7.4.2 Nitrogen Compounds

The N cycle is integral to functioning of the Earth system and to climate (Vitousek et al., 1997; Holland et al., 2005a). Over the last century, human activities have dramatically increased emissions and removal of reactive N to the global atmosphere by as much as three to five fold. Perturbations of the N cycle affect the atmosphere climate system through production of three key N-containing trace gases: N₂O, ammonia (NH₃) and NO_x (nitric oxide (NO) + nitrogen dioxide (NO₂)). Nitrous oxide is the fourth largest single contributor to positive radiative forcing, and serves as the only long-lived atmospheric tracer of human perturbations of the global N cycle (Holland et al., 2005a). Nitrogen oxides have short atmospheric lifetimes of hours to days (Prather et al., 2001). The dominant impact of NO_x emissions on the climate is through the formation of tropospheric ozone, the third largest single contributor to positive radiative forcing (Sections 2.3.6, 7.4.4). Emissions of NO_x generate indirect negative radiative forcing by shortening the atmospheric lifetime of CH₄ (Prather 2002). Ammonia contributes to the formation of sulphate and nitrate aerosols, thereby contributing to aerosol cooling and the aerosol indirect effect (Section 7.5), and to increased nutrient supply for the carbon cycle (Section 7.5). Ammonium and NO_x are removed from the atmosphere by deposition, thus affecting the carbon cycle through increased nutrient supply (Section 7.3.3.1.3).

Atmospheric concentrations of N₂O have risen 16%, from about 270 ppb during the pre-industrial era to 319 ppb in 2005 (Figure 7.16a). The average annual growth rate for 1999 to 2000 was 0.85 to 1.1 ppb yr^–1, or about 0.3% per year (WMO, 2003). The main change in the global N₂O budget since the TAR is quantification of the substantial human-driven emission of N₂O (Table 7.7; Naqvi et al., 2000; Nevison et al., 2004; Kroeze et al., 2005; Hirsch et al., 2006). The annual source of N₂O from the Earth’s surface has increased by about 40 to 50% over pre-industrial levels as a result of human activity (Hirsch et al., 2006). Human activity has increased N supply to coastal and open oceans, resulting in decreased O₂ availability and N₂O emissions (Naqvi et al., 2000; Nevison et al., 2004).

Figure 7.16. (a) Changes in the emissions of fuel combustion NO_x and atmospheric N₂O mixing ratios since 1750. Mixing ratios of N₂O provide the atmospheric measurement constraint on global changes in the N cycle. (b) Changes in the indices of the global agricultural N cycle since 1850: the production of manure, fertilizer and estimates of crop N fixation. For data sources see http://www-eosdis.ornl.gov/ (Holland et al., 2005b) and http://www.cmdl.noaa.gov/. Figure adapted from Holland et al. (2005c).

Since the TAR, both top-down and bottom-up estimates of N₂O have been refined. Agriculture remains the single biggest anthropogenic N₂O source (Bouwman et al., 2002; Smith and Conen, 2004; Del Grosso et al., 2005). Land use change continues to affect N₂O and NO emissions (Neill et al., 2005): logging is estimated to increase N₂O and NO emissions by 30 to 350% depending on conditions (Keller et al., 2005). Both studies underscore the importance of N supply, temperature and moisture as regulators of trace gas emissions. The inclusion of several minor sources (human excreta, landfills and atmospheric deposition) has increased the total bottom-up budget to 20.6 TgN yr^–1 (Bouwman et al., 2002). Sources of N₂O now estimated since the TAR include coastal N₂O fluxes of 0.2 TgN yr^–1 (±70%; Nevison et al., 2004) and river and estuarine N₂O fluxes of 1.5 TgN yr^–1 (Kroeze et al., 2005). Box model calculations show the additional river and estuarine sources to be consistent with the observed rise in atmospheric N₂O (Kroeze et al., 2005).

Top-down estimates of surface sources use observed concentrations to constrain total sources and their spatial distributions. A simple calculation, using the present-day N₂O burden divided by its atmospheric lifetime, yields a global stratospheric loss of about 12.5 ± 2.5 TgN yr^–1. Combined with the atmospheric increase, this loss yields a surface source of 16 TgN yr^–1. An inverse modelling study of the surface flux of N₂O yields a global source of 17.2 to 17.4 TgN yr^–1 with an estimated uncertainty of 1.4 (1 standard deviation; Hirsch et al., 2006). The largest sources of N₂O are from land at tropical latitudes, the majority located north of the equator. The Hirsch et al. inversion results further suggest that N₂O source estimates from agriculture and fertilizer may have increased markedly over the last three decades when compared with an earlier inverse model estimate (Prinn et al., 1990). Bottom-up estimates, which sum individual source estimates, are more evenly distributed with latitude and lack temporal variability. However, there is clear consistency between top-down and bottom-up global source estimates, which are 17.3 (15.8–18.4) and 17.7 (8.5–27.7) TgN yr^–1, respectively.

Concentrations of NO_x and reduced nitrogen (NH_x = NH₃ + ammonium ion (NH₄⁺)) are difficult to measure because the atmospheric lifetimes of hours to days instead of years generate pronounced spatial and temporal variations in their distributions. Atmospheric concentrations of NO_x and NH_x vary more regionally and temporally than concentrations of N₂O. Total global NO_x emissions have increased from an estimated pre-industrial value of 12 TgN yr^–1 (Holland et al., 1999; Galloway et al., 2004) to between 42 and 47 TgN yr^–1 in 2000 (Table 7.7). Lamarque et al. (2005a) forecast them to be 105 to 131 TgN yr^–1 by 2100. The range of surface NO_x emissions (excluding lightning and aircraft) used in the current generation of global models is 33 to 45 TgN yr^–1 with small ranges for individual sources. The agreement reflects the use of similar inventories and parametrizations. Current estimates of NO_x emissions from fossil fuel combustion are smaller than in the TAR.

Since the TAR, estimates of tropospheric NO₂ columns from space by the Global Ozone Monitoring Experiment (GOME, launched in 1995) and the SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY, launched in 2002) (Richter and Burrows, 2002; Heue et al., 2005) provide constraints on estimates of NO_x emissions (Leue et al., 2001). Martin et al. (2003a) use GOME data to estimate a global surface source of NO_x of 38 TgN yr^–1 for 1996 to 1997 with an uncertainty factor of 1.6. Jaeglé et al. (2005) partition the surface NO_x source inferred from GOME into 25.6 TgN yr^–1 from fuels, 5.9 TgN yr^–1 from biomass burning and 8.9 TgN yr^–1 from soils. Interactions between soil emissions and scavenging by plant canopies have a significant impact on soil NO_x emissions to the free troposphere: the impact may be greatest in subtropical and tropical regions where emissions from fuel combustion are rising (Ganzeveld et al., 2002). Boersma et al. (2005) find that GOME data constrain the global lightning NO_x source for 1997 to the range 1.1 to 6.4 TgN yr^–1. Comparison of the tropospheric NO₂ column of three state-of-the-art retrievals from GOME for the year 2000 with model results from 17 global atmospheric chemistry models highlights significant differences among the various models and among the three GOME retrievals (Figure 7.17, van Noije et al., 2006). The discrepancies among the retrievals (10 to 50% in the annual mean over polluted regions) indicate that the previously estimated retrieval uncertainties have a large systematic component. Top-down estimates of NO_x emissions from satellite retrievals of tropospheric NO₂ are strongly dependent on the choice of model and retrieval.

Figure 7.17. Tropospheric column NO₂ from (a) satellite measurements and (b) atmospheric chemistry models. The maps represent ensemble average annual mean tropospheric NO₂ column density maps for the year 2000. The satellite retrieval ensemble comprises three state-of-the-art retrievals from GOME; the model ensemble includes 17 global atmospheric chemistry models. These maps were obtained after smoothing the data to a common horizontal resolution of 5° × 5° (adapted from van Noije et al., 2006).

Knowledge of the spatial distribution of NO_x emissions has evolved significantly since the TAR. An Asian increase in emissions has been compensated by a European decrease over the past decade (Naja et al., 2003). Richter et al. (2005; see also Irie et al., 2005) use trends for 1996 to 2004 observed by GOME and SCIAMACHY to deduce a 50% increase in NO_x emissions over industrial areas of China. Observations of NO₂ in shipping lanes from GOME (Beirle et al., 2004) and SCIAMACHY (Richter et al., 2004) give values at the low end of emission inventories. Data from GOME and SCIAMACHY further reveal large pulses of soil NO_x emissions associated with rain (Jaeglé et al., 2004) and fertilizer application (Bertram et al., 2005).

All indices show an increase since pre-industrial times in the intensity of agricultural nitrogen cycling, the primary source of NH₃ emissions (Figure 7.16b and Table 7.7; Bouwman et al., 2002). Total global NH₃ emissions have increased from an estimated pre-industrial value of 11 TgN yr^–1 to 54 TgN yr^–1 for 2000 (Holland et al., 1999; Galloway et al., 2004), and are projected to increase to 116 TgN yr^–1 by 2050.

The primary sink for NH_x and NO_x and their reaction products is wet and dry deposition. Estimates of the removal rates of both NH_x and NO_x are provided by measurements of wet deposition over the USA and Western Europe to quantify acid rain inputs (Hauglustaine et al., 2004; Holland et al., 2005a; Lamarque et al., 2005a). Chemical transport models represent the wet and dry deposition of NO_x and NH_x and their reaction products. A study of 29 simulations with 6 different tropospheric chemistry models, focusing on present-day and 2100 conditions for NO_x and its reaction products, projects an average increase in N deposition over land by a factor of 2.5 by 2100 (Lamarque et al., 2005b), mostly due to increases in NO_x emissions. Nitrogen deposition rates over Asia are projected to increase by a factor of 1.4 to 2 by 2030. Climate contributions to the changes in oxidized N deposition are limited by the models’ ability to represent changes in precipitation patterns. An intercomparison of 26 global atmospheric chemistry models demonstrates that current scenarios and projections are not sufficient to stabilise or reduce N deposition or ozone pollution before 2030 (Dentener et al., 2006).