Deposition velocity concept does not apply to fluxes of ambient aerosol

Abstract

We demonstrate that the proportionality between a deposition flux and a corresponding concentration usually does not hold for ambient aerosol. Therefore the deposition velocity Vd, defined as the proportionality coefficient, while might exist for some components of the aerosol, is not applicable to aerosol as a bulk substance or to a size mode of it. Insufficient attention to the proportionality requirement leads to large discrepancies between field and wind-tunnel measurements of Vd of aerosols with aerodynamic diameters ranging from approximately 0.1 to 2 µm. In seemingly similar conditions, the deposition velocities reported in different experiments may differ by up to two orders of magnitude, with field measurements showing much higher values than experiments performed in controlled environments with known particle properties. We demonstrate that the bulk of the discrepancy can be explained by gas-particle partitioning in the immediate vicinity of the surface. With the chemistry-transport model SILAM equipped with gas-particle partitioning for ammonium nitrate, we demonstrate that in presence of even small amounts of ammonium nitrate, the vertical flux of total aerosol mass is not controlled by particle deposition but rather by aerosol-gas partitioning in the vicinity of the surface. Under these conditions, the deposition flux is not proportional to the concentration, and the concept of deposition velocity as a proportionality coefficient between concentration and deposition flux falls apart. Presence of other semi-volatile components in ambient aerosols may further complicate the case, but ammonium nitrate alone is sufficient to invalidate the concept for ambient aerosol. By simulating a renowned field experiment with the SILAM model, we are able to reproduce the magnitudes and temporal behaviors of ambient particle fluxes using the deposition parameterization derived from wind-tunnel studies. Combining these simulations with a set of computational experiments, we suggested guidelines for accounting for the relevant processes in regional atmospheric composition models.