Development and Evaluation of Chemistry‐Aerosol‐Climate Model CAM5‐Chem‐MAM7‐MOSAIC: Global Atmospheric Distribution and Radiative Effects of Nitrate Aerosol.

An advanced aerosol treatment, with a focus on semivolatile nitrate formation, is introduced into the Community Atmosphere Model version 5 with interactive chemistry (CAM5‐chem) by coupling the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) with the 7‐mode Modal Aerosol Module (MAM7). An important feature of MOSAIC is dynamic partitioning of all condensable gases to the different fine and coarse mode aerosols, as governed by mode‐resolved thermodynamics and heterogeneous chemical reactions. Applied in the free‐running mode from 1995 to 2005 with prescribed historical climatological conditions, the model simulates global distributions of sulfate, nitrate, and ammonium in good agreement with observations and previous studies. Inclusion of nitrate resulted in ∼10% higher global average accumulation mode number concentrations, indicating enhanced growth of Aitken mode aerosols from nitrate formation. While the simulated accumulation mode nitrate burdens are high over the anthropogenic source regions, the sea‐salt and dust modes respectively constitute about 74% and 17% of the annual global average nitrate burden. Regional clear‐sky shortwave radiative cooling of up to −5 W m−2 due to nitrate is seen, with a much smaller global average cooling of −0.05 W m−2. Significant enhancements in regional cloud condensation nuclei (at 0.1% supersaturation) and cloud droplet number concentrations are also attributed to nitrate, causing an additional global average shortwave cooling of −0.8 W m−2. Taking into consideration of changes in both longwave and shortwave radiation under all‐sky conditions, the net change in the top of the atmosphere radiative fluxes induced by including nitrate aerosol is −0.7 W m−2. Plain Language Summary: Atmospheric aerosols and aerosol‐cloud interactions continue to be a major source of uncertainty in global climate models that are used to assess the impacts of anthropogenic emissions on climate change. A notable fraction of aerosols is composed of ammonium nitrate, which forms in the atmosphere when ammonia combines with nitric acid produced from oxidation of nitrogen oxides. Both precursor gases are emitted in large amounts from anthropogenic activities as well as natural sources. However, a faithful numerical representation of nitrate aerosol in global models has been difficult owing to the semivolatile nature of ammonium nitrate. In this work, we introduce and evaluate an advanced and computationally efficient aerosol chemistry module in a state‐of‐the‐science global climate model to properly simulate the dynamics of nitrate aerosol formation and its interactions with the naturally occurring sea‐salt and dust aerosols. Inclusion of nitrate results in about 10% higher global average number concentrations of aerosols in the size range that efficiently interacts with solar radiation and acts as seeds upon which cloud droplets can form. Consequently, nitrate accounts for an additional radiative cooling, largely due to the changes in cloud formation. Key Points: A modal version of the advanced aerosol chemistry module MOSAIC is developed and introduced in a climate model to simulate nitrate aerosolMOSAIC provides an accurate and efficient treatment for dynamically partitioning semivolatile gases over to entire aerosol size distributionThe modeled global distribution of nitrate is in good agreement with observations and its impact on the radiative effects is quantified [ABSTRACT FROM AUTHOR]