Global modeling of nitrate and ammonium aerosol: Radiative effect and interaction with gas-phase chemistry and aerosols.

Abstract

Nitrate and ammonium aerosols are included in a global model. First, in order to examine the potential importance of sea salt and mineral dust aerosol on radiative effects by nitrate and ammonium, we used a thermodynamic equilibrium model to calculate aerosol composition, including nitrate and ammonium in aerosol, and water content. This equilibrium study finds that the presence of sea salt and dust aerosol could potentially increase nitrate aerosol burden by a factor of 4. As a result, global and annual average forcing by anthropogenic aerosols decreases by 12%, from -0.42 to -0.37 Wm<super>-2</super> for the present day and from -0.54 to -0.47 Wm<super>-2</super> for an IPCC projected emission scenario for 2100, respectively. Then, a more accurate hybrid dynamical approach is developed for use in the global chemistry-aerosol transport model to simulate nitrate and ammonium aerosols. In this study, gas-to-particle mass transport limitations are considered in partitioning of nitrate and ammonium between gas and aerosol phases. Simulated nitrate and ammonium aerosol concentrations are generally within a factor of 2 of observations. The global burden of nitrate and ammonium is 0.165 and 0.225 Tg N, with a lifetime of 4.85 and 3.41 days, respectively. 48% of nitrate and 90% of ammonium exist in the optically important size range. Nitrate formation could potentially decrease tropospheric ozone due to the decrease of NO<italic>_x</italic>. Several sensitivity studies were performed to analyze differences in nitrate and ammonium aerosol concentrations, between simplified methods (including the equilibrium model) for treating nitrate and ammonium in aerosol and the method developed here. It is indicated that these simplifications may result in an over-estimate of the reduction of tropospheric ozone and an overestimate of nitrate aerosol forcing, because they significantly overpredict the nitrate uptake by aerosols, especially that by dust aerosol. Finally, nitrate aerosol concentration is sensitive to the vertical representations of clouds in global models, since clouds have a large impact on the photo dissociation of its gaseous precursor, HNO₃, through interference with ultra-violet solar radiation. The effects of cloud overlap schemes on photolysis frequencies and tropospheric OH concentrations are thus examined in a global photochemistry model.