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

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 10 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, H2O2), aerosol types (mineral dust, soot, organic aerosol, sea salt aerosol), environments (urban, agricultural land, Amazon forest, water body), seasons, and mixing heights. 15 For all investigated gases, γeqv ranges from 10 ~ 10 in polluted urban environments to 10 ~ 10 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 Amazon forest are minor compared to the dry deposition. 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 is yet 20 to be measured at laboratory and evaluated. Given the fact that most models have considered their uptakes on the ground surface, we suggest also considering the N2O5 uptake by all types of aerosols, HNO3 and H2O2 uptakes by mineral dust, O3 uptake by liquid organic aerosols and NO2, SO2, HNO3 uptakes by sea salt aerosols in atmospheric models.

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 (γ eff ), whereas gas uptake on the ground is usually described by a deposition velocity (V d ).For efficient comparison, we introduce an equivalent uptake coefficient (γ eqv ) at which the uptake flux of aerosols would equal that on the ground surface.If γ eff is similar to or larger than γ eqv , 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 (O 3 , NO 2 , SO 2 , N 2 O 5 , HNO 3 and H 2 O 2 ), 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, γ eqv 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 (γ eff ≥ γ eqv ) 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 O 3 uptake on liquid organic aerosols which can be important, especially over regions with γ eff ≥ γ eqv .H 2 O 2 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: N 2 O 5 uptake by all types of aerosols, HNO 3 and SO 2 uptake by mineral dust and sea salt aerosols, H 2 O 2 uptake by mineral dust, NO 2 uptakes by sea salt aerosols and O 3 uptake by liquid organic aerosols.
A variety of ground surfaces, including vegetation, water, rock, roads and so on, can take up gaseous species via dry deposition, and, thus, have significant impacts on the budget of these reactive gases and on the physicochemical properties of the ground surface itself (Lelieveld and Dentener, 2000;Ashmore, 2005).Dry deposition is one of the major removal pathways for most gaseous species and has been extensively parameterized in atmospheric models (Wesely and Hicks, 2000;Zhang et al., 2002Zhang et al., , 2003)).A resistance model, which consists of the aerodynamic resistance, quasi-laminar resistance and surface resistance, has been widely applied to calculate the dry deposition flux in global and regional atmospheric models (see Fig. 1, Wesely and Hicks, 2000;Wesely, 2007).The dry deposition velocity, V d (in units of cm s −1 ) calculated as the reciprocal of the total resistance, is the key parameter to describe the uptake fluxes on the ground.
Since the late 1990s, the importance of reactive uptake of gases by aerosols has been commonly accepted (Ravishankara, 1997;Gard et al., 1998;Jacob, 2000).Gas uptake by aerosols not only influences the fate of reactive gases, but also changes the physiochemical properties of atmospheric aerosols (Kolb et al., 2010).Taking the multiphase chemistry into account has proven to be a key factor to explain the observations and improve the model performances (Zhang and Carmichael, 1999;Song and Carmichael, 2001;Liao and Seinfeld, 2005;Wang et al., 2006;McNaughton et al., 2009;X. Wang, et al., 2012;B. Zheng et al., 2015;Tang et al., 2017;Chen et al., 2018;Mu et al., 2018).Compared with dry deposition, the parameterization of gas uptake on aerosols is more challenging (Jacob, 2000;Pöschl and Shiraiwa, 2015).The mass transfer between gases and aerosols can be described by the resistance model in analogy with an electrical circuit that decoupled the physiochemical limitations in the gas phase, gas-surface interface and the bulk phase under (quasi-) steady-state conditions (Schwartz and Freiberg, 1981;Schwartz, 1986;Kolb et al., 1995).A simplified scheme, which relies on the formulation of an effective uptake coefficient (γ eff ) has been widely used in current atmospheric models (Jacob, 2000;Liao and Seinfeld, 2005;K. Wang et al., 2012).Growing numbers of laboratory studies have reported γ eff for various trace gases and aerosol particles that are potentially important for atmospheric chemistry, such as O 3 , NO 2 , SO 2 , N 2 O 5 , HNO 3 on mineral dust (Ullerstam et al., 2002;Mogili et al., 2006;Vlasenko et al., 2006;Wagner et al., 2008;Ndour et al., 2009), soot (Rogaski et al., 1997;Longfellow et al., 2000;Al-Abadleh and Grassian, 2000;Saathoff et al., 2001) and sea salt aerosols (Mochida et al., 2000;Gebel and Finlayson-Pitts, 2000;Hoffman et al., 2003;Thornton and Abbatt, 2005;Ye et al., 2010).A series of evaluations on the kinetic and photochemical data for the multiphase reactions were conducted afterwards (Crowley et al., 2010(Crowley et al., , 2013;;Ammann et al., 2013;Burkholder et al., 2015).Pöschl et al. (2007), and the follow-up studies (e.g., Shiraiwa et al., 2010Shiraiwa et al., , 2011) ) developed a comprehensive kinetic model framework, enabling consistent and unambiguous descriptions of mass transfer and chemical reactions in aerosol systems.
However, the question still remains as to which surface types are more important for gas uptake in the PBL.The answer is not straightforward due to the following reasons: 1. First, although the surface of the Earth seems to be much larger than that of tiny aerosols, its contribution is diluted by the large volume of the PBL, resulting in a surface to volume ratio close to that of aerosol; for example, for a PBL height of 1000 m, the corresponding surface to volume ratio is 1000 µm 2 cm −3 , which is comparable to aerosol surface area concentrations of 200-2000 µm 2 cm −3 for urban areas (Woo et al., 2001;Stanier et al., 2004;Wu et al., 2008Wu et al., , 2017;;Ma and Birmili, 2015) and 200-1000 µm 2 cm −3 for rural environments (Ma et al., 2014;Ma and Birmili, 2015;Wu et al., 2017;Held et al., 2008).
2. Second, different formulations also hinder the comparison.As illustrated above, different schemes, formulations and terminologies are applied to calculate the uptake fluxes on ground and aerosols.The dry deposition velocity (V d ) is the fundamental parameter to describe the deposition process on the ground, whereas the effective uptake coefficient (γ eff ) is used to describe the uptake fluxes on aerosols.
In this study, we conducted a comparative assessment of the gas uptake on both ground and aerosol surfaces.Our goal is to identify the prevailing multiphase process in the PBL, especially those processes that have not yet been sufficiently considered in atmospheric models.Section 2 describes the methods of calculation and comparison.We present and discuss the main results in Sect.3, which is followed by a summary of our major findings in Sect. 4.

Methods
In this work, we compared the relative importance of gas uptake by the ground and by aerosols based on their uptake fluxes.In this comparison, resistance models were applied to calculate uptake fluxes on both ground and aerosol surfaces (see Fig. 1) as detailed below.The uptake fluxes of six reactive gases (O 3 , NO 2 , SO 2 , N 2 O 5 , HNO 3 and H 2 O 2 ) were calculated and compared for four typical land use categories (urban areas, agricultural land, the Amazon forest and water) and five aerosol types (mineral dust, soot, solid organic aerosol, liquid organic aerosol and sea salt aerosol).These species were chosen considering their potential impor-Figure 1.Schematic illustration of gas uptake on the ground and on aerosols in the planetary boundary layer as characterized by resistance models.The relative importance of aerosol uptake and dry deposition on the ground is characterized by comparing the aerosol uptake coefficient (γ eff ) with an equivalent uptake coefficient (γ eqv ) corresponding to the deposition velocity (V d ).
tance regarding dry deposition on the ground and uptake on aerosols within the troposphere.

Ground gas uptake
Dry deposition fluxes were calculated following the scheme and parameters of Wesely (1989) and Zhang et al. (2003).As shown in Fig. 1, the resistance model applied to characterize the dry deposition process includes the aerodynamic resistance (R a ), quasi-laminar resistance (R b ) and surface resistance (R c ).The basic equations for the flux calculations are as follows: (1) where F grd represents the gas deposition fluxes on various ground surfaces (mol m −2 s −1 ); V d represents the deposition velocity (cm s −1 ); [X g ] is the averaged gas concentration (mol m −3 ); and R grd is the total resistance in the dry deposition process (s cm −1 ), composed of R a , R b and R c .The detailed equations and parameterization scheme for the determination of R a , R b and R c are provided in the Supplement.A neutral meteorological condition was assumed in the calculation.We present the key input parameters and the calculated V d in Tables S1 and S2, respectively.
2.2 Aerosol gas uptake and the effective uptake coefficient (γ eff ) The net flux of gas X from the gas phase to the condensed phase (J net , mol m −2 s −1 ) for one aerosol particle can be expressed as Eq. ( 3) under (quasi-) steady-state conditions (Pöschl et al., 2007): The effective uptake coefficient, γ eff , represents the number of gas molecules taken by the aerosol particle divided by the number of those impacting onto the particle surface (Pöschl et al., 2007); ω is the mean thermal velocity (m s −1 ) -we use a typical value of 300 m s −1 in this study; [X g ] is the averaged gas concentration far away from the aerosol surface (mol m −3 ).
As shown in Fig. 1, resistance models have been widely applied to quantify the mass transfer of gases to aerosol particles.For gas uptake on liquid droplets, following the resistance model as described by Eq. ( 4), the overall resistance 1/γ eff is composed of three resistor terms due to gas diffusion (1/ g ), interfacial mass transfer (1/α) and bulk diffusion and reaction (1/ b ) (Pöschl et al., 2007).The conductance of gas diffusion is commonly calculated based on g = 8D g ω −1 d −1 p , where D g is the diffusion coefficient of O 3 4.4 ×10 −9 -4.8 ×10 −5 1.0 ×10 −7 -1.6 ×10 −4 2.0 ×10 −6 -6.9 ×10 −5 1.1 gas X in the gas phase (m 2 s −1 ), and d p represents the aerosol particle diameter.For large particles and very fast uptake processes, the gas diffusion process can be a limiting factor for the overall uptake (Tang et al., 2014a).For atmospheric aerosols with a diameter of ∼ 0.2 µm, the related gaseous uptake tends to be limited by the free molecular collision rate (uptake rate → ωαA[X g ]/4) (Jacob, 2000).Thus, in the following analyses, we mainly focus on the discussion of γ eff , and neglect the diffusion resistance in the gas phase.Given a mixing height of h and an aerosol surface area density of A (particle surface area per unit volume of air, µm 2 cm −3 ), the total uptake flux of gas X by aerosols (F aer , mol m −2 s −1 ) is where 10 −6 is the unit conversion factor.We summarized the measured uptake coefficients for a variety of gas species and aerosol types at both the initial state and the steady state in Table 1 (details in Tables A1-A4).These coefficients are mainly derived from measured values in the literature, reviewed data from the IUPAC (International Union of Pure and Applied Chemistry) "Task Group on Atmospheric Chemical Kinetic Data Evaluation" (Crowley et al., 2010(Crowley et al., , 2013;;Ammann et al., 2013; A1-A4).As we focus on the PBL, γ eff values measured at room temperatures (∼ 298 K) are mainly presented.Gas uptakes at very low temperatures (e.g., in the polar region or stratosphere) are outside the scope of this study and should be explored in future work.
Although the initial and steady-state uptake coefficients are listed, it should be noted that the values at the initial state may not be appropriate for direct application in chemical transport models (CTMs) considering the subsequent surface saturation and depletion of reactants for several cases (e.g., on mineral dust and soot; Ndour et al., 2009;Stephens et al., 1986;Ammann et al., 1998;Kalberer et al., 1999).In general, the upper limit and lower limit are determined based on those derived using the geometric surface and the BET (Brunauer-Emmett-Teller) surface, respectively.Preferences are given to those measured at steady state using ambient aerosols, or values recommended by the IUPAC group with relatively high reliability.As shown in Table A1, variances of more than 3 orders of magnitude are found for SO 2 and O 3 uptake on mineral dust depending on the gas concentration and aerosol components (Michel et al., 2002(Michel et al., , 2003;;Mogili et al., 2006;Ullerstam et al., 2002Ullerstam et al., , 2003;;Li et al., 2006).Large discrepancies also exist for SO 2 and HNO 3 uptake on soot (Longfellow et al., 2000;Saathoff et al., 2001;Xu et al., 2015).For H 2 O 2 , limited measurements of γ eff have been conducted for aerosols apart from mineral dust.

Uptake coefficient at equivalent flux (γ eqv )
To help the evaluation, we define an uptake coefficient at equivalent flux γ eqv .Here, γ eqv is the effective uptake coefficient on aerosols when the ground flux equals the aerosol flux within the PBL.When γ eff > γ eqv , the aerosol surfaces are more important than the ground surfaces regarding the gas uptake and vice versa.By letting F grd equal F aer , we can derive the expression for γ eqv as follows: Relation between γ eqv and V d for a mixing height of 300 m and aerosol surface area densities (A) observed at different locations and under different conditions: Amazon forest (Pöschl et al., 2010), Leipzig/Melpitz (Ma et al., 2014;Ma and Birmili, 2015), Pittsburgh (Stanier et al., 2004), Wangdu with and without haze event (Wu et al., 2017) and Beijing (Wu et al., 2008).For each city/condition, the line is labeled with the corresponding aerosol surface area density.Aerosol particle number concentrations are also provided for orientation.
and at a typical mixing height of 300 m, we have According to Eq. ( 6), γ eqv is proportional to V d , and is inversely proportional to the aerosol surface area density and the mixing height.We calculated a series of γ eqv values for a variety of gas species, land use categories, seasons, aerosol surface area densities (A) and mixing heights (h).
As defined, γ eqv reflects the relative importance of gas uptake on aerosols compared to those on the ground surfaces.Larger γ eqv indicates a higher probability for gases to deposit on the ground rather than on aerosols for further chemical reactions on surface and bulk, and vice versa.Low dry deposition velocities and high loadings of aerosols providing large amounts of surface reaction sites can benefit gas uptake on aerosols.The derived γ eqv and γ eff values from laboratory measurements are compared in Sect.3.
b.Typical A (corresponding to the typical γ eqv in Figs.3-5): we use 1050 µm 2 cm −3 for the urban environment (Wu et al., 2017), 230 µm 2 cm −3 for agricultural regions (Held et al., 2008), 46 µm 2 cm −3 for the Amazon forest (Rissler et al., 2006) and 76 µm 2 cm −3 for SSA (canonical distribution at a wind speed of 10 m s −1 , Lewis and Schwartz, 2004).It should be noted that the above ranges and the typical values of A are derived from current available experiments to support our analyses and discussions in this study, but still cannot cover all cases of particle distributions in the world.
Figure 2 shows the calculated γ eqv over a range of dry deposition velocities and aerosol surface area densities at a mixing height of 300m.V d values for different scenarios were calculated based on the resistance scheme illustrated above, showing a range of 0.01-2.3cm s −1 , with the lowest for NO 2 and the highest for N 2 O 5 and HNO 3 (details in Table S2).Aerosol surface area densities covered a range of 8.6 to 2139 µm 2 cm −3 , from pristine rainforest to polluted megacities, respectively.We show the calculated γ eqv under typical conditions (typical A as described above, h = 300 m) by season in Table S3 and a detailed illustrated γ eqv for each gas species in the sections below.As shown in Fig. 2, γ eqv decreases with increasing A, which is closely related to the air pollution level, and increases with increasing V d .
For small V d (≤ 0.1 cm s −1 ), γ eqv values lie in the range of 10 −5 -10 −4 for clean regions, such as Leipzig, Melpitz and Pittsburgh, and decrease to 10 −6 -10 −5 in polluted cities including Beijing and Wangdu.This low dry deposition can be found for NO 2 above the urban ground (0.03 cm s −1 , seasonal mean), and O 3 , NO 2 , SO 2 and H 2 O 2 on water bodies (0.07, 0.01, 0.03 and 0.08 cm s −1 , respectively).The downward shift of γ eqv with larger aerosol surface area density suggests the increased importance of gas uptake in polluted areas compared with clean areas.
With the increase of V d (> 0.1 cm s −1 ), γ eqv increases to 10 −5 -10 −2 accordingly.In a pristine region of the Amazon forest, γ eqv can reach up to 10 −2 .The lowest γ eqv is 2.1 × 10 −5 during haze events with high concentrations of fine particulate matter and surface area in the PBL (A = 2139 µm 2 cm −3 ).In this study, this range of V d covers most of the investigated cases, including O 3 , SO 2 , H 2 O 2 on urban/Amazon forest/agricultural region, NO 2 on agricultural region/Amazon forest, and N 2 O 5 and HNO 3 over all land use types (see Table S2).Thus, we can derive a general conservative criterion of γ eff > 10 −5 for aerosol uptake to compete with the dry deposition.
In the following, we further compared γ eqv to the laboratory measurements of γ eff for different reactive gases (O 3 , NO 2 , SO 2 , N 2 O 5 , HNO 3 and H 2 O 2 ).The uptake coefficients at the initial state are generally 1-3 magnitudes higher than those at steady state (see Table 1 and Figs.3-5).Considering the timescale of gas uptake by aerosols in the real world and applications in models, we mainly focus on the comparisons of γ eqv and the steady-state γ eff in the following discussions.

O 3
Under typical conditions (typical A by land use, h = 300 m, as illustrated above), γ eqv values for O 3 between 9.2 × 10 −5 and 2.2 × 10 −3 are determined, with the lowest value in urban areas and the highest value in the Amazon forest.The extended range is 1.4 × 10 −5 -3.8 × 10 −2 , which varies with particle area densities and mixing heights (Fig. 3).There are overlaps between γ eqv and γ eff for liquid organic aerosols (OAs) among all investigated typical environments, and other types of aerosols under favorable circumstances for aerosol uptake in urban areas.γ eff values lie below γ eqv values for other combinations of aerosol types and land use categories.
We can only expect comparable uptake between ground and aerosol surfaces of mineral dust, soot, solid organic aerosol and SSA at high aerosol loadings in urban areas (e.g., A = 1400 µm 2 m −3 , Beijing) and/or high mixing layers (e.g., h = 1.0 km).Combined with the measured uptake coefficients which lie in the range of 1.0×10 −7 to 1.6×10 −4 for soot, 1.1 × 10 −5 to 3.0 × 10 −3 for liquid organic aerosols and 1.3 × 10 −6 to 1.0 × 10 −4 for SSA, we can expect high uptake fluxes of O 3 on these three kinds of aerosols when the corresponding γ eff value is larger than 10 −4 for ground surfaces other than urban.
Complexity comes from the organic aerosols, of which the phase state has a large impact on the uptake and is subject to the temperature, relative humidity and particle size (see Fig. 3) (Virtanen et al., 2010;Cheng et al., 2015).For liquid organic aerosols, the measured γ eff values show large variances from 10 −5 to 10 −3 and corresponding γ eqv values fall into this range, demonstrating that O 3 uptake on aerosols is comparable to that on the ground.Thus, multiphase reactions of O 3 on liquid organic aerosols should be included in atmospheric models.This is also consistent with the findings of Mu et al. (2018), which demonstrate the importance of the phase state of aerosols in multiphase reactions and the transport of polycyclic aromatic hydrocarbons to improve the model performances at both regional and global scales.Shiraiwa et al. (2017) show the global map of the SOA (secondary organic aerosol) phase state at the Earth's surface.SOA in southern China, the Amazon forest and South Africa are mainly in the liquid phase within the PBL.For these regions, the comparable uptake fluxes for O 3 on liquid organic aerosols compared with the dry deposition demonstrate the importance of aerosol uptake.Dry deposition is one of the major sinking pathways for O 3 (Ganzeveld and Lelieveld, 1995).The uptakes of O 3 by aerosols are expected to contribute comparable sink fluxes to dry deposition regionally.Inclusion of the O 3 uptake by organic aerosols in these regions will increase the deposition rate of O 3 on aerosols, affect its lifetime, and further affect the fate of HO x and NO x through chemical reactions in the gas phase.

NO 2
For NO 2 , γ eqv values are generally above the upper limit of γ eff in urban, agricultural and forest environments, as shown in Fig. 4, demonstrating that ground surfaces are of greater importance than aerosols.Overlaps are found for SSA on various land use types and also for liquid organic aerosols in the urban environment.
NO 2 tends to deposit on the ground surface instead of on mineral dust particles, soot and solid organic aerosols.As reviewed in Tables A1-A3, the effective uptake coefficient of NO 2 on these three kinds of aerosols are at magnitudes of < 10 −6 under steady-state conditions.For A values ranging from 46 µm 2 cm −3 (Amazon) to 1050 µm 2 cm −3 (Wangdu) and at a mixing height of 300 m, γ eqv values of NO 2 lie between 1.4 × 10 −5 and 1.3 × 10 −3 , and are 1-3 orders of magnitude larger than γ eff on these three kinds of aerosols.Increasing the PBL mixing height and aerosol surface area may reduce γ eqv values by ∼ 1-2 orders of magnitude, but they are still above the measured γ eff values at steady state.
The reactive uptake coefficients of NO 2 by SSA were quantified in the range of 10 −6 to 10 −4 , demonstrating the ability of ambient sea salt aerosols to take in chemical species like NO 2 (Harrison and Collins, 1998;Yabushita et al., 2009;Ye et al., 2010).The high uptake coefficients observed for SSA (6.0 × 10 −7 -3.0 × 10 −4 ) are probably attributed to the reactions of Cl − with dissolved NO 2 in the aqueous phase (Msibi et al., 1993;Harrison and Collins, 1998;Yabushita et al., 2009).The overlapped values of γ eqv and γ eff show that the NO 2 uptake by SSA is comparable to the uptake by the land surface or water bodies in coastal areas; therefore, it should be taken into account in atmospheric models.
Another important process is the NO 2 uptake on liquid organic aerosols (γ eff in the range of 2.2 × 10 −7 -7.0 × 10 −6 ) in urban areas of high A. As shown in Fig. 4, the lower limit of γ eqv in urban is ∼ 2.2 × 10 −6 , which lies within the range of γ eff .The uptake coefficients of NO 2 on pure water are estimated to be around 10 −7 -10 −6 , driven by low solubility and slow hydrolysis rates (Kleffmann et al., 1998;Gutzwiller et al., 2002;Ammann et al., 2005;Komiyama and Inoue, 1980).Harrison and Collins (1998) reported a high γ eff of 5.4-5.8 × 10 −4 for NO 2 uptake on ammonium sulfate aerosols at high relative humidity (RH; RH = 50 %, 85 %).The presence of reactants such as inorganics of HSO − 10 −4 (Msibi et al., 1993;Lee and Tang, 1998;Spindler et al., 2003;Ammann et al., 2005;Yabushita et al., 2009;Su et al., 2008a;Cheng et al., 2016).Multiple measurements and modeling work have also pointed out that the high alkalinity of aqueous aerosols is key to promote the reactions and further increase the NO 2 uptake rates (Ammann et al., 2005;Herrmann et al., 2015;Cheng et al., 2016).Therefore, the NO 2 uptake on alkaline aqueous aerosols containing organic/inorganic reactants is competitive in the urban atmosphere, and should be addressed in detail in models.In the Amazon forest, where A is too low (46 µm 2 cm −3 ), corresponding to a γ eqv value of the order of 10 −3 , even a high γ eff value of 10 −4 is not sufficient to compete with the uptake by the ground surfaces.

SO 2
The calculated γ eqv values of SO 2 vary between 1.0 × 10 −4 and 2.1 × 10 −3 for land surfaces and 1.7 × 10 −4 above water bodies under typical conditions.As shown in Fig. 4, the SO 2 uptake by mineral dust is comparable to the ground uptake in urban areas, and under favorable conditions over agricultural land and water bodies.For soot, aerosol uptake is magnitudes lower than those on the ground (γ eqv ≥ γ eff ); thus, this process is unimportant for SO 2 .For SSA, γ eff values of 3.2 × 10 −3 -1.7 × 10 −2 have been reported for SO 2 at an aerosol pH of 5.4-6.6, which is high enough to compete with dry deposition over most environments (Gebel et al., 2000).Additional reactions of SO 2 and O 3 in alkaline solutions are found to promote the SO 2 uptake and form sulfate on SSA at first stage (Laskin et al., 2003).However, aerosol acidification due to production of H + has been suggest to quickly suppress the oxidation process in the real world (Alexander et al., 2005).We suggest including both the SO 2 uptake on SSA and the aerosol acidification process in models.
The extended range of γ eqv is 1.6 × 10 −5 -1.6 × 10 −3 , 5.5 × 10 −5 -2.8 × 10 −3 and 1.9 × 10 −5 -1.9×10 −3 for urban areas, agricultural land and water bodies, respectively.The γ eff of mineral dust falls in this range under high aerosol loadings or high mixing heights.The wide range of γ eff values for mineral dust (1.5 × 10 −8 to 6.3 × 10 −4 ) is a big challenge re-garding its application in models, because it can be affected by the presence of oxidants, the phase state, the components of the tested dust and the use of surface area in calculation (Huss et al., 1982;Ullerstam et al., 2003;Li et al., 2006;Alexander et al., 2009;Zhang et al., 2018).We further discuss the SO 2 uptake on mineral dust under different conditions in the following.
In environments with a high RH, water can enhance or inhibit the uptake by affecting reactive sites, and this effect varies with experimental conditions (Huang et al., 2015;Zhang et al., 2018).Conversely, the uptake rate can be improved by several factors and/or aqueous chemical reactions, such as the presence of O 3 , H 2 O 2 and transition metal ions (TMIs), which strongly depend on the aerosol pH (Jayne et al., 1990;Li et al., 2006;Cheng et al., 2016;Zhang et al., 2018).The initial γ eff value of SO 2 on pure water can reach as high as 10 −3 -0.1, varying with pH (Gardner et al., 1987;Worsnop et al., 1989;Jayne et al., 1990;Ponche et al., 1993).Depending on aerosol pH and oxidant concentrations, the regimes of SO 2 uptake and sulfate formation may transit from a TMI-or H 2 O 2 -dominated regime to a NO 2 -or O 3dominated regime (Cheng et al., 2016).In this case, the SO 2 uptake on aqueous aerosols is expected to play a dominant role over dry deposition under specific circumstances, such as haze events (He et al., 2014;Cheng et al., 2016), which should be quantified by combining in situ measurements and atmospheric modeling.
As shown in the examples in Table S4, several model schemes adopt a γ eff value of ∼ 10 −4 (Liao and Seinfeld, 2005; K. Wang et al., 2012), around 1 order of magnitude higher than the measured values under low RH conditions (Usher et al., 2002;Ullerstam et al., 2003;Adams et al., 2005;Li et al., 2006).For example, in Liao and Seinfeld ( 2005), γ eff is 3.0×10 −4 for RH < 50 %, and 0.1 for RH ≥ 50 % (see Table S4 with references).Under low RH conditions, the uptake coefficient commonly used in models is based on the dry deposition measurement of SO 2 on calcareous soils, cements and Fe 2 O 3 , rather than on laboratory measured γ eff values recommended by IUPAC.The reason for this divergence is unclear, and we are in favor of using the IUPAC recommended γ eff (e.g., Zhu et al., 2010, as shown in Table S4).The high uptake coefficient in models under high RH conditions is based on two assumptions: fast oxidation of SO 2 by O 3 in the aqueous phase, and high alkalinity in the dust aerosols.Thus, this γ eff value should be applied with the caveat that these prerequisites have been fulfilled, especially when extending it for other type of aerosols (Zheng et al., 2015).3.4 N 2 O 5 , HNO 3 and H 2 O 2 N 2 O 5 , HNO 3 and H 2 O 2 demonstrate their high uptake ability on atmospheric aerosols, as shown in Fig. 5.For N 2 O 5 , the similar or higher values of γ eff over γ eqv demonstrate that the multiphase uptake by all types of aerosols is as important as or even more important than dry deposition.The N 2 O 5 uptake by aerosols has been widely included in models (Bauer et al., 2004;Liao and Seinfeld, 2005; K. Wang et al., 2012).The uptake of HNO 3 and H 2 O 2 by mineral dust and HNO 3 by SSA are important given the overlap between γ eff and γ eqv ; thus, this uptake should also be characterized in atmospheric models in detail.
The extended ranges of γ eqv for HNO 3 is 1.5 × 10 −4 -1.5×10 −2 (urban), 1.5×10 −4 -7.7×10 −3 (agricultural land), 4.2×10 −4 -3.7×10 −1 (Amazon forest) and 7.0×10 −4 -7.0× 10 −2 (water), which are within or below the range of γ eff for mineral dust and SSA.The higher γ eff of 1.0 × 10 −3 to 0.21 for mineral dust and of 5.0 × 10 −4 to 0.25 for SSA demonstrated the more important role of aerosol uptake than of the ground surfaces.The uptake of HNO 3 on soot and solid organic aerosols appears to be less important.The HNO 3 uptake on mineral dust has been implemented in current models with an uptake coefficient of 0.1, or between 1.1 × 10 −3 and 0.2, which is consistent with the range of experimentally determined γ eff values reviewed in this study (Liao and Seinfeld, 2005; K. Wang et al., 2012).
The study of the uptake of H 2 O 2 by aerosols is rather limited compared with the other aforementioned trace gases.The reported γ eff values on dust and ambient aerosol samples suggest that aerosol uptake is more important than that by the ground surface.The measured uptake coefficients of H 2 O 2 on mineral dust are in the range of 1.0 × 10 −5 -9.4 × 10 −4 and overlap with the calculated γ eqv of 1.5×10 −4 -3.0×10 −3 under typical conditions.Ambient aerosols collected in urban areas show similar γ eff values of H 2 O 2 (8.1 × 10 −5 -4.6 × 10 −4 ) to mineral dust (Wu et al., 2015).The aerosol chemistry of H 2 O 2 in the troposphere is complex and unclear (Liang et al., 2013;Li et al., 2016).In some cases, a net emission of H 2 O 2 from aerosol surfaces has been speculated instead of an uptake or adsorption as a result of HO x radicals cycling (Liang et al., 2013;Li et al., 2016).Most models only parameterize the H 2 O 2 uptake by dust particles (Dentener et al., 1996;K. Wang et al., 2012).The uptake by other aerosol types has not been considered due to limited experimental data.Thus, more laboratory kinetic measurements are needed.As ambient aerosol samples show a γ eff similar to that of dust particles (Wu et al., 2015;Pradhan et al., 2010ab;Zhou et al., 2016), we suggest adopting the γ eff of dust particles and applying it to all aerosol types before new kinetic data become available.

Discussion
In this section, we address several important issues based on the comparisons.The large variability found in the measured γ eff for SO 2 and NO 2 is discussed in Sect.4.1.How to apply the measured γ eff in atmospheric models to represent the reactivity of heterogeneous reactions still remains an open question.Regarding this, we discuss the underlying important factors that should be taken into account in Sect.4.2.Outlooks and the limitations of this work are provided in Sect.4.3.

Large variability of γ eff for SO 2 and NO 2
Notably, there is a large variability in the reviewed γ eff of SO 2 uptake by dust particles (as discussed in Sect.3.2).For SO 2 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 (O 3 , H 2 O 2 and NO 2 ), RH, measurement techniques and the surface area used in data processing (Ullerstam et al., 2003;Li et al., 2006;Huang et al., 2015).For example, a γ eff of 1.6×10 −4 was derived for SO 2 uptake on Al 2 O 3 powder (Usher et al., 2002).The uptake coefficient was reduced by 1 order of magnitude to 1.6-6.6×10−5 using ambient aerosols of Chinese loess/Saharan dust (Usher et al., 2002;Ullerstam et al., 2003;Adams et al., 2005), indicating that the particle substrate is key in investigating SO 2 uptake.Similarly, through cross-comparisons between the other different investigations shown in Table A1, we anticipate that the above factors can all contribute to this large discrepancy.As recommended by IUPAC, an uptake coefficient of 4 × 10 −5 based on airborne measurements is suggested for use in models under low RH conditions.For high RH, we suggest determining γ eff using information on aerosol pH due to the high correlation between these factors, as illustrated in Sect.3.3.

Initial vs. steady state and geometric vs. BET
Measurements of the effective uptake coefficients revealed the instantly fast uptake at the initial state and the gradual decline due to the saturation of surface reaction sites and loss of reactive substances (Hanisch and Crowley, 2003).The uptake at the initial state can be orders of magnitude faster than that at the steady state for aerosols (see Tables 1 and A1-A4).The timescale for reaching surface saturation/equilibrium is dependent on the reaction system.For a gas-aqueous particle surface, the timescale to establish equilibrium for the investigated species is less than 1 s (Seinfeld and Pandis, 2006, 554-557).For dust particles, it can take hours for complete saturation (Judeikis et al., 1978;Goodman et al., 2001).Fine particles with diameters < 10 µm have lifetimes of several days in the atmosphere (Prospero, 1999;Lee et al., 2009).Thus, using uptake coefficients at steady state maybe more representative in models, unless we can assume that the uptake process is not limited by surface accommodation and reactions (like HNO 3 ; Goodman et al., 2000), typically when the gas concentration is low enough that the surface passivation is negligible compared with the lifetime of aerosols in the atmosphere (Hanisch and Crowley, 2003).Gas uptake on fresh aerosols may reach or even surpass the level of the ground near emission sources.Using a uniform uptake coefficient in atmospheric models may not be enough to reflect the deactivation process of the multiphase gas uptake during aerosol aging, considering the large range of γ eff values varying with time.
In addition, the γ eff values are measured and reported based on the geometric surface and/or the BET surface.Differences of more than 3 orders of magnitude are derived depending on whether or not the pores within the microstructure of solid aerosol surface are considered (see Table A1).Using the same method to calculate the available surface area may reconcile these differences (Tang et al., 2017).In this study, γ eff values with a revised BET surface are generally used as the lower limit, and those using the geometric surface are used as the upper limit.Whether or not the BET area is used as a correction in the calculation of γ eff or not remains a discrepancy when applied in models (Hanisch and Crowley, 2001a, b;Underwood et al., 2001ab).This discrepancy from measurements may come from the differences in experimental samples (airborne particles vs. powder).To solve this issue, more studies on the reactive surface area for ambient aerosols are needed to guide the data processing and model parameterization.

Outlooks and limitations
We can conclude that the phase state is a crucial factor influencing the uptake rates.The uptake rates of O 3 and NO 2 in liquid organic aerosols are 1-3 orders of magnitude higher than on solid/semi-solid surfaces.In regions with high RH conditions and sufficient sources of organic compounds (e.g., the Amazon forest and southern China), the gas uptake is anticipated to have a considerable effect on concentrations.The effect is yet to be evaluated in combination with further model simulations.
Measurement of uptake by ambient aerosols is crucial to reconcile laboratory experiments and modeling results, especially for gases that have undergone limited investigation (e.g., H 2 O 2 ).Currently limited work has been undertaken to address the uptake of H 2 O 2 by aerosol particles other than mineral dust (Liao and Seinfeld, 2005;Pradhan et al., 2010a, b;K. Wang et al., 2012;Zhou et al., 2016).Because ambient aerosol samples show a γ eff value comparable to that of dust particles, we recommend similar γ eff values of 1.0 × 10 −5 -9.4 × 10 −4 for H 2 O 2 uptake by other types of aerosol, which will lead to a larger sink in the atmospheric budget of H 2 O 2 .
Considering the complexity of multiple factors affecting the uptake rates, such as temperature, RH, gas concentration, aerosol pH and aerosol state (fresh or aged), establishing a look-up table for γ eff based on the available factors mentioned above should be a feasible way to implement the gas uptake processes in atmospheric models (Mu et al., 2018).
There are limitations to the comparisons conducted in this study.We use a unified thermal velocity (300 m s −1 ) for all gases, which will introduce positive biases of +4 % to +30 % for O 3 , NO 2 , SO 2 , HNO 3 and H 2 O 2 , and a negative bias of −24 % for N 2 O 5 in calculations of γ eqv at the same temperature.The ambient parameters to calculate the dry deposition velocities (temperature and radiation) refer to the standard meteorological database for construction in northern China (Zhang, 2004), which may introduce uncertainties for analyses of other areas.In addition, the variability of aerosol surface area in each environment can also contribute to the variability of γ eqv .We mainly focused on the uptake fluxes at room temperature (∼ 298 K).The gas uptakes at very low temperatures (e.g., in the polar region and stratosphere) are outside the scope of this study but should be further explored concerning its potentially large impact.The real ambient multiphase processes are much more complex than the laboratory measurements; nevertheless, they use airborne aerosols.Ambient online measurements of γ eff will favor the model parameterization and improve our understanding of the multiphase processes within the PBL in the real world (Li et al., 2019).Moreover, more gaseous and aerosol species such as VOCs and bioaerosols should also be investigated (Zhou et al., 1996;Wagner et al., 2002;Fried et al., 2003;Beck et al., 2013;Li et al., 2014Li et al., , 2016;;Ouyang et al., 2016;Liu et al., 2017;Meusel et al., 2017).

Conclusions
In this work, we investigated the relative importance of gas uptake fluxes on the ground and aerosols for six reactive trace gases (O 3 , NO 2 , SO 2 , N 2 O 5 , HNO 3 and H 2 O 2 ), various environments, aerosol types and mixing heights.The purpose of the study was to identify the aerosol uptake process, which is equally or more important than dry deposition on ground surfaces but has not been adequately addressed in models.
For efficient comparison, we derived a criterion, γ eqv , to identify the dominant surface with respect to gas uptake.For investigated gas species, γ eqv values generally lie in the magnitude of 10 −4 , and can be extended to lower values in polluted areas and/or at low dry deposition velocities: γ eqv values lie in the range of 10 −6 -10 −4 in polluted urban environments and 10 −4 -10 −1 under pristine forest conditions.The effective uptake coefficient (γ eff ) values derived from experiments are reviewed and compared with γ eqv .Notably, the gas uptake by aerosols is comparable and should be considered in models when γ eff is equal to or higher than γ eqv .In urban environments, aerosol uptake is important for all combinations of gases and aerosols, which is favored by the high particle surface densities.On the contrary, the contribution of aerosol uptake is minor compared with dry deposition for gases in the Amazon forest.
The aerosol uptake of the following gases can be as important as the dry deposition processes and should be considered in atmospheric models: N 2 O 5 on all types of aerosols, HNO 3 and SO 2 on mineral dust and sea salt aerosols, H 2 O 2 on mineral dust, NO 2 on sea salt aerosols, and O 3 on liquid organic aerosols (γ eff ≥ γ eqv , as shown in Table 2).The gas uptake on mineral dust for most gases and sea salt aerosols' uptake of SO 2 and NO 2 have already been parameterized in a series of models.The processes of H 2 O 2 uptake on mineral dust and O 3 on liquid aerosols have unfortunately not received enough attention.For other combinations of gas species and aerosols, the ground tends to be the dominant surface rather than aerosols with respect to taking up trace gases within the PBL.
There are several indications from this work of processes that should be addressed in future measurements and model implementations: a.It is indicated that the multiphase processes for O 3 on liquid organic aerosols are underestimated in current atmospheric models.For regions with high RH conditions and the existence of organic aerosols in the liquid state such as southern China, the Amazon forest and South Africa, the multiphase uptakes of O 3 by aerosols are expected to contribute comparable sinking fluxes to dry deposition.Compared with the relatively low uptakes on (semi-) solid organic aerosols, we can conclude that the phase state is a crucial factor influencing the uptake rates.
Table 2. Gas uptake processes that are potentially important compared with dry deposition across various environments (marked using a √ ).

Gases
Mineral dust Soot Solid organic aerosol Liquid organic aerosol Sea salt aerosol b. Large uncertainties should be addressed for the comparison results of SO 2 and NO 2 .There are more than 3 orders of magnitude of variances in γ eff for SO 2 on mineral dust and NO 2 on aqueous aerosols.Under low RH circumstances, dry deposition tends to dominate the gas uptake rather than aerosols.However, for cases under high RH condition, the contributions of aerosols should be cautiously determined with full consideration of the aerosol component, aerosol pH and so on.
c. H 2 O 2 uptake on a variety of aerosols needs to be measured and evaluated.It is shown that the H 2 O 2 uptake on dust is comparable or even more important than that by the ground surface (γ eff ≥ γ eqv ).Measurements using ambient aerosols suggest that the uptake on aerosols other than mineral dust should be of a similar magnitude.
Data availability.All parameters used to calculate V d , the aerosol surface area densities (A) and the laboratory measurements of γ eff were derived from peer-reviewed literature or publicly available databases (as illustrated in the main text).
Appendix A

Figure 3 .
Figure 3. Equivalent uptake coefficients (γ eqv , a) and laboratory measurement values (γ eff , b) for O 3 on different ground types and aerosols.For γ eqv , the upper whiskers represent maximum values calculated at the lowest A and h (h = 100 m), the lower whiskers represent minimum values calculated at the highest A and h (h = 1 km), and boxes represent typical conditions (typical A as described in Sect.3.1, h = 300 m).For γ eff , the gray bar represents the range of initial values, and the blue bar represents the range of steady-state values observed in laboratory experiments.

Figure 4 .
Figure 4. Uptake coefficients (γ eqv , a, c; γ eff , b, d) for NO 2 and SO 2 on different ground types and aerosols.

Table 1 .
Aerosol uptake coefficients (γ eff ) observed in laboratory experiments a .
A detailed review with references for γ eff values is given in TablesA1-A4.bThefeature (initial or steady state) of the reported uptake coefficients are mainly derived from the literature.If no specific description is found, we assign the measurements on a timescale of milliseconds or seconds to the initial state, and those with a longer exposure time (∼ 1 h or longer) to the steady state.c "n/a" denotes not available. a

Table A1 .
Aerosol uptake coefficients (γ eff ) for reactive gases on mineral dust observed in laboratory experiments (T = 298 ± 2 K if not specified otherwise).

Table A2 .
Aerosol uptake coefficients (γ eff ) for reactive gases on soot observed in laboratory experiments (T = 298 ± 2 K if not specified otherwise).

Table A3 .
Aerosol uptake coefficients (γ eff ) for reactive gases on organic aerosols observed in laboratory experiments (T = 298 ± 2 K if not specified otherwise).

Table A4 .
Aerosol uptake coefficients (γ eff ) for reactive gases on sea salt aerosols observed in laboratory experiments (T = 298 ± 2 K if not specified otherwise).