Anthropogenic and Biogenic Emissions, and their Contributions to Summertime Haze in the Southeast U.S.: Results from the NOAA SENEX Study in 2013

Abstract:

The NOAA Southeast Nexus of Air Quality and Climate Change (SENEX) study was focused on biogenic and anthropogenic emissions in the Southeast United States, their interactions to form oxidants and aerosol, and the implications of these processes for air quality and climate. For SENEX, the NOAA WP-3D research aircraft was deployed out of a regional airport in Smyrna, Tennessee during June and July of 2013. The aircraft carried an extensive suite of instruments to characterize the gas- and aerosol-phase composition of the atmosphere, as well as the climate-relevant properties of the aerosol. The SENEX study was performed in close collaboration with several other studies in the framework of the Southeast Atmosphere Study.

Some highlights of the measurements will be presented with a particular focus on the processes by which anthropogenic and biogenic emissions interact to form secondary species including aerosol. The FLEXPART Lagrangian transport model was found to be a useful tool to quantify the relative contribution from anthropogenic and biogenic emissions to different trace species in the sampled air masses. Observations in power plant plumes provided constraints on the dependence of hydroxyl radical concentrations on nitrogen oxides (NOx). Oxidation rates were higher at enhanced concentrations of NOx, leading to faster production of products from biogenic volatile organic compounds (VOCs) in polluted conditions. Organic aerosol was formed from isoprene downwind from one power plant that had relatively high sulfur emissions. Nighttime flights were conducted to constrain organic aerosol formation from the reaction between biogenic VOCs and nitrate radicals. The volume of aerosol water depends on aerosol composition (e.g. sulfate versus organics), which has been proposed as a link between anthropogenic emissions and products of aqueous-phase chemistry. Vertical profiles of aerosol volume and composition provided evidence for aqueous-phase formation of sulfate aerosol, but formation of organic aerosol was not significant within the uncertainties of the analysis.