Meteorological controls on particle growth events in Beltsville, MD, USA during July 2011

During summer 2011, particle growth events were investigated in Beltsville, Maryland, USA where anthropogenic and biogenic hydrocarbon emissions heavily influenced the lower atmosphere. Aerosol sampling was made from a 5-m tower during July 2011. Ultrafine particle growth events occurred on 4 of the 25 days sampled. Regional particle growth events occurred after the passage of synoptic cold fronts. During regional particle growth events temperature and relative humidity were 2°Celsius lower and 3 % less than on non-event days and the local wind direction came from the northeast. With average growth rates of 3.5 nm hr^-1, regional particle growth events happened around 15:00 Universal Time Coordinated, shortly after increased mixing within the atmospheric boundary layer. Regional particle growth events were associated with cooler, drier, mostly sunny days that had low sulfur dioxide mixing ratio (0.6 parts per billion by volume). Regional particle growth events occurred when the atmospheric boundary layer was dominated by free convection and had lower sensible heat fluxes compared to short-lived events or non-particle growth event days.