Effects of arctic sulphuric acid aerosols on wintertime low-level atmospheric ice crystals, humidity and temperature at Alert, Nunavut

Simulations performed using a column model and analysis of observed data for the period of 1991-1994 at Alert (82° 30'N, 62°20'W) are used to assess the changes of the boundary layer cloud characteristics by sulphuric acid aerosols and the potential effect on arctic climate. Results show that aerosol acidification leads to depletion of the ice crystal number concentration and an increase of its mean size. As a result, low-level precipitating ice crystals occur more frequently than ice fog and thick nonprecipitating clouds during high concentration of pollution-derived aerosols. This result is in agreement with observations that indicate an increase by more than 50% of the frequency of precipitating ice crystals when the weight proportion of sulphuric acid is greater than its mean value of 20% of the total aerosol mass. Consequently, the ice crystal size increases and number decreases, and the sedimentation flux of ice crystals and the dehydration rate of the lower troposphere are accelerated in the presence of high sulphuric acid aerosol concentration. As a result, the infrared radiation flux reaching the surface and the greenhouse effect are decreased. This process is referred to as the dehydration-greenhouse feedback. One-dimensional simulation for Alert during the period of 1991 to 1994 shows that a negative cloud radiative forcing of −9 W m−2 may occur locally as a result of the enhanced dehydration rate produced by the aerosol acidity.