An Analysis of the Time Scales Associated with Aerosol Processes during Dry Deposition

Abstract

A fundamental assumption of air quality and process-level deposition models is that, with the exception of deliquescent behavior, the aerosol size distribution is invariant during the dry deposition process, and hence the deposition calculation can be effectively decoupled from the rest of the aerosol dynamics. However, this assumption may be violated due to the changing physical and chemical regimes close to the receptor surface. To determine whether aerosol dynamics might be responsible for violations of the constant flux layer assumption and thus the observed discrepancy between observed and model-predicted, size-resolved aerosol dry deposition velocities (V_d), we undertake a series of model experiments to quantify the time scales of aerosol dynamic processes relative to size-resolved aerosol V_d. The results indicate that using the model formulation of Slinn or alternative published formulations for the components of dry deposition, both intermodal coagulation and condensation of semivolatile compounds have time scales that are within an order of magnitude of the time scale for aerosol dry deposition to a forest canopy from 10 m above the canopy. This implies that there may be substantial evolution of the aerosol size distribution below typical flux measurement heights (or the mid-point of the lowest model layer) during dry deposition and hence that the transfer may exhibit flux divergence due to aerosol dynamics.