Articles | Volume 15, issue 20
Atmos. Chem. Phys., 15, 11807–11833, 2015

Special issue: Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5)...

Atmos. Chem. Phys., 15, 11807–11833, 2015

Research article 23 Oct 2015

Research article | 23 Oct 2015

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

W. W. Hu1,2, P. Campuzano-Jost1,2, B. B. Palm1,2, D. A. Day1,2, A. M. Ortega1,3, P. L. Hayes1,2,a, J. E. Krechmer1,2, Q. Chen4,5, M. Kuwata4,6, Y. J. Liu4, S. S. de Sá4, K. McKinney4, S. T. Martin4, M. Hu5, S. H. Budisulistiorini7, M. Riva7, J. D. Surratt7, J. M. St. Clair8,b,c, G. Isaacman-Van Wertz9, L. D. Yee9, A. H. Goldstein9,10, S. Carbone11, J. Brito11, P. Artaxo11, J. A. de Gouw1,2,12, A. Koss2,12, A. Wisthaler13,14, T. Mikoviny13, T. Karl15, L. Kaser14,16, W. Jud14, A. Hansel14, K. S. Docherty17, M. L. Alexander18, N. H. Robinson19,d, H. Coe19, J. D. Allan19,20, M. R. Canagaratna21, F. Paulot22,23, and J. L. Jimenez1,2 W. W. Hu et al.
  • 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 2Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA
  • 3Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA
  • 4School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
  • 5State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China
  • 6Earth Observatory of Singapore, Nanyang Technological University, Singapore
  • 7Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, The University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
  • 8Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
  • 9Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA, USA
  • 10Department of Civil and Environmental Engineering, University of California, Berkeley, CA, USA
  • 11Department of Applied Physics, University of Sao Paulo, Sao Paulo, Brazil
  • 12NOAA Earth System Research Laboratory, Boulder, CO, USA
  • 13Department of Chemistry, University of Oslo, Oslo, Norway
  • 14Institute for Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria
  • 15Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innsbruck, Austria
  • 16Atmospheric Chemistry Division (ACD), National Center for Atmospheric Research, Boulder, CO, USA
  • 17Alion Science and Technology, Research Triangle Park, NC, USA
  • 18Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, USA
  • 19School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester, UK
  • 20National Centre for Atmospheric Science, University of Manchester, Oxford Road, Manchester, UK
  • 21Aerodyne Research, Inc., Billerica, MA, USA
  • 22NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA
  • 23Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA
  • anow at: Department of Chemistry, Université de Montréal, Montréal, QC, Canada
  • bnow at: Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • cnow at: Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD, USA
  • dnow at: Met Office, Exeter, UK

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.

Short summary
This work summarized all the studies reporting isoprene epoxydiols-derived secondary organic aerosol (IEPOX-SOA) measured globally by aerosol mass spectrometer and compare them with modeled gas-phase IEPOX, with results suggestive of the importance of IEPOX-SOA for regional and global OA budgets. A real-time tracer of IEPOX-SOA is thoroughly evaluated for the first time by combing multiple field and chamber studies. A quick and easy empirical method on IEPOX-SOA estimation is also presented.
Final-revised paper