Modeling Aerosol Effects on Liquid Clouds in the Summertime Arctic

An investigation of aerosol effects on the Arctic summer clouds is carried out by using a fully-coupled version of GEM-MACH, an online air quality forecast model. Model simulations are carried out for the July 2014 NETCARE field campaign based from Resolute, Nunavut, Canada. For a pristine period, the model simulated thin water clouds, with liquid water path <50 g m(-2), at relatively low altitudes (<200 m). The model simulated more and smaller droplets with higher liquid water content (LWC) during a polluted period. Further sensitivity analysis indicates that the increased aerosol loading from long-range transport during the polluted period is responsible for the increase in CDNC and decreased droplet size, which also led to an increase in cloud liquid water content (LWC), a decrease in precipitation, and increased cloudiness in the area. The model agrees with observation showing that aerosols smaller than 100 nm are commonly activated in the summer Arctic, with even smaller aerosols (<50 nm) being activated during the pristine period. The impact of atmospheric dimethyl sulfide (DMS) upon the Arctic summer cloud microphysics was also investigated. The inclusion of DMS leads to an increase in CDNC and smaller droplet sizes. It also improves the modeled aerosol size distribution considerably compared to the observation, particularly in the size range between 60 and 200 nm. The enhancement of CDNC due to DMS is more significant (e.g., >50%) in the regions with higher DMS(g) mixing ratios, resulting in higher sulphate mass increases in these regions.