Hygroscopicity changes of organic aerosols by chemical reactions

Reactions of organic aerosols can lead to changes in their hygroscopicity and their CCN activity. As a result, these processes may change organic aerosols' roles in local, regional and global air quality and climate. While most organic aerosols are secondary and hence already at least slightly processed in the atmosphere, it is quite possible that during their atmospheric lifetime they will continue to change. While several previous studies suggested increase of hygroscopicity, other studies have shown that reactions of the organic matter either do not affect hygroscopicity or even decrease it. We will present recent laboratory studies demonstrating how reactions of ozone and ammonia with aerosol particles containing model organic compounds (organic acids, alkenes) and complex organic matter (HULIS and fulvic acids) affect their hygroscopicity. Specifically, we will discuss under which conditions these reactions occur most efficiently (concentration ranges, exposure times, and humidity), relevance to real atmospheric processes and possible reaction products.