**Preface to "Atmospheric Aqueous-Phase Chemistry"**

Liquid water in cloud and fog droplets and in moist aerosol particles is ubiquitous in the atmosphere. Dissolved species from the soluble aerosol fraction, as well as soluble trace gases, undergo chemical reactions in the aqueous phase via different mechanisms, usually yielding different products from those in the gas phase. In addition to their different reactivity, the chemical species solubility determines their fate in the atmosphere, i.e., their involvement in gas-phase or aqueous-phase chemistry.

Numerous studies confirm that the predominant fraction of atmospheric sulfate formed through multiphase oxidation of sulfur (IV) from fossil fuel combustion takes place in cloud droplets, yet there are still unresolved questions concerning sulfate formation under extremely polluted conditions, such as in China.

Recently, it has been recognized that secondary organic aerosol (SOA) mass may also be formed via chemical reactions, in cloud and fog droplets and moist aerosol particles. During atmospheric processing, the primary emitted organic pollutants become more oxidized, less volatile and more water-soluble. Consequently, within the pollutants' lifetime in the atmosphere, aqueous-phase chemistry becomes more and more important for their aging.

This Special Issue of *Atmosphere*, "Atmospheric Aqueous-Phase Chemistry", comprises ten original articles dealing with different aspects of chemistry in atmospheric liquid water. The articles can be divided into three groups: (i) the first group of four articles is mostly based on field measurements and/or combined field and modeling studies giving insights into the chemical characterization of different atmospheric liquid water samples from various environments; (ii) the second set of four articles is focused on studies of aqueous-phase reactivity of some important atmospheric organic compounds; and (iii) the final group comprises two articles based on predictive modeling and/or combined modeling, and laboratory studies providing insight into aqueous secondary organic aerosol (SOA) formation.

I thank all the authors for their valuable contributions, the reviewers for their constructive comments, and the staff of the editorial office at MDPI for their cooperation and support in publishing this issue. I hope that the articles of this Special Issue will stimulate further research to improve the current understanding of atmospheric aqueous-phase chemistry.

> **Irena Grgi´c** *Editor*
