Dry deposition velocities of particles govern the deposition and atmospheric lifetime of aerosols and their constituents and need to be parameterised in chemical transport models to predict air quality, climate change and ecosystem impacts. The same micrometeorological flux measurement approaches used to measure deposition rates can be used to quantify emissions from area sources such as urban emissions and resuspension. This paper reviews recent measurements of fluxes of particle number and, importantly, individual aerosol chemical compounds, made with wet-chemistry gradient systems and eddy-covariance using aerosol mass spectrometry. The results indicate that aerosol exchange tends to be bi-directional and is modulated by particle dynamics due to evaporation and condensation growth. Often, some aerosol components are emitted or chemically formed below the measurement height, while other components deposit or evaporate. This has important implications on the use of field measurements of number fluxes for the development of model parameterisations. For example, measurements over a range of surfaces show that ammonium nitrate volatilises during the deposition to semi-natural vegetation, forming associated gaseous precursors, which deposit faster than the aerosol species. This evaporation therefore enhances total nitrogen deposition, and models need to use enhanced effective deposition rates to account for the effect. Conversely, the effect of particle dynamics on fluxes can be used to study aerosol processing, such as the in-situ formation of inorganic and organic secondary aerosol and the volatilisation of primary combustion aerosol. Examples will be shown from agricultural, natural and urban environments. Finally, the paper will discuss future research priorities to help improve parameterisations of dry deposition velocities further
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