Index Geophysics

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 C₅H₆O⁺ (<I>m/z</I> 82) is found in PMF-resolved IEPOX-SOA spectra. To investigate the suitability of this ion as a tracer for IEPOX-SOA, we examine <I>f</I>_C₅H₆O (<I>f</I>_C₅H₆O= C₅H₆O⁺/OA) across multiple field, chamber, and source data sets. A background of ~ 1.7 ± 0.1 &permil; (&permil; = 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 &permil; are found in studies strongly influenced by monoterpene emissions. The average laboratory monoterpene SOA value (5.5 ± 2.0 &permil;) is 4 times lower than the average for IEPOX-SOA (22 ± 7 &permil;), which leaves some room to separate both contributions to OA. Locations strongly influenced by isoprene emissions under low-NO levels had higher <I>f</I>_C₅H₆O (~ 6.5 ± 2.2 &permil; on average) than other sites, consistent with the expected IEPOX-SOA formation in those studies. <I>f</I>_C₅H₆O in IEPOX-SOA is always elevated (12–40 &permil;) but varies substantially between locations, which is shown to reflect large variations in its detailed molecular composition. The low <I>f</I>_C₅H₆O (< 3 &permil;) 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 <I>triangle plot</I> of <I>f</I>_CO₂ vs. <I>f</I>_C₅H₆O. 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 <I>f</I>_C₅H₆O 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 <I>m/z</I> 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.

Hu, W. W.

Campuzano-Jost, P.

Palm, B. B.

Day, D. A.

Ortega, A. M.

Hayes, P. L.

Krechmer, J. E.

Chen, Q.

Kuwata, M.

Liu, Y. J.

Sá, S. S.

McKinney, K.

Martin, S. T.

Hu, M.

Budisulistiorini, S. H.

Riva, M.

Surratt, J. D.

Clair, J. M.

Isaacman-Van Wertz, G.

Yee, L. D.

Goldstein, A. H.

Carbone, S.

Brito, J.

Artaxo, P.

Gouw, J. A.

Koss, A.

Wisthaler, A.

Mikoviny, T.

Karl, T.

Kaser, L.

Jud, W.

Hansel, A.

Docherty, K. S.

Alexander, M. L.

Robinson, N. H.

Coe, H.

Allan, J. D.

Canagaratna, M. R.

Paulot, F.

Jimenez, J. L.

2018-09-06

Text

application/pdf

10.5194/acp-15-11807-2015

https://acp.copernicus.org/articles/15/11807/2015/

eISSN: 1680-7324

eng