Technical Note: An empirical algorithm estimating dry deposition velocity of fine, coarse and giant particles

摘要

An empirical algorithm is developed for calculating bulkdry deposition velocity (V_d) of fine (PM_2.5) – particles having adiameter of ≤ 2.5 μm), coarse (PM_2.5−10 – particles havinga diameter of 2.5–10 μm), and giant (PM₁₀₊ – particles having adiameter of > 10 μm) atmospheric particles. The algorithmis developed from an empirical fit of V_d data calculated using thesize-resolved V_d scheme of Zhang et al. (2001) with assumed lognormalsize distributions of PM_2.5, PM_2.5−10 and PM₁₀₊. In the newalgorithm, the surface deposition velocity (V_ds) is parameterized as asimple linear function of friction velocity (u_*) for PM_2.5 andas a polynomial function of u_* for both PM_2.5−10 andPM₁₀₊ over all the 26 land use categories (LUCs). An adjustment factoras an exponential function of u_* and leaf area index (LAI) is alsoapplied to V_ds of PM_2.5−10 and PM₁₀₊ over 9 of the 26 LUCsthat have variable LAI. Constant gravitational settling velocities areprovided for PM_2.5, PM_2.5−10 and PM₁₀₊. Aerodynamicresistance between a reference height and the surface can be calculatedusing available analytical formulas from the literature. The bulk V_d ofPM_2.5, PM_2.5−10 and PM₁₀₊ at the reference height can thenbe calculated by combining the gravitational settling velocity, aerodynamicresistance and the parameterized V_ds. V_d values calculated using thenew algorithm are within ±20% of those using the originalsize-resolved scheme for fine, coarse and giant particles. Uncertainties inV_d values from the new algorithm due to the pre-assumed sizedistributions are on the order of 20% for fine particles and on the orderof a factor of 2.0 for coarse and giant particles. The new algorithmprovides an alternative approach for calculating V_d of bulk aerosolparticles. V_d of any particulate species can be simply estimated usingthis scheme as long as the mass fractions in fine, coarse and giant particlesare known or can be assumed.