Differences in BVOC oxidation and SOA formation above and below the forest canopy

Abstract

Gas-phase biogenic volatile organic compounds (BVOCs) are oxidized in the troposphere to produce secondary pollutants such as ozone (O$_3$), organic nitrates (RONO$_2$), 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 (NO$_3$), above and below a forest canopy in rural Michigan. In both modeled environments (above and below the canopy), NO$_3$ mixing ratios are relatively small (<0.5pptv); however, daytime (08:00–20:00LT) mixing ratios below the canopy are 2 to 3 times larger than those above. As a result of this difference, NO$_3$ 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 RONO$_2$ 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 NO$_3$-derived RONO$_2$ production above the canopy in the morning (∼ 07:00). Modeled SOA mass loadings above and below the canopy ultimately differ by less than 0.5µgm$^{−3}$, 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 NO$_3$-derived α-pinene RONO$_2$ 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 RONO$_2$.