1. SOA derived from isoprene epoxydiols: Insights into formation, aging and distribution over the continental US from the DC3 and SEAC 4 RS campaigns Pedro Campuzano-Jost, Weiwei Hu, Brett B. Palm, Douglas A. Day, Amber M. Ortega, Jose-Luis Jimenez, Jin Liao, Karl D. Froyd, Fabien Paulot, Gabriel Isaacman, Allen Goldstein, Jason M. St. Clair, John D. Crounse, Paul O. Wennberg, Luke D Ziemba, Bruce E Anderson, Simone Meinardi, Barbara Barletta, Don Blake CU Boulder, NOAA ESRL, UC Berkeley, Princeton U, Caltech, NASA LarC, UC Irvine

2. What is “IEPOX-SOA”? 2 Paulot et al, 2009, Surrat et al, 2010, Robinson et al., 2011; Budisulistiorini et al., 2013  IEPOX (Isoprene epoxidiol) formed from oxidation of isoprene under low NO conditions  IEPOX uptake on acidic sulfate particles produces a variety of polyols, organosulfates and oligomers  The sum of all these is IEPOX-SOA  Despite the complexity of the chemistry, AMS spectra retrieved by PMF show a very specific and reproducible, yet slightly variable pattern IEPOX-SOA C4H5C4H5 C5H6OC5H6O C3H7O2C3H7O2

3. IEPOX-SOA as observed by the AMS 3 Hu et al, 2015, Isaacman et al, in prep SOAS Quite a few ground studies in the literature since 2011 IEPOX-SOA PMF Factor correlates well with molecular tracers SEAC 4 RS dataset unique in breadth and scope, ideally suited for testing the robustness of the assignment

4. Geographical distribution: SEAC 4 RS 4 IEPOXIEPOX-SOA SE US clearly stands out as a main source region, some in OR/CA as well There are however quite a few hotspots in the W US for both (and sometimes IEPOX-OS) 2013 global IEPOX model from Fabien Paulot

5. Vertical Distribution: SEAC 4 RS 5 EAST WEST Contribution varies between 20%- 50% Lower values at the beginning and end of the campaign (colder periods) This is almost twice as much as observed during SOAS Not an “isoprene volcano” artifact

6. Geographical distribution: DC3 6 IEPOXIEPOX-SOA Limited sampling, but SE IEPOX chemistry was still robust There is IEPOX in OK/CO/TX. Some IEPOX-SOA was found as well, might be anomaly

7. Vertical Distribution: DC3 7 EAST WEST IEPOX was higher than SEAC 4 RS, so substantial IEPOX-SOA production despite lower temps and less acidic aerosol not surprising. Some layering apparent, not only due to lower statistics.

8. IEPOX-SOA Formation: What can we learn from the field data? 8 Main factors affecting IEPOX-SOA formation according to our current understanding: -IEPOX -Acidity/Liquid water content -Sulfate availability Organosulfates are formed during IEPOX uptake. IEPOX-OS can be quantified by the PALMS instrument, and should hence be a good marker of fresh IEPOX-SOA Surrat et al, 2010, Froyd et al 2010, Nguyen et al 2015, Liao et al 2015

9. DC3: Close to sources, plenty of water 9 Look Rock 10:1 “1:1” More IEPOX-SOA formed at high RH/high liquid water/high sulfate This means mostly at higher altitudes/in the residual layer Little pH dependence, otherwise trends as expected

10. SEAC 4 RS:Many sources, different regimes 10

11. Variability of IEPOX-SOA Composition Only seen when doing vertical profiles This split is seen in six (of twelve) SEAC 4 RS flights, with the same MS signature in each case Enhanced C 3 H 7 O 2 (m/z 75) is seen in the higher up/CH x O rich factor every time 11 BL Factor O: C 0.70 RL Factor O: C 0.74 C5H6OC5H6OC3H7O2C3H7O2

12. C 3 H 7 O 2 as an RL IEPOX-SOA marker? 12 C 3 H 7 O 2 (m/z 75) enhancements relative to m/z 82 is a good tracer of IEPOX-SOA under more humid conditions aloft Hence this marker is significantly lower in ground measurements IEPOX-OS is formed under less humid conditions, hence some anticorrelation is expected

13. Conclusions  The contribution of IEPOX-SOA to SE OA was substantial for both DC3 and SEAC4RS (20-50%)  IEPOX, inorganic sulfate & aerosol water correlate with IEPOX-SOA.  A cleaner SE US will have more IEPOX (due to lower NO), but maybe not more IEPOX-SOA (as SO 4 is also lower)  Organosulfates correlate reasonably well with IEPOX-SOA at low liquid water content/high ionic strength  IEPOX-SOA in the residual layer has a distinct chemical signature  Formation under higher humidity or in-cloud? 13