ESTIMATION OF DRY DEPOSITION OF INORGANICS USING FILTER PACK DATA ANDINFERRED DEPOSITION VELOCITY

Low-volume filter packs were used at several sites in Minnesota to mea sure weekly averaged concentrations of sulfur dioxide, nitric acid vap or, and selected inorganic components of the small particles (SO42-, N O3-, NH4+, Ca, Mg, Na, K, and Cl). Hourly dry deposition velocities (V -d) were inferred using a multiple-layer canopy resistance model devel oped by the NOAA Air Resources Laboratory. Annual mean deposition velo cities in centimeters per second (cm/ s) for the years 1991-1993 range d from 0.83 to 1.46 for HNO3, from 0.28 to 0.42 for SO2, and from 0.09 to 0.15 for small particles. Deposition velocities were distinctly cy clical, both annually and diurnally, with maximum values in the summer time and at midday for all species. Weekly averaged deposition velocit ies were multiplied with the low-volume filter pack measurements of ai r concentrations to obtain dry deposition fluxes. Dry deposition of al l substances was generally highest in and near the Minneapolis - St. P aul metropolitan area and decreased with increasing distance from the metro area. Dry deposition of total sulfur and total nitrogen, respect ively, ranged from 0.98 and 0.53 kg ha(-1) yr(-1) at a remote site to 9.24 and 2.36 kg ha(-1) yr(-1) in the metro area. Dry deposition of Ca , K, Mg, and Na constituted a small fraction, typically less than 10%, of total (wet + dry) deposition of these substances; however, the low -volume filter pack sampling method does not capture the largest of th e coarse particles and may miss significant amounts of these elements. Dry deposition of sulfur and nitrogen, summed for all species contain ing these elements, averaged 22 and 14%, respectively, of the total (w et + dry) deposition of these elements.