Abstract
<p><strong>Abstract.</strong> Gas- and aerosol-phase measurements of oxidants, biogenic volatile organic compounds (BVOCs) and organic nitrates made during the Southern Oxidant and Aerosol Study (SOAS campaign, Summer 2013) in central Alabama show that a nitrate radical (NO<sub>3</sub>) reaction with monoterpenes leads to significant secondary aerosol formation. Cumulative losses of NO<sub>3</sub> to terpenes are correlated with increase in gas- and aerosol-organic nitrate concentrations made during the campaign. Correlation of NO<sub>3</sub> radical consumption to organic nitrate aerosol formation as measured by aerosol mass spectrometry and thermal dissociation laser-induced fluorescence suggests a molar yield of aerosol-phase monoterpene nitrates of 23–44 %. Compounds observed via chemical ionization mass spectrometry (CIMS) are correlated to predicted nitrate loss to BVOCs and show C<sub>10</sub>H<sub>17</sub>NO<sub>5</sub>, likely a hydroperoxy nitrate, is a major nitrate-oxidized terpene product being incorporated into aerosols. The comparable isoprene product C<sub>5</sub>H<sub>9</sub>NO<sub>5</sub> was observed to contribute less than 1 % of the total organic nitrate in the aerosol phase and correlations show that it is principally a gas-phase product from nitrate oxidation of isoprene. Organic nitrates comprise between 30 and 45 % of the NO<sub><I>y</I></sub> budget during SOAS. Inorganic nitrates were also monitored and showed that during incidents of increased coarse-mode mineral dust, HNO<sub>3</sub> uptake produced nitrate aerosol mass loading at a rate comparable to that of organic nitrate produced via NO<sub>3</sub> + BVOCs.</p>
Highlights
Secondary organic aerosol (SOA), formed from the oxidation of volatile organic compounds (VOCs) by ozone (O3), hydroxyl radical (OH), or nitrate radical (NO3), affects visibility as well as regional and global radiative climate forcing (Bellouin et al, 2011; Feng and Penner, 2007; Goldstein et al, 2009; Myhre et al, 2013)
During the Southern Oxidant and Aerosol Study (SOAS) campaign, we monitored reactant and product species indicative of NO3 + Biogenic volatile organic compounds (BVOCs), which may partition into the aerosol phase and serve as a source of first-generation SOA
The contribution of NO3 + BVOCs to SOA formation is found to be substantial in the terpene-rich southeastern United States (SEUS)
Summary
Secondary organic aerosol (SOA), formed from the oxidation of volatile organic compounds (VOCs) by ozone (O3), hydroxyl radical (OH), or nitrate radical (NO3), affects visibility as well as regional and global radiative climate forcing (Bellouin et al, 2011; Feng and Penner, 2007; Goldstein et al, 2009; Myhre et al, 2013). Nitrogen-containing oxidation products include alkyl nitrates (RONO2), peroxynitrates (RO2NO2) and nitric acid (HNO3) (Brown and Stutz, 2012; Perring et al, 2013), all of which may partition to the aerosol phase and contribute to SOA (via direct reaction or catalysis) (Kroll and Seinfeld, 2008). NO3 loss to BVOCs is calculated and compared to aerosol mass spectrometry (AMS), chemical ion mass spectrometry (CIMS), and thermal dissociation laser-induced fluorescence (TDLIF) measurements of aerosol-organic nitrates We compare this to an alternate fate of NOx, heterogeneous HNO3 uptake to produce inorganic nitrate aerosol, which is considered in detail in a second paper (Allen et al, 2015). Both pathways from NOx to nitrate aerosol shown in Scheme 1 are produced at various times in the SEUS
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