ABSTRACT
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 (râ¢OH) and the effects of hydroxyl radicals (â¢OH) were measured as well as the optical properties as a function of the photoaging of the extracts. râ¢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, â¢OH was seen to mostly originate from nitrate photolysis while for the summer extracts, a correlation was seen between râ¢OH and iron concentration. The extent of photobleaching of the extracts was correlated with râ¢OH, and the correlation also indicated that non-â¢OH processes took place. Using the â¢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.
