Biogenic VOC Multiphase Chemistry – From Cloud Scavenging to New Particle Formation

Development of a molecular-level understanding of the processes governing the evolution of organic aerosol mass has been a long running challenge. I will present new insights into the potential for biogenic volatile organic compounds (BVOC) to form new particles and contribute to organic aerosol formation. Specifically, I will illustrate the coupled roles of organic peroxy radical chemistry and shallow and deep convective clouds in transporting or processing BVOC and their oxidations products that impact aerosol particle formation and growth on scales that are typically unresolved in global scale chemical transport models. The fate of organic peroxy radicals from BVOC depends upon NO_x, with natural and anthropogenic sources, as well as temperature and therefore changes substantially with both altitude and region. Deep convection efficiently transports BVOC to the upper troposphere with significant decreases in temperature and, over land substantial NO_x from lightning and from co-transport of polluted boundary layer air. The fates of BVOC-derived organic peroxy radicals in the upper troposphere will therefore occur in conditions rarely probed experimentally, with implications for the formation of low volatility products. In addition, during transport through shallow or deep convective clouds, soluble BVOC oxidation products commonly considered important precursors to secondary organic aerosol (SOA) will partition and potentially react in the cloud water. Thus, the common occurrence of both shallow cumulus and deep convective clouds is a large but poorly represented lever on biogenic SOA formation. 

I will show results from studies of the above processes using a hierarchy of models, including parcel models run along trajectories from Large Eddy Simulation (LES) models of deep convective clouds, LES models of cumulus-topped boundary layers with online multi-phase chemistry, and global chemical transport simulations with online chemistry and implications for new particle formation and organic aerosol mass budgets. Chemical mechanisms are informed by recent laboratory studies of organic peroxy radicals, such as autoxidation, accretion product formation from cross-reactions, and organic nitrate formation, as well as the aqueous chemistry of isoprene-derived epoxy diols. Comparisons of these models to observations reveal the importance of scattered cumulus clouds to the fate of isoprene epoxy diols and thus its SOA formation potential, the role of lightning and soil NO_x in new particle formation from BVOC oxidation in the upper-tropospheric outflow of deep convective clouds, and the potential for isoprene oxidation at high NO and low temperature to serve as a key source of low volatility organics that drive new particle formation in deep convective outflow.