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

During summer 2011, particle growth events were investigated in Beltsville, MD, USA where anthropogenic and biogenic hydrocarbon emissions heavily influence the atmosphere. Ultrafine particle growth events were studied during this period and occurred on cooler (temperature 8°C less), drier (relative humidity 19% less), and clearer days than on the average July 2011 non-growth event day. These events occurred after cold air advection associated with cold frontal passages with significant precipitation. This dataset showed two interested types of cases: elevated ultrafine (<25 nm) number concentrations during the night-time hours (01 - 11 Universal Time Coordinated, UTC) and regional particle growth events during the day-time hours (11 - 00 UTC). At night, local sources of ammonium nitrate particles are present during low frictional velocities, high relative humidity, and lower temperatures. These events occurred throughout the campaign regardless of frontal passage and showed evidence of nucleating particles, growing to 30 nm. During the day, regional particle growth events grew to sizes around 110 nm at an average growth rate of 3.5 nm hr^-1. These results show that regional particle growth events occur shortly after an increase in turbulent mixing, an increase in temperature and a decrease in relative humidity, all consistent with the expansion of the boundary layer due to diurnal heating. Increased concentrations of sulfur dioxide (SO₂) occurred just prior to the particle growth events being detected at 15:00 UTC. The peak in SO₂ coincided with the breakdown of the nocturnal boundary layer and when the convective boundary layer began developing. More investigations using photochemical box-models will be presented to determine the significance of these chemical species on the elevated night-time ultrafine particles (ammonium nitrate) and regional particle growth events (SO₂).