Molecular Insights into the Impact of Anthropogenic Precursors on SOA Formation in Remote Pristine Environments

Secondary organic aerosol (SOA) has a significant effect on global air quality and the climate. However, due to the complexities of its precursor compounds and formation pathways, our present understanding of SOA formation is quite unclear, especially in remote locations that are pristine in nature. The present work investigates the molecular composition to unravel the formation mechanisms of water-soluble organic aerosols in the samples collected in campaign mode from January to December 2017 from a tropical rain forest location. The analysis showed that the molecular composition varied depending on the origin of the air masses. The sampling site was affected by three different air masses, with each displaying distinct molecular signatures and formation mechanisms. In samples of continental air masses, other than C₂, phthalic acid was the most prevalent, followed by succinic acid, but azelaic acid dominated in the samples of marine air masses. When anthropogenic precursors from continental sources arrived at the pristine location, the concentration of the stable end product, oxalic acid increased to two fold. Furthermore, the presence of precursor compounds from mixed sources impacted the formation of anthropogenic SOA, which included phthalic and terephthalic acids. Based on our experimental analysis, we proposed a new formation mechanism for terephthalic acid in a pristine environment. This work investigated and outlined the possible oxidative reaction mechanisms for the formation of dicarboxylic acids at the molecular level at a tropical rain forest location.