Development of a dry deposition model for atmospheric coarse particles

Abstract

Atmospheric inertial deposition of coarse particles has been quantified by the evaluation of particle dry deposition flux data collected simultaneously on the top and bottom surfaces of a smooth plate with a sharp leading edge that was pointed into the wind by a wind vane. The deposited particles were weighed and counted. The airborne concentration of coarse particles was measured with a Rotary Impactor. Deposition velocity was determined by dividing the mass flux (plate) by the airborne concentration (Rotary Impactor). The deposition velocity was considered to be due to gravitational settling ( V ST ) and inertial deposition ( V I ). Deposition to the upper plate surface ( V dU ) was given by: V dU = V ST + V I , while deposition to the lower plate surface ( V dL ) was given by: V dL = − V ST + V I . The inertial deposition velocity was defined as: V I = \\ ̄ ge AU ∗ , where \\ ̄ ge A is the particle effective inertial coefficient and U ∗ is friction velocity. Based on these equations, \\ ̄ ge A was evaluated as a function of particle size as: \\ ̄ ge A = 1.12e −30.36/d n , where d a is the particle aerodynamic diameter (μm). The correlation coefficient was 0.92, \\ ̄ ge A varied from 0.1 to 1.0 for particles between 5 and 100 μm diameter. The particle dry deposition fluxes obtained for the top and bottom surfaces of the plate were extended to the free atmosphere. A particle flux ratio ( F R ) was defined as: F R = V dL V dU . The mass median aerodynamic diameter MMD a for the atmospheric coarse particle size distribution correlated closely with the geometric mean values of ( F R ). The flux ratio was also related to the shape of the coarse particle mass distribution. The flux ratio was less than 0.1 for particles smaller than 3 μm diameter and did not increase significantly with wind speed. This corresponded to a minimum in the coarse particle mass distribution that was present for particles smaller than 3 μm diameter. The flux ratio was also small for particles larger than 50 μm diameter but increased rapidly with wind speed. This indicated that larger particles could remain suspended under high wind speed conditions. The measured mass distributions for atmospheric coarse particles showed an increase in larger particles with an increase in wind speed. This was in accordance with the increase in the particle flux ratio.