Mixed chloride aerosols and their atmospheric implications : a review

Abstract

Natural and anthropogenic chloride aerosols make up a significant fraction of atmospheric particulate matter and play important roles in the boundary layer chemistry. Here we provide a review of the mixing characteristics of chloride aerosols and the subsequent atmospheric implications, which are rarely considered in current field and modeling studies. Single-particle analytical techniques have shown that a large fraction of chlorides mix internally with other components, in particular inorganic salts and organic matters, instead of existing separately. In marine and coastal regions, high proportions of chloride aerosols usually mix with inorganic substances (e.g., Mg, Ca, K, N, S), while small quantities of them are coated by organic matter. In forest, grassland, and agricultural areas, most chlorides in biomass burning particles mix with or are coated by organics. In industrialized urban areas, the chloride aerosols often co-exist with heavy/transition metals (e.g., Zn, Pb) and are coated by organic materials in aged plumes. Moreover, secondary chlorides also mix with mineral dusts, nitrates, and sulfates. The mixing of chloride aerosols with insoluble substances can inhibit their hygroscopic properties, which in turn affects the cloud condensation nuclei activation and heterogeneous reactivity. The encasing of chloride aerosols within light-absorbing substances changes their optical properties and subsequently causes atmospheric warming. This paper emphasizes the complexity of the mixing of chloride aerosols, as well as the potential atmospheric implications thereof, and proposes some research topics deserving future study.