Heterogeneous nucleation of ice in (NH 4 ) 2 SO 4 ‐H 2 O particles with mineral dust immersions

Abstract

Using optical microscopy, we investigated the heterogeneous nucleation of ice in aqueous (NH 4 ) 2 SO 4 ‐H 2 O particles containing two types of mineral dusts, kaolinite and montmorillonite. The efficacy of montmorillonite and kaolinite to nucleate ice in (NH 4 ) 2 SO 4 ‐H 2 O particles is similar. The difference in freezing temperatures, compared to the homogeneous freezing temperatures, is found to vary from 8–20 K and it is larger for particles with concentrations greater than 27 wt %. Our freezing data shows that for temperatures ranging from 239 K to 198 K, ice super‐saturations between 1.35 and 1.51 are required for ice to heterogeneously nucleate in NH 4 SO 4 ‐H 2 O particles containing mineral dust immersions. Based on our results, we conclude mineral dust is an efficient nuclei for ice in NH 4 SO 4 ‐H 2 O aerosols and as a result, it can initiate the formation of upper tropospheric ice clouds at warmer temperatures and lower super‐saturations in comparison to homogeneous freezing.