UC Irvine

Understanding how gases interact with and are incorporated into atmospheric secondary organic aerosol particles is crucial for predicting particle effects on climate and human health. This work examined how three gaseous organic nitrates (ON) are taken up into viscous particles formed from the ozonolysis of α-pinene (AP). Experiments were performed in a flow reactor at room temperature under dry conditions, either with or without an OH scavenger present, with constant ozone and variable AP concentrations. Each ON was introduced independently into the flow reactor and was present during particle formation/growth. ON gas-phase concentrations were determined by gas chromatography−mass spectrometry, and particle phase concentrations were measured by high-resolution time-of-flight aerosol mass spectrometry. Partition coefficients (KSOA) for each ON were independent of the initial AP concentration, except for 2-ethylhexyl nitrate which was undetectable in the particles at the lowest AP concentration. Measured KSOA values were larger than those previously determined for equilibrium partitioning, which points to a potential burying mechanism for incorporation of ON during particle growth. Estimated effective net uptake coefficients (γΟΝ) were found to increase with initial AP concentration. Concentrations of gas-phase oxidation products (including dimers and autoxidation products) predicted using an updated master chemical mechanism increased with AP concentration, with little change in the overall species distribution, consistent with increased trapping/burying of ON during particle growth and thus increased values of γΟΝ. These results provide further evidence that kinetically controlled burying can contribute significantly to particle growth, provided that the incoming gas-phase molecules have sufficient residence time on the particle surface to become buried via subsequent gas−surface collisions.