Abstract
Abstract. An empirical algorithm is developed for calculating bulk dry deposition velocity (Vd) of fine (PM2.5) – particles having a diameter of ≤ 2.5 μm), coarse (PM2.5−10 – particles having a diameter of 2.5–10 μm), and giant (PM10+ – particles having a diameter of > 10 μm) atmospheric particles. The algorithm is developed from an empirical fit of Vd data calculated using the size-resolved Vd scheme of Zhang et al. (2001) with assumed lognormal size distributions of PM2.5, PM2.5−10 and PM10+. In the new algorithm, the surface deposition velocity (Vds) is parameterized as a simple linear function of friction velocity (u*) for PM2.5 and as a polynomial function of u* for both PM2.5−10 and PM10+ over all the 26 land use categories (LUCs). An adjustment factor as an exponential function of u* and leaf area index (LAI) is also applied to Vds of PM2.5−10 and PM10+ over 9 of the 26 LUCs that have variable LAI. Constant gravitational settling velocities are provided for PM2.5, PM2.5−10 and PM10+. Aerodynamic resistance between a reference height and the surface can be calculated using available analytical formulas from the literature. The bulk Vd of PM2.5, PM2.5−10 and PM10+ at the reference height can then be calculated by combining the gravitational settling velocity, aerodynamic resistance and the parameterized Vds. Vd values calculated using the new algorithm are within ±20% of those using the original size-resolved scheme for fine, coarse and giant particles. Uncertainties in Vd values from the new algorithm due to the pre-assumed size distributions are on the order of 20% for fine particles and on the order of a factor of 2.0 for coarse and giant particles. The new algorithm provides an alternative approach for calculating Vd of bulk aerosol particles. Vd of any particulate species can be simply estimated using this scheme as long as the mass fractions in fine, coarse and giant particles are known or can be assumed.
Highlights
The parameter known as dry deposition velocity (Vd) has been commonly used in chemical transport models as well as in monitoring networks to associate a chemical species’ mass flux density to the surface with its ambient concentration
Several size-resolved Vd schemes are available in the literature that can be applied to any particle species and over any different surfaces (Zhang and Vet, 2006)
The present study aims to fill this gap by developing a bulk Vd algorithm taking the size-resolved Vd scheme of Zhang et al (2001) as the benchmark
Summary
The parameter known as dry deposition velocity (Vd) has been commonly used in chemical transport models as well as in monitoring networks to associate a chemical species’ mass flux density to the surface with its ambient concentration (i.e., a species’ flux is a product of its Vd and its ambient concentration). In many environmental assessments the dry deposition flux of a pollutant or a group of pollutants of interest to various ecosystems is the only concern In this case, a simple empirical formula of Vd – or the so-called bulk Vd algorithm – combined with monitored air mass concentrations is sufficient. Ruijgrok et al (1997) generated a bulk Vd formula for water-soluble inorganic ions, which include species of both fine and coarse particles, using flux data over forest canopies, and Laumaud et al (1994) and Gallagher et al (2002) derived formulas for submicron particles. The new algorithm is expected to produce similar Vd values to the original sizeresolved scheme, but is much easier to implement at atmospheric deposition monitoring networks
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