Deposition velocity concept does not apply to fluxes of ambient aerosols

Abstract. The process of dry deposition in chemistry-transport models is usually implemented assuming a proportionality between the deposition flux and the corresponding concentration of a tracer at some reference height. The coefficient of proportionality, called deposition velocity, V_d, is to be parameterized and validated experimentally. We analyse large discrepancies between field and wind-tunnel measurements of V_d of aerosols with aerodynamic diameters ranging from approximately 0.1 μm 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 fast chemical reactions and a particle-to-gas conversion in the immediate vicinity of the surface. By applying the chemistry-transport model SILAM, equipped with gas-particle partitioning for ammonium nitrate, we demonstrate that in the 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. While there are many other non-conservative components in ambient aerosols apart from ammonium nitrate, we demonstrate that the abundance of ammonium nitrate alone is sufficient to render typical ambient aerosol into a non-conservative substance. 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. 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.