Abstract
Abstract. The formation of secondary organic aerosol (SOA) from the oxidation of β-pinene via nitrate radicals is investigated in the Georgia Tech Environmental Chamber (GTEC) facility. Aerosol yields are determined for experiments performed under both dry (relative humidity (RH) < 2 %) and humid (RH = 50 % and RH = 70 %) conditions. To probe the effects of peroxy radical (RO2) fate on aerosol formation, "RO2 + NO3 dominant" and "RO2 + HO2 dominant" experiments are performed. Gas-phase organic nitrate species (with molecular weights of 215, 229, 231, and 245 amu, which likely correspond to molecular formulas of C10H17NO4, C10H15NO5, C10H17NO5, and C10H15NO6, respectively) are detected by chemical ionization mass spectrometry (CIMS) and their formation mechanisms are proposed. The NO+ (at m/z 30) and NO2+ (at m/z 46) ions contribute about 11 % to the combined organics and nitrate signals in the typical aerosol mass spectrum, with the NO+ : NO2+ ratio ranging from 4.8 to 10.2 in all experiments conducted. The SOA yields in the "RO2 + NO3 dominant" and "RO2 + HO2 dominant" experiments are comparable. For a wide range of organic mass loadings (5.1–216.1 μg m−3), the aerosol mass yield is calculated to be 27.0–104.1 %. Although humidity does not appear to affect SOA yields, there is evidence of particle-phase hydrolysis of organic nitrates, which are estimated to compose 45–74 % of the organic aerosol. The extent of organic nitrate hydrolysis is significantly lower than that observed in previous studies on photooxidation of volatile organic compounds in the presence of NOx. It is estimated that about 90 and 10 % of the organic nitrates formed from the β-pinene+NO3 reaction are primary organic nitrates and tertiary organic nitrates, respectively. While the primary organic nitrates do not appear to hydrolyze, the tertiary organic nitrates undergo hydrolysis with a lifetime of 3–4.5 h. Results from this laboratory chamber study provide the fundamental data to evaluate the contributions of monoterpene + NO3 reaction to ambient organic aerosol measured in the southeastern United States, including the Southern Oxidant and Aerosol Study (SOAS) and the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study.
Highlights
Owing to their high emissions and high reactivity with the major atmospheric oxidants (O3, OH, NO3), the oxidation of biogenic volatile organic compounds (BVOCs) emitted by vegetation, such as isoprene (C5H8), monoterpenes (C10H16), and sesquiterpenes (C15H24), is believed to be the dominant contributor to global secondary organic aerosol (SOA) formation (e.g., Kanakidou et al, 2005)
The products at m/z 356 and 358 (MW = 229 and 231 amu) decrease over the course of the experiment. While this can be attributed to vapor phase wall loss, it is possible that these gas-phase compounds undergo further reaction
The oxidation process starts with Reaction (R1) for the sterically preferred addition of nitrate radical to the primary carbon (C1) in the double bond of β-pinene (Wayne et al, 1991)
Summary
Owing to their high emissions and high reactivity with the major atmospheric oxidants (O3, OH, NO3), the oxidation of biogenic volatile organic compounds (BVOCs) emitted by vegetation, such as isoprene (C5H8), monoterpenes (C10H16), and sesquiterpenes (C15H24), is believed to be the dominant contributor to global secondary organic aerosol (SOA) formation (e.g., Kanakidou et al, 2005) While this is supported by the observation that ambient organic aerosol is predominantly “modern” and biogenic in origin (Lewis et al, 2004; Schichtel et al, 2008; Marley et al, 2009), there exists an apparent contradiction because ambient organic aerosol is well correlated with anthropogenic tracers (de Gouw et al, 2005; Weber et al, 2007). Boyd et al.: Secondary organic aerosol formation from the β-pinene+NO3 system lution and the abundance of biogenic carbon in atmospheric aerosol
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