Characterization of the deposition of particles from the atmosphere to a flat plate

Abstract

The airborne concentration of coarse particles ( > 1 μm diameter) was measured with a Rotary Impactor simultaneously with the measurement of particle dry deposition flux with a smooth surrogate surface with a sharp edge, mounted on a wind vane. The deposition surface was designed to provide minimum air flow disruption and thus provide an estimate of the lower limit for dry deposition flux. The deposited particles were weighed and counted. Microscopic count data generated the size distribution of particles collected on the deposition plate. The results demonstrated that 99% of the mass deposited on the plate was due to particles > 2 μm diameter. Because the measured dry deposition mass was due to atmospheric coarse particles and the measured airborne concentration was also due to coarse particles, the two data sets were combined to evaluate the dynamics of atmospheric coarse particle deposition. Deposition velocity was calculated by dividing the particle flux by the airborne coarse particle concentration. The deposition flux was 3.0 μg m −2s −1 for 17 samples taken over a 1-month period in an urban area. The deposition velocity averaged 11.7cm s −1. The deposition flux, deposition velocity and coarse particle Mass Median Diameter (MMD) were all shown to increase as wind speed increased. There was a large difference between day and night samples. Night samples had lower airborne concentrations (47%), lower deposition flux (65%) and lower deposition velocities (40%). These differences were accompanied by a 47% decrease in wind speed and a 30% decrease in the MMD of airborne coarse particles. This is in agreement with deposition model results where deposition velocity decreases when wind speed and particle size decrease.