Estimation of Dry Deposition of Inorganics Using Filter Pack Data and Inferred Deposition Velocity

Abstract

Low-volume filter packs were used at several sites in Minnesota to measure weekly averaged concentrations of sulfur dioxide, nitric acid vapor, and selected inorganic components of the small particles (SO42-, NO3-, NH4+, Ca, Mg, Na, K, and Cl). Hourly dry deposition velocities (Vd) were inferred using a multiple-layer canopy resistance model developed by the NOAA Air Resources Laboratory. Annual mean deposition velocities in centimeters per second (cm/s) for the years 1991−1993 ranged 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 cyclical, both annually and diurnally, with maximum values in the summertime and at midday for all species. Weekly averaged deposition velocities were multiplied with the low-volume filter pack measurements of air concentrations to obtain dry deposition fluxes. Dry deposition of all substances was generally highest in and near the Minneapolis−St. Paul metropolitan area and decreased with increasing distance from the metro area. Dry deposition of total sulfur and total nitrogen, respectively, 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 the coarse particles and may miss significant amounts of these elements. Dry deposition of sulfur and nitrogen, summed for all species containing these elements, averaged 22 and 14%, respectively, of the total (wet + dry) deposition of these elements.