Detailed chemical characterization of unresolved complex mixtures in atmospheric organics: Insights into emission sources, atmospheric processing, and secondary organic aerosol formation

Abstract

[1] Recent studies suggest that semivolatile organic compounds (SVOCs) are important precursors to secondary organic aerosol (SOA) in urban atmospheres. However, knowledge of the chemical composition of SVOCs is limited by current analytical techniques, which are typically unable to resolve a large number of constitutional isomers. Using a combination of gas chromatography and soft photoionization mass spectrometry, we characterize the unresolved complex mixture (UCM) of semivolatile aliphatic hydrocarbons observed in Pasadena, California (~16 km NE of downtown Los Angeles), and Bakersfield, California, during the California Research at the Nexus of Air Quality and Climate Change 2010. To the authors' knowledge, this work represents the most detailed characterization of the UCM in atmospheric samples to date. Knowledge of molecular structures, including carbon number, alkyl branching, and number of rings, provides important constraints on the rate of atmospheric processing, as the relative amounts of branched and linear alkanes are shown to be a function of integrated exposure to hydroxyl radicals. Emissions of semivolatile branched alkanes from fossil fuel-related sources are up to an order of magnitude higher than those of linear alkanes, and the gas-phase OH rate constants of branched alkanes are~30% higher than their linear isomers. Based on a box model considering gas/particle partitioning, emissions, and reaction rates, semivolatile branched alkanes are expected to play a more important role than linear alkanes in the photooxidation of the UCM and subsequent transformations into SOA. Detailed speciation of semivolatile compounds therefore provides essential understanding of SOA sources and formation processes in urban areas.Citation: Chan, A. W. H., et al. (2013), Detailed chemical characterization of unresolved complex mixtures in atmospheric organics: Insights into emission sources, atmospheric processing, and secondary organic aerosol formation, J. Geophys.

A naming convention for atmospheric organic aerosol

“…Chemical transport models have implemented this framework and explored the sensitivity of OA mass predictions to uncertainties in volatility of emissions and effects of oxidation Hodzic et al, 2010;Murphy and Pandis, 2010;Fountoukis et al, 2011;Murphy et al, 2011;Ahmadov et al, 2012;Bergström et al, 2012). Observations of organic aerosol volatility have been refined as well through the use of gas chromatography (Isaacman et al, 2011;Presto et al, 2012;May et al, 2013a, b;Chan et al, 2013) and thermodenuder methods (Baltensperger et al, 2005;An et al, 2007;Saleh et al, 2008;Grieshop et al, 2009a, b;Hildebrandt et al, 2009;Huffman et al, 2009). Because of the physical importance of volatility to gas-particle partitioning and the widespread attention volatility has received, it makes sense to anchor a proposed scheme to this property.…”

“…The proposed framework specifically identifies the current volatility of organic compounds, a property already measured and reported by many field and lab campaigns Lee et al, 2010;Isaacman et al, 2011;Chan et al, 2013;May et al, 2013a, b). As thermodenuder, dilution, and gas chromatography experiments become more common in the future, it will be critical to report current volatility in a consistent, succinct way.…”

Particle size dependence of biogenic secondary organic aerosol molecular composition

“…On a molecular level, ambient organic aerosol is very complex, encompassing hundreds to thousands of individual compounds (Chan et al, 2013;Goldstein and Galbally, 2007;Hallquist et al, 2009;Mutzel et al, 2015;Putman et al, 2012). Laboratory secondary organic aerosol (SOA) is similarly complex, and a variety of oxidation and degradation pathways have been proposed to explain the product distributions (Daumit et al, 2013;Hall and Johnston, 2012;Hallquist et al, 2009;Perraud et al, 2012).…”

Comparison of advanced offline and in situ techniques of organic aerosol composition measurement during the CalNex campaign

“…Details of the analysis method are described in Chan et al (2013). In brief, filter punches (total area of 1.6 cm 2 ) were thermally desorbed at 320 • C under helium (TDS3, Gerstel) to a two-dimensional gas chromatograph (Agilent 7890).…”