PNNL

Abstract

Isoprene-epoxydiols-derived secondary organic aerosol (IEPOX-SOA) can contribute substantially to organic aerosol (OA) concentrations in forested areas under low NO conditions, hence significantly influencing the regional and global OA budgets, accounting, for example, for 16-36% of the submicron OA in the southeastern United States (SE US) summer. Particle evaporation measurements from a thermodenuder show that the volatility of ambient IEPOX-SOA is lower than that of bulk OA and also much lower than that of known monomer IEPOX-SOA tracer species, indicating that IEPOX-SOA likely exists mostly as oligomers in the aerosol phase. The OH aging process of ambient IEPOXSOA was investigated with an oxidation flow reactor (OFR). New IEPOX-SOA formation in the reactor was negligible, as the OFR does not accelerate processes such as aerosol up-take and reactions that do not scale with OH. Simulation results indicate that adding similar to 100 mu g m(-3) of pure H2SO4 to the ambient air allows IEPOX-SOA to be efficiently formed in the reactor. The heterogeneous reaction rate coefficient of ambient IEPOX-SOA with OH radical (k(OH) /was estimated as 4.0 +/- 2.0 x 10(-13) cm(3) molec(-1) s(-1), which is equivalent to more than a 2-week lifetime. A similar k(OH) was found for measurements of OH oxidation of ambient Amazon forest air in an OFR. At higher OH exposures in the reactor (> 1 x 10(12) molec cm(-3) s), the mass loss of IEPOX-SOA due to heterogeneous reaction was mainly due to revolatilization of fragmented reaction products. We report, for the first time, OH reactive uptake coefficients (gamma(OH) = 0.59 +/- 0.33 in SE US and gamma(OH) = 0.68 +/- 0.38 in Amazon) for SOA under ambient conditions. A relative humidity dependence of k(OH) and gamma(OH) was observed, consistent with surface-area-limited OH uptake. No decrease of k(OH) was observed as OH concentrations increased. These observations of physicochemical properties of IEPOX-SOA can help to constrain OA impact on air quality and climate.