Real-time chemical characterization of atmospheric organic aerosol in the southeastern United States by aerosol mass spectrometry

Secondary organic aerosol (SOA) produced from reactive uptake and multiphase chemistry of isoprene epoxydiols (IEPOX) has been found to contribute substantially (upward of 33%) to the fine organic aerosol mass over the Southeastern U.S. Brown carbon (BrC) in rural areas of this region has been linked to secondary sources in the summer when the influence of biomass burning is low. We demonstrate the formation of light-absorbing (290 < λ < 700 nm) SOA constituents from reactive uptake of trans-β-IEPOX onto preexisting sulfate aerosols as a potential source of secondary BrC. IEPOX-derived BrC generated in controlled chamber experiments under dry, acidic conditions has an average mass absorption coefficient of ∼300 cm g−1. Chemical analyses of SOA constituents using UV−visible spectroscopy and high-resolution mass spectrometry indicate the presence of highly unsaturated oligomeric species with molecular weights separated by mass units of 100 (C5H8O2) and 82 (C5H6O) coincident with the observations of enhanced light absorption, suggesting such oligomers as chromophores, and potentially explaining one source of humic-like substances (HULIS) ubiquitously present in atmospheric aerosol. Similar light-absorbing oligomers were identified in fine aerosol collected in the rural Southeastern U.S., supporting their atmospheric relevance and revealing a previously unrecognized source of oligomers derived from isoprene that contributes to ambient fine aerosol mass. ■ INTRODUCTION Atmospheric fine aerosol significantly impacts Earth’s climate directly through scattering and absorbing solar radiation and indirectly through aerosol−cloud interactions. The combined effects of atmospheric aerosol on total radiative forcing (RF) constitute a major source of uncertainty in climate models, with the consequence that confidence in assessments of the effects on climate change is lacking. Secondary organic aerosol (SOA) produced by atmospheric transformation of gaseous hydrocarbon precursors is generally not considered as a source of light-absorbing materials and is therefore not well-represented in current climate models. However, recent laboratory and field observations indicate SOA composed of either humic-like substances (HULIS) or nitrogen-containing species derived from anthropogenic and biogenic sources have significant light absorbance in the near-ultraviolet (UV) and visible range associated with brown carbon (BrC). Such observations establish the need to better identify and quantify sources of light-absorbing SOA and suggest that the warming contribution of organic carbonaceous aerosol is likely to be underestimated in models which do not incorporate a realistic treatment of this potentially important source of positive RF. Recent investigations of the atmospheric transformations of isoprene (2-methyl-1,3-butadiene) emissions from terrestrial vegetation have demonstrated the importance of relative humidity (RH) as well as anthropogenically mitigated levels of oxides of nitrogen (NOx) and aerosol acidity in isoprene SOA formation. Substantial isoprene SOA (upward of 33% of the organic mass fraction in fine aerosol) has been observed in recent field studies in the Southeastern U.S. where biogenic SOA formation is enhanced by anthropogenic influences. During summer, when emissions from biomass burning are low in the rural southeast, secondary BrC from biogenic sources was suggested to contribute a major fraction of soluble BrC.Nevertheless, light-absorbing properties of SOA from isoprene photooxidation products and the extent to which isoprene-derived SOA might contribute to BrC aerosol have not been considered to date in evaluating the environmental impact of atmospheric BrC. We report here that SOA derived from the reactive uptake of isoprene epoxydiols (IEPOX), second-generation photochemical oxidation products of isoprene under low-NOx conditions, onto acidified sulfate seed aerosol yields SOA with a chemical composition similar to HULIS and may be a substantial contributor to the absorbance of tropospheric UV and visible Received: June 30, 2014 Revised: September 10, 2014 Accepted: September 16, 2014 Published: September 16, 2014 Article