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
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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.
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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
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EPA/600/SR-95/105
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