Characterization of a real-time tracer for isoprene epoxydiols-derived secondary organic aerosol (IEPOX-SOA) from aerosol mass spectrometer measurements
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 C_5H_6O^+ (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_5H_6O (fC_5H_6O= C_5H_6O+/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 fC_5H_6O (~ 6.5 ± 2.2 ‰ on average) than other sites, consistent with the expected IEPOX-SOA formation in those studies. fC_5H_6O 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 fCO_2 vs. fC_5H_6O. 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 fC_5H_6O 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.
© Author(s) 2015. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 03 Mar 2015. Published in Atmos. Chem. Phys. Discuss.: 16 Apr 2015. Revised: 16 Aug 2015. Accepted: 25 Sep 2015. Published: 23 Oct 2015. Edited by: A. Carlton. This study was partially supported by NSF AGS-1243354 and AGS-1360834, NASA NNX12AC03G, DOE (BER/ASR) DE-SC0011105, and NOAA NA13OAR4310063. B. Palm and J. Krechmer are grateful for fellowships from EPA STAR (FP-91761701-0 and FP-91770901-0) and CIRES. A. Ortega is grateful for a CU-Boulder Chancellor's and DOE SCGF (ORAU/ORISE) fellowship. A. Wisthaler and T. Mikoviny were supported by the Austrian Federal Ministry for Transport, Innovation and Technology (BMVIT) through the Austrian Space Applications Programme (ASAP) of the Austrian Research Promotion Agency (FFG), and the Visiting Scientist Program at the National Institute of Aerospace (NIA). G. Isaacman-Van Wertz is grateful for an NSF Fellowship (DGE-1106400). U. C. Berkeley was supported by NSF AGS-1250569. We acknowledge the logistical support from the LBA Central Office at INPA (Instituto Nacional de Pesquisas da Amazonia). P. Artaxo acknowledges support from FAPESP grants 2013/05014-0 and 2014/05238-8 and CNPq support from grants 457843/2013-6 and 307160/2014-9. We acknowledges this work was funded by the US Environmental Protection Agency (EPA) through grant number 835404. The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of the US EPA. Further, the US EPA does not endorse the purchase of any commercial products or services mentioned in the publication. The US EPA through its Office of Research and Development collaborated in the research described here. It has been subjected to agency review and approved for publication, but may not necessarily reflect social agency policy. The authors would also like to thank the Electric Power Research Institute (EPRI) for their support. M. Riva and J. D. Surratt wish to thank the Camille and Henry Dreyfus Postdoctoral Fellowship Program in Environmental Chemistry for their financial support. We thank J. Crounse and P.Wennberg from Caltech for gas-phase IEPOX data in SOAS-CTR and DC3, under support from NASA NNX12AC06G. We thank Lu Xu and Nga Lee Ng from Georgia Tech for providing data from their studies. We acknowledge funding from the UK Natural Environment Research Council through the OP3 and SAMBBA projects (grant refs. NE/D002117/1 and NE/J010073/1).
Published - acp-15-11807-2015.pdf