4.2.3.3 Nitrogen oxides (NO_x)

Nitrogen oxides (NO_x = NO + NO₂) do not directly affect Earth’s radiative balance, but they catalyse tropospheric O₃ formation through a sequence of reactions, e.g.,

OH + CO + O₂ CO₂ + HO₂
HO₂ + NO NO₂ + OH
NO₂ + h NO + O(³P)
O(³P) + O₂ + M O₃ + M

net: CO + 2O₂ + h CO₂ + O₃

By rapidly converting HO₂ to OH, NO enhances tropo-spheric OH abundances and thus indirectly reduces the atmospheric burdens of CO, CH₄, and HFCs. Much of recent understanding of the role of NO_x in producing tropospheric O₃ and changing OH abundances is derived from in situ measurement campaigns that sample over a wide range of chemical conditions in the upper troposphere or at the surface (see Section 4.2.6 on tropospheric OH). These atmospheric measurements generally support the current photochemical models. There is substantial spatial and temporal variability in the measured abundance of NO_x, which ranges from a few ppt near the surface over the remote tropical Pacific Ocean to >100 ppb in urban regions. The local chemical lifetime of NO_x is always short, but varies widely throughout the troposphere, being 1 day or less in the polluted boundary layer, day or night, and 5 to 10 days in the upper troposphere. As with VOC, it is not possible to derive a global burden or average abundance for NO_x from measurements of atmospheric abundances.

Most tropospheric NO_x are emitted as NO, which photochemically equilibrates with nitrogen dioxide (NO₂) within a few minutes. Significant sources, summarised in Table 4.8, include both surface and in situ emissions, and only a small amount is transported down from the stratosphere. NO_x emitted within polluted regions are more rapidly removed than those in remote regions. Emissions directly into the free troposphere have a disproportionately large impact on global greenhouse gases. The major source of NO_x is fossil fuel combustion, with 40% coming from the transportation sector. Benkovitz et al. (1996) estimated global emissions at 21 TgN/yr for 1985. The NO_x emissions from fossil fuel use used in model studies here for year 2000 are considerably higher, namely 33 TgN/yr. The large American and European emissions are relatively stable, but emissions from East Asia are increasing by about +4%/yr (Kato and Akimoto, 1992). Other important, but more uncertain surface sources are biomass burning and soil emissions. The soil source recently derived from a bottom-up compilation of over 100 measurements from various ecosystems is 21 TgN/yr (Davidson and Kingerlee, 1997), much higher than earlier estimates. Part of the discrepancy can be explained by the trapping of soil-emitted NO in the vegetation canopy. Inclusion of canopy scavenging reduces the NO_x flux to the free troposphere to 13 TgN/yr, which is still twice the flux estimated by another recent study (Yienger and Levy, 1995). Emissions of NO_x in the free troposphere include NO_x from aircraft (8 to 12 km), ammonia oxidation, and lightning (Lee et al., 1997). Estimates of the lightning NO_x source are quite variable; some recent global estimates are 12 TgN/yr (Price et al., 1997a,b), while other studies recommend 3 to 5 TgN/yr (e.g., Huntrieser et al., 1998; Wang et al., 1998a). Recent studies indicate that the global lightning frequency may be lower than previously estimated (Christian et al., 1999) but that intra-cloud lightning may be much more effective at producing NO (DeCaria et al., 2000). In total, anthropogenic NO_x emissions dominate natural sources, with fossil fuel combustion concentrated in northern industrial regions. However, natural sources may control a larger fraction of the globe. Overall, anthropogenic NO_x emissions are expected to undergo a fundamental shift from the current dominance of the Northern Hemisphere to a more tropical distribution of emissions. Asian emissions from fossil fuel are expected to drive an overall increase in NO_x emissions in the 21st century (Logan, 1994; Van Aardenne et al., 1999).

Table 4.8: Estimates of the global tropospheric NOx budget (in TgN/yr) from different sources compared with the values adopted for this report.
Reference:	TAR	Ehhalt (1999)	Holland et al. (1999)	Penner et al. (1999)	Lee et al. (1997)
Base year	2000	~1985	~1985	1992
Fossil fuel	33.0	21.0	20 - 24	21.0	22.0
Aircraft	0.7	0.45	0.23 -0.6	0.5	0.85
Biomass burning	7.1	7.5	3 - 13	5 - 12	7.9
Soils	5.6	5.5	4 - 21	4 - 6	7.0
NH3 oxidation	-	3.0	0.5 - 3	-	0.9
Lightning	5.0	7.0	3 - 13	3 - 5	5.0
Stratosphere	<0.5	0.15	0.1 - 0.6	-	0.6
Total	51.9	44.6			44.3
Anthropogenic emissions by continent/region	Y2000	Y2100(A2p)
Africa	2.5	21.8
South America	1.4	10.8
Southeast Asia	1.2	6.8
India	1.7	10.0
North America	10.1	18.5
Europe	7.3	14.3
East Asia	5.6	24.1
Australia	0.5	1.1
Other	2.3	2.6
Sum	32.6	110.0
The TAR column was used in OxComp model calculations for year 2000; fossil fuel includes bio-fuels, but surface sources only; stratospheric source in TAR is upper limit and includes HNO3 ; the range of values used in modelling for IPCC aviation assessment (Penner et al. 1999) is given.

The dominant sink of NO_x is atmospheric oxidation of NO₂ by OH to form nitric acid (HNO₃), which then collects on aerosols or dissolves in precipitation and is subsequently scavenged by rainfall. Other pathways for direct NO_x removal occur through canopy scavenging of NO_x and direct, dry deposition of NO_x, HNO₃, and particulate nitrates to the land surface and the ocean. Dry deposition can influence the surface exchanges and can thus alter the release of NO_x and N₂O to the atmosphere. Peroxyacetyl nitrate (PAN), formed by the reaction of CH₃C(O)O₂ with NO₂, can transport HO_x and NO_x to remote regions of the atmosphere due to its stability at the cold temperatures of the upper troposphere. In addition tropospheric aerosols provide surfaces on which reactive nitrogen, in the form of NO₃ (nitrate radical) or N₂O₅, is converted to HNO₃ (Dentener and Crutzen, 1993; Jacob, 2000).

Some CTM studies argue against either the large soil source or the large lightning source of NO_x. A climatology of NO_x measurements from aircraft was prepared by Emmons et al. (1997) and compared with six chemical transport models. They found that the processes controlling NO_x in the remote troposphere are not well modelled and that, of course, there is a paucity of global NO_x measurements. For short-lived gases like NO_x, resolution of budget discrepancies is even more challenging than for the long-lived species, because the limited atmospheric measurements offer few real constraints on the global budget. However, an additional constraint on the NO_x budget is emerging as the extensive measurements of wet deposition of nitrate over Northern Hemisphere continents are compiled and increasing numbers of surface measurements of dry deposition of HNO₃, NO₂, and particulate nitrate become available, and thus allow a much better estimate of the NO_x sink.

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