Abstract
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 IEPOX-SOA 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 uptake and reactions that do not scale with OH. Simulation results indicate that adding ∼ 100 µg 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 (kOH) was estimated as 4.0 ± 2.0 × 10−13 cm3 molec−1 s−1, which is equivalent to more than a 2-week lifetime. A similar kOH was found for measurements of OH oxidation of ambient Amazon forest air in an OFR. At higher OH exposures in the reactor (> 1 × 1012 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 (γOH = 0.59 ± 0.33 in SE US and γOH = 0.68 ± 0.38 in Amazon) for SOA under ambient conditions. A relative humidity dependence of kOH and γOH was observed, consistent with surface-area-limited OH uptake. No decrease of kOH 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.
Highlights
Organic aerosol (OA), which comprises 10–90 % of ambient submicron aerosol mass globally, has important impacts on climate forcing and human health (Kanakidou et al, 2005; Zhang et al, 2007; Hallquist et al, 2009)
Lopez-Hilfiker et al (2016) have shown that oligomer decomposition for isoprene-derived epoxydiols (IEPOX)-secondary OA (SOA) upon heating at ∼ 90◦ was important during the southeastern United States (SE US) study, but that process will only make the measured volatility of IEPOX-SOA in TD higher than it should be
We investigated volatility and aging processes of IEPOXSOA during the late spring and early summer of SE US and the dry season of central Amazonia with a field-deployed thermodenuder and an oxidation flow reactor
Summary
Organic aerosol (OA), which comprises 10–90 % of ambient submicron aerosol mass globally, has important impacts on climate forcing and human health (Kanakidou et al, 2005; Zhang et al, 2007; Hallquist et al, 2009). Several studies have shown that factor analysis of real-time aerosol mass spectrometer (AMS) data provides a method to obtain the total amount, overall fraction contribution, and properties of IEPOX-SOA (Robinson et al, 2011; Budisulistiorini et al, 2013; Chen et al, 2015). Ambient gas and aerosol species were sampled through an oxidation flow reactor (OFR) and a thermodenuder (TD) to investigate heterogeneous oxidation and evaporation of ambient IEPOX-SOA, respectively. These systems included an AMS and other online instruments measuring both gas and aerosol species inflow and outflow. IEPOX-SOA aging during the dry season of 2014 in central Amazonia as part of the Green Ocean Amazon (GoAmazon2014/5, IOP2) experiment, using the same OFR experimental setup, was compared to the SOAS results
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