes.

Nitrous Oxide

Nitrous oxide (N2O) is produced from natural and anthropogenic sources. It is chemically stable and has little impact on human health and other living organisms at its normal atmospheric concentration, however, it is a significant greenhouse gas with approximately 296 times the global warming potential of CO2 over a 100-year time horizon.

The nitrous oxide concentration in the atmosphere has increased by about 18% over the last two hundred years, from 275 parts per billion by volume (ppbV) in pre-industrial times to 311 ppbV in 1992 (IEA, 2000).

This increase was mainly due to anthropogenic emissions, including agricultural soil management, manure management, production of nitric acid and adipic acid, wastewater, and stationary and mobile combustions.

The top five contributors for N2O emissions in the United States are agricultural soil management (67.5%), mobile combustion (11.0%), manure management (4.6%), nitric acid production (4.3%), and human sewage (4.1%).

The contributors for N2O emissions in California, in the order of magnitude, are agricultural soil management (57.5%), mobile combustion (35.3%), human sewage (3.2%), manure management (2.7%), stationary combustion (0.6%), nitric acid production (0.5%), field burning of agricultural residues (0.2%), and municipal solid waste combustion (0.1%).

N2O emissions in California occur from all six major sectors: energy, industrial processes, solvent use, agricultural, land-use change and forestry, and waste.

The top TWO significatnt sources of NOX in California are Agricultural Soil (60%) and Mobile combustion (35%).

The agricultural sector is currently and will remain the dominant source of nitrous oxide emissions in both California and the United States in the foreseeable future.

Nitrous oxide is produced naturally in soils through the microbial processes of nitrification (the aerobic microbial oxidation of ammonium to nitrate) and denitrification (the anaerobic reduction of nitrate to nitrogen and/or nitrous oxide). N2O is one of the intermediate products in both the nitrification and denitrification processes.

Several agricultural activities increase mineral nitrogen availability in soils for nitrification and denitrification and ultimately increase the amount of N2O emissions. These activities increase soil nitrogen availability directly or indirectly. Activities that will directly increase the nitrogen availability include (USEPA, 2006a):

  • Fertilization
  • Application of managed livestock manure or other organics such as wastewater sludge
  • Deposition of manure by domestic animals in pastures, rangelands, and paddocks
  • Production of nitrogen-fixing crops and forages
  • Retention of crop residues
  • Cultivation of organic soils
  • Others including irrigation, drainage, tillage practices, and fallowing of land

Technological Solutions

With regards to improving nitrogen utilization efficiencies to reduce N2O emission from agricultural soil, many technological options and practices have been mentioned in literature. However, many of them were mentioned without detailed discussion and information. In addition, very few studies include cost data for implementing mitigation options. The economic potential for nitrous oxide emission reduction probably is low, except perhaps for efficient fertilizer use.

Methane Emissions from Manure Management

Livestock manure can produce N2O emissions, as part of the nitrogen cycle through nitrification and denitrification of organic nitrogen compounds in manure and urine. The extent of N2O production depends on the composition of the manure and urine, types of bacteria involved in the process, moisture, and oxygen content in the manure management system. As mentioned, N2O is one of the intermediate products in the nitrification and denitrification process. The N2O emissions are most likely to occur in dry manure handling systems that have aerobic conditions for nitrification, but also contain pockets of anaerobic conditions, due to water saturation, for denitrification (USEPA, 2006a). The N2O emissions in the United States from manure management were 17.7 MMTCO2-Eq. in 2004; a 9% increase from 1990. The emission rates depend heavily on the population of the livestock and types of manure management systems (liquid vs. dry). Poultry (7.4 MMTCO2-Eq.), beef cattle (5.7 MMTCO2-Eq.), and dairy cattle (3.8 MMTCO2-Eq.) accounted for more than 90% of the total, 17.7 MMTCO2-Eq., while swine, horses, sheep, and goat made the balance (USEPA, 2006a).

The N2O emissions from manure management in California were 0.89 MMTCO2-Eq. in 2004, 2.7% of the state’s total nitrous oxide emissions

Technological Solutions

Technological options and practices include:
  • Optimizing the crude protein/energy ratio in animal diets Nitrification and urease inhibitors can be used to reduce N2O emissions from livestock manure.
  • Waste storage – A shift towards anaerobic storage rather than aerobic storage of manures may reduce N2O losses by a factor of 10
  • Use of cattle feed-pads during winter months – By keeping cattle on feed-pads during autumn/winter period, excretes can be collected and utilized as fertilizer later
  • Reducing the number of animals by increasing their productivity Optimizing manure management and limiting grazing

Methane Emissions from Field Burning of Agricultural Residues

Large quantities of agricultural crop residues are produced from farming activities. The crop types whose residues are typically burned in the United States are wheat, sugarcane, corn, barley, soybeans, peanuts, and rice.

Less than 5% of these residues are burned each year, with the exception of a significantly higher percentage for rice straw.

In the United States and California, nitrous oxide emissions from field burning of agricultural residues has been a very small fraction of the total nitrous oxide emissions,.

Technological Solutions

The mitigation options for reducing N2O emissions from agricultural residue include improved fire management practices, plowing under, or composting.

Methane Emissions from Energy

N2O emissions from mobile combustion in California account for 27.5% of the nationwide emissions in this sector.

Mobile combustion is the dominant contributor to N2O emissions in the energy sector in California, at 98.2%.

N2O emissions from mobile sources depend on characteristics of fuel, air-fuel ratios, combustion temperatures, maintenance and operation practices, and usage of pollution control equipment.

Emissions from mobile combustion are often grouped by transport mode (e.g., highway, air, rail), fuel type (e.g., motor gasoline, diesel fuel, jet fuel), and vehicle type (e.g., passenger cars, light-duty trucks). Road transport accounted for the majority of mobile source fuel consumption, and, hence, the majority of mobile combustion emissions.

Mobile combustion is the second largest source of N2O in the United States, at 11%.

HISOTRY LESSON: N2O emissions from vehicles have only recently been studied in detail and they mainly come from the catalytic converters. Present converters, using so-called three-way catalysts, are only designed to reduce emissions of ozone precursors such as volatile organic compounds (VOCs), CO, and nitrogen oxides (NOx), thereby increasing N2O emissions with respect to uncontrolled emissions (Lucas et al., 2006). Use of these catalytic converters resulted in an increase of N2O emission in the United States between 1990 and 1998. However, N2O emissions have subsequently declined from mobile sources as improvements in emission control technologies employed on new vehicles.

As a result, N2O emissions from mobile sources in 2004 were 1% lower than that in 1990 in the United States (USEPA, 2006a).

N2O Emissions from Mobile Sources in California and the USA


USA (2004)

California (2004)

Gasoline Highway 90.2% 31.0%
Diesel Highway 0.7% 69.0%
Non-Highway 8.6%
Ships and Boats, Locomotives, Farm Equipment, Construction Equipment, Aircraft

Technological Solutions

The technological options for reducing N2O emission from mobile sources include the following:
  • Improve catalytic converter performance
  • Use of N2O-decomposition catalyst – A future catalytic converter may consist of a traditional three-way catalyst (for NOx CO and VOC), followed by a N2O-decomposition catalyst. But there are technical obstacles to overcome.
  • Use of alternative technologies for NOx-emission reduction. Increased use of low-VOC and low-NOx engines may replace the traditional three-way catalyst controlled engines.
  • Alternative fuel – Technological breakthroughs, such as fuel cell, will also greatly reduce the level of NOx emissions. Fuel substitutes, such as use of hybrid, electric, ethanol, and natural gas vehicles, will also reduce N2O emissions.

Methane Emissions from Stationary Combustion

Stationary combustion includes all the combustion activities except waste incineration, transportation (mobile combustion), and biomass burning for non-energy purposes. For stationary sources, nitrous oxide may result from the incomplete combustion of fuels. In the USEPA GHG inventory report, the sectors for N2O emissions from stationary sources are categorized into five groups. The amounts of emissions from these five sub-sectors in 2004 were (USEPA, 2006a):
Electric power 9.4
Industrial 3.0
Commercial 0.3
Residential 0.8

Nitrous oxide emissions from stationary combustion are closely related to fuel types (coal, fuel oil, natural gas, or wood) and characteristics, combustion temperatures, and characteristics of pollution control equipment, and ambient environmental conditions. In general, lower combustion temperatures cause higher N2O emissions. Emissions also vary with operation and maintenance practices.

Technological Solutions

Technological options for emission reduction of N2O may be categorized into three groups:
  • Reduced emissions from fluidized bed combustion
  • Use of selective catalytic reduction
  • Fuel shift and reduction in fossil fuel consumption

Methane Emissions from Municipal Solid Waste Combustion

About 7 to 17% of the municipal solid wastes (MSW) in the United States are managed by combustion. Almost all combustion of MSW occurs at waste-to-energy facilities where energy is recovered, while N2O is a by-product of the combustion process.

Nitrous oxide emissions from this sector depend on a variety of factors, including types of waste as well as combustion temperature.

Nitrous oxide emissions from MSW combustion in California were a very small fraction of the state’s total N2O emissions.

Technological Solutions

The emission from this sector can be effectively reduced from source reduction, reuse, and recycling of municipal solid waste.

Methane Emissions from Industrial Processes

In California, nitric acid production is the only industrial process that generates a reportable amount of nitrous oxide emissions. Nitric acid (HNO3) is used in production of synthetic fertilizers, adipic acid, and explosives. The nitrous oxide emissions from nitric acid production in California were a very small fraction of the state’s total nitrous oxide emissions.

The viable technological options with further development could include High- and low temperature catalytic reduction; Non-selective catalytic reduction (NSCR); Thermal decomposition; Photo-catalytic conversion; and Biofiltration of off-gases using denitrifying bacteria

Methane Emissions from Solvent and Other Product Uses

Nitrous oxide is a clear, colorless, oxidizing gas with a slightly sweet odor. It is often used with oxygen in carrier gases to administer more potent inhalation anesthetics for general anesthesia and as an anesthetic in various dental and veterinary applications. It is also commonly used as a propellant in pressure and aerosol products with pressure-packaged whipped cream as the largest application. Small quantities of nitrous oxide are also used in the following applications:
  • Oxidizing agent and etchant used in semiconductor manufacturing
  • Oxidizing agent used, with acetylene, in atomic absorption spectrometry
  • Production of sodium azide, which is used to inflate airbags
  • Fuel oxidants in auto racing
  • Oxidizing agent in blow torches used by jewelers and others

Approximately 90% of the total production/usage in 2004 ended up in the atmosphere.

Production of N2O has stabilized since 1990 because medical industries have found other alternatives for anesthetics. In addition, more medical procedures are being performed on an out-patient based using local anesthetics (N2O is not required).

Use of N2O as a propellant for whipped cream has also stabilized due to the increase use of reusable plastic tubes in packaging of cream products (USEPA, 2006a).

Technological Solutions

No practical technological options for reducing nitrous oxide emissions from this sector were found from the literature search.

Methane Emissions from Land-use Change and Forestry

In California there are three relevant emission sources:
(1) forestland remaining forestland,
(2) settlement remaining settlement, and
(3) forest fires.

The category of Forest Land Remaining Forest Land refers to forest areas that have been forests for at least 20 years. Less than 1% of the fertilizers applied to soils in the United States are added to the forest soils. Application rates are similar to those for the cropped soils, but in any given year, only a small proportion of total forest receives fertilizer. This is because the forests are typically fertilized twice in their entire 40-year growth cycle.

Settlements Remaining Settlements refers to all classes of urban tree formations, focusing primarily on urban trees grown along streets, in gardens, and parks, in lands that have been in use as settlements. Of the fertilizers applied to soils in the United States, approximately 10 percent are applied to lawns, golf courses, and other landscaping occurring within the settled areas. Application rates are less than those on cropped soils, and, consequently, account for a smaller proportion of N2O emissions per unit area. The 2004 N2O emissions from this source were 15% higher than in 1990. The increase is due to a general increase in the application of synthetic fertilizer.

Nitrous oxide emissions from this sub-sector are not included in the most recent inventory report by CEC. No specific technological options for emission reduction were found from the literature search.

Methane Emissions from Waste

Waste management is one of the minor sources of N2O emissions. The emissions can come from domestic wastewater and industrial wastewater.

In contrast to methane emission reduction technologies, which are primarily focused on untreated wastewater and on-site small wastewater treatment plants, N2O reduction should be more focused on N2O emission from denitrification in large-scale, centralized plant. Under optimal operating conditions, N2O formation can be reduced by up to one-third during nitrification and two-thirds during denitrification

Technological Solutions

When nitrogen removal in wastewater treatment is not necessary and the application of wastewater sludge to agricultural land as a nitrogen source is allowable, the net N2O emission from wastewater sector may be reduced.

Nitrous oxide is an intermediate by-product of decomposition of organic nitrogen compounds, such as protein and urea, in industrial wastewater. N2O generation and emission mechanisms are not well understood. No specific technological options for emission reduction were found from the literature search.

High-GWP Gases

Hydrofluorocarbons (HFCs) and, to a lesser extent, perfluorocarbons (PFCs) are used as alternatives to several classes of ozone-depleting substances (ODS) that have been or are being phased out under the terms of the Montreal Protocol and the Clean Air Act Amendments of 1990; however, these compounds, along with sulfur hexafluoride (SF6), are potent greenhouse gases with high global warming potentials (GWP).

The GWPs of these gases range from 120 to 22,200 times the global warming capability of CO2 over a 100-year time horizon and they have long life spans in the atmosphere, in some cases for hundreds and thousands of years.

Emission sources of these high-GWP gases in the United States include HCFC-22 production, electrical transmission and distribution systems, semiconductor manufacturing, aluminum production, and magnesium production and processing.

Emissions of high-GWP gases in California account for approximately 9.9% of the nationwide emissions of high GWP-gases.

Substitution of Ozone Depleting Substances (ODS) is the dominant emission source of high-GWP gases in California and in the USA; 88.8% and 72.3% of total emissions, respectively.

Electrical transmission and distribution (7.2%) and semiconductor manufacture (4.0%) are the other two significant emission sources of high-GWP gases in California.

There are three significant differences between these high-GWP fluorinated compounds (HFCs, PFCs, and SF6) with the other major greenhouse gases (CO2, CH4, and N2O). Unlike other greenhouse gases, the fluorinated gases have few or no natural sources. Most of the end-uses of these fluorinated compounds are in enclosed systems so that their potential emissions can occur years after production and consumption, but emissions reductions through containment, recovery, and recapture are feasible. Most of these fluorinated compounds are for use in applications that use large amount of energy; thus, the energy efficiency becomes an important factor in considering options for their emission reductions

Substitution of Ozone Depleting Substances Use of HFCs has allowed the rapid phase-out of halons, chlorofluorocarbons (CFCs), and hydrochlorofluorocarbons (HCFCs). HFCs are generally selected for applications where they can provide superior reliability or safety (e.g., low toxicity and flammability). They are used in various industrial applications including the following (USEPA, 2001; USEPA, 2004; USEPA, 2006a):

  • Refrigeration and air conditioning equipment
  • Solvent cleaning
  • Foam production
  • Sterilization
  • Fire extinguishing
  • Aerosols
The end-use sectors that contribute most toward emissions of HFCs and PFCs as ODS substitutes in the United States include (USEPA, 2006a):
  • Refrigeration and air conditioning – 85.6%
  • Technical aerosols – 10.7%
  • Solvents – 1.5%
The increasing trend is expected to continue in the near term and will accelerate over the next decade as HCFCs, interim substitutes for CFCs in many applications, are themselves phased-out under the provisions of the Copenhagen Amendments to the Montreal Protocol.

Technological Solutions

Refrigerant options – Some “natural refrigerants”, such as hydrocarbons (HCs) or ammonia, with no or low GWP can be alternative substitutes. Carbon dioxide is another alternative

Alternative technologies – There are a number of alternative technologies mentioned in literature. Most of them are under evaluation and are not available as commercial products in a wide range soon; they include Joule-Brayton air cycle, Stirling cooling engines, Peltier coolers, thermo-acoustics, electro-osmosis, evaporative cooling, and thermoelectric refrigeration.

Distributed refrigeration puts refrigeration equipment closer to the food display cases they serve and reduces the excessive refrigerant piping (US Climate Change, 2005). Refrigerant charges for a distributed system can be smaller than that used in a comparable conventional direct expansion system. Reduced charge sizes and increased energy efficiency could effectively decrease global warming impacts.

In desiccant cooling, a desiccant removes air moisture and the dry air is then cooled. Desiccant cooling may replace the latent cooling by coolers and motor vehicle air conditioners. Desiccant is thermally regenerated. New automobiles are energy efficient and do not produce enough waste heat to regenerate desiccants; therefore, it is only feasible where there is a large heat source as in a truck or bus.

Leakage control – Reducing leaks, recovering and recycling refrigerants during servicing, and capturing, recycling, or destroying refrigerant at decommissioning of equipment have led to significant emission reductions in this sector.

Recovery and recycling – Recovery involves use of a device that transfer refrigerant to an external storage container prior to servicing of the equipment, and the recovered refrigerant may then be recharged back to the equipment, cleaned through the use of recycling devices, sent to a reclamation facility to be purified, or disposed of through incineration.

One method to reduce venting of refrigerant is to increase reclamation of waste refrigerant and properly dispose of the refrigerant that cannot be reclaimed

The US refrigeration and air-conditioning sector includes nine major end-uses:

  • Household refrigeration
  • Residential air-conditioning and heat pumps
  • Motor vehicle air-conditioning (MVAC)
  • Chillers
  • Retail food refrigeration
  • Cold storage warehouse
  • Refrigerated transport
  • Industrial process refrigeration
  • Commercial unitary air-conditioning systems

Technological Solutions

Although the potential of HFC emission from this end-use is relatively small, there are many technological options for emission reduction available. Hydrocarbons have the same good thermal properties as HFCs; however, the flammability of the hydrocarbons makes redesigns in the manufacturing process necessary. Since the handling of the refrigerant mainly takes place at the manufacturing site, the conversion to hydrocarbon in hermetic systems has proved easier than expected. Consequently, approximately 45% of new household refrigeration equipment uses hydrocarbon as the refrigerant in Europe. In Northern Europe, essentially all new appliances are charged with hydrocarbons.

Heat pumps use the cold vapor compression cycle to absorb heat at a low temperature level and transfer it to a higher temperature level. Most of these units are window units and central air conditioners. Carbon dioxide is under research as an alternative fluid for heat pumps (IEA, 2001). Smaller units for residential use with hydrocarbon are available from manufactures in Northern Europe (UNEP, 1998).

Leak repair and refrigerant recovery/recycling are considered as viable technological options for this end-use. Leak repair is much more cost effective than refrigerant recovery, delivering approximately $3 of benefits to $1.69 benefits for recovery, abased on similar capital costs.

Commercial chillers: Large capacity screw and centrifugal chillers account for over 150,000 units in the United States. Relative to most AC and refrigeration equipment, chillers are built for longer service lifetime. Leak repair is considered a viable technological option for emission reduction in this sector.

Refrigerated equipment found in food service such as supermarkets, convenience store, and restaurants. There are about 1.6 million retail food refrigeration systems in the United States.

Annual emission rates are estimated to fall in the range from 15 - 30% for direct expansion systems; new installations after 1998 can have loss rates of 3% if carefully designed and maintained.

Distributed refrigeration systems offer the ability to reduce the refrigerant charge and minimize the need for a dedicated mechanical room containing multiple compressor racks.

Alternative systems, such as using CO2, ammonia, hydrocarbons, or a combination of them as refrigerants, can be used, but the capital cost is high. Hydrocarbons and ammonia are best used in decentralized refrigeration units with secondary carrier loops. Because these systems isolate customers from the refrigerant, ammonia and hydrocarbons can be used.

Cold storage warehouses are used to store perishable goods such as meat, produce, and dairy products. There are about 2,000 cold storage warehouses in the United States. The technological options for reducing emissions are very similar to those for retail food refrigeration in principle, but with different levels of technical applicability.

Technical aerosol end-uses that contribute most toward emissions of HFCs as technical aerosols in the United States include:

  • Metered dose inhalers (MDIs) – treatment of asthma and chronic obstructive pulmonary disease, will account for one-third of all aerosol HFCequivalent emissions by 2010 (USEPA, 2001).
  • Consumer products – hairsprays, mousse, deodorants and anti-perspirants, household products, spray paints, and automotive products. Many of these products use HFCs to comply with regulations that reduce allowable VOC content.
  • Specialty products – Specialty aerosol end-uses include tire inflators, electronics cleaning products, dust removal, freeze spray, signaling devices, and mold release agents. HFCs are often used when flammability issues cannot be overcome.

Technological Solutions

Although hydrocarbons can be used as propellants in many commercial aerosols, they have not been found acceptable for use in MDIs (USEPA, 2004). Nitrogen is another alternative as the propellant (IEA, 2001). The main technological option for reducing HFCs from end-use of MDIs is dry powder inhalers (DPIs).

A trend is developing for novel oral treatment that would be swallowed, rather than inhaled. They may become available in the next 10 to 20 years, but they would not completely replace inhaled MDI therapy.

There are several technological options for reducing HFCs emissions from the non-MDI aerosol enduses, mainly as consumer products and specialty products:

  • Substitution with lower GWP HFCs – HFC-134a is often the propellant of choice for products that must be non-flammable, e.g., dust-removing agent for electronic equipment and long reaching insecticide products used on high-voltage power lines and transformers (IEA, 2001). Replacement of HFC-134a with a lower GWP HFC, such as HFC-152a which posses only moderate flammability hazards, will greatly reduce emissions from the aerosol sectors
  • Not-in-kind (NIK) alternatives – They include finger-trigger pumps, powder formulations, sticks, rollers, brushes, nebulizers, and bag-in-can/piston-can systems. They often prove to be better and more cost-effective than HFC-propelled aerosols
  • Hydrocarbon aerosol propellants – They are usually mixtures of propane, butane, and isobutane. Their costs are typically less than one-tenth that of HFCs. However, flammability and VOC emission are of major concern.
  • Compressed gases – Noninflammable gases including CO2, N2, compressed air, or even nitrous oxide can be used in aerosol applications.

High-GWP Gases from Solvents

HFCs, especially HFC-4310mee, and PFCs are used as solvents for various industrial cleaning applications, including precision, electronics, and metal cleaning. HFC emissions from the precision and electronics cleaning end-uses currently dominate the GWP-weighted emissions from this sector (UNEP, 1999b; USEPA, 2004). The emissions of these GWP gases from solvent uses account for ~1.5% of their emissions from the sector of ODS substitutes in the United States.

Technological Solutions

There are several technological options for reducing HFC/PFC emissions from solvent uses:
  • Improved equipment and cleaning processes using existing solvent – Better engineering control (e.g., increasing freeboard height, installing freeboard chillers, less drag-out losses, and using automatic hoists) and improved containment (e.g., better solvent bath enclosure and better vapor condensing systems) will minimize emissions and losses of existing solvents
  • Recycle and recovery – For cases where HFCs and PFCs continue to be used for performance reasons, the emission can be minimized from recycle and reuse. In many cases, the reduced costs of solvent disposal offset the purchase cost of solvent reclamation equipment
  • Not-in-kind (NIK) technologies – Aqueous and semi-aqueous NIK replacement options can displace HFC and PFC usage in some solvent applications. Aqueous cleaning uses a waterbased cleaning solution that often contains detergents. The products are then rinsed with water. Although the material costs are lower, the energy cost is often higher and subsequent wastewater treatment or disposal is needed. They have good cleaning ability, suppressed vapor pressure, and reduced evaporative loss; however, wastewater treatment or disposal is needed along with concerns of flammability and VOC emissions
  • Alternative solvents – Alternative organic solvents with lower GWPs, such as hydrofluoroethers (HFEs), hydrocarbons, alcohols, volatile methyl siloxanes, brominated solvents, non-ODS chlorinated solvents, are being used in electronics, metal, and some precision cleaning end-uses. HFE solvents are gaining acceptance in the U.S. industry due to their availability, safety, and effectiveness

Methane High-GWP Gases from Foams

Foams are used for thermal and sound insulation as well as cushioning. Various HFCs, such as HFC- 134a, HFC-152a, HFC-245fa, and HFC-365mfc, are used as the blowing agents during the manufacture of foams. These blowing agents might be emitted to the atmosphere during foam manufacturing or on-site foam application, while foams are in use, and when foams are disposed of. The foams can be categorized by the composition (e.g., polyurethane, polystyrene, or polyolefin), type of cell (open vs. closed), manufacturing process (spray vs. extrusion; thermoset vs. thermoplastic), and the properties (rigid vs. flexible).

Technological Solutions

Technological options to reduce HFC emissions from foams include the following
  • Alternative blowing agents – Hydrocarbons (propane, butane, pentane, and hexane), waterblown CO2, and liquid CO2 are alternatives to HFCs as the blowing agents. Due to the flammability, stringent safety precautions and specialized equipment may be required. In addition, most of these hydrocarbons are part of the VOC family, emission control may be needed and it will increase the cost of conversion. Some of the hydrocarbons may not yield comparable insulating values of HFCs, and, consequently, a thicker form may be required.
  • Lower-GWP HFC substitution – HFC-134a is the HFC that is most-commonly used in the foam industry, but some other lower GWP HFCs, such as HFC-245fa, are viable alternatives. However, choices of blowing agents are usually driven by performance and economic considerations
  • Alternative insulation materials and technologies – Alternative insulation technologies are available in some construction applications include fiberboard, fiberglass, and cellular glass. However, HFC foams, despite higher costs, are still used often because of superior properties in fire resistance, structural rigidity, moisture resistance, and insulation effectiveness
  • Direct emission reduction – Various direct emission reduction measures can be adopted in production, usage, and decommissioning of the foams. Examples include vapor capture at the “head”, use of low-permeability facing, recycle and recovery, and destruction by incineration

High-GWP Gases from Fire Extinguishing

HFCs (HFC-227ea, HFC-236fa, HFC-23) and perfluromethane (CF4) are the principal greenhouse gases emitted from the fire extinguishing systems. Basically there are two types of fire fighting systems: portable fire extinguishers and total flooding applications. Portable fire extinguishers are used widely; foam, water, CO2, or dry powder is commonly used. Market penetration of HFCs and PFCs in this sector is very limited

Technological Solutions

The technological options for reducing HFC and PFC emissions from the fire protection sector include the use of alternative fire protection agents and alternative technologies and practices.

Carbon dioxide has been used in total flooding systems for many years; however, safety standards regulate its use in occupied areas because lethal concentrations of carbon dioxide are needed during fire fighting. Water mist and inert gases systems are alternatives to some HFC uses in total flooding systems

Improved fire prevention technologies may reduce emissions of HFCs and PFCs. The viable options include early warning smoke detection, infrared cameras to distinguish real fires from false alarm, and technologies to reduce the amount of agent discharge to prevent a fire.

High-GWP Gases from Electrical Transmission and Distribution

Sulfur hexafluoride (SF6) is a colorless, odorless, non-toxic, and nonflammable gas, but it has a GWP that is 22,200 times that of CO2 for a 100-yr time horizon and a lifetime of 3,200 years in the atmosphere. It has been used by the electric power industry since the 1950s because of its dielectric strength and arc-quenching characteristics. The largest use of SF6 in the United States is as an electrical insulator and interrupter in equipment that transmits and distributes electricity. SF6 has replaced flammable insulating oils in many applications and allows for more compact substations in populated urban areas.

Typical applications of SF6 in high voltage technology include gas-insulated switchgear (GIS), circuit breakers, and gas insulated lines (GIL). SF6 can escape from gas-insulated substations and switchgear through gasket seals, flanges, and threaded fittings, especially from older equipment. The gas can also be released during equipment manufacturing, installation, servicing, SF6 analysis, and disposal. Emissions of SF6 from electrical transmission and distribution were 13.8 MMTCO2-Eq. in 2004 in the United States. It represents a 52% decrease from 1990. Several plausible reasons for the decrease, including (1) increases of price of SF6 during 1990s, (2) growing awareness of the environmental impact of SF6 emissions, and (3) programs such as EPA’s SF6 Emission Reduction Partnership for Electric Power System.

The 2004 high GWP gases emissions from electrical transmission and distribution in California were 7.2% of the state’s total high GWP gases emissions.

Technological Solutions

The technological options for reducing SF6 emissions from electric power transmission and distribution can be grouped into the following categories:
  • Equipment replacement/refurbishment – Equipment replacement/refurbishment addresses the need, when the leakage losses are large and beyond the LDAR-based repairs. 50% or more of all emissions from older equipment could be avoided if the equipment were replaced by new “tighter” unit
  • Use of recycling equipment – Recycling equipment can capture and recycle SF6 gas during equipment maintenance and retirement. It was estimated that recycling could reduce 10% of total SF6 emissions from the electric power systems in the U.S.
  • Leak detection and repair (LDAR) – This option aims to identify and reduce the SF6 leakage from the gas-insulated equipment. The leak detection is accomplished through various techniques, including sniffing with SF6 gas sensors and using a laser-based remote sensing technology. It was estimated that the practice could reduce 20% of total SF6 emissions from this sector in the U.S.

High-GWP Gases from Semiconductor Manufacture

Several long-life fluorinated gases (CF4, C2F6, C3F8, C4F8, HFC-23, NF3, and SF6) are currently used in the semiconductor industry. A fraction of these gases is emitted from two frequently used manufacturing processes to produce semi-conductor products: plasma etching of thin films and cleaning of plasma-enhanced chemical vapor deposition (PECVD) chambers. Chemical vapor deposition (CVD) chamber cleaning is estimated to account for 80% of the emissions from semiconductor manufacturing; while the etching process is estimated to account for 20% (USEPA, 2006b).

Technological Solutions

The technological options that can reduce HFCs, PFCs, and SF6 from semiconductor manufacturing can be grouped into the following categories:
  • NF3 remote clean technology – Perfluoroethane (C2H6) has been the primary CVD chamber clean gas in the semiconductor manufacturing processes. Two basic NF3 clean technologies are available as alternatives.
  • C3F8 replacement – C3F8 has a smaller 100-yr global warming potential than C2F6 (7,000 vs. 9,200) and a much shorter atmospheric lifetime (2,600 vs. 10,000 years). In addition, C3F8 is more efficiently used or consumed during the CVD chamber cleaning process than C2F6. It is assumed that the C3F8 replacement would yield an emission reduction efficiency of 85%, and this option should be applicable to all fabrication facilities
  • Point-of-use (POU) plasma abatement system – The system is to reduce emissions from the etching process and it should be applicable to all fabrication facilities
  • Thermal destruction/thermal processing units (TPU) – This can be used to reduce PFC emissions from both the etching and the CVD chamber cleaning processes and should be applicable to all fabrication facilities
  • Catalytic decomposition system – The catalytic systems are very similar to the thermal destruction units, but operate at lower temperatures because the presence of the catalyst. They produce little or no NOx emissions and demand lower volumes of cooling water (Applied Materials, 1999). It was reported that the Hitachi system using catalytic oxidation technology can achieve more than 99% destruction efficiency for CF4, C2F6, C4F8, and SF6
  • Facility-wide solutions – These solutions include PFC capture/recovery and process optimization. The capture/recovery membrane can be used to reduce PFC emissions from both the etching and the CVD chamber cleaning processes and should be applicable to all fabrication facilities.

Black Carbon

Black carbon aerosols are particulates formed by incomplete combustion of carbon-containing fuels. They have been identified as having potentially significant impacts on climate change, especially at a regional scale. Black carbon is generally considered to have both a direct warming effect by absorbing the incoming solar irradiation in the atmosphere and an additional warming effect by reducing the albedo (reflectivity) of snow and ice. Black carbon is the main light-absorbing component of soot.

Some believe that control of fossil fuel soot (BC + OC) may be the fastest way of slowing warming for a specific period.

Black carbon emissions are seldom quantified, so data on the emission rates of black carbon are scarce in literature.

US EPA developed inventories for BC as part of the National Ambient Air Quality Standards (NAAQS) Regulation Impact Analyses (USEPA, 2006).

The emission rate of PM2.5 EC from the South Coast Air Basin in September of 1996 is 7.31 tons/day, comparable to that from the San Joaquin Valley. However, the relative contributions from various sectors are different; they are

  • light-duty industrial diesel equipment (45%)
  • on-road vehicles (27%)
  • industrial processes (8%)
  • trains (6%)
  • fuel combustion (4%),
  • paved road dust (4%)

Mobile sources account for more than 50% of BC emissions in the United States.

Diesel engines result in more black carbon emissions than gasoline engines. The best gasoline vehicles in the United States emit less black carbon than the best diesel vehicle can, even with a particle trap

In the United States, on-road and non-road diesel vehicles are the largest sources of BC emissions.

One estimate on BC emissions from a light-duty diesel vehicle is 4 mg/mile from exhaust, 1 mg/mi from brakes, and 3 mg/mi from tire wear.

The U.S. EPA has regulated emissions of particulate matter (PM) from on-road mobile sources for many years. The PM emissions from properly operating gasoline automobiles have decreased from 300 mg/mile to 1 mg/mile from 1970 to 2004; while those from diesel vehicles have decreased from about 2,500 mg/mile to 20 mg/mile.

Since mobile sources, especially those associate with diesel, are responsible for most of the BC emissions, most technological options for BC emission reduction found from the literature search are for diesel vehicles and engines. Basically, BC is removed in the process that is mainly aimed for removal of particulate matter.

Technological Solutions

To date, most of the diesel PM reduction efforts have been focused on either new engine replacements or retrofitting engines with post-combustion emission control. Specific technological options to reduce BC emissions from mobile sources include:
  • Diesel particle filters (DPFs) – DPFs, also known as “trap”, remove PM from the diesel exhaust through physical filtration.
  • Catalyst-based DPFs – The added catalyst effectively lowers the temperature required for regeneration of the filters. The catalyst can be poisoned by sulfur; therefore, this type of DPFs can only be used with diesel fuel of ultra low sulfur content
  • Diesel oxidation catalysts (DOCs) – DOCs use the same type of catalyst material as that in the catalyst-based DPFs, but applied to a flow-through monolith, without the physical filter
  • Closed crankcase emissions filtration device – In many diesel engines, crankcase emissions are released from the engine through the “road draft tube” without passing through the exhaust system. A closed crankcase filtration device, by rerouting crankcase ventilation back to the engine, can be fitted to school buses and eliminate these emissions
  • Alternatives to diesel – Biodiesel is mono-alkyl ester oxygenated fuel made from vegetable (soybean or canola) or animal fats. It has been demonstrated that using biodiesel can reduce emissions of particulate matter
  • Engine modifications – Particulates emissions can also be reduced through improvements to the basic engine such as turbo-charging, after-cooling, high-pressure fuel injection, retarding injection timing, and optimizing combustion chamber design
  • Proper maintenance of diesel engines
  • Reduce idling of diesel engines
  • Replace gas lawn mowers with electrical mowers
  • Reduce fuel consumption
  • Reduce vehicle use

Stationary Sources

The stationary sources of black carbon emissions include:
  • Open burning (wildfires, prescribed burning, agricultural field burning, land clearing, other waste burning)
  • Residential combustion (wood burning, yard waste burning)
  • Utility combustion
  • Industrial combustion
  • Commercial combustion
  • Incineration
  • Fugitive dust
  • Livestock
Biomass burning accounts for approximately 25% of BC emissions in the United States

Technological Solutions

Specific technological options to reduce BC emissions from stationary sources include the following:
  • Mitigation measures for diesels – diesel particulate filters on diesel-fueled compression-ignition engines can achieve up to 90% reduction in fine particulate matter. Other measures such as engine modification, alternative fuels, reducing idle time, and proper maintenance should also reduce BC emissions.
  • PM control measures for area sources – Specific controls exist for stationary area sources, including catalytic oxidizers on conveyorized char-broilers at restaurants that can reduce PM emissions by 80%. Also, replace older woodstoves with those using the New Source Performance Standard (NSPS) for residential wood combustion.
  • Apply the end-of pipe control on utility and non-energy generating utilities (non-EGU) point sources – Use ESPs, bag houses, or wet scrubbers for particulate removal. Upgrade the existing systems to better remove finer particles may be needed: one example is to add more collector plates in an ESP system to increase its removal efficiency (USEPA, 2006).
  • Alternatives to open biomass burning – change the frequency and conditions of prescribed burning and reducing open waste burning


Intergovernmental Panel on Climate Change - IPCC (2003) “Good Practice Guidance for Land Use, Land-use Change, and Forestry”, approved in November, 2003.

U.S. Environmental Protection Agency (2006) “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 to 2004”, Office of Atmospheric Programs, United States Environmental Protection Agency, EPA-430-R-06-002, June 2006.

California Energy Commission (2006) “Inventory of California Greenhouse Gas Emissions and Sinks: 1990 to 2004”, final staff report, CEC-600-2006-013, December 22, 2006.

Edited by Carolyn Allen, owner/editor of California Green Solutions
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