Isotopic constraints on atmospheric sulfate formation pathways in the Mt. Everest region, southern Tibetan Plateau

As an important atmosphere constituent, sulfate aerosols exert profound impacts on climate, the ecological environment, and human health. The Tibetan Plateau (TP), identified as the “Third Pole”, contains the largest land ice masses outside the poles and has attracted widespread attention for its environment and climatic change. However, the mechanisms of sulfate formation in this specific region still remain poorly characterized. An oxygen-17 anomaly (117O) has been used as a probe to constrain the relative importance of different pathways leading to sulfate formation. Here, we report the 117O values in atmospheric sulfate collected at a remote site in the Mt. Everest region to decipher the possible formation mechanisms of sulfate in such a pristine environment. Throughout the sampling campaign (April–September 2018), the 117O in non-dust sulfate show an average of 1.7‰± 0.5 ‰, which is higher than most existing data on modern atmospheric sulfate. The seasonality of 117O in non-dust sulfate exhibits high values in the premonsoon and low values in the monsoon, opposite to the seasonality in 117O for both sulfate and nitrate (i.e., minima in the warm season and maxima in the cold season) observed from diverse geographic sites. This high 117O in non-dust sulfate found in this region clearly indicates the important role of the S(IV)+O3 pathway in atmospheric sulfate formation promoted by conditions of high cloud water pH. Overall, our study provides an observational constraint on atmospheric acidity in altering sulfate formation pathways, particularly in dust-rich environments, and such identification of key processes provides an important basis for a better understanding of the sulfur cycle in the TP.

seasonality over the TP since the 1870s (Duan et al., 2017). Correspondingly, a deep understanding of the formation mechanisms of sulfate in this region is crucial for an accurate assessment of the environmental impacts as well as cryospheric extent variation in the TP region.
The mass-independent oxygen-17 anomaly (D 17 O) is any deviation from the linear approximation of δ 17 O = 0.52 ´ δ 18 O, a relationship that describes the mass-dependent process, and can be quantified as D 17 O = δ 17 O -0.52 ´ δ 18 O (Thiemens, 1999), 70 wherein δ 17,18 O = [( 17,18 O/ 16 O)sample/( 17,18 O/ 16 O)VSMOW -1]. Since it was discovered to be produced during the chemical formation of ozone (O3) for the first time in 1980s, (Thiemens and Heidenreich, 1983), the D 17 O has been studied extensively and proven to be a powerful tool in discerning the formation mechanisms of atmospheric sulfate (e.g., Ishino et al., 2017;Alexander et al., 2002Alexander et al., , 2005Alexander et al., , 2009Alexander et al., , 2012Li et al., 2013;Jenkins and Bao, 2006;He et al., 2018;McCabe et al., 2006;Lee and Thiemens, 2001;Dominguez et al., 2008;Walters et al., 2019;Lin et al., 2017;Lin et al., 2020). Because the oxidants 75 transfer unique D 17 O signal to the produced sulfate (Savarino et al., 2000), D 17 O in sulfate  )) reflects the relative importance of various oxidation pathways involved in its formation. Once emitted, SO2 quickly exchanges its oxygen atoms with abundant water vapor (D 17 O = 0‰) in the atmosphere (Lyons, 2001), and any source signature in the oxygen isotopes is erased (Holt et al., 1981). Thus, unlike δ 18 O values that integrate both the δ 18 O values of reactants (SO2 and oxidants) and oxygen isotopic kinetic and/or equilibrium fractionation effects during the oxidation processes, atmospheric transport and 80 deposition, D 17 O is fairly insensitive to mass-dependent fractionations and solely depend on the oxidation pathway of SO2 to sulfate, which render D 17 O a powerful tool to investigate the formation processes of sulfate. Laboratory experiments demonstrated that the mass-independent sulfate in the troposphere mainly originates from oxygen atom transfer from O3 (D 17 O = 25.6 ± 1.3 ‰ for bulk tropospheric O3 (Ishino et al., 2017;Savarino et al., 2000;Vicars and Savarino, 2014)) and H2O2 (D 17 O = 1.6 ± 0.3 ‰ (Savarino and Thiemens, 1999) during oxidation of SO2, while oxidation by OH (D 17 O = 0‰) as well as 85 TMI-catalyzed oxidation by O2 (D 17 O ≈ −0.3‰) produce sulfate with D 17 O at or near 0‰ (Vicars and Savarino, 2014;Savarino et al., 2000;Lyons, 2001). Based on the transfer of the D 17 O signature from the oxidant to the produced sulfate (Savarino et al., 2000), S(IV) oxidation by other oxidants (e.g., NO2 and hypohalous acids) is expected to produce sulfate with D 17 O near 0‰ as described in He et al. (2018) and Chen et al. (2016). Besides, the primary sulfate, including natural (mineral dust and sea salt) and anthropogenic (e.g., fossil fuel combustion) sources, also possesses a D 17 O value of 0‰ (Dominguez et al., 2008;Lee and 95 Thiemens, 2001). As a non-labile oxyanion, once produced, sulfate in the atmosphere does not undergo further oxygen isotope exchange with ambient species. Theoretically, D 17 O(SO [2][3][4] ) values in the real atmosphere can be predicted using the estimated https://doi.org/10.5194/acp-2020-1279 Preprint. Discussion started: 12 January 2021 c Author(s) 2021. CC BY 4.0 License.
fractional contribution of each formation pathway and corresponding D 17 O values by atmospheric chemical transport models (e.g., McCabe et al., 2006;Sofen et al., 2011). By comparing in-situ observations with modeling results, the missing processes involved in sulfate formation can be quantified. However, existing observations of D 17 O(SO 2-4 ) values provide sparse spatial 100 coverage, particularly in remote regions such as the TP.
Although the TP is one of the most climatically important regions in the world, until now there are almost no observational studies focusing on the mechanisms of sulfate formation in this region, which partly due to the harsh environmental conditions.
Here, for the first time, we present relatively long term Δ 17 O observations in atmospheric sulfate in this region, which is an important addition to the global sulfate isotope dataset. Using the APCC (Atmospheric Pollution and Cryospheric Changes) 105 monitoring network (Kang et al., 2019), aerosols samples (total suspended particulates, TSP) were collected from a remote site located in the northern slope of Mt. Everest (27.98°N,86.92°E;8844.43 m a.s.l.) from April to September 2018. Located at the boundary of the Indian Monsoon and the southern edge of the TP, the Mt. Everest region is a possible receptor of atmospheric pollutants transported directly from the Indian subcontinent to the TP. By characterizing the observed Δ 17 O data combined with model simulations (GEOS-Chem global three-dimensional atmospheric chemical transport model), we 110 decipher the possible mechanisms of sulfate formation in the Mt. Everest region, with important implications for the sulfur cycle, atmospheric oxidation processes and models of climatic change in the TP region.

Observation site and aerosol sampling
The field sampling was conducted at the Qomolangma Station for Atmospheric and Environmental Observation and Research,115 Chinese Academy of Sciences (QOMS; 28.36°N, 86.95°E; 4300 m a.s.l.) (Fig. 1). The sampling site has been previously described in detail (e.g., Ma et al., 2011). In brief, the QOMS is located in an S-shaped valley (Fig. 1b) where the surface is covered by sandy soil with sparse vegetation and gravel, and the surrounding region has limited human activity. Almost all precipitation occurs during the monsoon season, and temperature and relative humidity (RH) also show clear seasonal patterns with higher values in the monsoon season than in the non-monsoon seasons (Fig. 1c). According to the measured 120 meteorological parameters, mainly precipitation, the entire year of 2018 in the Mt. Everest region can be divided into four seasons, i.e., pre-monsoon (March to May), monsoon (June to August), post-monsoon (September to November), and winter (December to February).
The TSP samples were collected by a high-volume air sampler (Laoying-2031, LAOYING Institute, China) mounted on the roof of an instrument room (Fig. 1d). Each sample was collected on a pre-baked (450 °C for 4 h) quartz filters (Whatman Inc., 125 UK), and covered 4-7 d at a flow rate of 1.05 m 3 min -1 . After sampling, the filters were preserved properly (wrapped using aluminum foil, sealed in polyethylene bags and stored in a clean refrigerator at -20 °C), and eventually shipped to Tokyo Institute of Technology, Japan for further chemical and isotopic analyses. A filter was subjected to the same chemical analyses https://doi.org/10.5194/acp-2020-1279 Preprint. Discussion started: 12 January 2021 c Author(s) 2021. CC BY 4.0 License. but without turning pump on when sampling for blank test. A few samples with insufficient amount of sulfate were combined with adjacent samples to obtain enough sulfates to run isotopic measurements. 130

Chemical and isotopic analyses
A detailed description of the method for chemical analysis of water-soluble inorganic ions can be found in Wang et al. (2020a).
Briefly, a small portion of each filter was soaked in 30 mL of deionized water in a 50 mL centrifuge tube under ultrasonic conditions for 20 min. Then the sample solution was separated from insoluble materials and the filter by a centrifugal filter unit centrifuged for 10 min. This method can recover more than 98% of the initial water volume. The major anions (e.g., SO 135 2-4 and NO -3 ) were quantified by an ion chromatography (Dionex ICS-2100, Thermo Fisher Scientific) while the cations (e.g., Ca 2+ and K + ) were detected by another ion chromatography (881 Compact IC pro, Metrohm). The uncertainty of both instruments was approximately 4% as determined by repeated measurements of standards. The reported concentrations of ions in this study are corrected by a measured field blank.
Organic material, which may reduce the precision and accuracy of oxygen isotopic measurements, was removed by applying 140 a high-temperature heating method . Briefly, the sample precipitates were heated at 450 °C for 2 h prior to isotopic measurements, and the organic materials were largely removed as detected by the ion chromatography. As demonstrated by Xie et al. (2016), the effect of oxygen isotopic exchange during the heating is negligible. After the removal of organic materials, the oxygen isotopic compositions of sulfate were measured using a pyrolysis technique in a continuous flow system as introduced by Savarino et al. (2001) with further modifications described in several later studies (Geng et al., 145 2013;Schauer et al., 2012). Sulfate was firstly separated from other chemical impurities via ion chromatography (Dionex Integrion, Thermo Fisher Scientific). About 1 µmol of sulfate in acidic form was then chemically converted into sodium salt (Na2SO4) using a cation-exchange resin. 1 mL of H2O2 (30%) was then added to the Na2SO4 samples and dried in a vacuum centrifuge with the aim to remove any remaining organic materials. The re-dissolved Na2SO4 was subsequently converted into silver salt (Ag2SO4) by passing it through the cation-exchange resin that was in silver form. After being dried in the vacuum 150 centrifuge, the Ag2SO4 powder contained in a custom-made quartz cup was thermally decomposed to O2 and SO2 at a temperature of 1000 °C. The gas products were carried by ultra-pure helium through a cleanup trap held at -196 °C to remove byproducts (mainly SO2 and trace SO3). The O2 was then cryofocused by a liquid nitrogen trap with Molecular Sieve 5Å. After thawing, the released O2 was further purified through a gas chromatography, and finally the obtained O2 was carried into the isotope ratio mass spectrometer (MAT253, Thermo Fisher Scientific) for oxygen isotopic measurement. Since oxygen isotope 155 (δ 17 O and δ 18 O) exchange between the produced O2 and quartz materials (D 17 O = 0‰) occurring during the pyrolysis process shifts the D 17 O values (Savarino et al., 2001;Schauer et al., 2012), the raw D 17 O values were corrected by estimating the magnitude of the oxygen isotope exchange using inter-laboratory calibrated standards, as described in Ishino et al. (2017) and Gautier et al. (2019). Due to the unknown δ 17 O and δ 18 O values of each quartz material used in this study, it is difficult to get reliable corrected δ 17 O and δ 18 O values, thus we don't discuss these values in the following sections. The 1σ precision of 160 corrected D 17 O was ± 0.1‰ based on replicate analyses (n = 20) of the standard B (D 17 O(SO 2-4 ) = 2.4‰) with five independent runs of this study.

Model description
GEOS-Chem is a global 3-D model of atmospheric composition (www.geos-chem.org) originally developed by Bey et al. (2001). In this study, we use GEOS-Chem (version 12.5.0, DOI: 10.5281/zenodo.3403111) driven by assimilated 165 meteorological fields from MERRA-2 reanalysis data product from NASA Global Modeling and Assimilation Office's GEOS-5 Data Assimilation System. We simulate aerosol-oxidant tropospheric chemistry containing detailed HOX-NOX-VOC-ozone-BrOX chemistry (Bey et al., 2001;Pye et al., 2009;Sherwen et al., 2016). The model was run at 4° × 5° horizontal resolution and 47 vertical levels up to 0.01 hPa, and spun up for 1 year before each simulation. In the model, sulfate is produced from gas-phase oxidation of SO2 by OH, aqueous-phase oxidation of S(IV) by H2O2, O3, HOBr, metal-catalyzed O2, and 170 heterogeneous oxidation on sea-salt aerosols and dust aerosols by O3 (Alexander et al., 2012;Chen et al., 2017;Fairlie et al., 2010). To examine the importance of sulfate formation on dust aerosol, we tested the model simulation with or without sulfate formation on mineral dust (Fairlie et al., 2007;. The parameterization of the metal-catalyzed S(IV) oxidation is described in Alexander et al. (2009) For pH-dependent S(IV) partitioning, bulk cloud water pH is calculated as described in Alexander et al. (2012). We use the 180 parameterization as described in Yuen et al. (1996) to account for the effect of heterogeneity of cloud water pH on S(IV) partitioning and subsequent aqueous phase sulfate formation (Alexander et al., 2012). Sulfate formed from each oxidation pathway was treated as a different "tracer" in the model as described elsewhere Sofen et al., 2011).

Meteorological and black carbon data
The meteorological information was recorded by a Vantage Pro2 weather station (Davis Instruments) located at the QOMS.
Measured meteorological parameters include temperature, RH, wind speed and air pressure with a precision of 0.1 °C, 1%, 0.1 m s −1 and 0.1 hPa, respectively. Precipitation data presented here was collected by manual measurements after each 200 precipitation event. The airborne black carbon (BC) concentrations at the sampling site were measured by a newly developed Aethalometer model AE-33 (Magee Scientific), which was operated at an airflow rate of 4 L min −1 with a 1 min time resolution.
By incorporating a patented DualSpot™ measurement method, the instrument can provide accurate real-time BC measurements. The detailed information on the BC observations is described in Chen et al. (2018).

O3 mixing ratios 205
Since observations of O3 at the sampling site are unavailable, the surface O3 concentration in 2013 presented in this study are from an upwind region of our sampling site, i.e., NCO-P (Nepal Climate Observatory at Pyramid, 27.95°N, 86.80°E), which has been reported by Putero et al. (2018). This dataset was obtained from GAW/WDCRG (Global Atmosphere Watch programme/World Data Centre for Reactive Gases) hosted by EBAS data infrastructure at NILU (the Norwegian Institute for Air Research). Located in the southern slope of Mt. Everest, NCO-P is not far from our sampling site (~50 km), and the O3 210 measurements there have been continuously performed with a UV-photometric analyzer (Thermo Scientific-Tei 49C) since year 2006 (Cristofanelli et al., 2010). In addition to O3 concentration at NCO-P, the O3 reanalysis data (at the level of 500 hPa) in 2018 for our sampling region (i.e. QOMS) was obtained from the ERA-Interim reanalysis (Dee et al., 2011) to further clarify the seasonality of O3 over the southern TP.

Solar radiation and RH along with backward trajectories 215
Apart from the meteorological data directly observed at the sampling site, the solar radiation (SR) fluxes and RH along with backward trajectories arriving at the sampling sites during the sampling campaigns were also calculated by NOAA's HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) atmospheric transport and dispersion model, and averaged for each day (https://ready.arl.noaa.gov/HYSPLIT_traj.php) (Stein et al., 2015). The model calculation was forced by archived GDAS (Global Data Assimilation System) meteorological data obtained from NOAA Air Resource Laboratory with 1° ´ 1° latitude 220 and longitude horizontal resolution. The calculated backward trajectories were clustered using TrajStat, an air mass trajectory statistical analysis tool contributed by Wang et al. (2009). Since the lifetime of sulfate aerosol is on the order of 4-5 d https://doi.org/10.5194/acp-2020-1279 Preprint. Discussion started: 12 January 2021 c Author(s) 2021. CC BY 4.0 License. (Alexander et al., 2012), the total run time of 120 h with time intervals of 3 h were adopted for each backward trajectory. Since the topography is characterized by huge relief differences in the Mt. Everest region, the arrival height of trajectories was set to 1000 m above the surface to reveal the long-range transport of air masses. 225 3 Results and discussion

Ionic characteristics and potential sulfate sources
Over the entire sampling campaign, concentrations of the main water-soluble ionic species (e.g., SO 2-4 , NO -3 , Ca 2+ , K + ) extracted from TSP samples in the Mt. Everest region show very similar and clear seasonal variations (Fig. 2a). That is, concentrations in the pre-monsoon seasons are 3-4 times higher than those during the monsoon season. It is likely the seasonal 230 variations in the main ionic concentrations mainly result from the scavenging effect of precipitation in the monsoon season and more frequent dust storms in the pre-monsoon season (Kang et al., 2000). Our measurements indicate that SO 2-4 was the most abundant inorganic anion species followed by NO -3 , while for cations, Ca 2+ was the most abundant species. Although the Mt. Everest region is far from human activities, and is one of the most pristine areas in the world, we observed much higher The elevated [SO 2-4 ] in our study might suggest a significant contribution of long-range transported polluted air masses with sulfur sources. 240 Being located at high elevation and ~750 km away from the ocean, the Mt. Everest region is considered to possess a negligible amount of sea-salt sulfate in its air masses, which was also suggested by Cong et al. (2015a). In addition, the contribution of primary sulfate emitted from anthropogenic activity has been considered to be insignificant as compared to those from the secondary sulfate produced through oxidation of SO2 (Berresheim et al., 1995). Thus, mineral dust is considered to be the only significant source of primary sulfate in our study due to frequently occurring dust storms (Fig. 3)  Here we adopt a k value of 0.18, corresponding to a molar ratio of 0.075, which has been widely used in previous studies for the estimation of the terrigenous sulfate (Lin et al., 2020;Kunasek et al., 2010;Patris et al., 2002). Note that the k value of 0.59, corresponding 255 to a molar ratio of 0.246, is known to be an upper limit as discussed in Kaufmann et al. (2010) Table 1. The average 260 contribution of terrigenous sulfate to the bulk sulfate is correspondingly calculated to be 6.3 ± 3.0%.
To identify the possible sources of air masses arriving at the Mt. Everest region during the sampling campaign, we analyzed the five-day backward trajectory of air masses (Fig. 1a). The results showed that the northward trajectories account for less than 10% of the total trajectories, and the air masses originated mainly from Nepal, northern/northeastern India, and Bangladesh. Previous studies have indicated that atmospheric brown clouds, basically layers of atmospheric pollution 265 consisting of aerosols such as BC, dust, sulfate, and nitrate, extend from South Asia and accumulated on the southern slope of the Himalayas (Ramanathan et al., 2005;Kang et al., 2019;Wang et al., 2014). These South Asia-sourced pollutants can be transported into the TP region via the larger-scale atmospheric circulation and/or south-north-trending valley wind system (Cong et al., 2015a;Chen et al., 2018;Cong et al., 2015b;Xia et al., 2011). As a product of incomplete combustion of fossil fuel and biomass, BC deposited in the TP region has attracted much attention, since it can accelerate glacier melting, although 270 the magnitude of this effect is uncertain  and references therein). Li et al. (2016) indicated that BC aerosols originating from Indo-Gangetic Plain can be transported to the Himalayas and even further to the southern TP. As shown in Fig. 2c, the [nd-SO 2-4 ] showed a consistent seasonality similar to [BC] during the sampling period, especially for the premonsoon season during which time a significant correlation (r = 0.806, p < 0.01) exists. Note that the insignificant correlation (r = 0.434, p > 0.05) in the monsoon season might be due to the much lower scavenging ratio of BC than that of SO 2-4 (Cerqueira 275 et al., 2010). As a marker of biomass burning, [K + ] also showed significant correlations with [nd-SO 2-4 ] in both seasons (r = 0.858, p < 0.01 for pre-monsoon; r = 0.977, p < 0.01 for monsoon) (Fig. 2d). Thus, it reasonable to suggest that combustion sources in South Asia contribute significantly to the atmospheric sulfate level in the Mt. Everest region, and even the entire southern TP.

D 17 O signatures of sulfate and comparison with other sites 280
As listed in Table 1 averaged at 1.2 ± 0.6‰), which is obviously lower than that of 2.0 ± 0.5‰ (weighted averaged at 1.9 ± 0.7‰) in the monsoon season (n = 11) with a range from 1.3‰ to 3.0‰ (Fig. 4a). Additionally, no apparent correlation is observed between D 17 O(SO  (Wang et al., 2020a) shows a typical seasonality (Fig. 4) reflecting the sunlight-driven seasonal changes in the photochemical oxidants, which has been reported in many previous studies conducted 315 from diverse geographic sites (e.g., Michalski et al., 2003;Savarino et al., 2007;Guha et al., 2017;Wang et al., 2019b). Thus, the clear seasonal trend of high D 17 O(SO 2-4 )SAS in warm season and low D 17 O(SO 2-4 )SAS in cold season observed in the Mt. Everest region imply the existence of characteristic factor controlling sulfate formation.

Contribution of stratospheric intrusion 320
Sulfate in the stratosphere, mainly produced by the reaction of SO2 with OH (high D 17 O in stratosphere due to lack of liquid water for isotopic exchange), has been suggested as a potential contributor to the relatively higher D 17 O(SO 2-4 ) (Jenkins and Bao, 2006). Additionally, O3 in stratosphere also has a higher D 17 O value (34.2 ± 3.7‰ for bulk oxygen atoms) as compared with that in the troposphere (Krankowsky et al., 2000). Numerical simulations and field-based observations show that the Himalayas are a global hot spot for deep stratospheric intrusion (SI) in the springtime (pre-monsoon) (Škerlak et al., 2014;Lin 325 et al., 2016;Lin et al., 2020). A recent study carried out at the southern TP and its downwind Southwest China observed high  (Lin et al., 2020). We therefore first evaluate the potential stratospheric influence on the relatively higher D 17 O(SO 2-4 )SAS values in our study. Previous studies suggest that SI frequency at the southern TP is generally lower in summer than spring (Priyadarshi et 330 al., 2014;Zheng et al., 2011;Yin et al., 2017). D 17 O(SO 2-4 )SAS in our study, however, displays higher values in summer than spring. Consequently, the observed high D 17 O(SO 2-4 )SAS values, especially in summer, may not be predominately explained by the stratospheric influences. The stratospheric contribution to each individual sample, which may be quantified and constrained in the future by simultaneous measurements of chemical tracers for stratospheric air masses, is beyond the scope of this study.
In the ensuing sections, we focus on the role of sulfate formation mechanisms in the troposphere in elevating D 17 O(SO 2-4 )SAS 335 values.  7) and (8)) (Walters et al., 2019), we calculated the maximum contribution of oxidation by O3 (fO3, max) to SAS production by assuming no contribution from oxidation by H2O2, and the minimum contribution (fO3, min) was estimated by assuming that H2O2 is the only other oxidant. 5‰ for pre-monsoon and 2.0 ± 0.5‰ for monsoon) and estimation of fO3 by the above isotope mass-balance method, the fO3 fell in a range of 10 ± 9% to 21 ± 350 8% and 21 ± 9% to 31 ± 8% for the pre-monsoon and monsoon seasons, respectively. The relative contribution of aqueous oxidation by O3 is significantly higher in the monsoon than in the pre-monsoon season.

Importance of S(IV) + O3 oxidation pathways
In Fig. 6 aqueous oxidation of S(IV) by O3. We therefore discuss the importance of atmospheric acidity in the aqueous phase chemistry as factor controlling the S(IV) + O3 oxidation pathway in the next section.

Importance of atmospheric acidity for sulfate formation pathways
The aqueous oxidation of S(IV) by O3 is only significant at high pH conditions greater than pH = 5, due to the strong pH dependence of S(IV) species in solutions (Calvert et al., 1985;Seinfeld and Pandis, 2016). At the pH below 5, H2O2 is 370 considered to dominate aqueous sulfate production. Since the pH in modern cloud water is typically within the range of 3 to 5 (Pye et al., 2020 and references therein), the aqueous oxidation of S(IV) by O3 is thus considered to be unimportant when compared to the oxidation by H2O2. However, our observed high D 17 O(SO 2-4 )SAS values suggest the importance of sulfate formation by aqueous oxidation of S(IV) by O3, indicating that sulfate formation is occurring at relatively high pH conditions. Indeed, the pH in fog, rain and snow in the TP region were reported to be high, respectively, pH = 6.4, 6.2, and 5.96 ± 0.54 375 (Wang et al., 2019a;Kang et al., 2002), and the modeled cloud water pH in the Mt. Everest region and the South Asia showed approximately pH = 6 (Shah et al., 2020;Pye et al., 2020). Such high pH condition favors S(IV) oxidation by O3. We suggest that the frequently occurring dust storms, not only in the Mt. Everest region but also in South Asia (Fig. 3 & Prospero et al., 2002), are likely to play an important role for promoting the S(IV) oxidation by O3 through the high cloud water pH conditions. Consequently, our findings provide an important observational basis for better constraints on the importance of pH in sulfate 380 formation pathways.
To examine the importance of cloud water pH and sulfate formation on dust surface, we conducted model simulations by GEOS-Chem in three cases including (i) fixed cloud water pH (= 4.5) (Fig. 7a). The increases of modeled D 17 O(SO 2-4 )SAS when calculating cloud water pH are due to increases of the modeled fO3 from 5 ± 1% in case (i) to 24 ± 6% in cases (ii) and/or (iii). 390 In addition to the magnitude of D 17 O(SO 2-4 ), fO3 are also well reproduced by the model simulations in cases (ii) and (iii). During the sampling period, the modeled monthly-mean fO3 in cases (ii) and (iii) varied from 14% to 31% with an average of 24 ± 6%, which shows a good agreement with fO3 calculated based on Eqs. (7) and (8) (15 ± 10% for fO3, min and 25 ± 9% for fO3, max, respectively). It is important to note that the modeled fO3 in cases (ii) and/or (iii) is even higher than the modeled fH2O2 (averaged at 20 ± 7%). Also, as shown in Fig. 7d, the modeled heterogeneous S(IV) oxidation on dust surface also play a role for 395 producing high D 17 O(SO 2-4 ) values, particularly in the pre-monsoon season, but the modeled relative contribution of this pathway does not exceed 5% to the total sulfate production.
Our result with high D 17 O(SO 2-4 )SAS found in the Mt. Everest region, in turn, highlights observational evidence that atmospheric acidity plays an important role in controlling sulfate formation pathways particularly for dust-rich environments. For typical earlier studies until the 1990s, pH of cloud droplet was considered too low to promote the S(IV) + O3 reaction for sulfate 400 formation (Chin et al., 1996;Koch et al., 1999). Although recent studies have proposed the importance of the cloud water pH promoting the S(IV) + O3 pathway to estimate atmospheric sulfate formation (Paulot et al., 2014;Banzhaf et al., 2012), even the latest study (Turnock et al., 2019) still prescribes a constant cloud pH in the estimation for radiative forcing effect via aerosols. Consequently, accurate estimate of cloud water pH in the model simulation and elucidation of its relation to https://doi.org/10.5194/acp-2020https://doi.org/10.5194/acp- -1279

Possible importance of atmospheric humidity on D 17 O(SO 2-4 )
Although the model in this study predicted minor relative contribution of heterogeneous oxidation of SO2 by O3 on dust surface ( Fig. 7d), it is worth noting the possible importance of this pathway. In a laboratory study, Ullerstam et al. (2002) observed a 47% increase in the amount of sulfate formed via SO2 oxidation by O3 on dust surface when dust samples were exposed to a 410 RH of 80% as compared to an experiment without the intermittent water exposure. Taken together with the co-variation between D 17 O(SO 2-4 )SAS and RH as described above (Figs. 7a and 7b), there is a possibility that the ubiquitous alkaline dust aerosol extending from South Asia to the Mt. Everest region as reported in previous studies (e.g., Wang et al., 2020b;Prospero et al., 2002) render a suitable condition for S(IV) oxidation by O3 especially during the monsoon season. Given that soluble Ca 2+ and Mg 2+ , those being thought to provide alkalinity, mostly exist as coarse-mode particles larger than 1 µm (Yang et al., 415 2016), size distributions of sulfate concentration and D 17 O(SO 2-4 )SAS for dust-rich environments would provide a detailed consequence for the importance of heterogeneous oxidation of SO2 by O3 on the dust surface in the future study.

Conclusions
In this study, we report observations of sulfate concentrations and D 17 O(SO 2-4 ) in aerosols samples collected from the Mt. Everest region over a period including pre-monsoon and monsoon seasons in 2018. The combustion tracers (i.e., BC and K + ) 420 as well as the backward trajectories of air masses suggest a combustion of sources (e.g., fossil fuel and biomass combustion) in South Asia contributed significantly to the observed sulfate. The average D 17 O(SO 2-4 )SAS value of 1.7 ± 0.5‰ observed from the Mt. Everest region is higher than the published data reported from the Earth's mid-latitude continents which have relatively and Ev-K2-CNR Chartered Association for providing O3 dataset at NCO-P and managing/supporting the NCO-P station as well as the on-site scientific operation. The authors also thank European Centre for Medium-range Weather Forecast for the ERA-Interim reanalysis dataset (https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era-interim). The authors 450 gratefully acknowledge the NOAA Air Resources Laboratory for the provision of the HYSPLIT transport and dispersion model and/or READY website (https://www.ready.noaa.gov) used in this publication. This study was also supported by Japan Society  Table 1 Concentrations and D 17 O values of atmospheric sulfate collected in the Mt. Everest region for individual sampling durations as well as the corresponding meteorological parameters including relative humidity (RH) and temperature. The relative contribution of SO2 oxidation by O3 to SAS for each sample is also shown as fO3, min and fO3, max. Apr. 10-Apr. 14 8.47 8.14 1.6 ± 0.1 1.7 ± 0.