Effects of heterogeneous reactions on global tropospheric chemistry

Abstract. This study uses a chemistry-climate model CHASER (MIROC) to explore the roles of heterogeneous reactions (HRs) in global tropospheric chemistry. Three distinct HRs of N2O5, HO2, and RO2 are considered for surfaces of aerosols and cloud particles. The model simulation is verified with EANET and EMEP stationary observations, R/V MIRAI ship-based data, ATOM1 aircraft measurements, satellite observations by OMI, ISCCP, and CALIPSO-GOCCP, and reanalysis data JRA55. The heterogeneous chemistry facilitates improvement of model performance with respect to observations for NO2, OH, CO, and O3, especially in the lower troposphere. The calculated effects of heterogeneous reactions cause marked changes in global abundances of O3 (−3 %), NOx (−2.2 %), CO (+3.3 %), and global mean CH4 lifetime (+5.9 %). These global effects were contributed mostly by N2O5 uptake onto aerosols in the middle troposphere. At the surface, HO2 uptake gives the largest contributions, with a particularly significant effect in the North Pacific region (−24% O3, +68 % NOx, +8 % CO, and −70 % OH), mainly attributable to its uptake onto clouds. The RO2 reaction has a small contribution, but its global-mean negative effect on O3 is not negligible. In general, the uptakes onto ice crystals and cloud droplets that occur mainly by HO2 and RO2 radicals cause smaller global effects than the aerosol-uptake effects by N2O5 radicals (+1.34 % CH4 lifetime, +1.71 % NOx, −0.56 % O3, +0.63 % CO abundances). Nonlinear responses of tropospheric O3, NOx, and OH to the N2O5 and HO2 uptakes are found in the same modelling framework of this study (R > 0.93). Although all HRs showed negative tendencies for OH and O3 levels, the effects of HR(HO2) on the tropospheric abundance of O3 showed a small increment with an increasing loss rate. However, this positive tendency turns to reduction at higher rates (> 5 times). Our results demonstrate that the HRs affect not only polluted areas but also remote areas such as the mid-latitude sea boundary layer and upper troposphere. Furthermore, HR(HO2) can bring challenges to pollution reduction efforts because it causes opposite effects between NOx (increase) and surface O3 (decrease).



Introduction
Heterogeneous reactions (HRs) on the surfaces of atmospheric aerosols and cloud droplets are regarded as playing crucial roles in atmospheric chemistry. They affect ozone (O3) concentrations in various pathways via the cycle of odd hydrogens (HOx) and nitrogen oxides (NOx) (Jacob, 2000). Tropospheric ozone, an important greenhouse gas, causes damage to human health, https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License.
crop, and ecosystem productivity (Monks et al., 2015). Although tropospheric O3 was recognized as a critical oxidant species; 30 its global distribution has not been adequately captured to date because of the limited number of observations. Whereas many sites in the heavily polluted regions of eastern Asia show ozone increases since 2000 (Liu and Wang, 2020), many sites in other regions show decreases (Gaudel et al., 2018). Moreover, O3 responds to changes of multiple pollutants such as NOx and VOCs in different ways, which challenge the local pollutant control policy. For instance, since the Chinese government released the Air Pollution Prevention and Control Action Plan in 2010 (Zheng et al., 2018), the targets of SO2, NOx, and 35 particulate matter (PM) decreased drastically, but urban ozone pollution has been worsening (Liu and Wang, 2020). Indeed, the O3 responses are controlled by several mechanisms including heterogeneous effects of HO2 and N2O5 onto aerosols Taketani et al., 2012;Li et al., 2019;Liu and Wang, 2020).
Stationary observations and laboratory experiments are important for enhancing the understanding of the tropospheric chemistry of O3 and other essential components (NOx, HOx). However, direct observation of vertical O3 distribution including 40 upper tropospheric O3 was not available before 1970. It has been deployed only at limited sites of the globe. Global atmospheric modelling is a useful method to reanalyze or to forecast the past and future changes in O3 and their effects on human health and climate. To serve this task, atmospheric models use both laboratory and observational data to help achieve accurate simulations of O3 and its precursors (HOx, NOx, hydrocarbons). To date, many modelling studies have suggested that heterogeneous chemistry be included in a standard model for tropospheric chemistry (Jacob, 2000;Evans, 2010, 45 2011;de Reus et al., 2005).
One fundamentally important HR in the troposphere is the uptake of N2O5 onto aqueous aerosols, known as a removal pathway for NOx at night (Platt et al., 1984). Actually, NOx plays crucially important roles in the troposphere because it controls the cycle of HOx and the production rate of tropospheric O3 (Logan et al., 1981;Riemer et al., 2003). The morning photochemistry can be affected by NO3 and N2O5, which are important nocturnal oxidants. Since the early 1980s, the role of 50 urban NOx chemistry in Los Angeles pollution (National Research Council, 1991) has been acknowledged, but the proclamation of nighttime radicals remained sparse. It was only recognized in the past decade that N2O5 radical chemistry could have a much more perceptible effect stemming from reasons counting a refined understanding of heterogeneous processes occurring at night (Brown and Stutz, 2012). The HR of N2O5 was revealed under different meteorological conditions in the US, Europe, and China (photosmog, high relative humidity (RH), or seasonal variation) for particles of various types: 55 ice, aqueous aerosols with organic-coating, urban aerosols, dust, and soot (Apodaca et al., 2008;Lowe et al., 2015;Qu et al., 2019;Riemer et al., 2003Riemer et al., , 2009Wang et al., 2018;Wang et al., 2017;Xia et al., 2019). The uptake of N2O5 can markedly enhance nitrate concentration in nocturnal chemistry or PM2.5 explosive growth events in summer, decrease NOx, and either increase or decrease O3 concentrations in different NOx conditions (Dentener and Crutzen, 1993;Qu et al., 2019;Riemer et al., 2003;Wang et al., 2017). Even during daytime, N2O5 in the marine boundary layers can enhance the NOx to HNO3 60 conversion, and chemical destruction of O3 (Osthoff et al., 2006). The 10-20 ppbv reduction of O3 because of N2O5 uptake in the polluted regions of China has also been reported . At mid-to high latitudes, N2O5 uptakes on sulfate aerosols https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License. could engender 80% and 10% NOx reduction, respectively, in winter and summer, leading to approximate 10% reduction of O3 in both seasons .
Another vital process taking place on particles is the HRs of peroxy radicals (HO2 and RO2). Peroxy radicals are the 65 primary chain carriers driving O3 production in the troposphere. Moreover, it can drive the hydrocarbons and NOx concentration which are important for nocturnal radical chemistry (Geyer et al., 2003;Richard, 2000;Salisbury et al., 2001).
In the past, the HR(HO2) effects have been well considered in the laboratory (Macintyre and Evans, 2011) and field observations (Kanaya et al., 2001(Kanaya et al., , 2002a(Kanaya et al., , 2002b(Kanaya et al., , 2007Taketani et al., 2012), but many technical problems (e.g., detecting HO2) have created difficulties that challenge its reported importance in the troposphere, as asserted from recent 70 studies (Liao and Seinfeld, 2005;Martin et al., 2003;Tie et al., 2001). More recently, global modelling reports have described that the inclusion of HO2 uptake can affect atmospheric constituents strongly by the increment in tropospheric abundances for carbon monoxide (CO) and other trace gases because of reduced oxidation capacity (Lin et al., 2012;Macintyre and Evans, 2011). The HOx loss on aerosols can reduce O3 concentrations by up to 33% in remote areas and up to 10% in a smog episode (Saathoff et al., 2001;Taketani et al., 2012). The HOx loss on sea-salt, sulfate, and organic carbon in various environments can 75 decrease respectively HO2 levels by 6-13%, 10-40%, and 40-70% (Martin et al., 2003;Taketani et al., 2008Taketani et al., , 2009Tie et al., 2001). For RO2 with a typical representative of CH3CO.O2 (peroxyacetyl radical, PA), it plays a big role in the long-range transport of pollution (VOC, NOx) (Richard, 2000;Villalta et al., 1996). It can bring NOx from polluted domains as PAN to remote regions in the ocean and higher altitudes (Qin et al., 2018;Richard, 2000). The concentrations of HO2 and RO2 at nighttime in the marine boundary layer were measured and confirmed (Geyer et al., 2003;Salisbury et al., 2001). Moreover, 80 some evidence suggests uptake of HO2 and PA on clouds, aqueous aerosols, and other surfaces in high humidity conditions, although the mechanism is uncertain (Geyer et al., 2003;Jacob, 2000;Kanaya et al., 2002b;Liao and Seinfeld, 2005;Lin et al., 2012;Richard, 2000;Salisbury et al., 2001). The predominance of peroxy uptake to clouds results from the ubiquitous existence and larger SAD maxima of cloud droplets in the atmosphere. Indeed, aqueous-phase chemistry might represent an important sink for O3 (Lelieveld and Crutzen, 1990). Also, PA loss on aqueous particles can mediate the loss of PAN 85 (CH3CO.O2NO) in fog (Villalta et al., 1996). Some modelling studies indicate that HOx loss (including HO2 loss) on aqueous aerosols causes about 2% reduction, 7% and 0.5% increments, respectively, in the annual mean global burden of OH, CO, and O3 (Huijnen et al., 2014). However, in a coastal environment in the Northern Hemisphere it increases 15% OH and reduces 30% HO2 Thornton et al., 2008).
Although the contributions of each uptake category to tropospheric chemistry differ and must be considered both 90 separately and as a whole, few studies have provided a global overview of heterogeneous chemistry the comprehensively examines the uptakes of N2O5, HO2, and RO2 on widely various particles. For instance, both uptakes of N2O5 and HO2 tend to reduce O3 in particular environments Saathoff et al., 2001;Taketani et al., 2012), but the HO2 loss on clouds can increase the tropospheric O3 burden (Huijnen et al., 2014). The latter trend is not widely suggested yet because the cloud chemistry is still neglected in many O3 models (Stadtler et al., 2018;Thornton et al., 2008). The predominant effects of HO2 95 uptake on aerosols compared to the effect by N2O5 were reported during the summer smog condition (Saathoff et al., 2001(Saathoff et al., ), https://doi.org/10.5194/gmd-2020 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License. but with lack of confirmation on a global scale. Moreover, the heterogeneous effects of RO2 have been investigated only insufficiently (Jacob, 2000). In this study, we examine these uncertainties using the global model CHASER to perceive the respective and total effects of the HRs of N2O5, HO2, and RO2 on the tropospheric chemistry. For the interface of HRs in the atmosphere, we tentatively consider surfaces of cloud particles and those of aerosols and discuss details of its effects in this 100 study. In the following text, the research method, including model description and configuration, is described in section 2. In section 3.1, our model is verified with available observations including ground stations, ship/aircraft and satellite measurements, particularly addressing the roles of the HRs. The global effects of N2O5, HO2, and RO2 uptake are discussed in section 3.2 to elucidate cloud-particles and aerosol effects. Section 3.3 will discuss sensitivities of tropospheric chemistry to the magnitudes of HRs. Section 4 presents a summary and concluding remarks. 105

Global chemistry model
The global chemistry model used for this study is CHASER (MIROC-ESM) (Sudo et al., 2002(Sudo et al., , 2007Watanabe et al., 2011), which considers detailed photochemistry in the troposphere and stratosphere. The chemistry component of the model, based on CHASER-V4.0, calculates the concentrations of 92 chemical species and 262 chemical reactions (58 photolytic, 183 kinetic, 110 and 21 heterogeneous reactions including reactions on PSCs); more details on CHASER can be found in an earlier report of the literature (Morgenstern et al., 2017). Its tropospheric chemistry considers the fundamental chemical cycle of Ox-NOx-HOx-CH4-CO along with oxidation of non-methane volatile organic compounds (NMVOCs). Its stratospheric chemistry simulates chlorine and bromine-containing compounds, CFCs, HFCs, OCS, NO2, and the formation of polar stratospheric clouds (PSCs) and heterogeneous reactions on PSC surfaces. In the framework of MIROC-Chem, CHASER is coupled with 115 the MIROC-AGCM atmospheric general circulation model (ver. 4;Watanabe et al., 2011). The meteorological fields simulated by MIROC-AGCM were nudged toward the six-hourly NCEP FNL data. For this study, the spatial resolution of the model was set as T42 (about 2.8° × 2.8° grid spacing) in horizontal and L36 (surface to approx. 50 km) in vertical. Anthropogenic emissions for O3 and aerosol precursors like NOx, CO, VOCs, and SO2 are specified using the HTAP-II inventory for 2008 (http://edgar.jrc.ec.europa.eu/htap_v2/), with biomass burning emissions derived from the MAC reanalysis system. 120 In the model, the aerosol concentrations for BC/OC, sea-salt, and soil dust are handled by the SPRINTAR module, which is also based on the CCSR/NIES AGCM (Takemura et al., 2000). The bulk thermodynamics for aerosols are applied, including SO4 2chemistry (SO2 oxidation with OH, O3/H2O2, cloud-pH dependent) SO4 2--NO3 --NH4 + and SO4 2--dust interaction.

Heterogeneous reactions in the chemistry-climate model (CHASER)
The CHASER-V4 model considers HRs in both the troposphere and stratosphere. In this work, we particularly examine HRs 125 in the troposphere. In the current version of CHASER, tropospheric HRs are considered for N2O5, HO2, and RO2, using uptake coefficients for the distinct surfaces of aerosols (sulfate, sea-salt, dust, and organic carbons) and cloud particles (liquid/ice) as https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License. listed in Table 2. Although some other views incorporate the catalysis of transition metal ions (TMI) Cu(I)/Cu(II) and Fe(II)/Fe(III) for the HO2 conversion on aqueous aerosols Mao et al., 2013;Taketani et al., 2012), this mechanism remains uncertainties (Jacob, 2000). The TMI mechanism might lead to either H2O2 (Jacob, 2000) or H2O product 130 (Mao et al., 2018). However, this may not cause any significant difference since recycling HO2 from H2O2 is ineffective . For this study, the uptake of HO2 is affirmed with H2O2 as the product (Loukhovitskaya et al., 2009;Taketani et al., 2009), generally used in many atmospheric models such that this is not counted as a terminal sink for HO2 (Jacob, 2000;Lelieveld and Crutzen, 1990;Morita et al., 2004;Thornton et al., 2008). The RO2 uptakes are assumed with inert products, as suggested by Jacob (2000). The heterogeneous pseudo-first-order loss rate for the species i is given using the theory of 135 Schwartz (Dentener and Crutzen, 1993;Jacob, 2000;Schwartz, 1986), in which it is simply treated with the mass transfer limitations operating two conductances represented free molecular and continuum regimes for tropospheric clouds and aerosols, in addition to using reactive uptake coefficient ( ) instead of the mass accommodation coefficient as Therein, stands for the mean molecular speed (cm s -1 ) of species i, is the gaseous mass transfer (diffusion) coefficient 140 (cm 2 s -1 ) of species i for particle type j, and expresses the surface area density (cm 2 cm -3 ) for particle type j. In the model, the particle size and effective radius for aerosols are calculated as a function of RH (Takemura et al., 2000). The aerosol concentrations are based on SPRINTAR for BC/OC, sea-salt, and dust (Takemura et al., 2000). The surface area density (SAD) for aerosols ( ) is estimated using lognormal distributions of particle size ( ) with mode radii variable with the RH (Sudo et al., 2002) as 145 where represents number density (cm -3 ), signifies the effective radii (cm) of particle type j. To calculate SAD for cloud particles, the liquid water content (LWC) and ice water content (IWC) in the AGCM are converted using the cloud droplet distribution of Battan and Reitan (1957) and the relation between IWC and the surface area density for ice clouds (Lawrence and Crutzen, 1998;McFarquhar and Heymsfield, 1996). 150 In those equations, represents the cross-section area for ice crystals (cm 2 cm -3 ). For liquid clouds, the following holds.
The uptake coefficient parameter ( ) is defined as the net probability that a molecule X undergoing a gas-kinetic collision with 155 a surface is actually taken up onto the surface. Although several recent model studies that consider dependency of on RH and/or T, majority of the earlier studies uses constant values which only vary with aerosol particle compositions Evans and Jacob, 2005;Evans, 2010, 2011). For one study, 2 for the uptake onto aqueous aerosols is https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License.
considered with pH dependence (Thornton et al., 2008). However, another study demonstrated that the uptake is large, irrespective of the solubility in cloud water or pH (Morita et al., 2004). Therefore, we instead choose 2 as fix values 160 depending on the type of particle. Indeed, from Eq. (1) it is apparent that uptake coefficients should be unimportant for uptake onto large particles such as cloud droplets. In this study, for cloud particles of liquid and ice phases are given based on suggestions from earlier reports (Dentener and Crutzen, 1993;Jacob, 2000). One study (Dentener and Crutzen, 1993) used a constant 2 5 of 0.1 for uptake on seasalt, sulfate, and cloud particles. They also revealed that smaller 2 5 of 0.01, which had been reported as laboratory measurements, has insensitivity to effects on tropospheric oxidant components. Results of 165 another study (Jacob, 2000) indicated constants 2 5 = 0.1 and 2 = 0.2 for uptakes on both liquid clouds and aerosols, the later aims to involve the HO2 scavenging by clouds without accounting for details of aqueous-phase chemistry. For ice crystals, Jacob suggested 2 = 0.025 based on a report by Cooper and Abbatt (1996). Jacob recommended using 2 = 0.1 for hydroxy-RO2 group produced by oxidation of unsaturated hydrocarbons and 2 = 4 × 10 -3 for PA. The values for aerosols are assumed to be fundamentally the same as those for liquid cloud particles in this study. It is noteworthy that the values 170 for cloud particles are given tentatively in this study and are adjusted based on evaluation of the resulting species concentrations of O3, NOy, and OH with the observations.

Experiment setup
In this study, simulations of two types were conducted to isolate the distinct effects of each HR for the surface types considered in the model (Table 3 and Table S 1). Whereas a control simulation STD considers all HRs, noHR cases intentionally ignore 175 one or all of the HRs to calculate effects of individual HRs. The sensitivity runs that turned off the separate HRs onto clouds (liquid and ice) and aerosols were also added to exploit the separate aerosol-heterogeneous and cloud-heterogeneous effects, as suggested in many earlier studies (Apodaca et al., 2008;Jacob, 2000;Crutzen, 1990, 1991;Morita et al., 2004). The HR effects are determined as the differences between noHR cases and STD simulation as Eq. (5).
Therein, STDi stands for the concentration of investigated atmospheric component i in the STD run; and noHR( ) denotes the concentration of component i in the sensitivity run in which the HRs of/onto j was ignored (j could be N2O5, HO2, RO2, clouds, aerosols).
An additional sensitivity test was run to examine the sensitivity of the troposphere's responses with the amplified HRs magnitudes (Table S 1). These simulations only apply for HR(N2O5) and HR(HO2) to verify some uncertainties that have been 185 argued among earlier studies Evans and Jacob, 2005;Evans, 2010, 2011).

Observation data for model evaluation
Model simulations with and without HRs are evaluated distinctively with stationary, ship-based, aircraft-based, and satellite-195 based measurements. The observational information and locations of the surface site and ship/aircraft tracks for the observations used for this study are summarized in Table 4

Cloud verification
For this study, we tentatively consider HRs on the cloud particle surface. Given the great uncertainties related to the reaction 225 coefficient () Evans, 2010, 2011), the cloud distributions must be examined adequately in the model to the greatest extent possible. The model-calculated cloud distributions were verified using satellite observation data ISCCP D2, CALIPSO-GOCCP, and reanalysis data JRA55.
For the entire troposphere, the calculated cloud fraction was generally underestimated against the satellite observations and reanalysis data (Fig. 2, the first row). At the North Pacific region in JJA (Fig. 2, the second row), when the cloud fraction 230 peaked in the region, the model was able to reproduce the satellite observations (ISCCP and CALIPSO). However, for the lower troposphere over the region, the cloud fraction calculated using CHASER in JJA appears to be overestimated (Fig. 2, the fourth row), suggesting that the resulting HR effects would also be exaggerated to some extent. . Color-bars are the same for all panels. In ISCCP and CALIPSO data, the pressure boundary layer of the low troposphere is > 680 hPa. In JRA55, the low troposphere was defined as 850-1100 hPa of pressure.

Verification with stationary observations 240
Verifications with EANET and EMEP stationary observations were conducted to assess the model performance on land domains of eastern Asia and Europe, particularly addressing the roles of the heterogeneous reactions considered for this study.
The mass concentrations of particulate matter (PM2.5), sulfate (SO4 2-), nitrate (NO3 -), aerosols and gaseous HNO3, NOx, O3, and CO (CO only for EMEP) of 2010-2016 were evaluated (see Fig. S3 to S10 for monthly concentrations and Fig. S12: for correlations). In general, the model can moderately reproduce the PM2.5, SO4 2-, and NO3aerosol concentrations at these 245 Table 5), although PM2.5 was underestimated, sulfate was overestimated slightly. Nitrate was underestimated for EANET and overestimated for EMEP. It is noteworthy that the model performance for EMEP stations was better than that for EANET. The PM2.5 concentration was better estimated with the inclusion of N2O5 and HO2 uptakes (bias reduction in Table 5). The high negative biases for NO3are significant at urban sites, e.g. at Tokyo (Fig. 3), which can be associated with undervaluation for NOx and which can thereby lessen the effects of N2O5 uptake. 250 Nitric acid in both regions was overestimated. The correlations, biases, and normalized root mean square error (NRMSE) of the model for SO4, NO3, and HNO3 are in the ranges as reported in a multi-model study by Bian et al. (2017) (Table 6).
The NOx concentration for eastern Asia and Europe was underestimated, with significant bias for Asian polluted locations.
The increasing effects of NOx attributable to heterogeneous reactions, although minor, mitigated these underestimations. In Fig. 3, although NOx was partly reduced via uptake of N2O5, the NOx level was mostly increased because of HO2 and RO2 255 uptake. CO for EMEP was underestimated by the model. This undervalue was mitigated by increasing effects because of HRs of N2O5 and HO2. The uptakes of RO2, in contrast, minorly reduced CO levels so that the model bias was worsened slightly.
For O3, whereas the model tends to overestimate this tracer for both regions, O3 reduction effects of all HRs also alleviated the model overestimates, especially in June, July, and August (JJA). In December, January, and February (DJF), the model tended to underestimate O3 levels at some stations. The reduction effects on O3 extended this undervalue. In general, STD simulation 260 with coupled HRs partly improved the agreement related to the particulate and gaseous species, showing less bias than that of simulations without HRs (Table 5).
https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License.  Table 5: Model correlations and biases with EANET/EMEP observations: three-sigma-rule outlier detection is applied for each station before calculating all data. For NOx, all data were filtered once more using the two-sigma-rule. Bias of the sensitivity run is shown as bold if it is higher than the bias of the STD run.    Table 7 shows correlation coefficients (plotted in Fig. S13), indicating that the CHASER simulations for CO and O3 are in good agreement with MIRAI observations (R = approx. 0.6). However, the model still shows some discrepancies for both CO and O3 concentrations. In general, the estimated CO and O3 are both reduced for T1, T4-6 as compared to observations whereas superior for the data located in 20° S-20° N during T2-3. Overestimations for CO and O3 occurring in the region with considerably low levels of these species might be attributed to the lack of halogen chemistry in the model, as also discussed 285 for the nearby region in a past report (Kanaya et al., 2019). Undervalues for CO and O3 levels in the higher latitudes (T1, T4-6) are ascribable to the insufficient downward mixing process of stratosphere O3 in the model (Kanaya et al., 2019).
The negative biases in the noHR simulations for CO are lower in the STD run for all cruises, as they are for the North Pacific region (second versus third/fourth/fifth data rows for CO, Table 7). The CO-increasing effects by N2O5 and HO2 uptakes are consistent with the comparison for EMEP. So are CO-reduction effects because of HRs(RO2). Whereas the effects by N2O5 290 and HO2 reduce the model bias, the CO-reducing effects by HRs(RO2) exaggerated the CO bias (second versus sixth data rows for CO in Table 7), which is already apparent for comparison with EMEP (last column, Table 5).
For O3 level, the model undervalues (Table 7) are in the opposite direction to the O3 overestimates for EANET and EMEP stations ( Table 5). The lower panels presented in Fig. 4 show marked O3 reduction with all HRs, mostly contributed from the HO2 uptake onto cloud particles (gaps between red and green lines). This marked reduction of the O3 level is evident at some 295 https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License. points during the cruises, especially in the North Pacific region (the shaded areas), especially for T4. Unlike comparisons for land-domain data (Table 5), O3 reduction because of HRs extends the model underestimates during the MIRAI cruises. It is noteworthy that one cannot necessarily confirm whether the STD run better simulates these species than the noHR does because tropospheric CO and O3 levels are controlled by a complicated chemical mechanism and an interplay of emission, transport, deposition, and local mixing in the boundary layers. As discussed later in Sect. 3.2, the surface aerosols concentration in the 300 West Pacific Ocean mostly dominated by liquid clouds (exceed 50,000 m 2 cm -3 during JJA) and sulfate aerosols (approximately 75 m 2 cm -3 in JJA). The model improvements in reproducing CO by adding N2O5 and HO2 uptake indicate that the appropriate mechanisms of these processes onto cloud droplets and sulfate aerosols are well-established in the model.
For HRs(RO2), which induce the smallest and opposite effects on CO compared with the effects of N2O5 and HO2 uptakes, it can be stated in general for the total HR effects that including all three HRs partially improves the model during MIRAI cruises. 305  https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License.

Verification using aircraft measurements
To verify the model performance in the free troposphere, we used ATom1 aircraft measurements in August of 2016 (for NO2, OH, CO, and O3). The spatial and temporal concentrations are available in Fig. S11. Correlations are shown in Fig. S14. 315 In general, the model simulations for NO2, OH, CO, and O3 adequately agree with aircraft measurements with R>0.5 (Fig.   S14). However, NO2 and CO still tend to be underestimated by the model, which is consistent with comparisons for EANET/EMEP and MIRAI observations. In Fig. 5, the CO-increasing effects, mostly by the uptake of N2O5 and HO2, mitigated the negative bias of the model. This CO bias reduction was visible for all flight altitudes, the lower troposphere, and North Pacific region (Table 8). Both N2O5 and HO2 uptakes show improvements for CO reproduction of the model. However, 320 RO2 uptake seems to worsen the model's CO bias, which is consistent with the MIRAI comparison.
For the O3 level, the model generally overestimates O3 when calculating for all altitudes or lower troposphere, which is similar to the EANET/EMEP observations. In the North Pacific region with P > 600 hPa (40-60° N, 198-210° W), the model bias for O3 in STD run turns to underestimate (second data row -second column from the right, Table 8), which might be similar with MIRAI data verification for the western North Pacific (143° E-193° W). However, for the underlayers (>700 hPa) 325 show overestimation again (second data row -last column, Table 8). As MIRAI and Atom1 data show, the underestimates for O3 exist at the marine boundary layer in the western North Pacific and extend to the upper troposphere (<700 hPa) of the east side, might be ascribed to the insufficient downward mixing process of stratosphere O3 in the model as discussed previously.
The HR effects on O3 are generally negative effects (all-flight mean concentration is 78.17 ppb by STD and 80.178 ppb by noHR runs), although they are small and barely recognizable in Fig. 5, which mitigates the model bias in the noHR run. 330 This model improvement is consistent for all flight altitudes, the low troposphere, and the North Pacific region (second versus third data rows in Table 8). Both HR(N2O5) and HR(RO2) typically contribute to this improvement (second versus fourth, fifth data rows in Table 8). In contrast, HR(HO2) seems only to reduce the model bias in the ground layer, which is > 800 hPa for all flights and > 700 hPa for the North Pacific region (second versus sixth rows in Table 8). At the bottommost layers in this region, the model's overestimates for O3 are reduced by the negative effects of HO2 uptake. The extension of model bias 335 because of HO2 uptake above 800 hPa is attributable to its increasing effect on O3 level: the all-flight mean concentration is 78.17 ppb by STD and 77.96 ppb by noHR_ho2 runs. We recognize that this O3 increase effect is opposite to the effects for EANET/EMEP and MIRAI comparisons, which is discussed in Sect. 3.2 for HO2 uptake effects.
The vertical means of model biases for all four species (NO2, OH, CO, O3) are presented in Fig. 6. In general, the STD run reduces model biases for all four species, with better performance for broader regions (all flight-pressures and Northern 340 Hemisphere) than for the smaller region (North Pacific). In the North Pacific region, the negative bias for O3 is observed only for the 500-900 hPa layers (right-bottom panel of Fig. 6). The model bias is apparently extended in this region. However, the inclusion of HR(HO2) reduces O3 bias in this region (red line versus green line in the same panel), which might indicate that the O3 increase effect by HR(HO2) is verified particularly in 500-900 hPa layers during ATom1. https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License.
We also verify the total uptake of N2O5, HO2, and RO2 onto ice and liquid clouds using data obtained from ATom1 flights 345 within the free troposphere. As Table 8 and Fig. 6 show, the inclusion of HRs onto clouds reduces the model biases for CO and O3 in all calculations. The vertical-mean biases for NO2, OH, CO, and O3 species are all reduced by the inclusion of HRs onto clouds. However, Fig. 6 shows model worsening at 500-900 hPa, which coincides with the area in which O3 is underestimated as described above. This result might prove that cloud overestimation for the North Pacific, as revealed at the beginning of this section, affects the model bias in this region. 350 Table 8: Model correlations and biases with ATom1: three-sigma-rule was applied for CO and O3. NP denotes North Pacific region (140-240º E, 40-60º N). The bias of sensitivity run, which is higher than the bias of STD run, is presented as bold.

Verification with OMI satellite observation for TCO
We also tested STD and noHR simulations using the tropospheric column ozone (TCO) derived from the OMI satellite instrument ( Fig. S1 and Fig. 7). In a large area of the Northern Hemisphere, the inclusion of HRs (STD run) generally improved the consistency with the OMI TCO (Fig. 7), particularly enhancing the winter minima (first and second panels in Fig. S1). 365 This improvement in DJF is attributed mostly to the reductive effects of HR(N2O5) and HR(RO2) in the lower (800 hPa) and middle troposphere (500 hPa), respectively (see Fig. 10 for vertical profiles of HR(N2O5) on O3 and Fig. 14 for vertical profiles of HR(RO2)). In the North Pacific, HRs appeared to extend O3 underestimates, especially for latitudes higher than 40° N (Fig.   7) during the first half of the year (third panel, Fig. S1). However, such a discrepancy, which was also observed from comparison for R/V MIRAI observations (Fig. 4), might result from other factors such as deposition or vertical mixing rather 370 than by HRs. https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License.

HR effects 375
This section presents a discussion of the global effects of HRs calculated using CHASER with their spatial distributions in the troposphere using standard (STD) and sensitivity simulations (noHR_n2o5, noHR_ho2, noHRs_ro2, noHR) for the meteorological year of 2011. Aside from the main simulations described in Table 3, additional runs that separately turned off the uptakes onto clouds or aerosols for each HR are also conducted to exploit the contributions of effects to the troposphere.

Distribution of clouds and aerosols surface aerosol density (SAD) 380
To obtain the parameters for uptake to clouds and aerosols, SAD estimations are used together with cloud fraction and aerosols concentration. Hereinafter, we discuss SAD distributions for total aerosol, ice clouds, and cloud droplets, which are estimated for the model using Eqs. (2), (3), and (4), respectively.
In Fig. 8, total surface area concentrations of liquid clouds and aerosols are both much lower aloft than at the surface (as counted on the dry and wet depositions). The liquid cloud SAD results are two orders of magnitude larger than ice cloud SAD 385 and total aerosol SAD. The ice cloud SAD, distributed at the middle and upper troposphere, is enhanced for N/S middle latitudes in wintertime. Liquid cloud SAD concentrates mainly at the surface with distributions extending to 500 hPa, and https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License. maximized at approx. 800 hPa over the mid-latitude storm tracks and in tropical convective systems, especially at 60° N in JJA. Total aerosol SAD was derived mainly from pollution sources at 40° N during both seasons with higher concentrations apparent for DJF, with a greater spatial spread observed for JJA. Sulfate aerosols are becoming the dominant source of aerosol 390 surface area in the model above 600 hPa (approx. 20 m 2 cm -3 ) in addition with organic carbons and soil dust (both are approx. 10 m 2 cm -3 in JJA) for the Northern Hemisphere.
In Fig. S2, showing the SAD distribution at the surface, the SAD for liquid clouds is dominant in JJA reaching approx. 50,000 m 2 cm -3 for middle-latitude and high-latitude ocean regions. Liquid clouds are the most contribution to the SAD at the surface. Our model performance for aerosols SAD shows agreement with that presented in an earlier report (Thornton et 395 al., 2008). Sulfate aerosols are prevalent in the northern mid-latitudes near industrial bases, maximize at the surface in DJF for the Chinese region (exceeding 1,000 m 2 cm -3 ), NE U.S. (approx. 500 m 2 cm -3 ), western Europe, and transport to the North Pacific region in JJA (approx. 250 m 2 cm -3 ). Soil dust aerosol SAD dominate in the regions of the Sahara and Gobi deserts, reaching annual average values exceeding 100 m 2 cm -3 . Organic carbon (OC) is a dominant source of aerosol SAD over biomass burning regions in China (up to 1,000 m 2 cm -3 in DJF), South Africa (up to 800 m 2 cm -3 in JJA), West Europe, and 400 South America. The black carbon (BC) surface area can reach values exceeding 600 m 2 cm -3 in DJF for the region of China or other significant industrial areas (India reaches 75 m 2 cm -3 , NE U.S., Europe) or over tropical forests, primarily in Africa.
Sea salt aerosols are most important in high-latitude oceans during winter. However, the maximum contributions only reach 2 m 2 cm -3 in our model, which is much underestimated compared to Thornton's work (75 m 2 cm -3 ) (Thornton et al., 2008). In brief, SAD for aerosols of all types contributes the most during DJF, whereas during JJA, the SAD for liquid clouds and sulfate 405 aerosols are dominant, particularly for the northern high-latitude and mid-latitude oceans. The total aerosols SAD in this region are approx. 75 m 2 cm -3 , which is consistent with estimation by Thornton (2008).
In Fig. 9, the changes in OH, NOx, O3, and CO are most significant in the middle troposphere (400-600 hPa). These changes are attributed mostly to uptakes of N2O5 onto aerosols, preferably onto clouds (apparent through correlation among 415 effects by all HR(N2O5) and that by the HR(N2O5) onto total aerosols, Fig. 9). Marked negative effects on NOx concentration are apparent for DJF in the middle troposphere (600-700 hPa) of the 60° N and the Arctic region (>-20% at 700 hPa). The effects are probably associated with high concentrations of sulfate aerosols, organic carbons or soil dusts in the middle troposphere (see the paragraph above) and are also related to a long chemical lifetime of NOy in the middle-upper troposphere in winter. When it comes to JJA, these negative effects become significant at higher altitudes around the 30 o N/S (>-10% at 420 400 hPa). At the surface, HR(N2O5) causes negative effects on NOx, O3, OH concentrations (up to -23%, -4.5% and -7.5% respectively) and positive effects on CO concentration (up to +4.1%), also mainly attributable to the N2O5 uptake on aerosols.
In Fig. 10, the latitude-longitude means of HR(N2O5) effects are calculated for each pressure range (pressure ranges are defined as in Fig. 6). The global NOx decrease is up to -8.5% at 300-400 hPa. This decrease causes correspondent reductions in O3 and OH, which are calculated as about -3% and -7% at 400-600 hPa, respectively, for global mean O3 and OH. About 425 3.5-4% global mean CO increment throughout the entire troposphere responds to decreased OH.
The small effects of HR(N2O5) on O3 in the lower troposphere are consistent with findings from an earlier study (Riemer et al., 2003). Reductions in O3 and NOx concentrations also well agree with the collective knowledge summarized in work reported by Brown and Stutz (2012). Despite a considerable HR(N2O5) effect calculated in the middle troposphere, its effect in the whole troposphere is apparently not as great as reported to date. Another study assessed HR(N2O5) effects on annual 430 burdens of NOx, O3, and OH, respectively as -11%, -5%, and -7% when using a similar 2 5 value (0.1) (Macintyre and Evans, 2010). Although the effects of magnitude estimated in our work (Table 9) are almost half less than this earlier study (probably because of differences in NOx emissions, estimation of SAD, and chemical mechanism), the effect tendencies are similar. A strong increase of ozone attributed to N2O5 uptake under high-NOx conditions calculated using box models was reported from an earlier study (Riemer et al., 2003), but this is only slightly apparent in our global model. Our results revealed 435 that the HR(N2O5) effect might help clean up NOx pollutant. However, it increases the concentration of other pollutants (such as CO) because of the effects of reducing oxidizing agents in the atmosphere.
As Fig. 11 shows, the zonal-mean effects of HR(HO2) on NOx, OH, and O3 are more widespread in DJF, but are more 450 concentrated at the surface in JJA because of the high level of HO2. The most substantial effects by HR(HO2) are calculated in JJA at the surface of North Pacific (140-240° E, 40-60° N) by as much as +68.7% (NOx), +7.29% (CO), -70% (OH), and -21% (O3), which are more significant than those of HR(N2O5) at the surface. These effects are primarily attributable to HR(HO2) in clouds rather than to aerosols (which is opposite to N2O5 uptake). These OH and O3 reduction effects go along with past studies in which approx. 50% OH and approx. 10% O3 reductions are calculated for the low troposphere of northern 455 mid-latitude region ascribed to aqueous-phase HOx sink in clouds Crutzen, 1990, 1991). The efficient scavenge of HO2 radical by cloud droplets might associate with acid-base dissociation HO2/O2and electron transfer of O2to HO2 to produce H2O2 (Jacob, 2000). Furthermore, cloud droplets SAD in our model are two orders of magnitude higher than total aerosol SAD (Fig. 8), which also contributes to the preference of the aqueous-phase HO2 sink. Our large calculated effects for https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License.
the North Pacific region are new findings from other models, which have considered only aqueous aerosols (Stadtler et al., 460 2018;Thornton et al., 2008), because cloud particles are dominant at remote marine areas in addition to sulfate and aqueous sea salt particles (discussed at the beginning of Sect. 3.2). The HO2 uptake onto aerosols is minor; it is observed only in DJF at the Arctic region and polluted areas (China and US), with apparent changes of up to +17% NOx, -40% OH, and -14% O3 at the local surface (panel b , Fig. 11). The aerosol negative-effect of HR(HO2) on surface O3 concentration is significant at the Chinese area, which might be in line with other studies of the Chinese O3 trend Li et al., 2019;Liu and 465 Wang, 2020;Taketani et al., 2012), which suggests that the observed recent O3 increases can be attributed to reduced HO2 uptake under aerosol (PM) decreases brought about by the new Chinese Air Pollution policy.
In Fig. 10, vertical profiles show that the latitude-longitude (lat-long) averaged effect of HR(HO2) on OH is -9% in the lower troposphere. As a result, the lat-long mean CO level increases +2.5% at the surface. Additionally, the daytime NOx oxidation by OH is suppressed. Also, NOx might be preserved in clouds (Dentener, 1993), which increases the lat-long 470 averaged NOx level by +6% at 900 hPa. The lat-long mean O3 is reduced by -1% at the surface, but it is increased at higher altitudes (about +0.2% at 300 hPa). The reduction of O3 associates with HO2 depletion in clouds and aqueous aerosols as described above, coupled with the NOx preservation in clouds, which enhance the NO/NOx ratio. The preserved NOx in clouds might remain available for O3 production after the cloud evaporates (Dentener, 1993), along with the low SAD for both liquid clouds and aerosols at higher altitudes (Fig. 8), thereby increasing O3 in places other than aqueous phase. The O3 increment 475 might be trivial in DJF, but enhanced in JJA. As a result, the Northern Hemisphere-mean O3 in JJA exhibits only positive effects. In contrast, for the North Pacific region in JJA, because of its large cloud fraction, an O3 reduction effect is apparent in this region. The effects in JJA for this region show changes of -25% OH, +35% NOx, -12% O3, and +5% CO at 900-100 hPa as the most remarkable HR(HO2) effects as described above. In general, the regional mean effects of HR(HO2) in the North Pacific region are enhanced in JJA, but the mean global effects of HR(HO2) are slightly favored in DJF because of the 480 additional effects of aerosols during this season. Macintyre and Evans (2011) also found a similar contrast between the behaviors of HR(N2O5) and HR(HO2): the uptake of N2O5 produces both regional and global effects on O3, whereas the uptake of HO2 affects O3 at regional scales more strongly than on a global scale (Macintyre and Evans, 2011). Such features are generally consistent with our results.

Effects of RO2 heterogeneous reactions (HRs(RO2))
Effects of HR(RO2) increase the global mean methane lifetime by +0.15% and change tropospheric abundances of NOx (+0.52%), O3 (-0.93%), and CO (-1.78%) ( Table 9). In Fig. 12, significant latitudinal contrasts exist in the NOx changes: large 490 NOx increases at high latitudes with decreases at lower latitudes. These NOx changes probably reflect the reduced formation of PANs which decreases NOx transport from source regions to remote areas and from the surface to the upper troposphere (Villalta et al., 1996). The model calculated especially large NOx increases (>50%) for high latitudes around the Arctic sea in JJA, indicating reduction in the formation of PANs (NO2 + RO2 → PANs), which is linked tightly to the enhanced biogenic emissions of VOCs such as isoprene and terpenes in summer. The corresponding changes in OH concentration (because of the 495 reduced NOx levels) at the surface are in the range of -4.6% to +20.4%. The effects of HR(RO2) are primarily attributed to the heterogeneous reaction on clouds rather than on aerosols, although this cloud-effect is far smaller than the cloud-effect to the HO2 uptake. Although it is proper to expect the high solubility of RO2 (e.g. CH3O2) from its peroxy substituent (Betterton, https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License. 1992;Shepson et al., 1996), it is much less soluble than HO2 because of its lower polarity, therefore the lower Henry law constant (Jacob, 2000). Consequently, the possible accumulation of CH3O2 in the cloud is rather attributable to suppression of 500 its gas-phase sink with HO2 (Jacob, 1996).
Fig. 14 a-c show latitude-longitude means of HR(RO2) effects calculated for the respective pressure ranges: the latitudelongitude are constrained for the entire globe, the Northern Hemisphere, and North Pacific region. For the entire glob, the contrast effects of HR(RO2) between the lower and higher troposphere on NOx and OH are shown clearly (+3.5% NOx and +0.55% OH at 900 hPa, but -2.5% NOx and -0.75% OH at 400-500 hPa annually). As a result, the annual mean O3 and CO 505 levels decreased throughout the troposphere, reaching their lowest at -1.6% O3 and -1.5% CO at the surface. In JJA, the global effects by HR(RO2) are more concentrated in the lower troposphere, especially in the North Pacific (+3% OH, +10% NOx, -3% O3, -2% CO at 900-1000 hPa). In DJF, the HR(RO2) effects are observed mostly in the middle and higher troposphere, especially when considering the Northern Hemisphere (-1.25% OH, -4% NOx, -2% O3 at 500-800 hPa). https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License.

Total effects of all HRs
As discussed above, different heterogeneous reactions affect tropospheric chemistry differently. However, their effects can either augment or negate others in performing for the atmospheric chemistry. HR(N2O5) is the greatest contributor to reduction 515 of tropospheric OH, O3, and NOx abundances, which is more active in the middle troposphere. HR(HO2) reduces OH, but increases the abundances of O3 and NOx globally, whereas it exposes a negative effect on O3 level at the surface of the North Pacific region. HR(RO2), similarly, has a smaller distribution to the total heterogeneous effects but its global-mean negative effects for O3 are not negligible. The uptake of N2O5 mainly takes place to aerosols, whereas the uptakes of HO2 and RO2 occur more to liquid and ice clouds. Overall, the total effects of all HRs for the whole troposphere are +5.9% for global mean 520 CH4 lifetime, -2.2% for NOx (tropospheric abundance), -2.96% for O3, and +3.3% for CO (Table 9). At the surface, the annual effects ranged from -52.7 to +2.3% for OH, -13.1 to +51.1% for NOx, -13.1 to -1.5 for O3, and -0.3 to +5.8% for CO (Fig. 13).
As Fig. 14 d-f show for the vertical profiles of HR effects, the change of OH largely concentrated in the lower troposphere (-10% OH at 900 hPa, calculated for the entire globe) is associated with the HO2 uptake. By contrast, the NOx change is more intensive at higher altitudes (-9% NOx at 400 hPa, calculated for the entire globe), associated with N2O5 and RO2 uptakes. The 525 global-mean HR effects on O3 and CO are vertically even, with the highest effects reaching -4.5% O3 and +3.8% CO at the surface. Globally, HR effects on atmospheric oxidants (OH and O3) are enhanced in DJF because of the higher pollution in the Northern Hemisphere. However, the largest HR effects are apparent for JJA at the surface of the North Pacific (-25% OH, +38% NOx, -14% O3, +6% CO as calculated for the 950-1000 hPa layer). These effects are mostly ascribed to HO2 uptake onto clouds. This finding is also apparent from Fig. S15-c: these effects reach -66% for OH, +120% for NOx, -23% for O3, 530 and +4.4% for CO at the surface. They were able to extend up to 400 hPa in the atmosphere. These substantial effects are readily apparent for the large reduction of O3 level during MIRAI observation (red line versus green line in T5 bottom panel, Fig. 4). However, the major contribution of HR(HO2) to these effects is only partially verified by the ATom1 measurements in this study (red versus green lines in the bottom-right panel, Fig. 6). Because of model overestimates of cloud fraction in JJA for the North Pacific region, these effects of HR(HO2) should have existed at some smaller magnitude. For HR effects in the 535 middle to upper troposphere, the N2O5 uptake on aerosols are mostly ascribed, which is intensive in both DJF and JJA.

Sensitivities of tropospheric chemistry respond to heterogeneous reactions
From the discussion presented above, marked effects of HRs on global tropospheric chemistry are apparent. Here we examine how the tropospheric chemistry responds to the magnitude of HRs. To do this, we introduced a factor F for application to the first-order loss rate shown in Eq. (1) for artificially manipulating the HR magnitude.
For this sensitivity test, we only specifically examine HR(HO2) and HR(N2O5) and consider factors of 0-10 to the STD (Table   S 1). This test might help to show the effective-oxidation sensitivity of the troposphere because future pollution and climate change might enhance the activities of these HRs.
For both effects, we performed nonlinear function fitting with their uptake loss rates, which yielded correlation coefficients higher than 0.93 (Fig. 15). Although both HRs showed negative tendencies for OH and O3 levels, the effect of HR(HO2) on 555 the tropospheric abundance of O3 showed only a small increment with an increasing loss rate (maxima at around F = 3), and turned to reduction at higher rates (F>5). As discussed along with HR(HO2) effects, the O3 level is expected to be reduced primarily only in JJA at the surface of the North Pacific region. At the same time, O3 will be increased gradually elsewhere because of the persistent NOx increment. This behavior produces a positive global mean effect. Fig. 10 (dashed lines) shows that manipulation of the HR(HO2) loss rate ten factors higher will effectively increase the negative HR(HO2) effects on O3 in 560 DJF (blue dashed versus solid blue lines, third row -fourth column panel), which results in a higher tendency of negative values for global-mean effects. This sensitivity in DJF might be attributable to the HO2 uptake to aerosols rather than to clouds during this polluted period, which is apparent through comparison of Fig. 11 and Fig. S16 for notable events. In DJF, as amplifying a factor of 10 to HO2 uptake loss rate, the effects for the polluted Chinese area (because of HO2 uptake onto aerosols) significantly magnify from -18% (third row -first column in panel b, Fig. 11) to -47% (third row -first column in 565 panel b, Fig. S16). In contrast, effects at the surface O3 level in JJA for the North Pacific region (because of HO2 uptake onto clouds) only enhance from -21% (third row-second column in panel b, Fig. 11) to -29% (third row -second column in panel b, Fig. S16).
As amplifying a factor of 10 to HR(N2O5), the sensitivities of global effects show no seasonal variation. The HR(N2O5) effects are more sensitive in DJF for the North Pacific region, which link to the higher concentration of aerosol in this season. 570 Otherwise, the HR(N2O5) effects for the generic Northern Hemisphere tend to be more sensitive in JJA as a result of pollutant transportation to the higher troposphere.
Consequently, we suggest that the sensitivity of tropospheric chemistry to HR(N2O5) and HR(HO2) might be attributable to loss activities to aerosols rather than to clouds. The sharp-curved effect on O3 because of amplification of HR(HO2) makes sense in plans for ozone pollution control when increased pollution or climate change factors cause the rate of HRs in the 575 future to increase by 3-5 times or more. https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License.

Conclusion
The "CHASER" chemistry-climate model was used to investigate global effects of N2O5, HO2, and RO2 uptake. Verification of the model with observations from inland and ocean domains showed adequate agreement for PM2.5, SO4 2-, NO3particles, gaseous HNO3, NOx, OH, CO, and O3 concentrations. R, bias, and NRMSE values for SO4 2-, NO3 -, and HNO3 at EANET and EMEP stations are comparable with other models. Inclusion of HR reduced model bias for OH, NO2, CO, and O3, especially 585 in the low troposphere. However, verification with satellite and reanalysis data showed deterioration by HRs for TCO, and an overestimate for cloud fraction in the North Pacific region.
The total effects of HRs are important for the tropospheric chemistry that might change +5.9% CH4 lifetime, -2.19% NOx, -2.96% O3, and +3.3% CO abundances. Global effects are -9% NOx at 400 hPa, -10% OH at 900 hPa, -4.5% O3 and +3.8% CO at the surface. Global HR effects tend to be enhanced in DJF because of greater amounts of pollution in the Northern 590 Hemisphere.
Total HR effects are contributed mainly by HR(N2O5) onto aerosols in the middle troposphere. At the surface, HR(HO2) is more active and leaves a remarkable disturbance in JJA at the North Pacific region with changes of -70% for OH, -24% for O3, +68% for NOx, and +8% for CO. These effects were attributed to the uptake of HO2 on cloud particles, which were partially https://doi.org/10.5194/gmd-2020-335 Preprint. Discussion started: 5 November 2020 c Author(s) 2020. CC BY 4.0 License. verified with ATOM1 observations. However, the effect magnitude requires further investigation because of model 595 overestimates for cloud fractions in this region.
The sensitivity of tropospheric chemistry with the HR magnitude was determined as nonlinear functions. The increasing effect for the global O3 abundance by HR(HO2) will sharply change to a decreasing effect when the uptake rate is amplified by more than three times. This turning is ascribed to the uptake onto aerosols in DJF. In general, uptake to aerosols is more responsive to the heterogeneous loss rate than uptake to clouds. 600 Overall, the N2O5 and HO2 uptakes will sweep away atmospheric oxidants, thereby enhancing concentrations of pollutants.
Our results reveal that although HRs are reported to be associated with polluted regions, the global effects of HRs reach further remote regions such as the marine boundary layer at the middle latitude and the upper troposphere. For ground-based studies of polluted regions such as China, it should be considered that HR(HO2) and HR(RO2) were able to contribute respectively to the NOx increment in DJF and JJA. Moreover, the HR(HO2) effect might hinder efforts at reducing environmental pollution in 605 urban areas because it increases NOx but decreases O3 at the surface. Therefore, if this reaction is minimized because of a decrease in particulate matter, then the surface ozone level might increase.

Code availability
The source code for CHASER V4.0 and input data to reproduce results in this work can be obtained from the repository at http://doi.org/10.5281/zenodo.4153452 (Ha et al., 2020).