Fog/cloud processing of atmospheric aerosols from a single particle perspective: a review of field observations

Fog/Cloud processing of atmospheric particles changes their physiochemical properties, with profound impacts on atmospheric compositions, climate, and human health. These processes involve intricate physical and chemical processes of individual particles, which are often inadequately represented in current models that assume uniformly mixed aerosol compositions. The activation of particles as cloud condensation nuclei (CCN) or ice nuclei (IN), as well as their subsequent modification during fog/cloud processing, are greatly influenced by the unique physiochemical properties of individual particles. Since the 1990s, numerous observations have been conducted to investigate the physiochemical properties of individual droplet residual particles—those particles remaining after droplet evaporation. These particles serve as valuable tracers for understanding the fog/cloud processing of aerosols. In this review, we first compile the various sampling methods employed to collect individual fog/cloud residual particles and summarize the current understanding of the diverse particle types observed in various regions, including urban/industrial, marine/coastal, dusty, and remote areas. Specifically, we explore the size and chemically segregated activation of these particles, as well as the potential role of complex compositions in the aqueous formation of secondary components. We highlight the impact of mixed secondary compositions on the activation of particle types, including soot-containing, organic matters, sea salt, and dust particles. These works also emphasize the importance of understanding the heterogeneous compositions across individual particles for accurately assessing the formation of secondary species during fog/cloud processing. Furthermore, this review underscores the modification of microphysical properties of individual particles and their potential implications for optical properties and CCN/IN activities. Finally, we outline the limitations of current research on individual fog/cloud residual particles and identify areas for future investigation.