Modeling of the Effects of Acidic Aerosols on Arctic Cloud Microstructure and Surface Radiative Budget during Winter

The effect of pollution-derived sulphuric acid aerosols on the aerosol-cloud-radiation interactions is investigated. Observations suggest that acidic aerosols can decrease the heterogeneous nucleation rate of ice crystals and lower the homogeneous freezing temperature of haze droplets. Based on these observations, we hypothesize that the cloud thermodynamic phase is modified in polluted air mass (Arctic haze). Cloud ice number concentration is reduced, thus promoting further ice crystal growth by the Bergeron-Findeisen process. Hence, ice crystals reach larger sizes and low-level ice crystal precipitation from mixed-phase clouds increases. Enhanced dehydration of the lower troposphere contributes to decrease the water vapour greenhouse effect and cool the surface. A positive feedback is created between surface cooling and air dehydration, accelerating the cold air production. This process is referred to as the dehydration-greenhouse feedback (DGF). Simulations using 0D, 1D and 3D models are performed to assess the potential effect of the DGF on the Arctic cloud microstructure and surface radiative budget. Results with the explicit size bin 0D model show that acidic aerosols promote the formation of larger ice crystals in smaller concentration, which enhance precipitation and air dehydration. Results with the 1D and 3D model show that the DGF has an important effect on cloud, atmospheric dehydration, and temperature over the Central and Eastern Arctic, which is the coldest part of the Arctic. Cloud ice is significantly reduced and the total atmospheric water path is decreased by as much as 12%. This results in a surface cooling ranging between 0 and –3K. Moreover, the lower tropospheric cooling over the Eastern and Central Arctic strengthens the atmospheric circulation at upper level, thus increasing the aerosol transport from the mid-latitudes and enhancing the DGF. Over warmer areas, the increased aerosol concentration (caused by the DGF) leads to longer cloud lifetime, which contributes to warm these areas.