The Role of Clouds in the Tropospheric NOx Cycle: A New Modeling Approach for Cloud Chemistry and Its Global Implications

Abstract

AbstractWe present a new method for simulating heterogeneous (surface and multiphase) cloud chemistry in atmospheric models that do not spatially resolve clouds. The method accounts for cloud entrainment within the chemical rate expression, making it more accurate and stable than other approaches. Using this “entrainment‐limited uptake,” we evaluate the role of clouds in the tropospheric NOx cycle. Past literature suggests that on large scales, losses of N2O5 and NO3 in clouds are much less important than losses on aerosols. We find, however, that cloud reactions provide 25% of tropospheric NOx loss in high latitudes and 5% of global loss. Homogeneous, gas phase hydrolysis of N2O5 is likely 2% or less of global NOx loss. Both clouds and aerosols have similar impacts on global tropospheric O3 and OH levels, around 2% each. Accounting for cloud uptake reduces the sensitivity of atmospheric chemistry to aerosol surface area and uptake coefficient since clouds and aerosols compete for the same NO3 and N2O5.