Secondary organic aerosol (SOA) derived from isoprene epoxydiols: Insights into formation, aging and distribution over the continental US from the DC3 and SEAC4RS campaigns

Wednesday, 17 December 2014: 1:55 PM
Pedro Campuzano Jost1,2, Brett B Palm1,2, Douglas A Day1,2, Weiwei Hu1,2, Amber M Ortega1,2, Jose L Jimenez1,2, Jin Liao2,3, Karl D Froyd2,3, Ilana B Pollack3, Jeff Peischl3, Thomas B Ryerson3, Jason Michael St Clair4, John Crounse4, Paul O Wennberg4, Tomas Mikoviny5, Armin Wisthaler5,6, Luke D Ziemba7 and Bruce E Anderson7, (1)University of Colorado Boulder, Boulder, CO, United States, (2)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (3)NOAA ESRL, Boulder, CO, United States, (4)California Institute of Technology, Pasadena, CA, United States, (5)University of Oslo, Department of Chemistry, Oslo, Norway, (6)University of Innsbruck, Institute of Ion Physics and Applied Physics, Innsbruck, Austria, (7)NASA Langley Research Center, Hampton, VA, United States
Isoprene-derived SOA formation has been studied extensively in the laboratory. However, it is still unclear to what extent isoprene contributes to the overall SOA burden over the southeastern US, an area with both strong isoprene emissions as well as large discrepancies between modeled and observed aerosol optical depth. For the low-NO isoprene oxidation pathway, the key gas-phase intermediate is believed to be isoprene epoxide (IEPOX), which can be incorporated into the aerosol phase by either sulfate ester formation (IEPOX sulfate) or direct hydrolysis. As first suggested by Robinson et al, the SOA formed by this mechanism (IEPOX-SOA) has a characteristic fragmentation pattern when analyzed by an Aerodyne Aerosol Mass Spectrometer (AMS) with enhanced relative abundances of the C5H6Oion (fC5H6O). Based on data from previous ground campaigns and chamber studies, we have developed a empirical method to quantify IEPOX-SOA and have applied it to the data from the DC3 and SEAC4RS aircraft campaigns that sampled the SE US during the Spring of 2012 and the Summer of 2013. We used Positive Matrix Factorization (PMF) to extract IEPOX-SOA factors that show good correlation with inside or downwind of high isoprene emitting areas and in general agree well with the IEPOX-SOA mass predicted by the empirical expression. According to this analysis, the empirical method performs well regardless of (at times very strong) BBOA or urban OA influences. On average 17% of SOA in the SE US boundary layer was IEPOX-SOA.

Overall, the highest concentrations of IEPOX-SOA were typically found around 1-2 km AGL, several hours downwind of the isoprene source areas with high gas-phase IEPOX present. IEPOX-SOA was also detected up to altitudes of 6 km, with a clear trend towards more aged aerosol at altitude, likely a combination of chemical aging and physical airmass mixing. The unique instrument package aboard the NASA-DC8 allows us to examine the influence of multiple factors (aerosol acidity, aerosol water content, sulfate mass fraction, isoprene and terpene source strength) to the relative and absolute contribution of IEPOX-SOA to the total OA burden. In particular, the IEPOX-sulfate measurement from the PALMS instrument was used to estimate the relative contribution of the organosulfate channel to the IEPOX-SOA formation.