AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Oxidation of BVOCs and SOA Formation above and below a Forest Canopy
Benjamin Schulze, Henry Wallace, ROBERT GRIFFIN, Rice University
Abstract Number: 363 Working Group: Effects of NOx and SO2 on BVOC Oxidation and Organic Aerosol Formation
Abstract Oxidation of biogenic volatile organic compounds (BVOCs) produces organic nitrates (RONO$_2) and secondary organic aerosol (SOA). RONO$_2 may be formed via oxidation by nitrate radical (NO$_3) or other oxidants (such as hydroxyl radical, OH) in the presence of nitrogen oxides (NO$_x). NO$_3 is especially reactive towards isoprene and monoterpenes such as alpha-pinene but also undergoes rapid photolysis. It is hypothesized that shading provided by forest canopies enhances daytime oxidation of BVOCs by NO$_3. A zero-dimensional model has been developed to investigate oxidation of and SOA production from these two BVOCs in forested areas under rural and polluted cases. Because of the light gradient caused by forest canopy shading, specific attention is paid to conditions both above and below the canopy. Daytime concentrations of NO$_3 below the canopy are two-to-three times larger than those above, and relative contributions of NO$_3 to daytime alpha-pinene oxidation double under more polluted scenarios. Oxidation of isoprene is almost entirely dominated by reaction with OH. The most significant first-generation RONO$_2 formation mechanism varies significantly between scenarios and canopy locations. Nonetheless, in every scenario, daytime production of first-generation RONO$_2 from NO$_3-alpha-pinene reactions is more significant below the canopy than above. While SOA mass loadings are moderate (2 micrograms per cubic meter or less), total SOA production is consistently enhanced below the canopy, due to the combined effects of elevated isoprene and reduced NO$_x concentrations relative to above the canopy. Below-canopy daytime production of RONO$_2 SOA through NO$_3 oxidation of alpha-pinene, while small in absolute terms, is more than double above-canopy production under every scenario. This work emphasizes the probability of daytime oxidation of BVOCs by NO$_3, the relative importance of OH/NO$_x versus NO$_3 oxidation in leading to RONO$_2 formation, and the need for further investigation of how BVOC and anthropogenic species interact to form SOA.