Limited Effect of Anthropogenic Nitrogen Oxides on Secondary Organic Aerosol Formation

Abstract:

Globally secondary organic aerosol (SOA) is mostly formed from biogenic vegetation emissions and as such is regarded as natural aerosol that cannot be reduced by emission control legislation. However, recent research implies that human activities facilitate SOA formation by affecting the amount of precursor emission, the chemical processing and the partitioning into the aerosol phase. Among the multiple human influences, nitrogen oxides (NO + NO₂ = NO_x) have been assumed to play a critical role in the chemical formation of low volatile compounds. The goal of this study is to improve the SOA scheme in the global NCAR Community Atmospheric Model version 4 with chemistry (CAM4-Chem) by implementing an updated 4-product Volatility Basis Set (VBS) scheme, and apply it to investigate the impact of anthropogenic NO_x on SOA. We first compare three different SOA parameterizations: a 2-product model and the updated VBS model both with and without a SOA aging parameterization. Secondly we evaluate predicted organic aerosol amounts against surface measurement from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network and Aerosol Mass Spectrometer (AMS) measurements from 13 aircraft-based field campaigns. We then perform sensitivity experiments to examine how the SOA loading responds to a 50% reduction in anthropogenic NO_x in different regions. We find limited SOA reductions of -2.3%, -5.6% and -4.0% for global, southeastern U.S. and Amazon NO_x perturbations, respectively. To investigate the chemical processes in more detail, we also use a simplified box model with the same gas-phase chemistry and gas-aerosol partitioning mechanism as in CAM4-Chem to examine the SOA yields dependence on initial precursor emissions and background NO_x level. The fact that SOA formation is almost unaffected by changes in NO_x can be largely attributed to buffering in chemical pathways (low- versus high-NO_x pathways, OH versus NO₃-initiated oxidation) and to offsetting tendencies in the biogenic versus anthropogenic SOA responses.