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We update and evaluate the treatment of nitrate aerosols in theGeophysical Fluid Dynamics Laboratory (GFDL) atmospheric model(AM3). Accounting for the radiative effects of nitrate aerosolsgenerally improves the simulated aerosol optical depth, althoughnitrate concentrations at the surface are biased high. This bias canbe reduced by increasing the deposition of nitrate to account forthe near-surface volatilization of ammonium nitrate or by neglectingthe heterogeneous production of nitric acid to account for theinhibition of N2O5 reactive uptake at high nitrateconcentrations. Globally, uncertainties in these processes canimpact the simulated nitrate optical depth by up to 25 %, muchmore than the impact of uncertainties in the seasonality of ammoniaemissions (6 %) or in the uptake of nitric acid on dust(13 %). Our best estimate for fine nitrate opticaldepth at 550 nm in 2010 is 0.006 (0.005–0.008). In wintertime, nitrate aerosolsare simulated to account for over 30 % of the aerosol optical depth overwestern Europe and North America. Simulated nitrate optical depth increasesby less than 30 % (0.0061–0.010) in response to projected changes inanthropogenic emissions from 2010 to 2050 (e.g., −40 % for SO2 and+38 % for ammonia). This increase is primarily driven by greaterconcentrations of nitrate in the free troposphere, while surface nitrateconcentrations decrease in the midlatitudes following lower concentrations ofnitric acid. With the projected increase of ammonia emissions, we show thatbetter constraints on the vertical distribution of ammonia (e.g., convective transportand biomass burning injection) and on the sources and sinks of nitric acid(e.g., heterogeneous reaction on dust) are needed to improve estimates offuture nitrate optical depth

Sensitivity of nitrate aerosols to ammonia emissions and to nitrate chemistry: implications for present and future nitrate optical depth