Abstract
Abstract. Secondary organic aerosol (SOA) formation from the reaction of isoprene with nitrate radicals (NO3) is investigated in the Caltech indoor chambers. Experiments are performed in the dark and under dry conditions (RH&lt10%) using N2O5 as a source of NO3 radicals. For an initial isoprene concentration of 18.4 to 101.6 ppb, the SOA yield (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) ranges from 4.3% to 23.8%. By examining the time evolutions of gas-phase intermediate products and aerosol volume in real time, we are able to constrain the chemistry that leads to the formation of low-volatility products. Although the formation of ROOR from the reaction of two peroxy radicals (RO2) has generally been considered as a minor channel, based on the gas-phase and aerosol-phase data it appears that RO2+RO2 reaction (self reaction or cross-reaction) in the gas phase yielding ROOR products is a dominant SOA formation pathway. A wide array of organic nitrates and peroxides are identified in the aerosol formed and mechanisms for SOA formation are proposed. Using a uniform SOA yield of 10% (corresponding to Mo≅10 μg m−3), it is estimated that ~2 to 3 Tg yr−1 of SOA results from isoprene+NO3. The extent to which the results from this study can be applied to conditions in the atmosphere depends on the fate of peroxy radicals in the nighttime troposphere.
Highlights
Isoprene is the most abundant non-methane hydrocarbon emitted into the atmosphere with a global emission of ∼500 Tg yr−1 (Guenther et al, 1995; Guenther et al, 2006)
To study the mechanism of Secondary organic aerosol (SOA) formation, in several experiments the experimental protocols are slightly modified: (1) An excess amount of isoprene is injected into the chamber to prevent the further reaction of first-generation gas-phase products, allowing these products to be detected more readily; (2) After the addition of isoprene, pulses of N2O5 are introduced into the chamber to study the evolution of different intermediate gas-phase products; (3) With isoprene well mixed in the chamber, N2O5 is introduced slowly to maximize the self-reaction of peroxy radicals
About 2.5 μg m−3 of inorganic nitrate is measured by PILS/ion chromatography (IC), which agrees well with the amount of nitrates detected by quadrupole Aerosol Mass Spectrometer (Q-AMS)
Summary
Isoprene is the most abundant non-methane hydrocarbon emitted into the atmosphere with a global emission of ∼500 Tg yr−1 (Guenther et al, 1995; Guenther et al, 2006). C5-nitrooxycarbonyl is identified as the major first-generation gas-phase reaction product (Jay and Stieglitz, 1989; Skov et al, 1992; Kwok et al, 1996; Berndt and Boge, 1997). Other compounds such as C5-hydroxynitrate, C5nitrooxyhydroperoxide, and C5-hydroxycarbonyl have been identified (Kwok et al, 1996); C5-hydroxynitrate has been measured in ambient air with concentrations in the lower ppt range at a few ng m−3 (Werner et al, 1999). Mechanisms for SOA formation are proposed and chemical composition data of the SOA formed are presented
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