On the Relationship between Acidic Aerosols and Types of Ice Clouds Over the North Slope of Alaska

Ice clouds play an important role in the Arctic weather and climate system. Consequently, it is essential to understand their properties and especially their formation process. Remote sensing observations over the Arctic have revealed the existence of two types of ice clouds (TIC). The first type, TIC-1, is characterized by a high concentration of small ice crystals and is typically observed in non-polluted areas. On the other hand, TIC-2 is characterized by a low concentration of larger precipitating ice crystals. In this study, it is hypothesised that TIC-2 are linked to highly polluted environments. Past field experiments have shown that most aerosols in the accumulation mode are coated by sulphuric acid in polluted episodes in the Arctic during winter. Recent laboratory experiments have shown that sulphuric acid coating can alter the efficiency of ice nuclei (IN) to nucleate ice crystals. In this study, we hypothesize that the resulting lower IN concentration found in polluted air masses leads to the decrease of the ice crystal concentration. Since there is in less competition for the available moisture, ice crystals reach precipitating sizes leading to the formation of TIC-2. This research aims to better understand the formation process of these two types of TIC through in-situ observations of TICs during the ISDAC campaign and modeling. Microphysical properties of observed cases of TICs will be presented. Modeling of 4 of these TICs will also be presented using an ice nucleation parameterization based on a combination of results from laboratory studies and the classical heterogeneous nucleation theory. Results show that the parameterization of ice nucleation on sulphuric acid-coated dust particles better reproduces TIC-2 while TIC-1 are better reproduced by the parameterization of ice nucleation on uncoated dust particles. ISDAC observations of TICs also confirm the presence of both TICs and support the relationships between (1) TIC-2 and acidic aerosols and (2) TIC-1 and pristine aerosols.