Abstract View
SOA Production from Chlorine Radical Oxidation of Alkanes: Effects of Structural Branching, NOx, and Relative Humidity
LEIF JAHN, Dongyu S. Wang, Surya Venkatesh Dhulipala, Felipe Cardoso Saldaña, Lea Hildebrandt Ruiz, University of Texas at Austin
Abstract Number: 576
Working Group: Aerosol Chemistry
Abstract
Atmospheric chlorine radicals (Cl) have recently been observed in a variety of continental as well as coastal regions at concentrations significant for the formation of secondary organic aerosol (SOA). Experiments have shown that Cl oxidizes most VOC precursors more rapidly than hydroxyl (OH) radicals and that Cl-oxidation often results in higher SOA yields. This effect is especially pronounced for alkanes, however comparatively little is known about the mechanism and products of the reactions of chlorine radicals with alkanes of different structures. We investigated the Cl-oxidation of a series of C10 hydrocarbon precursors with linear (n-decane), branched (2-methylnonane and 3,3-dimethyloctane), and branched cyclic (butylcyclohexane) structures through laboratory smog chamber experiments utilizing an iodide-adduct chemical ionization mass spectrometer (I- CIMS) with a filter inlet for gases and aerosols (FIGAERO) and an aerosol chemical speciation monitor (ACSM). Experiments were performed with or without added NOx and at low (<5%) or medium (40-50%) relative humidity. Under all examined conditions SOA production decreased on the order butylcyclohexane > n-decane > 2-methylnonane > 3,3-dimethyloctane. Lower SOA production observed for branched hydrocarbons is consistent with increased fragmentation to higher volatility molecules and was observed through early-generation gas-phase fragmentation products. Patterns in fragmentation products of the branched alkanes also suggest that chlorine radicals are able to abstract primary hydrogen atoms, confirming prior theory; this likely contributes to increased SOA production. More SOA was produced under low-NOx conditions, while experiments under low and medium RH conditions produced comparable amounts of SOA but some differences in particle-phase composition were observed. Further details on the products and mechanisms contributing to these trends will also be discussed.