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4 Nitrous Oxide Emissions
Overview
Estimated U.S. anthropogenic nitrous oxide emissions totaled 1.2 million
metric tons in 2005, or 366.6 million metric tons carbon dioxide equivalent
(MMTCO2e), 1.9 percent more than in 2004 and 9.9 percent above 1990 levels
(Table 25). The 2005 total for nitrous oxide emissions represents 5.1 percent
of all U.S. greenhouse gas emissions for the year. Most of the increase
in U.S. nitrous oxide emissions for 2005 can be attributed to emissions
from agricultural sources, which increased by 7.0 MMTCO2e.
A downward trend in U.S. nitrous oxide emissions that began in 1995, after
emissions of nitrous oxide peaked at 374.5 MMTCO2e in 1994, ended in 2003.
Over the past 2 years, nitrous oxide emissions have increased. In 2004, annual
U.S. emissions of nitrous oxide (359.9 MMTCO2e) were higher than their
1990 level (333.5 MMTCO2e) for the first time since 2001, and in 2005 they
were only 2.1 percent below their 1994 peak value.
Sources of U.S. nitrous oxide emissions include energy use, agriculture,
waste management, and industrial processes. The largest component of U.S.
anthropogenic nitrous oxide emissions is emissions from agricultural activities,
at 279.9 MMTCO2e or 76 percent of total nitrous oxide emissions in 2005.
Nitrogen fertilization of agricultural soils represents 78 percent of emissions
from agricultural activities. Most of the remainder (22 percent) is from
the handling of animal waste in managed systems. Small quantities of nitrous
oxide (0.2 percent of nitrous oxide emissions from agricultural activities)
are also released from the burning of crop residues. Estimated emissions
of nitrous oxide from agricultural sources in 2005 were 2.6 percent above
2004 levels and 12 percent above 1990 levels (Figure 3).
The second-largest source of anthropogenic nitrous oxide emissions is energy
consumption, which includes mobile source combustion from passenger cars,
buses, motorcycles, and trucks; and stationary source combustion from commercial,
residential, industrial, and electric power sector energy use. Energy use
was responsible for 67.3 MMTCO2e of nitrous oxide emissions in 2005 (18
percent of total U.S. nitrous oxide emissions). The 2005 level of emissions
from energy sources is 0.6 percent higher than the 2004 level and 33 percent
higher than in 1990.
Industrial production of adipic and nitric acid, which releases nitrous
oxide as a byproduct, accounted for emissions of 13.2 MMTCO2e in 2005 (3.6
percent of total U.S. nitrous oxide emissions), 5.3 percent lower than
the 2004 level and 54 percent lower than in 1990. The large decline in
emissions of nitrous oxide from adipic acid production since 1990 is a
result of the continuing utilization of emissions control technology at
three of the four adipic acid plants operating in the United States.
Nitrous oxide emissions from activities related to waste management in
2005 totaled 6.2 MMTCO2e, or 1.7 percent of all U.S. anthropogenic nitrous
oxide emissions (Table 25). During 2005, emissions from human sewage in
wastewater accounted for 94 percent of estimated nitrous oxide emissions
from this source, and the remainder was associated with waste combustion.
Agriculture
Nitrous oxide emissions from agricultural activities increased by 7.0 MMTCO2e
(2.6 percent) in 2005 to a total of 279.9 MMTCO2e, compared with 272.9 MMTCO2e
in 2004. Since 1990, nitrous oxide emissions from agricultural activities
have increased by 12.3 percent. Agricultural activities were responsible
for 76 percent of U.S. nitrous oxide emissions in 2005, smaller than the
86-percent share that agricultural practices contribute to nitrous oxide
emissions globally.63 Nitrous oxide emissions from agricultural activities
represent 3.9 percent of total U.S. greenhouse gas emissions.
Nitrogen fertilization of agricultural soils accounted for 78 percent of
U.S. agricultural emissions of nitrous oxide in 2005. Nearly all the remaining
agricultural emissions (22 percent) can be traced to the management of
the solid waste of domesticated animals. The disposal of crop residues
by burning also produces nitrous oxide that is released into the atmosphere;
however, the amount is relatively minor, at 0.6 MMTCO2e or 0.2 percent
of total U.S. emissions of nitrous oxide from agricultural sources in 2005.
Nitrogen Fertilization of Agricultural Soils
EIA estimates that 218.1 MMTCO2e of nitrous oxide was released into the
atmosphere as a result of direct and indirect emissions associated with
fertilization practices in 2005 (Table 26). Estimated emissions increased
by 2.9 percent compared with 2004 levels and were 16.7 percent higher than
in 1990.
Nitrogen uptake and nitrous oxide emissions occur naturally as a result
of nitrification and denitrification processes in soil and crops, generally
through bacterial action. When nitrogen compounds are added to the soil,
bacterial action is stimulated, and emissions generally increase, unless
the application precisely matches plant uptake and soil capture.64 Nitrogen
may be added to the soil by synthetic or organic fertilizers, nitrogen-fixing
crops, and crop residues. Nitrogen-rich soils, called “histosols,” may
also stimulate emissions. Direct emissions in 2005 (171.0 MMTCO2e) represented
78 percent of total emissions from nitrogen fertilization, with the primary
components including the biological fixation of nitrogen in crops (70.7
MMTCO2e), nitrogen fertilizers (58.6 MMTCO2e), and crop residues (37.3
MMTCO2e).
Indirect emissions from nitrogen fertilization result from adding excess
nitrogen to the soil, which in turn enriches ground and surface waters,
such as rivers and streams, and results in emissions of nitrous oxide.
This source is referred to as “soil leaching.” Additional indirect emissions
occur from “atmospheric deposition,” in which soils emit other nitrogen
compounds that react to form nitrous oxide in the atmosphere. Indirect
emissions in 2005 (47.1 MMTCO2e) represented 22 percent of total emissions
from nitrogen fertilization, with soil leaching accounting for 40.0 MMTCO2e
and atmospheric deposition totaling 7.1 MMTCO2e.
There are significant uncertainties associated with estimating the amount
of emissions produced by adding nitrogen to agricultural soils. Models
used to estimate
the amount are based on limited sources of experimental data.65 The uncertainty
increases when moving from emissions associated with animal manure to soil
mineralization and atmospheric deposition, where both estimating the amount
of emissions and segmenting anthropogenic from biogenic sources become
increasingly difficult.
Solid Waste of Domesticated Animals
Estimated 2005 nitrous oxide emissions from animal waste management were
61.2 MMTCO2e, up by 1.4 percent from 2004 levels but 1.2 percent lower
than 1990 levels (Table 27), making animal waste management the second-largest
U.S. agricultural source of nitrous oxide emissions, after nitrogen fertilization
of soils. Nitrous oxide emissions from animal waste are dominated by emissions
from cattle waste, which in 2005 accounted for 92 percent of emissions
from the solid waste of domesticated animals (a total of 56.4 MMTCO2e in
2005).
Nitrous oxide is released as part of the microbial denitrification of animal
manure. The total volume of nitrous oxide emissions is a function of animal
size and manure production, the amount of nitrogen in the animal waste,
and the method of managing the animal waste. Waste managed by a solid storage
or pasture range method may emit 20 times more nitrous oxide per unit of
nitrogen content than does waste managed in anaerobic lagoon and liquid
systems. Generally, solid waste from feedlot beef cattle is managed with
the solid storage or pasture range method, accounting for the majority
of nitrous oxide emissions. Solid waste from swine is generally managed
in anaerobic lagoons and other liquid systems. Anaerobic digestion yields
methane emissions but only negligible amounts of nitrous oxide. Thus, changes
in estimated emissions result primarily from changes in cattle populations.
For example, beef cattle populations grew during the first half of the
1990s, leading to higher emissions through 1995, but then declined slowly
through 2004, lowering emissions to below 1990 levels. In 2005, U.S. cattle
populations increased slightly from their 2004 level.
Crop Residue Burning
In 2005, estimated emissions of nitrous oxide from crop residue burning
were 0.6 MMTCO2e, 3.0 percent below the 2004 level (Table 25). The decrease
is attributable to a nearly across-the-board decrease in U.S. crop production.
Emissions from this source remain very small, at 0.2 percent of all U.S.
nitrous oxide emissions. When crop residues are burned, the incomplete
combustion of agricultural waste results in the production of nitrous oxide,
as well as methane (discussed in Chapter 3).
Energy Use
The energy use category includes nitrous oxide emissions from both mobile
and stationary sources as byproducts of fuel combustion. Estimated 2005
energy-related emissions were 67.3 MMTCO2e, or 18 percent of total U.S.
anthropogenic nitrous oxide emissions (Table 25). Emissions from energy
use are dominated by mobile combustion (78 percent of nitrous oxide emissions
from energy use in 2005).
Mobile Combustion
Nitrous oxide emissions from mobile source combustion in 2005 were 52.6
MMTCO2e, an increase of 1.0 percent from the 2004 level of 52.0 MMTCO2e
(Table 28). In addition to emissions from passenger cars and light-duty
trucks, emissions from air, rail, and marine transportation and from farm
and construction equipment are also included in the estimates. Motor vehicles,
however, are the predominant source, accounting for 92 percent of nitrous
oxide emissions from mobile combustion.
Nitrous oxide emissions from motor vehicles are caused primarily by the
conversion of nitrogen oxides (NOx) into nitrous oxide (N2O) by vehicle
catalytic converters. The normal operating temperature of catalytic converters
is high enough to cause the thermal decomposition of nitrous oxide. Consequently,
it is probable that nitrous oxide emissions result primarily from “cold
starts” of motor vehicles and from catalytic converters that are defective
or operating under abnormal conditions. This implies that the primary determinant
of the level of emissions is motor vehicle operating conditions; however,
different types of catalytic converters appear to differ systematically
in their emissions, and emissions probably vary with engine size. Thus,
emissions also depend on the “mix” of vehicle age and type on the road.
Nitrous oxide emissions from mobile sources grew rapidly from 1990 to 1996
due to increasing motor vehicle use, the shifting composition of the light-duty
vehicle fleet toward light trucks that have lower fuel economy and higher
per-mile emission factors, and the gradual replacement of low emitting
pre-1983 vehicles that did not use catalytic converters with higher emitting
post-1983 vehicles that do use catalytic converters. This growth moderated
between 1996 and 1999 due to the introduction of more advanced, lower-emitting
catalytic converters. After peaking in 1999, U.S. emissions of nitrous
oxide from mobile sources declined slowly through 2002, as vehicle turnover
led to a fleet dominated by the more advanced catalytic converters. Since
2002, emissions have increased as both the number of motor vehicles on
U.S. roadways and emissions from other mobile sources have grown.
Stationary Combustion
In 2005, estimated nitrous oxide emissions from stationary combustion sources
were 14.7 MMTCO2e, 1.1 percent (0.2 MMTCO2e) lower than in 2004 and 9.9
percent (1.3 MMTCO2e) higher than in 1990 (Table 29). The increase in emissions
from this source from 1990 to 2005 can be attributed principally to coal-fired
combustion systems. Nitrous oxide emissions from coal-fired combustion
systems increased by 19 percent over the period, from 8.0 MMTCO2e in 1990
to 9.5 MMTCO2e in 2005.
Coal-fired combustion systems produced 65 percent of the 2005 emissions
of nitrous oxide from stationary combustion. Other fuels—including fuel
oil (2.3 MMTCO2e), wood (2.2 MMTCO2e), and natural gas (0.6 MMTCO2e) —accounted
for the balance. During combustion, nitrous oxide is produced as a result
of chemical interactions between nitrogen oxides (mostly NO2) and other
combustion products. With most conventional stationary combustion systems,
high temperatures destroy almost all nitrous oxide, limiting the quantity
that escapes; therefore, emissions from these systems typically are low.
Industrial Sources
Emissions of nitrous oxide from industrial sources were 13.2 MMTCO2e in
2005, a decrease of 0.7 MMTCO2e (5.3 percent) from 2004 and a decrease of
15.3 MMTCO2e (54 percent) since 1990. Nitrous oxide is emitted as a byproduct
of certain chemical production processes. Table 30 provides estimates of
emissions from the production of adipic acid and nitric acid, the two principal
known sources.
Nitric Acid Production
The 6.3 million metric tons of nitric acid manufactured in 200566 resulted
in estimated nitrous oxide emissions of 10.3 MMTCO2e (Table 30). This estimate
was 5.6 percent lower than 2004 levels and 12 percent lower than 1990 levels.
The emissions factor used to estimate nitrous oxide emissions from the
production of nitric acid was based on measurements at a single DuPont
plant, which indicated an emissions factor of 2 to 9 grams of nitrous oxide
emitted per kilogram of nitric acid manufactured, suggesting a significant
range of uncertainty.67 Nitric acid, a primary ingredient in fertilizers,
usually is manufactured by oxidizing ammonia (NH3) with a platinum catalyst.
Nitrous oxide emissions are a direct result of the oxidation.
Adipic Acid Production
Emissions from adipic acid production in 2005 were 2.9 MMTCO2e, 4.2 percent
lower than in 2004. Nitrous oxide emissions from this source in 2005 were
83 percent (13.9 MMTCO2e) lower than in 1990.
Adipic acid is a fine white powder that is used primarily in the manufacture
of nylon fibers and plastics, such as carpet yarn, clothing, and tire cord.
Other uses of adipic acid include production of plasticizer for polyvinyl
chloride and polyurethane resins, lubricants, insecticides, and dyes. In
the United States, three companies, which operate four plants, manufacture
adipic acid by oxidizing a ketone-alcohol mixture with nitric acid. Nitrous
oxide is an intrinsic byproduct of this chemical reaction. For every metric
ton of adipic acid produced, 0.3 metric ton of nitrous oxide is created.68 Emissions from adipic acid manufacture grew by 23 percent from 1990 to
1996, reaching 20.7 MMTCO2e before dropping sharply to 7.8 MMTCO2e in 1997.
Before 1997, two of the four plants that manufacture adipic acid controlled
emissions by thermally decomposing the nitrous oxide. This technique eliminates
98 percent of potential nitrous oxide emissions from the process.69 During
the first quarter of 1997, a third plant installed emissions controls,
increasing the share of adipic acid production employing emissions abatement
controls from 74 percent in 1996 to 92 percent in 1997. In 1998, with emissions
controls in place for the full year, 97 percent of emissions from U.S.
adipic acid production were controlled.70
Estimated emissions of nitrous oxide from uncontrolled adipic acid production
decreased from 19.6 MMTCO2e in 1996 to 2.0 MMTCO2e in 1998 and remained
fairly stable through 2002, before dropping to 1.4 MMTCO2e per year in
2003 (Table 30). Emissions of nitrous oxide from controlled plants have
remained relatively constant from 1998 through 2005, in a range of 1.4
to 1.6 MMTCO2e. With the share of adipic acid production employing abatement
controls now at 98 percent, future changes in nitrous oxide emissions from
this source are expected to result primarily from changes in plant production
levels in response to market demand.
Waste Management
Nitrous oxide emissions from waste management in 2005 are estimated at
6.2 MMTCO2e, or 1.7 percent of all U.S. anthropogenic nitrous oxide emissions
(Table 25). During 2005, emissions from human sewage in wastewater were
responsible for 94 percent of the estimated emissions from this source,
and the remainder was associated with waste combustion. Estimated emissions
from waste management increased by 1.1 percent from 2004 to 2005 and by
26 percent from 1990 to 2005. Because of the lack of reliable data and
an effective
estimation method, no estimate of emissions from industrial wastewater
was calculated, leaving estimated emissions from waste management lower
than they otherwise would be had a viable estimation method been available.
Human Sewage in Wastewater
In 2005, nitrous oxide emissions from wastewater were 5.8 MMTCO2e, a 1.0-percent
increase from 2004 levels and a 27-percent increase from the 1990 level
(Table 25). Estimates of nitrous oxide emissions from human waste are scaled
to population size and per capita protein intake. U.S. population has grown
by 19 percent since 1990.71 U.S. per capita protein intake rose steadily
from 1990 to 1999, then declined from 2000 to 2002. U.S. per capita protein
intake in 2002 was 6.5 percent above the 1990 level.72
Nitrous oxide is emitted from wastewater that contains nitrogen-based organic
materials, such as those found in human or animal waste. Two natural processes—nitrification
and denitrification—combine to produce nitrous oxide. Nitrification, an
aerobic process, converts ammonia into nitrate; denitrification, an anaerobic
process, converts nitrate to nitrous oxide. Factors that influence the
amount of nitrous oxide generated from wastewater include temperature,
acidity, biochemical oxygen demand (BOD),73 and nitrogen concentration.
Waste Combustion
In 2005, estimated nitrous oxide emissions from waste combustion were 0.3
MMTCO2e, up by 2.8 percent from the 2004 level and 15 percent above the
1990 level. Data on the amount of waste generated in the United States in
2005 were not available in time for this report; therefore, EIA scaled
the 2005 estimate for waste
combustion to the growth in U.S. gross domestic product. The share of waste
burned is estimated to have remained nearly stable between 1998 and 2005,
but the total volume of waste generated is estimated to have risen steadily.
The total volume of waste generated in the United States increased by 79
percent from 1990 to 2005; however, the share of waste burned in 2005 was
just 7.4 percent, compared with 12 percent in 1990.74
Chapter 4. Notes and Sources
Tables 25-30 |
