Isoprene Epoxydiols Derived Secondary Organic Aerosol (IEPOX-SOA): Insights from Aerosol Mass Spectrometer Field Measurements

Wednesday, 17 December 2014: 2:10 PM
Weiwei Hu1, Pedro Campuzano Jost1, Brett B Palm1, Douglas A Day1, Amber M Ortega1, Patrick L Hayes1, Qi Chen2, Mikinori Kuwata2,3, Yingjun Liu2, Suzane S. de Sá2, Scot T Martin2, Min Hu4, Sri Hapsari Budisulistiorini5, Jason D Surratt5, Kenneth Docherty6, Gabriel A Isaacman7, Allen H Goldstein7, Jason Michael St Clair8, John Crounse8, Paul O. Wennberg8 and Jose-Luis Jimenez1, (1)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (2)Harvard University, Cambridge, MA, United States, (3)Nanyang Technological University, Earth Observatory of Singapore, Singapore, Singapore, (4)Peking University, Beijing, China, (5)University of North Carolina at Chapel Hill, Chapel Hill, NC, United States, (6)Environmental Protection Agency Research Triangle Park, Research Triangle Park, NC, United States, (7)University of California Berkeley, Berkeley, CA, United States, (8)California Institute of Technology, Pasadena, CA, United States
Secondary organic aerosol (SOA) derived from isoprene epoxydiols (IEPOX), produced from isoprene oxidation under low-NO conditions, can account for a substantial fraction of organic aerosol (OA) in biogenic dominated areas. In this study, IEPOX-SOA was identified from measurements at the forested southeast US supersite (Centreville, AL) during the SOAS campaign using Positive Matrix Factorization (PMF) of aerosol mass spectrometer (AMS) measurement. By combining SOAS results with other field and lab studies, characteristics of IEPOX-SOA spectra in AMS are systematically evaluated. fC5H6O (The fraction of OA measured at C5H6O+, the major ion at m/z 82) in AMS spectra is shown to be a good tracer for IEPOX-SOA. We observed statistically higher fC5H6O in OA from regions with strong isoprene emissions (3×10-3 - 25×10-3) vs. that from urban and biomass-burning plumes (0 - 3.5×10-3, an avg. of 1.75 ×10-3). In isoprene-influenced areas, fC5H6O in ambient OA decreases with OA aging. The decay of fC5H6O in fCOvs fC5H6O scatter plot suggests that physical mixing of air masses might play a more important role in ambient atmosphere. Analyzing the decay of IEPOX-SOA in an oxidation flow reactor results in an empirical kOH for IEPOX-SOA chemical aging of 5.310-13 cm3 molec.−1 s−1, equivalent to an OH lifetime of 2 weeks (OH avg. of 1.5×106 molec. cm-3). The thermograms of IEPOX-SOA suggest similar or lower volatility than for the bulk OA, indicating that most of the IEPOX-SOA will not evaporate under any atmospheric conditions. This contrasts with the small semivolatile molecules that have been identified so far as IEPOX-SOA components. Finally, we develop a simplified method to estimate ambient IEPOX-SOA mass concentrations as a function of C5H6O+, which is shown to perform well compared to the full PMF method. When only unit mass resolution data is available the method performs less well due to increased interferences from other ions at m/z 82. Estimated IEPOX-SOA concentrations in the southeast US from an aircraft campaign correlate well with IEPOX-related species detected by other techniques, which confirms the validity of this method applied here. IEPOX-SOA accounts for 14-17% of the OA in the SE US. We explore the dependence of IEPOX-SOA on key parameters such as acidity, sulfate, and gas-phase IEPOX concentrations.