United States Environmental Protection Agency National Kisk Management Research Laboratory Cincinnati, OH 45268 Research and Development EPA/600/SR-95/105 February 1996 & EPA Project Summary Characterization of Nitrous Oxide Emission Sources Rebecca L. Peer, Eric P. Epner, and Richard S. Billings Nitrous oxide (N2O) is both a green- house gas and a precursor of nitric oxide (NO) which destroys strato- spheric ozone. This study presents a global N2O inventory based on re-evalu- ation of previous estimates and addi- tions of previously uninventoried source categories. The best estimate of anthropogenic N2O is 5.7 teragrams per year (Tg/yr), which is still much lower than natural source emissions. Much uncertainty remains about esti- mates for many source categories. In- adequate data are available for some categories, while others are limited by a lack of reliable functional models of factors affecting emission rates. This Project Summary was developed by the National Risk Management Re- search Laboratory's Air Pollution Pre- vention and Control Division, Research Triangle Park, NC, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction Nitrous Oxide (N2O) is the largest source of stratospheric nitric oxide (NO), which destroys stratospheric ozone, and the fourth most significant greenhouse gas, which contributes to global warming. Lev- els of atmospheric N2O are increasing by about 0.2% per year. Previous studies have attempted to quantify the global N2O budget for the larger sources. Many of these estimates were based on limited data and fairly broad assumptions. The objectives of this study were to re-evaluate previous estimates using new data and more refined ap- proaches, and to quantify emissions from some of the smaller source categories. In addition, country-specific estimates were developed for the anthropogenic sources. For a few categories, new data were gath- ered from industry and the government. The information contained in this report is based on original calculations and analy- ses of existing data. In addition, individu- als recognized as experts in various fields were contacted, and the information they provided was either used directly or to identify literature references not already reviewed. For the most part, the revised emissions estimates presented in this re- port are based on a re-evaluation of previ- ously published data, but new data are included in a few cases. Results Fourteen source categories, classified as anthropogenic and biogenic (or natu- ral), were identified as potentially substan- tial contributors of global N2O. These source categories are presented in Table 1, along with their estimated N2O emis- sions and, where possible, an estimated emissions range. Natural sources were found to be by far the largest emitters of N2O. Although anthropogenic sources emit smaller amounts of N2O, these sources can be controlled to some extent. The overall result of this study was to revise the previous best estimate of N2O emissions from all anthropogenic sources of 8 to 5.7 Tg/yr. However, this lower estimate is still within the 5-10 Tg/yr previ- ous best estimate range. Estimates of N2O emissions associated with adipic acid production were reduced ------- Table 1. Summary of Global N2O Emissions by Source Category Source Category N2O Emissions (Tg/yr)a Anthropogenic Sources Biomass Burningb Aquifer Contamination11 Municipal Wastewater Stationary Combustion Sources Livestock (agricultural) Adipic Acid (Nylon) Production Fertilizer Use Climatic Feedbacks'1 Mobile Combustion Sources Nitric Acid Production Municipal Waste Combustion Sewage Sludge Incineration Total 1.5 (1.3-1.6) 1.1 (0.7-1.5) 0.7(0.2-1.2) 0.5 (0.3-0.9) 0.5 (0.3-1) 0.4 (NA) 0.3(0.1-1.9) 0.3 (0.0-1.0) 0.2 (NA) 0.2 (0.07-0.30) 0.012 (0.012-0.013) 0.005 (NA) 5.7 Natural Sources Soils Oceans6 Total 17.3(11.2-21.9) 3.3 (2.2-4.4) 20.6 " Variability or range of estimated emissions is shown in parantheses; in some cases (shown as NA), available data were not sufficient to estimate a range. Lack of a range does not imply greater certainty in the estimate. * Estimates calculated by other than preparers of this report by 40% based on new data from U.S. facilities. Previous estimates may not have included the effects of emissions controls now in place at many of these facilities. Emisons from mobile sources may also have been overestimated by using mea- surements from automobiles in California. California (and the U.S. in general) have a greater proportion of controlled vehicles, which in some cases emit more N2O than uncontrolled vehicles. Thus, the new esti- mates for this category are also much lower. Average emissions estimates for sta- tionary coal combustion and fertilizer use were also reduced. New emission factors and 95% confidence intervals were devel- oped for these categories. The estimates for municipal wastewater emissions in this report are lower than previous estimates, which assumed that municipal wastewa- ter systems are universal. In fact, a large population uses other methods of waste disposal, most of which do not result in conditions conducive to N2O formation. N2O emissions from nitric acid produc- tion, municipal solid waste (MSW) com- bustion, and sewage sludge incinerators are quantified here for the first time. MSW combustors and sludge incinerators are both relatively small sources. However, they may become important in the future as waste management practices change. The largest anthropogenic source of N2O is biomass burning, accounting for about 27% of global N2O emissions. However, given the uncertainty of the estimates for most catgories, the relative importance of the anthropogenic source categories is dif- ficult to specify. Recommendations for improving future emissions inventories are included in this study. Estimates for many source catego- ries could benefit from more field studies to obtain better emission factors and ac- tivity data. Some of these source catego- ries, such as municipal wastewater and aquifer contamination, are potentially large contributors to global N2O emissions. For some categories, such as fertilizer use, obtaining more data is unlikely to signifi- cantly reduce the range of possible emis- sions. This will be accomplished only by identifying the main factors affecting emis- sion rates and developing functional rela- tionships. This, in turn, will require devel- opment of better activity data sets that quantify the key variables. ------- R. Peer and R. Billings are with Radian Corp., Morrisville, NC 27560; and E. Epner is with Radian Corp., Rochester, NY 14623. Lee L. Beck is the EPA Project Officer (see below). The complete report, entitled "Characterization of Nitrous Oxide Emission Sources," (Order No. PB95-246252; Cost: $27.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Air Pollution Prevention and Control Division National Risk Management Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency National Risk Management Research Laboratory (G-72) Cincinnati, OH 45268 Official Business Penalty for Private Use $300 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 EPA/600/SR-95/105 ------- |