Abstract
Abstract. We performed a systematic intercomparison study of the chemistry and yields of secondary organic aerosol (SOA) generated from OH oxidation of a common set of gas-phase precursors in a Potential Aerosol Mass (PAM) continuous flow reactor and several environmental chambers. In the flow reactor, SOA precursors were oxidized using OH concentrations ranging from 2.0 × 108 to 2.2 × 1010 molec cm−3 over exposure times of 100 s. In the environmental chambers, precursors were oxidized using OH concentrations ranging from 2 × 106 to 2 × 107 molec cm−3 over exposure times of several hours. The OH concentration in the chamber experiments is close to that found in the atmosphere, but the integrated OH exposure in the flow reactor can simulate atmospheric exposure times of multiple days compared to chamber exposure times of only a day or so. In most cases, for a specific SOA type the most-oxidized chamber SOA and the least-oxidized flow reactor SOA have similar mass spectra, oxygen-to-carbon and hydrogen-to-carbon ratios, and carbon oxidation states at integrated OH exposures between approximately 1 × 1011 and 2 × 1011 molec cm−3 s, or about 1–2 days of equivalent atmospheric oxidation. This observation suggests that in the range of available OH exposure overlap for the flow reactor and chambers, SOA elemental composition as measured by an aerosol mass spectrometer is similar whether the precursor is exposed to low OH concentrations over long exposure times or high OH concentrations over short exposure times. This similarity in turn suggests that both in the flow reactor and in chambers, SOA chemical composition at low OH exposure is governed primarily by gas-phase OH oxidation of the precursors rather than heterogeneous oxidation of the condensed particles. In general, SOA yields measured in the flow reactor are lower than measured in chambers for the range of equivalent OH exposures that can be measured in both the flow reactor and chambers. The influence of sulfate seed particles on isoprene SOA yield measurements was examined in the flow reactor. The studies show that seed particles increase the yield of SOA produced in flow reactors by a factor of 3 to 5 and may also account in part for higher SOA yields obtained in the chambers, where seed particles are routinely used.
Highlights
Laboratory and field studies over the last decade have shown that organic components of atmospheric particles constitute 20 to 50 % of the fine particle mass (PM) in the continental mid-latitudes, though the organic content can be higher in tropical forested regions (Kanakidou et al, 2005)
We performed a systematic intercomparison study of the chemistry and yields of secondary organic aerosol (SOA) generated from OH oxidation of a common set of gas-phase precursors in several environmental chambers and a flow reactor
The most significant experimental parameters that varied between chambers and the flow reactor were OH concentration, residence time, and use of seed particles to promote condensation of oxidized vapors
Summary
Laboratory and field studies over the last decade have shown that organic components of atmospheric particles constitute 20 to 50 % of the fine particle mass (PM) in the continental mid-latitudes, though the organic content can be higher (up to 90 %) in tropical forested regions (Kanakidou et al, 2005). Aerosol flow reactors have been developed to study SOA formation and evolution equivalent to multiple days of atmospheric OH exposure In these reactors OH concentrations are typically ∼ 109 molec cm−3 or greater, with reactor residence times of seconds to minutes (Kang et al., 2007; Hall IV et al, 2013; Keller and Burtscher, 2012; Lambe et al, 2011a; Slowik et al, 2012). Other studies have used a combination of aerosol flow reactors and environmental chambers to characterize heterogeneous uptake of organics on seed particles (Jang et al, 2003), SOA formation potential (Kang et al, 2007; Bernard et al, 2012), and evolution of functional groups in SOA with aging (Ofner et al, 2010); in general, similar results are obtained in reactors and chambers These comparisons need to be extended over a wider range of reactants and experimental conditions than are currently available.
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