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

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