Impacts of Chemical and Physical Processes on Particle Growth Events in a Semi-Urban Region during July 2011

Many meteorological and chemical factors influence formation and growth of secondary particles in the atmosphere, including temperature, relative humidity, and the abundance of precursor gases. During July 2011 in Beltsville, MD, as part of DISCOVER-AQ, aerosol size and number distributions of ultrafine particles were measured at the surface and characterized during differing air mass histories and meteorological regimes. The key findings were that well-defined regional scale particle growth events occurred, even during the summer months, on days when cold air advection passed over the region and there was slightly higher boundary layer mixing than during non-event days. On days of particle growth events temperatures were cooler, relative humidity was drier, and cloud conditions were clearer than on non-event days suggesting that there are predominant and favorable synoptic, regional, and even local meteorological conditions needed to promote particle growth within the boundary layer. However, exploring the meteorology only provided insight into the physical processes and did not help to explain the chemical processes and to what extent the chemistry was significant to these events. With a limited chemical and aerosol composition observational dataset, global and regional chemical models were used to suggest key chemical processes that can be contributing to aerosol number, size, and composition. Using a photochemical Box Model with organic and inorganic chemical mechanisms represented (RACM2), weighing and sorting of significant chemical species and reactions were explored. Sensitivity studies of gas phased mixing ratios of sulfur dioxide and the hydroxyl radical were analyzed to determine if the chemical reaction with sulfur dioxide and other photochemical dependent species were correlated to the timing of particle growth events. Using a global chemical transport model that accounts for meteorology and include size-resolved particle microphysics (GEOS-Chem/APM) gave an opportunity to explore specific parameters and processes, like the role of sulfur dioxide on particle growth or boundary layer mixing, and detailed changes in aerosol properties relative to changes in the meteorology, boundary layer processes, and chemistry. Given the complex nature of the aerosols present in semi-urban region, influenced by both anthropogenic and biogenic sources, the two models provided key insight into which processes were significant to the particle growth during the summer of 2011.