Abstract
Abstract. 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 fC5H6O (fC5H6O= C5H6O+/OA) across multiple field, chamber, and source data sets. A background of ~ 1.7 ± 0.1 ‰ (‰ = 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 ‰ are found in studies strongly influenced by monoterpene 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 both contributions to OA. Locations strongly influenced by isoprene emissions under low-NO levels had higher fC5H6O (~ 6.5 ± 2.2 ‰ on average) than other sites, consistent with the expected IEPOX-SOA formation in those studies. fC5H6O in IEPOX-SOA is always elevated (12–40 ‰) but varies substantially between locations, which is shown to reflect large variations in its detailed molecular composition. The low fC5H6O (< 3 ‰) 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 fCO2 vs. fC5H6O. 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 ~ 2, if the fC5H6O 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.
Highlights
Isoprene (2-methyl-1,3-butadiene, C5H8) emitted by vegetation is the most abundant non-methane hydrocarbon emitted to the Earth’s atmosphere (∼ 440–600 TgC year−1) (Guenther et al, 2012)
The latter site is classified in this category because (i) high isoprene concentrations (e.g. 3 ppb in average peaks in the afternoon) were observed during the study; (ii) the impact of biogenic secondary organic aerosol (SOA) formed during 1000 km where the air travels over the pristine forest upwind of Manaus; (iii) positive matrix factorization (PMF) results indicate an important impact of isoprene epoxydiols (IEPOX)-SOA at this site; (iv) PTRMS results indicate a substantial concentration of the isoprene hydroperoxyde formed by low-NO chemistry, Borneo rain forest in Malaysia, and flight data from SE US flights from aircraft campaign (SEAC4RS). (3) Studies strongly influenced by monoterpene emissions in a pine forest in the Rocky Mountains and a European boreal forest
In practice the uncertainty in IEPOX-SOAPMF concentration is dominated by the larger uncertainty on the aerosol mass spectrometer (AMS) concentrations arising from the collection efficiency and relative ionization efficiency (Middlebrook et al, 2012)
Summary
Isoprene (2-methyl-1,3-butadiene, C5H8) emitted by vegetation is the most abundant non-methane hydrocarbon emitted to the Earth’s atmosphere (∼ 440–600 TgC year−1) (Guenther et al, 2012). The complete molecular composition of IEPOX-SOA has not been elucidated, several molecular species that are part of IEPOX-SOA have been identified through gas chromatography/mass spectrometry (GC/MS), liquid chromatography/mass spectrometry (LC/MS), and particle analysis by laser mass spectrometry (PALMS) They include 2-methyltetrols (and oligomers that contain them) (Surratt et al, 2010; Lin et al, 2014), C5alkene triols (Wang et al, 2005), 3-methyltetrahydrofuran3,4-diols (Lin et al, 2012), and an IEPOX-organosulfate (Froyd et al, 2010; Liao et al, 2014). If f82 in AMS spectra (and/or fC5H6O in HR-AMS spectra) is dominated by IEPOX-SOA, f82 would be a convenient, high time resolution, and potentially quantitative tracer for IEPOX-SOA It will be very useful for investigating the impacts of SOA formation from isoprene with AMS/ACSM measurements, which have become increasingly common in recent years including some continental-scale continuous networks (Fröhlich et al, 2015). While this method is no substitute for a detailed IEPOXSOA identification via PMF, it is a simple method to estimate IEPOX-SOA concentrations (or its absence) in real time from AMS or ACSM measurements or under conditions in real time, or where PMF analysis is not possible or is difficult to perform
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