Estimating dry deposition of atmospheric aerosols to urban surfaces by rain washoff

Atmospheric aerosols accumulate onto urban surfaces by dry deposition and are subsequently washed off by rain. If there is a sufficiently long period of dry weather, it may be possible to estimate the dry deposition rate of a chemical species by measuring its concentration in runoff from the surface and its concentration in fresh rain. This hypothesis was tested for horizontal surrogate surfaces exposed to the atmosphere in Syracuse, New York. In each experiment, two flat disks 1 m diameter with 2.54 cm rims and two disks of similar size but without rims were exposed to the atmosphere on the roof of Hinds Hall at Syracuse University to collect dry deposition of aerosol fluoride, chloride, sulfate, potassium, calcium, and strontium. Airborne concentrations were measured simultaneously to calculate dry deposition velocities. Dry deposited material on the flat disks was removed using a Teflon scraper after a few days of exposure, just prior to a rainstorm. In the subsequent storm, sequential samples of runoff from the rimmed disks and fresh rain were collected. The concentrations in fresh rain were subtracted from the concentrations in the runoff to calculate dry deposition fluxes, which were compared to fluxes measured by scraping the flat disks prior to rain. Results showed that sufficient removal of dry deposited material was possible to estimate dry deposition fluxes. Ratios of the mean dry deposition flux onto the rimmed disks to the mean dry deposition flux onto the flat disks had an overall average of 0.89 for all chemical species. This ratio mean and standard deviation was 0.94 +/- 0.30 for fluoride, 0.59 +/- 0.54 for chloride, 1.0 +/- 0.10 for sulfate, 0.80 +/- 0.28 for potassium, 1.0 +/- 0.03 for calcium, and 0.93 +/- 0.12 for strontium. However, some dry deposition fluxes to the rimmed disks were smaller than those to the flat disks in cases where the wet deposition flux was at least one half of the contribution to the total flux. This suggests the importance of having antecedent dry periods of at least several days to use this method to estimate dry deposition of aerosols and to minimize the role of contaminants in the fresh rain contributing significantly to the total concentration in the runoff samples. Future work includes testing this method on rooftops and other horizontal urban surfaces, which may enable reliable estimates of dry deposition rates to urban areas.