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Characterization of a real-time tracer for isoprene epoxydiols-derived secondary organic aerosol (IEPOX-SOA) from aerosol mass spectrometer measurements
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2015
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Source: Atmospheric Chemistry and Physics, 15(20), 11807-11833.
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Journal Title:Atmospheric Chemistry and Physics
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Description:Substantial amounts of secondary organic aerosol (SOA) can be formed from isoprene epoxydiols (IEPOX), which are oxidation products of isoprene mainly under low-NO conditions. Total IEPOX-SOA, which may include SOA formed from other parallel isoprene oxidation pathways, was quantified by applying positive matrix factorization (PMF) to aerosol mass spectrometer (AMS) measurements. The IEPOX-SOA fractions of organic aerosol (OA) in multiple field studies across several continents are summarized here and show consistent patterns with the concentration of gas-phase IEPOX simulated by the GEOS-Chem chemical transport model. During the Southern Oxidant and Aerosol Study (SOAS), 78% of PMF-resolved IEPOX-SOA is accounted by the measured IEPOX-SOA molecular tracers (2-methyltetrols, C5-Triols, and IEPOX-derived organosulfate and its dimers), making it the highest level of molecular identification of an ambient SOA component to our knowledge. An enhanced signal at C5H6O+ (m/z 82) is found in PMF-resolved IEPOX-SOA spectra. To investigate the suitability of this ion as a tracer for IEPOX-SOA, we examine fC(5)H(6)O (fC(5)H(6)O = C5H6O+ / OA) across multiple field, chamber, and source data sets. A background of similar to 1.7 +/- 0.1 parts per thousand (parts per thousand = parts per thousand) is observed in studies strongly influenced by urban, biomass-burning, and other anthropogenic primary organic aerosol (POA). Higher background values of 3.1 +/- 0.6 parts per thousand are found in studies strongly influenced by monoterpene emissions. The average laboratory monoterpene SOA value (5.5 +/- 2.0 parts per thousand) is 4 times lower than the average for IEPOX-SOA (22 +/- 7 parts per thousand), which leaves some room to separate both contributions to OA. Locations strongly influenced by isoprene emissions under low-NO levels had higher fC(5)H(6)O (similar to 6.5 +/- 2.2 parts per thousand on average) than other sites, consistent with the expected IEPOX- SOA formation in those studies. fC(5)H(6)O in IEPOX- SOA is always elevated (12-40 parts per thousand) but varies substantially between locations, which is shown to reflect large variations in its detailed molecular composition. The low fC(5)H(6)O (< 3 parts per thousand) reported in non-IEPOX-derived isoprene-SOA from chamber studies indicates that this tracer ion is specifically enhanced from IEPOX- SOA, and is not a tracer for all SOA from isoprene. We introduce a graphical diagnostic to study the presence and aging of IEPOX- SOA as a triangle plot of f(CO2) vs. fC(5)H(6)O. Finally, we develop a simplified method to estimate ambient IEPOX- SOA mass concentrations, which is shown to perform well compared to the full PMF method. The uncertainty of the tracer method is up to a factor of similar to 2, if the fC(5)H(6)O of the local IEPOX- SOA is not available. When only unit mass-resolution data are available, as with the aerosol chemical speciation monitor (ACSM), all methods may perform less well because of increased interferences from other ions at m/z 82. This study clarifies the strengths and limitations of the different AMS methods for detection of IEPOX- SOA and will enable improved characterization of this OA component.
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Source:Atmospheric Chemistry and Physics, 15(20), 11807-11833.
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Rights Information:CC BY
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