Ambient Measurements of Highly Oxidized Gas-Phase Molecules during the Southern Oxidant and Aerosol Study (SOAS) 2013

We present measurements of highly oxidized multifunctional molecules (HOMs) detected in the gas phase using a high-resolution time-of-flight chemical ionization mass spectrometer with nitrate reagent ion (NO₃ ^- CIMS). The measurements took place during the 2013 Southern Oxidant and Aerosol Study (SOAS 2013) at a forest site in Alabama, where emissions were dominated by biogenic volatile organic compounds (BVOCs). Primary BVOC emissions were represented by isoprene mixed with various terpenes, making it a unique sampling location compared to previous NO₃ ^- CIMS deployments in monoterpene-dominated environments. During SOAS 2013, the NO₃ ^- CIMS detected HOMs with oxygen-to-carbon (O:C) ratios between 0.5 and 1.4 originating from both isoprene (C₅) and monoterpenes (C₁₀) as well as hundreds of additional HOMs with carbon numbers between C₃ and C₂₀. We used positive matrix factorization (PMF) to deconvolve the complex data set and extract information about classes of HOMs with similar temporal trends. This analysis revealed three isoprene-dominated and three monoterpene-dominated PMF factors. We observed significant amounts of isoprene- and monoterpene-derived organic nitrates (ONs) in most factors. The abundant presence of ONs was consistent with previous studies that have highlighted the importance of NO_x-driven chemistry at the site. One of the isoprene-dominated factors had a strong correlation with SO₂ plumes likely advected from nearby coal-fired power plants and was dominated by an isoprene-derived ON (C₅H₁₀N₂O₈). These results indicate that anthropogenic emissions played a significant role in the formation of low-volatility compounds from BVOC emissions in the region.