Nitrous Oxide Emissions: Sources, Global Warming Effects, and Mitigation Strategies

Nitrous Oxide Emissions: Sources, Global Warming Effects, and Mitigation Strategies 

Nitrous oxide (N2O) made up about 5% of U.S. anthropogenic greenhouse gas emissions in 2013. This makes it the third most abundant greenhouse gas after CO2 and methane. The source of 74% of those emissions was “agriculture and soil management” according to the EPA. 5% of the emissions were sourced by “manure management.” That makes nearly 80% sourced from the agriculture sector. Industry, transportation, chemical production, and stationary combustion make up about 16%. The EPA also notes that N2O has an avg. staying time in the atmosphere of 114 years (compared to about 10 years for methane). This gives it 300 times the warming power in weight equivalence to CO2. About 40% of global N2O emissions are thought to derive from human activities. Since the Industrial Revolution nitrous oxide concentrations in the atmosphere have risen by about 15%. Variations in naturally emitted N2O were not addressed in the EPA report.

N2O is the same gas used as a dental anesthetic (so-called laughing gas), an oxidation agent, and a food additive. Nitrous oxide is distinct from nitric oxide (NO) and nitrogen dioxide (NO2), but all three are produced during reactions from combustion. NO, NO2, and N2O react to form smog, acid rain, and tropospheric ozone, or ground level ozone, none of which are desirable.

  

One issue I found annoying in the EPA report was the section on – Emissions and Trends – where they stated that there was an 8% increase in emissions since 1990 (from the graph it looked like it increased about 8% from 1990 to 1991). Technically this true but emissions since 1991 have been close to flat overall. Presentation of data and statistics should avoid being misleading, if possible. Emissions of N2O are projected to rise 5% by 2020.

The transportation sector makes up about two-thirds of non-agricultural N2O emissions. Stationary combustion from coal and gas power plants makes up a much smaller amount of N2O emissions as does biomass burning. An even smaller amount is released in nitrogen fertilizer manufacture. Domestic waste water treatment is another minor source.

Mitigation Strategies

Under-utilized nitrogen-based synthetic fertilizer is the biggest source of atmospheric N2O. Mitigation strategies such as organic farming could theoretically help but crop yields would be reduced and the use of manure-based fertilizer would increase, also increasing N2O emissions from manure management. More land use would also be required to make up for the decrease in crop yields resulting in reduced carbon sink potential. Better management and more efficient use of synthetic nitrogen-based fertilizer is perhaps a better mitigation strategy. This could also decrease fertilizer runoff which is a serious problem around the world as nitrogen and particularly phosphorous runoff into bodies of water is the main source of dangerous algae blooms, red tides, and de-oxygenated dead zones where rivers meet seas.

In Crop Farming

Nitrogen (N) from fertilizer, whether synthetic or organic (typically manure) is often mobile. Synthetic fertilizer often has N in inorganic form which is more readily available to plants. Organic fertilizer contains organic N that converts to inorganic N over time. N can be lost as nitrate to groundwater or in gaseous form as nitrous oxide (N2O), dinitrogen (N2), or ammonia (NH4). It is typical that about half of applied fertilizer is taken up by the crops for which it is destined. Soil microbes produce the N2O from the N during both aerobic nitrification and anaerobic de-nitrification. The anaerobic process is thought to make the most N2O. Thus one important mitigation strategy is simply to try to reduce the amount of waterlogged soils where anaerobic microbial functions can occur. Strategies to reduce N2O formation involve avoiding the formation of inorganic N by basically using the N by increasing the NUE, or N use efficiency. By tweaking the application rate, fertilizer formulation, timing of application, and placement, the N2O produced can be reduced. Rate of application depends on the crop as different crops take up fertilizer at different rates. Formulation can also depend on crops – whether to use anhydrous ammonia or urea ammonium nitrate. Additives can also reduce some N2O emissions by inhibiting nitrification.  Timing of application can be tweaked to when it is most readily taken up by the plants. Adding fertilizer in the fall or spreading manure on frozen fields can lead to big nitrate and N2O losses. Placement may involve concentrating the fertilizer nearest the plant roots where it is needed rather than spreading it across the fields. Carbon reduction credits as incentives are also a potential reward of targeting fertilizer to reduce N2O emissions.

In Automobiles

In automobiles N2O emissions can be reduced by lowering the operating temperature of the engine through exhaust heat recirculation which employs the exhaust gas recirculation (EGR) valve to recirculate part of the hot exhaust gases to perform other functions, several of which can help power the hybrid batteries, keep the engine and fuel warm, help warm the interior, and improve gas mileage, all while reducing N2O emissions. This technology is used extensively in hybrid vehicles to help charge the Lithium batteries. 

Simply increasing MPG in vehicles to reduce overall fuel consumption will decrease N2O emissions. Catalytic converters and other pollution control technologies can also reduce N2O emissions.

In Dairy Farming

Cows fed on grass release more urea in urine than in dung so mitigation strategies can involve helping cows to have more efficient digestion. Applying nitrification inhibitors as a spray to fields where cows pee can reduce nitrous oxide emissions from urine patches by 60-90%. The sprays also tend to increase nitrogen availability and thus fertility of the soils. Avoiding of grazing on wet soils can trigger less anaerobic N2O production. Better soil drainage, improved irrigation management, and effluent management (applying effluent dry rather than wet) are other strategies that can reduce N2O emissions. 

In Industry

In the power generation industry one simple way to reduce N2O emissions is to switch fuels from coal to natural gas since natural gas produces far less when burned than coal. Natural gas power plants emit 7% of the nitrogen oxides (NO, NO2, N2O) emitted by coal plants so that is a pretty dramatic difference. In manufacture of nitrogen fertilizer some fiber materials such as nylon, N2O is emitted in the production of nitric acid for fertilizers and adipic acid for making materials. EPA lists “technological upgrades” as a means to decrease emissions in these industries, which may involve capturing and reusing the gas.    

References: 

Overview of Greenhouse Gases: Nitrous Oxide Emissions – U.S. EPA (www3.epa.gov)

Global Mitigation of Non-CO2 Greenhouse Gases, 2010-2030 – U.S. EPA, EPA-430-R-13-011, September 2013

Mitigation of Non-CO2 Greenhouse Gases in the United States: 2010 to 2030 – U.S. EPA, EPA-430-S1-4-002, April 2014

Management of Nitrogen Fertilizer to Reduce Nitrous Oxide (N2O) Emissions From Field Crops -  by Neville Millar, Julie E. Doll, and G. Phillip Robertson, Michigan State University Extension Bulletin E3152, November 2014 

How Exhaust Heat Recovery and Recirculation Works – by Christopher Lampton – Auto/Hybrid Technology, at howstuffworks.com

Reducing Nitrous Oxide: Options for Reducing Nitrous Oxide Emissions from Dairy Farms, at dairyaustralia.com.au

Emissions of Greenhouse Gases in the U.S. – U.S. Energy Information Administration (EIA), March 31, 2011

What Are the Main Sources of Nitrous Oxide Emissions? -  from whatsyourimpact.org

Switch to Gas Slashed Power-Plant Emissions, Study Finds – article by Douglas Fischer, in the Daily Climate, Jan. 10, 2014