Photochemical Aging of Atmospheric Particulate Matter in the Aqueous Phase.

This study focused on the photoaging of atmospheric particulate matter smaller than 2.5 μm (PM2.5) in the aqueous phase. PM2.5 was collected during a winter, a spring, and a summer campaign in urban and rural settings in Colorado and extracted into water. The aqueous extracts were photoirradiated using simulated sunlight, and the production rate (r•OH) and the effects of hydroxyl radicals (•OH) were measured as well as the optical properties as a function of the photoaging of the extracts. r•OH was seen to have a strong seasonality with low mean values for the winter and spring extracts (4.8 and 14 fM s-1 mgC-1 L, respectively) and a higher mean value for the summer extracts (65.4 fM s-1 mgC-1 L). For the winter extracts, •OH was seen to mostly originate from nitrate photolysis while for the summer extracts, a correlation was seen between r•OH and iron concentration. The extent of photobleaching of the extracts was correlated with r•OH, and the correlation also indicated that non-•OH processes took place. Using the •OH measurements and singlet oxygen (1O2) measurements, the half-life of a selection of compounds was modeled in the atmospheric aqueous phase to be between 1.9 and 434 h.