Chemical transformations in monoterpene-derived organic aerosol enhanced by inorganic composition

Secondary organic aerosol (SOA) is known to impact both climate and air quality, yet molecular-level composition measurements remain challenging, hampering our understanding of SOA formation and evolution. Here, we reveal the importance of underestimated reaction pathways for the (trans)formation of SOA from monoterpenes, one of the largest SOA precursors globally. Utilizing mass spectrometric techniques to achieve a comprehensive characterization of molecular-level changes in the SOA, we were able to link the appearance of high-molecular weight (HMW) organic molecules to the concentration and level of neutralization of particulate sulfate. Interestingly, this oligomerization coincided with a decrease of highly oxygenated molecules (HOMs). Our findings highlight the role of particle-phase processing, and the underestimated importance of sulfate aerosol for monoterpene-SOA formation. The observations of these processes directly in the atmosphere reveal the need to account for the formation of HMW oligomers to fully understand the physicochemical properties of organic aerosol. Distant pollution drives formation of secondary organic aerosols Sulfate aerosols produced by industrial activities provide a key substrate for the formation of secondary organic aerosols (SOA) hundreds of km away. An international research group led by Matthieu Riva, now at the Université de Lyon, applied detailed field observations and atmospheric transport modeling to show that SOA are produced when sulfate aerosols interact with monoterpenes — a volatile organic compound emitted by forests. Specifically, along the atmospheric transport pathway from the Russian pollution source to the research station in Finland, oxidized monoterpenes condensed onto sulfate particles, forming SOA. The team used laboratory experiments to confirm a powerful influence of sulfate levels on SOA formation. The research extends prior work showing similar pathways for isoprene-related SOA formation, and cements the importance of SOA formation through multiphase chemistry across of range of biogenically-derived atmospheric compounds.