On the applicability of the deposition velocity concept for ambient aerosols

Dry deposition is an important process of removal of various airborne substances from the atmospheric boundary layer. In many applications it is convenient to assume that the deposition flux of a substance is proportional to the near-surface concentration, and that the proportionality coefficient does not depend on particle concentration. This assumption is based on the idea of a constant-flux layer between the reference height and the surface, and holds for substances that have no sources/sinks in the layer. The deposition velocity concept is a core part of dry deposition schemes of atmospheric transport models.

We address large discrepancies between field and wind-tunnel measurements of deposition velocities of aerosols with aerodynamic diameter between approximately 0.1µm and 2µm. In seemingly similar conditions, deposition velocities derived from field measurements are in range of 1-10 cm/s, while wind-tunnel measurements show a fraction of a millimeter per second. This difference translates to the discrepancy in dry deposition parametrizations.

 

SILAM chemistry transport model features a dry deposition scheme for particles by Kouznetsov and Sofiev (2012, https://doi.org/10.1029/2011JD016366) that predicts 'low' deposition velocities. With such a scheme, simulations that explicitly account for aerosol transformations are able to reproduce the ambient observed fluxes and agree well with the 'high' apparent deposition velocity. A regional simulation covering the period of the Gallagher (2007, https://doi.org/10.1016/S1352-2310(96)00057-X) field campaign was capable of reproducing both magnitude and temporal evolution of aerosol fluxes measured over a forest.

 

We demonstrate that the conservation of aerosol mass in the immediate vicinity of the surface is not fulfilled for ambient aerosols when the aerosols include a fraction of ammonium nitrate. For such a mixture the fluxes of ambient aerosols are not controlled by particle deposition but rather by gas-particle partitioning in the vicinity of the surface and by the deposition flux of nitric acid. The particle flux does not depend on particle concentrations in quite a wide concentration range. For such a mixture the entire concept of deposition velocity is inapplicable.

 

Simulations of atmospheric aerosol composition show that the presence of ammonium nitrate as a part of aerosol is rather common in many places of the world. Moreover, we are not aware of any publication that demonstrates a linear dependency between the flux and concentration for ambient accumulation-mode aerosols. Based on our findings and the results of wind tunnel measurements we suggest that field campaigns could observe detectable fluxes of aerosol only if the fluxes were caused by aerosol processes in air. Therefore, such measurements cannot be used to directly infer particle deposition velocities, and the measurements with known conservative particles should be used instead. Parametrizations of deposition velocities that are based on the field-measured fluxes do not predict flux-concentration relation for particles if ammonium nitrate is present, and strongly over-deposit conservative aerosols. Therefore, the parametrizations based on wind-tunnel measurements with calibrated particles should be used instead, despite high-vegetation cases are not covered by such experiments.