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Substantial amounts of secondary organic aerosol (SOA) can be formed fromisoprene epoxydiols (IEPOX), which are oxidation products of isoprene mainlyunder low-NO conditions. Total IEPOX-SOA, which may include SOA formed fromother parallel isoprene oxidation pathways, was quantified by applyingpositive matrix factorization (PMF) to aerosol mass spectrometer (AMS)measurements. The IEPOX-SOA fractions of organic aerosol (OA) in multiple field studies acrossseveral continents are summarized here and show consistent patterns with theconcentration of gas-phase IEPOX simulated by the GEOS-Chem chemicaltransport model. During the Southern Oxidant and Aerosol Study (SOAS),78 % of PMF-resolved IEPOX-SOA is accounted by the measured IEPOX-SOAmolecular tracers (2-methyltetrols, C5-Triols, and IEPOX-derived organosulfateand its dimers), making it the highest level of molecular identification ofan ambient SOA component to our knowledge. An enhanced signal atC5H6O+ (m/z 82) is found in PMF-resolved IEPOX-SOA spectra.To investigate the suitability of this ion as a tracer for IEPOX-SOA, weexamine fC5H6O(fC5H6O=C5H6O+/OA) across multiple field,chamber, and source data sets. A background of~ 1.7 ± 0.1 ‰ (‰ = parts per thousand) isobserved in studies strongly influenced by urban, biomass-burning, and otheranthropogenic primary organic aerosol (POA). Higher background values of3.1 ± 0.6 ‰ are found in studies strongly influenced bymonoterpene emissions. The average laboratory monoterpene SOA value(5.5 ± 2.0 ‰) is 4 times lower than the average for IEPOX-SOA(22 ± 7 ‰), which leaves some room to separate bothcontributions to OA. Locations strongly influenced by isoprene emissionsunder low-NO levels had higher fC5H6O(~ 6.5 ± 2.2 ‰ on average) than other sites, consistentwith the expected IEPOX-SOA formation in those studies.fC5H6O in IEPOX-SOA is always elevated(12–40 ‰) but varies substantially between locations, which isshown to reflect large variations in its detailed molecular composition. Thelow fC5H6O (< 3 ‰) reported innon-IEPOX-derived isoprene-SOA from chamber studies indicates that thistracer ion is specifically enhanced from IEPOX-SOA, and is not a tracer forall SOA from isoprene. We introduce a graphical diagnostic to study thepresence and aging of IEPOX-SOA as a triangle plot of fCO2vs. fC5H6O. Finally, we develop a simplifiedmethod to estimate ambient IEPOX-SOA mass concentrations, which is shown toperform well compared to the full PMF method. The uncertainty of the tracermethod is up to a factor of ~ 2, if thefC5H6O of the local IEPOX-SOA is notavailable. When only unit mass-resolution data are available, as with theaerosol chemical speciation monitor (ACSM), all methods may perform less wellbecause of increased interferences from other ions at m/z 82. This studyclarifies the strengths and limitations of the different AMS methods fordetection of IEPOX-SOA and will enable improved characterization of this OAcomponent

atmospheric chemistry and physics

Characterization of a real-time tracer for isoprene epoxydiols-derived secondary organic aerosol (IEPOX-SOA) from aerosol mass spectrometer measurements