2.4.4.5 Nitrate Aerosol

Atmospheric ammonium nitrate aerosol forms if sulphate aerosol is fully neutralised and there is excess ammonia. The direct RF due to nitrate aerosol is therefore sensitive to atmospheric concentrations of ammonia as well as NO_x emissions. In addition, the weakening of the RF of sulphate aerosol in many regions due to reduced emissions (Section 2.4.4.1) will be partially balanced by increases in the RF of nitrate aerosol (e.g., Liao and Seinfeld, 2005). The TAR did not quantify the RF due to nitrate aerosol owing to the large discrepancies in the studies available at that time. Van Dorland (1997) and Jacobson (2001a) suggested relatively minor global mean RFs of –0.03 and –0.05 W m^–2, respectively, while Adams et al. (2001) suggested a global mean RF as strong as –0.22 W m^–2. Subsequent studies include those of Schaap et al. (2004), who estimated that the RF of nitrate over Europe is about 25% of that due to sulphate aerosol, and of Martin et al. (2004), who reported –0.04 to –0.08 W m^–2 for global mean RF due to nitrate. Further, Liao and Seinfeld (2005) estimated a global mean RF due to nitrate of –0.16 W m^–2. In this study, heterogeneous chemistry reactions on particles were included; this strengthens the RF due to nitrate and accounts for 25% of its RF. Feng and Penner (2007) estimated a large, global, fine-mode nitrate burden of 0.58 mg NO₃ m^–2, which would imply an equivalent of 20% of the mean anthropogenic sulphate burden. Surface observations of fine-mode nitrate particles show that high concentrations are mainly found in highly industrialised regions, while low concentrations are found in rural areas (Malm et al., 2004; Putaud et al., 2004). Atmospheric nitrate is essentially non-absorbing in the visible spectrum, and laboratory studies have been performed to determine the hygroscopicity of the aerosols (e.g., Tang 1997; Martin et al., 2004 and references therein). In the AeroCom exercise, nitrate aerosols were not included so fewer estimates of this compound exist compared to the other aerosol species considered.

The mean direct RF for nitrate is estimated to be –0.10 W m^–2 at the TOA, and the conservative scattering nature means a similar flux change at the surface. However, the uncertainty in this estimate is necessarily large owing to the relatively small number of studies that have been performed and the considerable uncertainty in estimates, for example, of the nitrate τ_aer. Thus, a direct RF of –0.10 ± 0.10 W m^–2 is tentatively adopted, but it is acknowledged that the number of studies performed is insufficient for accurate characterisation of the magnitude and uncertainty of the RF.