Understanding Chemical and Physical Processes of Biogenic Volatile Organic Compounds in The Atmospheric Boundary Layer

Abstract

Biogenic volatile organic compounds (BVOC) are emitted naturally from the terrestrial biosphere and are oxidized quickly within the troposphere. These compounds play crucial roles in the formation of ozone and secondary organic aerosols (SOA) in the atmospheric boundary layer (ABL), affecting both air quality and climate. Isoprene (C5H8) is a relatively well-studied BVOC, emitted in large quantities with a very short atmospheric lifetime (~10 min) that is comparable to mixing timescales within the ABL. However, atmospheric regional models have difficulties in reproducing the vertical distributions of many different BVOC species. This dissertation uses two different model tools to understand the roles of chemical oxidation and turbulent transport in BVOC-dominated regions. The National Center for Atmospheric Research’s Large-Eddy Simulation (LES) model is used to simulate distinct convective environments to understand the role of boundary layer turbulence on the atmospheric chemistry of key BVOC species and their oxidation products during the 2011 NASA DISCOVER-AQ field campaign. Convection plays an important role in mixing oxygenated volatile organic compounds (OVOC) into the upper ABL, therefore, modifying the vertical structure of atmospheric oxidation capacity. I implement a new chemical mechanism in the LES to include both gas- and aqueous-phase reactions to provide the first detailed code to account for aqueous chemistry within clouds, and find that the inclusion of aqueous chemistry reduces HCHO in the cloud layer by up to 18% and increases isoprene mixing ratios, promoting segregation of reactants and reducing reaction rates in cloud layers. Using the LES as an evaluation standard, I implement a regional atmospheric chemistry model, the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) to understand BVOC chemistry in the ABL across spatial scales. Although the WRF-Chem model is widely used to study atmospheric chemistry at regional scales, it simulates weaker turbulence than the LES, which leads to stronger segregation of isoprene and OH in the WRF-Chem simulations. Overall, this work elucidates the role of turbulence on BVOC chemistry in the ABL and suggests that competing chemical and physical processes are key for simulating BVOC oxidation in the ABL. Regional models such as WRF-Chem have difficulties in simulating these complex processes for BVOC and this may influence how we simulate and understand the oxidation capacity of the troposphere.