Abstract
Abstract. 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 (γeff), whereas gas uptake on the ground is usually described by a deposition velocity (Vd). For efficient comparison, we introduce an equivalent uptake coefficient (γeqv) at which the uptake flux of aerosols would equal that on the ground surface. If γeff is similar to or larger than γeqv, 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 (O3, NO2, SO2, N2O5, HNO3 and H2O2), 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, γeqv ranges from magnitudes of 10−6–10−4 in polluted urban environments to 10−4–10−1 under pristine forest conditions. In urban areas, aerosol uptake is relevant for all species (γeff≥γeqv) 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 O3 uptake on liquid organic aerosols which can be important, especially over regions with γeff≥γeqv. H2O2 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: N2O5 uptake by all types of aerosols, HNO3 and SO2 uptake by mineral dust and sea salt aerosols, H2O2 uptake by mineral dust, NO2 uptakes by sea salt aerosols and O3 uptake by liquid organic aerosols.
Highlights
Given the fact that most models have considered the uptakes of these species on the ground surface, we suggest considering the following processes in atmospheric models: N2O5 uptake by all types of aerosols, HNO3 and SO2 uptake by mineral dust and sea salt aerosols, H2O2 uptake by mineral dust, NO2 uptakes by sea salt aerosols and O3 uptake by liquid organic aerosols
The lowest γeqv is 2.1 × 10−5 during haze events with high concentrations of fine particulate matter and surface area in the planetary boundary layer (PBL) (A = 2139 μm2 cm−3). This range of Vd covers most of the investigated cases, including O3, SO2, H2O2 on urban/Amazon forest/agricultural region, NO2 on agricultural region/Amazon forest, and N2O5 and HNO3 over all land use types
For SO2 uptake by dust particles, differences of more than 3 orders of magnitude are found for its uptake by mineral dust (10−8–10−4, steady state), which may be attributed to several factors such as the experimental particle substrates, co-existing oxidants (O3, H2O2 and NO2), relative humidity (RH), measurement techniques and the surface area used in data processing (Ullerstam et al, 2003; Li et al, 2006; Huang et al, 2015)
Summary
Multiphase processes play an essential role in atmospheric chemistry and atmosphere–biosphere exchange (Ravishankara, 1997; Ammann et al, 1998, 2013; Gard et al, 1998; Usher et al, 2003; Bauer et al, 2004; Fowler et al, 2009; Kolb et al, 2010; Su et al, 2011, 2013; Herrmann, 2003; Herrmann et al, 2015; Pöhlker et al, 2012; Oswald et al, 2013; Andreae et al, 2015; George et al, 2015; McNeill, 2015; Pöschl and Shiraiwa, 2015; Quinn et al, 2015; Weber et al, 2015; Cheng et al, 2016; Fröhlich-Nowoisky et al, 2016; Lappalainen et al, 2016; Tang et al, 2016; Meusel et al, 2018) They affect the atmospheric trace gas concentrations and modify the properties of condensed phases, commonly known as the aging process (Song and Carmichael, 1999; Cheng et al, 2006, 2012; Rudich et al, 2007; Andreae 2009; Jimenez et al, 2009; Gunthe et al, 2011; Ditas et al, 2018). Li et al.: Relative importance of gas uptake on aerosol and ground surfaces ies, different formulations have been used to describe and parameterize the gas uptake processes (Wesely, 1989, 2007; Ravishankara, 1997; Jacob, 2000; Wesely and Hicks, 2000; Zhang et al, 2003; Ammann and Pöschl, 2007; Pöschl et al, 2007)
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