Nitrogencontaining Compounds

The strong triple bond of the N=N molecule makes it practically inert; it is extremely stable chemically and is not involved in the chemistry of the troposphere or stratosphere. The important nitrogen-containing trace species in the atmosphere are nitrous oxide (N20), nitric oxide (NO), nitrogen dioxide (N02), nitric acid (HN03), and ammonia (NH3). The first of these, nitrous oxide (N20), is a colorless gas that is emitted almost totally by natural sources, principally by bacterial action in the soil. The gas is employed as an anesthetic and is commonly referred to as "laughing gas." The second, nitric oxide (NO), is emitted by both natural and anthropogenic sources. Nitrogen dioxide (N02) is emitted in small quantities from combustion processes along with NO and is also formed in the atmosphere by oxidation of NO. The sum of NO and N02 is usually designated as NOA, Nitric oxide is the major oxide of nitrogen formed during high-temperature combustion, resulting from both the interaction of nitrogen in the fuel with oxygen present in the air and the chemical conversion of atmospheric nitrogen and oxygen at the high temperatures of combustion. Other oxides of nitrogen, such as N03 and N205, exist in the atmosphere in relatively low concentrations but nonetheless participate importantly in atmospheric chemistry. Nitric acid is an oxidation product of N02 in the atmosphere. Ammonia (NH3) is emitted primarily by natural sources. Finally, nitrate and ammonium salts are not emitted in any significant quantities but result from the atmospheric conversion of NO, N02, and NH3.

Nitrogen is an essential nutrient for all living organisms. The primary source of this nitrogen is the atmosphere. However, N2 is not useful to most organisms until it is

"fixed" or converted to a form that can be chemically utilized by the organisms. (Nitrogen fixation refers to the chemical conversion of N2 to any other nitrogen compound.) The "natural" fixation of N2 occurs by two types of processes. One is the action of a comparatively few microorganisms that are capable of converting N2 to ammonia, ammonium ion (NH|), and organic nitrogen compounds. The other natural nitrogen fixation process occurs in the atmosphere by the action of ionizing phenomena, such as cosmic radiation or lightning, on N2. This process leads to the formation of nitrogen oxides in the atmosphere, which are ultimately deposited on the Earth's surface as biologically useful nitrates.

In addition to natural nitrogen fixation, human activities have led to biological and industrial fixation and fixation by combustion. Humans have increased the cultivation of legumes, which have a symbiotic relationship with certain microorganisms capable of nitrogen fixation. Legumes provide an increase in the soil nitrogen and serve as a valuable food crop. Industrial nitrogen fixation consists primarily of the production of ammonia for fertilizer use. Combustion can also lead to the fixation of nitrogen as NO t. In the process of nitrification, ammonium is oxidized to N02 and NOJ by microbial action. N20 and NO are byproducts of nitrification; the result is the release of N20 and NO to the atmosphere. Reduction of NO^" to N2, N02, N20, or NO is called denitrification. Denitrification is accomplished by a number of bacteria and is the process that continually replenishes the atmosphere's N2. Figure 2.2 depicts the atmospheric nitrogen cycle.

Stratosphere

Transport

Troposphere

Transport

Stratosphere

Transport

Troposphere

Transport

n2o

n7

Combustion^

no

o3,

no2

oh

hno3

Lightning

Denitrification

Fixation oh.

nh3

h2o>

nh4

no3

Deposition

Deposition ♦

Deposition

Assimilation:

nh il rn

Soil and Marine Fixed Nitrogen

Ammonification: RN — NH3

Nitrification: NH4 — N03 Denitrification: N03 Fixation: [sj2 -

NH, n02

no nh3

FIGURE 2.2 Processes in the atmospheric cycle of nitrogen compounds. A species written over an arrow signifies reaction with the species from which the arrow originates.

TABLE 2.5 Estimates of the Global N20 Budget (in TgN/yr) and Values Adopted by IPCC (2001)

Reference: Mosier et al. (1998b) Olivier

TABLE 2.5 Estimates of the Global N20 Budget (in TgN/yr) and Values Adopted by IPCC (2001)

Reference: Mosier et al. (1998b) Olivier

Base Year:

1994

Range

1990

Range

1990

Sources

Ocean

3.0

1-5

3.6

2.8-5.7

Atmosphere (NH3

0.6

0.3-1.2

0.6

0.3-1.2

oxidation)

Tropical soils

Wet forest

3.0

2.2-3.7

Dry savannas

1.0

0.5-2.0

Temperate soils

Forests

1.0

0.1-2.0

Grasslands

1.0

0.5-2.0

All soils

6.6

3.3-9.9

Natural subtotal

9.6

4.6-15.9

10.8

6.4-16.8

Agricultural soils

4.2

0.6-14.8

1.9

0.7-4.3

Biomass burning

0.5

0.2-1.0

0.5

0.2-0.8

Industrial sources

1.3

0.7-1.8

0.7

0.2-1.1

Cattle and feedlots

2.1

0.6-3.1

1.0

0.2-2.0

Anthropogenic subtotal

8.1

2.1-20.7

4.1

1.3-7.7

6.9

Total sources

17.7

6.7-36.6

14.9

7.7-24.5

Imbalance (trend)

3.9

3.1^1.7

3.8

Total sinks (stratospheric)

12.3

9-16

12.6

Implied total source

16.2

16.4

Source: IPCC (2001).

Source: IPCC (2001).

2.3.1 Nitrous Oxide (N20)

Nitrous oxide (N20) is an important atmospheric gas that is emitted predominantly by biological sources in soils and water (Table 2.5). Although by comparison to C02 and HzO, N20 has a far lower concentration, it is an extremely influential greenhouse gas. This is a result of its long residence time and its relatively large energy absorption capacity per molecule. Per unit mass the global warming potential of N20 (see Chapter 23) is about 300 times that of C02. Tropical soils are the most important individual sources of N20 to the atmosphere. N20 is also emitted in smaller quantities by a large number of other sources, such as biomass burning, degassing of irrigation water, agricultural activities, and industrial processes. The oceans are significant N20 sources. The total preindustrial N20 source was approximately 10Tg(N)yr_1. The current flux of N20 into the atmosphere that results from anthropogenic activities is estimated by IPCC (2001) to be e^Tg^yr""1.

Nitrous oxide is inert in the troposphere; its major atmospheric sink is photodissociation in the stratosphere (about 90%) and reaction with excited oxygen atoms, 0('D) (about 10%). Oxidation of N20 by O('D) yields NO, providing the major input of NO to the stratosphere. We will return to this process in Chapter 5. Sources of N20 exceed estimated sinks by 3.8Tg(N)yr_1.

Estimates for the atmospheric lifetime of N20 come from stratospheric chemical transport models that have been tested against observed N20 distributions. The best

FIGURE 2.3 Atmospheric abundance of N20 over the last millennium, as determined from ice cores, firn, and whole-air samples (IPCC 2001). Sources of data are indicated, references for which are given in IPCC. The inset contains deseasonalized global averages.

current estimate for the lifetime of N20 is 120 years. Because of its long lifetime, N20 exhibits more or less uniform concentrations throughout the troposphere. Ice core records of N20 show a preindustrial mixing ratio of about 276 ppb. N20 levels have risen approximately 15% since preindustrial times, reaching 315 ppb in 2000 (Figure 2.3). This observed atmospheric increase is consistent with a difference of 3.8Tg(N)yr~' excess of sources over sinks, which is in reasonable agreement, given the uncertainties, with the mismatch based on attempting to estimate sources and sinks independently.

The oxides of nitrogen, NO and N02, are among the most important molecules in atmospheric chemistry. We will devote in this book considerable attention to their chemistry. Estimated global emissions of NO, are given in Table 2.6. Aircraft emissions are listed separately in Table 2.6 because they are released predominantly in the free

TABLE 2.6 Estimate of Global Tropospheric NO* Emissions in TgNyr 1 for Year 2000

Sources

Emissions, Tg N yr 1

Fossil fuel combustion

33.0

Aircraft

0.7

Biomass burning

7.1

Soils

5.6

NH3 oxidation

Lightning

5.0

Stratosphere

<0.5

Total

51.9

Source: IPCC (2001).

Source: IPCC (2001).

troposphere at altitudes of 8-12km rather than at the surface, and although such emissions are only a small fraction of the total combustion source, they are potentially responsible for a large fraction of the NO, found at those altitudes at northern midlatitudes.

2.3.3 Reactive Odd Nitrogen (NO,)

Reactive nitrogen, denoted NO,, is defined as the sum of the two oxides of nitrogen (NO, = NO -I- NO2) and all compounds that are products of the atmospheric oxidation of NO,. These include nitric acid (HN03), nitrous acid (HONO), the nitrate radical (N03), dinitrogen pentoxide (N205), peroxynitric acid (HNO4), peroxyacetyl nitrate (PAN) (CH3C(0)00N02) and its homologs, alkyl nitrates (R0N02), and peroxyalkyl nitrates (ROONOz). Nitric acid (HN03) is the major oxidation product of NO* in the atmosphere. Because of its extreme water solubility, HN03 is rapidly deposited on surfaces and in water droplets. Also, in the presence of NH3, HN03 can form an ammonium nitrate (NH4N03) aerosol. The nitrate radical (NO3) is an important constituent in the chemistry of the troposphere, especially at night. N03 is present at night at mixing ratios ranging up to 300 ppt in the boundary layer. Nitrous oxide (N20) and ammonia (NH3) are not considered in this context as reactive nitrogen compounds.

Measurement of total NO, in the atmosphere provides an important measure of the total oxidized nitrogen content. Concentrations of individual NO, species relative to the total indicate the extent of interconversion among species. NO, is indeed closer to a conserved quantity than any of its constituent species (Roberts 1995).

Only during the past two decades have techniques been available with sufficient sensitivity and range of detectability to measure NO, in nonurban locales (NO, concentrations below 1 ppb), and as a result the size and reliability of the database needed to define nonurban NOA concentrations are limited. Measurements taken at isolated rural sites tend to be significantly lower than concentrations measured at less-isolated rural sites and generally range from a few tenths to 1 ppb. Measurements of NO, in the atmospheric boundary layer and lower free troposphere in remote maritime locations have generally yielded mixing ratios of 0.02-0.04 ppb (20-40 ppt). Although the database is still quite sparse, mixing ratios in remote tropical forests (not under the direct influence of biomass burning) appear to range from 0.02 to 0.08 ppb (from 20 to 80 ppt); the somewhat higher NOx concentrations found in remote tropical forests, as compared with those observed in remote marine locations, could result from biogenic NOx emissions from soil.

A summary of the NOx measurements made in the four regions of the globe mentioned above is presented in Table 2.7. NO, concentrations decrease sharply as one moves from

TABLE 2.7 Typical Boundary-Layer NO; Mixing Ratios

Region

NOx, ppb

Urban-suburban Rural

Remote tropical forest Remote marine

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