Relative importance of gas uptake on aerosol and ground surfaces characterized by equivalent uptake coefficients

Li, Meng; Su, Hang; Li, Guo; Ma, Nan; Pöschl, Ulrich; Cheng, Yafang

Quantifying the relative importance of gas uptake on the ground and aerosol surfaces helps to determine which processes should be included in atmospheric chemistry models. Gas uptake by aerosols is often characterized by an effective uptake coefficient (inline-formulaγeff), whereas gas uptake on the ground is usually described by a deposition velocity (inline-formulaVd). For efficient comparison, we introduce an equivalent uptake coefficient (inline-formulaγeqv) at which the uptake flux of aerosols would equal that on the ground surface. If inline-formulaγeff is similar to or larger than inline-formulaγeqv, aerosol uptake is important and should be included in atmospheric models. In this study, we compare uptake fluxes in the planetary boundary layer (PBL) for different reactive trace gases (inline-formulaO3, inline-formulaNO2, inline-formulaSO2, inline-formulaN2O5, inline-formulaHNO3 and inline-formulaH2O2), aerosol types (mineral dust, soot, organic aerosol and sea salt aerosol), environments (urban areas, agricultural land, the Amazon forest and water bodies), seasons and mixing heights.

For all investigated gases, inline-formulaγeqv ranges from magnitudes of 10inline-formula−6–10inline-formula−4 in polluted urban environments to 10inline-formula−4–10inline-formula−1 under pristine forest conditions. In urban areas, aerosol uptake is relevant for all species (inline-formulaγeffγeqv) and should be considered in models. On the contrary, contributions of aerosol uptakes in the Amazon forest are minor compared with the dry deposition. The phase state of aerosols could be one of the crucial factors influencing the uptake rates. Current models tend to underestimate the inline-formulaO3 uptake on liquid organic aerosols which can be important, especially over regions with inline-formulaγeffγeqv. inline-formulaH2O2 uptakes on a variety of aerosols are yet to be measured under laboratory conditions and evaluated.

Given the fact that most models have considered the uptakes of these species on the ground surface, we suggest also considering the following processes in atmospheric models: inline-formulaN2O5 uptake by all types of aerosols, inline-formulaHNO3 and inline-formulaSO2 uptake by mineral dust and sea salt aerosols, inline-formulaH2O2 uptake by mineral dust, inline-formulaNO2 uptakes by sea salt aerosols and inline-formulaO3 uptake by liquid organic aerosols.



Li, Meng / Su, Hang / Li, Guo / et al: Relative importance of gas uptake on aerosol and ground surfaces characterized by equivalent uptake coefficients. 2019. Copernicus Publications.


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