Aerosol nitrate is closely tied to the relative abundances of ammonium and
sulphate. If ammonia is available in excess of the amount required to neutralise
sulphuric acid, nitrate can form small, radiatively efficient aerosols. In the
presence of accumulation-mode sulphuric acid containing aerosols, however, nitric
acid deposits on larger, alkaline mineral or salt particles (Bassett and Seinfeld,
1984; Murphy and Thomson, 1997; Gard et al., 1998). Because coarse mode particles
are less efficient per unit mass at scattering light, this process reduces the
radiative impact of nitrate (Yang et al., 1994; Li-Jones and Prospero, 1998).
Until recently, nitrate has not been considered in assessments of the radiative effects of aerosols. Andreae (1995) estimated that the global burden of ammonium nitrate aerosol from natural and anthropogenic sources is 0.24 and 0.4 Tg (as NH4NO3), respectively, and that anthropogenic nitrates cause only 2% of the total direct forcing. Jacobson (2001) derived similar burdens, and estimated forcing by anthropogenic nitrate to be -0.024 Wm-2. Adams et al. (1999) obtained an even lower value of 0.17 Tg (as NO3-) for the global nitrate burden. Part of this difference may be due to the fact that the latter model does not include nitrate deposition on sea salt aerosols. Another estimate (van Dorland et al., 1997) suggested that forcing due to ammonium nitrate is about one tenth of the sulphate forcing. The importance of aerosol nitrate could increase substantially over the next century, however. For example, the SRES A2 emissions scenario projects that NOx emissions will more than triple in that time period while SO2 emissions decline slightly. Assuming increasing agricultural emissions of ammonia, it is conceivable that direct forcing by ammonium nitrate could become comparable in magnitude to that due to sulphate (Adams et al., 2001).
Forcing due to nitrate aerosol is already important at the regional scale (ten Brink et al., 1996). Observations and model results both show that in regions of elevated NOx and NH3 emissions, such as Europe, India, and parts of North America, NH4NO3 aerosol concentrations may be quite high and actually exceed those of sulphate. This is particularly evident when aerosol sampling techniques are used that avoid nitrate evaporation from the sampling substrate (Slanina et al., 1999). Substantial amounts of NH4NO3 have also been observed in the European plume during ACE-2 (Andreae et al., 2000).
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