Dry deposition fluxes and deposition velocities of seven trace metal species at five sites in Central Taiwan - a summary of surrogate surface measurements and a comparison with model estimation

Abstract

Abstract. Daily air concentrations and dry deposition fluxes of seven metal species were monitored at five sites in Central Taiwan for five or six days each month from September 2009 to August 2010. Annual average concentrations at the five sites were in the range of 2.8 to 3.6 ng m−3 for As, 25 to 82 ng m−3 for Mn, 1900 to 2800 ng m−3 for Fe, 69 to 109 ng m−3 for Zn, 18 to 33 ng m−3 for Cr, 60 to 110 ng m−3 for Cu, and 25 to 40 ng m−3 for Pb. Annual average dry deposition fluxes were on the order of 3, 20, 400, 50, 25, 50 and 50 μg m−2 day−1 for As, Mn, Fe, Zn, Cr, Cu and Pb, respectively. Annual average dry deposition velocities (Vd) for the seven metal species ranged from 0.18 to 2.22 cm s−1 at these locations. Small seasonal and geographical variations, e.g., from a few percent to a factor of 2 for different species and/or at different locations, were found for measured concentrations, fluxes and Vd. Measured fluxes and air concentrations had moderate to good correlations for several species at several sites, but had weak or no correlations for other species or at other sites, the latter cases were believed to have large uncertainties in flux measurements using surrogate surfaces. Sensitivity tests were conducted for particle Vd using a size-segregated particle dry deposition model, assuming various combinations of three lognormal size distributions representing fine particles (PM2.5), coarse particles (PM2.5-10) and super size particles (PM10+), respectively. It was found that measured dry deposition fluxes can be reproduced reasonably well using the size-segregated particle dry deposition model if the mass fractions of metal species in PM2.5, PM2.5-10 and PM10+ were known. Significant correlations between modeled and measured daily fluxes were found for those cases that were believed to have small uncertainties in flux measurements.