Abstract
Abstract. Gas-phase biogenic volatile organic compounds (BVOCs) are oxidized in the troposphere to produce secondary pollutants such as ozone (O3), organic nitrates (RONO2), and secondary organic aerosol (SOA). Two coupled zero-dimensional models have been used to investigate differences in oxidation and SOA production from isoprene and α-pinene, especially with respect to the nitrate radical (NO3), above and below a forest canopy in rural Michigan. In both modeled environments (above and below the canopy), NO3 mixing ratios are relatively small (< 0.5 pptv); however, daytime (08:00–20:00 LT) mixing ratios below the canopy are 2 to 3 times larger than those above. As a result of this difference, NO3 contributes 12 % of total daytime α-pinene oxidation below the canopy while only contributing 4 % above. Increasing background pollutant levels to simulate a more polluted suburban or peri-urban forest environment increases the average contribution of NO3 to daytime below-canopy α-pinene oxidation to 32 %. Gas-phase RONO2 produced through NO3 oxidation undergoes net transport upward from the below-canopy environment during the day, and this transport contributes up to 30 % of total NO3-derived RONO2 production above the canopy in the morning (∼ 07:00). Modeled SOA mass loadings above and below the canopy ultimately differ by less than 0.5 µg m−3, and extremely low-volatility organic compounds dominate SOA composition. Lower temperatures below the canopy cause increased partitioning of semi-volatile gas-phase products to the particle phase and up to 35 % larger SOA mass loadings of these products relative to above the canopy in the model. Including transport between above- and below-canopy environments increases above-canopy NO3-derived α-pinene RONO2 SOA mass by as much as 45 %, suggesting that below-canopy chemical processes substantially influence above-canopy SOA mass loadings, especially with regard to monoterpene-derived RONO2.
Highlights
Organic compounds account for a substantial fraction of total atmospheric aerosol mass (Zhang et al, 2007; Jimenez et al, 2009) and have significant implications for health, visibility, and climate
Mixing ratios under 1 pptv agree well with previous results from Mogensen et al (2015) in a boreal forest setting and Ayres et al (2015) in rural Alabama. While this concentration is much lower than previous nighttime observations in polluted urban environments (Brown et al, 2011), model results have shown that NO3 can dominate the total nighttime oxidative strength of a forest environment, even at concentrations of less than 1 pptv (Mogensen et al, 2015)
The model of Pratt et al (2012) includes more biogenic volatile organic compounds (VOCs) (BVOCs) but less detail in terms of subsequent oxidation chemistry, and the results do not extend below the canopy, preventing a comparison
Summary
Organic compounds account for a substantial fraction of total atmospheric aerosol mass (Zhang et al, 2007; Jimenez et al, 2009) and have significant implications for health, visibility, and climate. Rather than being emitted directly, nearly 70 % of this material is thought to be secondary organic aerosol (SOA) formed from the oxidation of volatile organic compounds (VOCs) (Hallquist et al, 2009). Many of the relevant VOCs are biogenic in origin, causing naturally emitted compounds to contribute substantially to tropospheric aerosol burdens (Seinfeld and Pankow, 2003). Isoprene (C5H8) and monoterpenes (C10H16) are important biogenic VOCs (BVOCs) due to their significant rates of emission and reactivity. Many questions remain regarding the mechanisms of SOA production, and current large-scale atmospheric models often underpredict organic aerosol (OA) mass loadings (Heald et al, 2005; Volkamer et al, 2006; Pye and Seinfeld, 2010)
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