Comparison of Isoprene-Derived Secondary Organic Aerosol Formation Pathways at an Urban and a Forest Site

Atmospheric oxidation of nonmethane hydrocarbons (NMHCs) under low nitrogen oxide conditions plays a critical role in the formation of oxygenated volatile organic compounds (OVOCs), yet measurements reflecting an accurate representation of these processes remain challenging. This study investigates the oxidation products of nbutane and 1-butene (C4 oxidation) under both low and high NOx regimes and of isoprene in a low NOx regime using the NSF NCAR atmospheric simulation chamber. Measurements were obtained using the Trace Organic Gas Analyzer (TOGA) and a proton-transfer reaction mass spectrometer (PTR-MS) and compared with predictions from a box model using the Master Chemical Mechanism (MCMv3.3.1). Our results show that under low NOx conditions, C4 hydroperoxides convert on the PTR-MS instrument surfaces to carbonyl artifacts, precluding an accurate picture of atmospheric composition. The PTR-MS conversion efficiencies for n-butane hydroperoxides, 1-butene hydroxy hydroperoxides, and ISOPOOH to carbonyl products were found to be 35 ± 1%, 67 ± 5%, and 24 ± 2%, respectively. TOGA exhibited minimal bias due to its inert internal surfaces. To further investigate surface effects, this study assessed the relative conversion of hydroperoxides to carbonyl products during analyte transmission through both stainless steel (SS) tubing and tubing treated to improve inertness (Restek Sulfinert) at different temperatures. We found that the conversion efficiency increases with temperature for hydroperoxides formed from both isoprene and C4 oxidation and that the treated surface tubing is far superior to that of untreated SS in preventing these conversion reactions. These findings highlight the potential for significant error from the reported low NOx oxidation products of the many other hydrocarbons in historical VOC data sets, apart from the previously studied isoprene. Accurate quantification of OVOCs in these environments is essential for refining atmospheric models and understanding chemical cycling in the changing NOx landscape.