Chemical characteristics of submicron particles at the central 1 Tibet Plateau : influence of long-range transport 2 3

Recent studies have revealed a significant influx of air pollution from south Asia to Himalayas and Tibet Plateau (TP) during pre-monsoon period. In order to characterize the chemical composition, sources, and transport mechanism of polluted air mass in this pristine area, we performed a field study during June 2015 by deploying a suite of online instruments including an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) and a multi-angle absorption photometer (MAAP) at Nam Co Station (90°57' E, 30°46 ' N 4730 m a.s.l.) at the central of the TP. The measurements were made at a time when the transition from pre-monsoon to monsoon occurred. The average ambient mass concentration of submicron particulate matter (PM 1 ) over the whole campaign period was 2.0 µg m −3 , with organics accounting for 64 %, followed by sulfate (16 %), black carbon (9 %), ammonium (8 %), and nitrate (3 %). This mass loading and composition were comparable with most of AMS results in remote sites worldwide. Air pollution episodes were observed during the pre-monsoon period, while consistently low aerosol concentrations were observed during the monsoon period. However, the chemical composition of aerosol during the air pollution episodes in the pre-monsoon season was on a case-by-case basis, depending on the prevailing meteorological conditions and air mass transport routes. Most of the chemical species exhibited significant diurnal variations with higher values occurring during afternoon and lower values during early morning time whereas nitrate peaked during early morning in association with higher relative humidity and lower air temperature. Organic aerosol (OA) was more oxidized with an oxygen-to-carbon ratio (O / C) of 0.94 during the pre-monsoon period than during monsoon (average O / C of 0.48). The average O / C of OA was 0.88 over the entire campaign period. Positive matrix factorization of the high resolution mass spectra of OA identified two oxygenated organic aerosol (OOA) factors: a less oxidized OOA (LO-OOA) and a more oxidized OOA (MO-OOA). The MO-OOA dominated during the pre-monsoon period, while LO-OOA dominated during the monsoon. The sensitivity of air pollution transport with synoptic process was also evaluated with a 3-D chemical transport model.


Introduction
The Tibet Plateau (TP) and Himalayas is a vast and elevated highland in Central Asia that extends over the area of 27˗45°N, 70˗105°E with a mean elevation of more than 4000 m above sea level (a.s.l.).It is a sparsely populated area with minimal local pollution.The TP is an ideal area for observations of free tropospheric air masses and pollutants from polluted areas surrounding the TP after long-range transport.
Determination of the chemical characteristics of aerosol particles in TP is important for assessment on their influence on atmospheric chemistry and climate (Li et al., 2016), which are so far poorly understood due to harsh conditions and logistic limitation.
Over recent decades, an increasing number of field studies have been conducted in these regions to characterize aerosol physical and chemical features from mountain observatories, e.g., the Nepal Climate Observatory-Pyramid (5079 m), which are set up for long-term monitoring and synchronous observation (Bonasoni et al., 2010;Liu et al., 2017).There are significant seasonal variations in aerosol loading in the southern TP and Himalayas.Higher aerosol concentration was often found during pre-monsoon due to less precipitation and favorable atmospheric circulation (Bonasoni et al., 2010;Marinoni et al., 2010;Marinoni et al., 2013;Zhao et al., 2013).For example, the concentration of carbonaceous species at the Qomolangma (Mt.Everest) Station (4276 m a.s.l.), northern Himalayas during pre-monsoon was found to be 3 -5 times higher than that during the monsoon periods (Cong et al., 2015).Since this seasonal variation of aerosol loading is consistent at both the southern and northern Himalayas (Xu et al., 2014), polluted air mass was thought to be able to across the Himalayas, a condition which is also supported by model results (Lu et al., 2012;Lüthi et al., 2015;Zhang et al., 2015).During monsoon period, ambient aerosol at the upwind sources is significantly scavenged before long-range transport and air mass are mainly originated from marine which had chemical difference between pre-monsoon and monsoon.
Most of the studies conducted in these regions focused on some specific species, such as black carbon, which has strongly light absorption.Burning of biomass fuels and wildfires in the south Himalaya and South Asia are thought to be important sources for the black carbon (Stone et al., 2010;Engling et al., 2011;Kumar et al., 2011).However, a recent study shows that sources of black carbon in the region of South Asia are highly complex, including emissions from low efficient transport tools and cooking using cow dung and biogas as well (Stockwell et al., 2016).These burning activities also emit other species such as organic and inorganic particulate species and volatile organic compounds, which generate a well-mixed Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-587Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 1 September 2017 c Author(s) 2017.CC BY 4.0 License.aerosol plume eventually via processes such as coagulation, evaporation, oxidation, and condensation.Fang et al. (2015) recently suspected that biogenic aerosol could also be an important contribution for aerosol in the TP during summer.However, these mixed plumes have been rarely, if ever, characterized by comprehensive field measurements.
Filter-based sampling method with a low time resolution (days) were widely adopt in these remote regions due to logistical difficulties with deployment of real-time instruments.The low time resolution made the understanding of chemical processes of aerosol during transport challenging.Secondary species such as sulfate and water soluble organic carbon (WSOC) are normally the dominated species in aerosol.For example, WSOC accounted for about 60% of OC and the ratio between OC and EC could be up to 10 (Zhao et al., 2013;Cong et al., 2015), suggesting a dominant contribution of secondary organic aerosol (SOA) to carbonaceous aerosol loading in the TP.The ambient conditions at high elevation regions are characterized by higher solar radiation and concentrations of oxidants such as OH radicals and O 3 , which make photochemical processing in this high elevation remote region intense and likely dynamic.Therefore high-resolution measurement is necessary in these regions for detecting short-term events and the evolution of pollution events.In addition, the high time resolution data are useful for constraining atmospheric chemical transport models.
The aerodyne aerosol mass spectrometer (AMS) has been widely used to study the chemical composition of non-refractory submicron particle.There are two merits of the AMS including its high time resolution and bulk measurement.The high time resolution is usually minute which has advanced our understanding of fundamental chemical composition at different regions of the world (Canagaratna et al., 2007).The bulk measurements make the observation obtain more information of aerosol in one time.The organic aerosol, which is often the most important component of aerosol, can be further analyzed for their mass spectra to determine the average elemental ratios and by positive matrix factorization (PMF) analysis to determine the sources and atmospheric processes of aerosol (Ulbrich et al., 2009;Zhang et al., 2011).The atomic elemental ratios of oxygen and hydrogen with carbon (O:C and H:C) calculated from the OA mass spectra can provide information about the sources and evolution processes of OA in the atmosphere (Aiken et al., 2008;Heald et al., 2010;Kroll et al., 2011;Ng et al., 2011).They also often correlate with key OA properties such as hygroscopicity, material density, and phase separation (Jimenez et al., 2009;Bertram et al., 2011).In addition, due to high sensitivity, AMS has been successfully deployed at many remote sites with low aerosol loading such as Antarctica (Schmale et al., 2013;Giordano et al., 2017).However, there are not AMS measurement in the TP and Himalayas so far.
The study here presents an analysis of comprehensive measurements using AMS acquired at the central of the TP during the transitional season from pre-monsoon to monsoon.The study was designed to During the campaign, multiple real-time instruments were deployed.

Site description
The field study was conducted between May 31 and July 1, 2015 at a high altitude observatory, i.e., Nam Co Station for Multisphere Observation and Research, Chinese Academy of Sciences (Nam Co station, 90°57′E, 30°46′N 4730m a.s.l), at the central part of the TP (Figure 1).The Nam Co station is located nearby Nam Co lake (Figure 1b), the second largest inland lake in the TP (area: 1920 km 2 ) which is located at the northern border of Nyainqentanglha Mountains.The melted glacier from Nyainqentanglha Mountains supply water to the lake each year during warm seasons.This is a pristine region except for a small county for local people that is about 10 km to west of the station.In the past several years, tourism for this beautiful lake has growth.A highway for tourists was built about 1 km south of the station.The capital city (Lhasa) of the Tibet Autonomous Region is about 100 km southeast of the station with an average elevation of 3600 m a.s.l. between which is Nyainqentanglha Mountains (higher than 6000 m a.s.l) (Figure 1c).The closest town, Dangxiong, is along the famous highway, Qinghai-Tibet Highway, and about 70 km east of the station with an average elevation of 4200 m a.s.l.Overall, the station is around by the mountains in the south and east, and the lake at the west.The ecology of surrounding area is semi-arid type land dominated by alpine meadow and barren areas.The precipitation is mainly occurred during summer monsoon period.
The cooking and heating at the station is maintained by the power and natural gas.

Instrument setup
The study was conducted at the observatory field of the station using a customer-made trailer with inlet stepped out of the top with the height of ~5m above ground.All the instruments were arranged in the trailer where the air temperature is controlled at 20°C by two conditions.Figure 1 shows the set-up of the instruments.The total inlet was maintained by a vacuum pump with a flowrate of 10 m/s, following by a high-resolution time-of-flight aerosol mass spectrometer (HR-AMS, Aerodyne Research Inc., Billerica, MA, USA), a custom-made scanning mobility particle sizer (SMPS), a soot particle Aerodyne mass spectrometer (SP-AMS, Aerodyne Research Inc., Billerica, MA, USA), a single-particle intracavity laserinduced incandescence photometer (SP2, DMT, Inc., Boulder, CO, USA), and a seven wavelength Athelometer (model AE31, Magee Scientific, Berkeley, CA, USA).The total flowrate of this inlet was maintained at ~16 m/s and a PM 2.5 cyclone at the beginning of the inlet (model URG-2000-30EH, URG Corp., Chapel Hill, NC, USA).Several additional instruments were also co-located with their own inlet, such as a Nephelometer (model 3563, TSI, Shoreview, MN, USA), a PM 2.5 sampler (model PQ200, BGI, USA), and a multi-angle absorption photometer (MAAP, model 5012, Thermo Electron Cooperation, USA).
The operations of the instruments used in this study were described below.

HR-AMS
HR-AMS was used to measure non-refractory PM 1 (NR-PM 1 ) which was thermal vaporized at ~600 °C, ionized with a 70eV electron impact and measured using a time of flight mass spectrometry.The details on the instrument have been described elsewhere (DeCarlo et al., 2006).For this study, HR-AMS was only setup with V-mode with 5 min resolution due to the low aerosol loading.Due to damage of chopper, the size distribution of NR-PM 1 was not determined.The HR-AMS was calibrated for ionization efficiency (IE) with the ammonium nitrate following standard procedures (Jayne et al., 2000) at the beginning, in the middle and end of the study.Particulate-free air was sampled two times during the study to adjust the interaction of air on the fragmentation table and the detection limitation (DLs) of species.The DLs were calculated as 3 times the standard deviations (3σ) of the measured values during this period.The 2.5-min DLs for organic, sulfate, nitrate, ammonium, and chloride were determined at 0.108, 0.014, 0.007, 0.002, 0.010, which are comparable to the values reported in previous studies.

Other instruments
The Aethalometer was used to measure the equivalent black carbon mass concentration using seven wavelength lights.The data for BC is commonly calculated from measurement at 880 nm using recommended MAC from factory.The instrument was operated at the time resolution of 5 min with a flow rate of 5 L/min, which was calibrated at a week frequency.PM 2.5 sampler was operated at the top of the trailer.Filter samples were collected between 2 June and 1 July 2015.Each aerosol sample was collected on a 47 mm quartz filter (Whatman, Maidstone, UK) using the aerosol sampler.The meteorological data was recorded at the tower of the Nam Co station at a height of 20 m above ground.

Data processing
The HR-ToF-AMS data were processed using the standard software of SQUIRREL (v1.56) and PIKA (v1.15c) (http://cires.colorado.edu/jimenez-group/ToFAMSResources/ToFSoftware/index.html) to determine the mass concentrations of the NR-PM 1 species and the ion-speciated mass spectra of organics, written in IGOR (Wavemetrics, Inc., Lake Oswego, OR, USA).An empirical particle collection efficiency (CE) of 0.5 was used, which has been widely used in field studies employing AMS with a dryer installed in front of the equipment's particle inlet.This CE value was further validated by the consistency and reasonable slope between HR-AMS measured mass concentrations and SMPS-determined particle volumes (section 3.1.2,R 2 = 0.9, slope = 1.48).The elemental ratios of OA (O:C, H:C, and organic matter to carbon (OM:OC)) for this study was determined using the "improved-ambient" method (Canagaratna et al., 2015).
The source decomposition of organics was analysed by PMF with the robust engine.First, organic matrix was analysed using the PMF2.exealgorithm in robust mode (Paatero and Tapper, 1994) and explored using the PMF Evaluation Toolkit (PET) (Ulbrich et al., 2009).The PMF solution was evaluated following the procedures outlined in Table 1 of (Zhang et al., 2011) including modification of the error matrix and downweight of low S/N ions.Moreover, based on the AMS fragmentation table, some organic ions were not directly measured but scaled to the organic signal at m/z 44, which were downweighted by increasing their errors by a factor of 3. A two-factor solution with fPeak = 0 was chosen in this study, as it is able to reconstruct the total OA mass and temporal profiles very well.The results of three-factor solution with fPeak = 0 are shown in supplementary material (Figure S1), which show splitting in the solutions.

The meteorological conditions during the study
During the field study, the meteorological conditions were generally cold and windy (Figure 2a  (http://imdpune.gov.in/Clim_RCC_LRF/Climate_Diagnostic_Bulletins/cdbi_apr_2015.pdf), the precipitation from Indian monsoon had covered most part of India on June 12 to June 14, and these days are treated as the onset of India monsoon.Indeed, precipitation was recorded at the Nam Co station on June 13 and lasted for several days (Figure 2a).Based on this weather condition, the period before June 14 was classified as the pre-monsoon period and afterwards as the monsoon period.The RH during the premonsoon and monsoon periods were 48.4 ± 19.2% and 58.8 ± 16.5%, respectively.The originations of the air masses were also different during these two periods.Figure 3 shows the air mass from west during premonsoon periods accounted about 30-40%, while it was only 7% during monsoon period of which ~80% was from south.

S3
).Although the sampling season and elevation of these sampling sites are different (Table 1), the results of these studies could basically represent the level of pollution at these sits due to their relatively long sampling spans (from one month to ten months).The contribution of OA at these high-elevation sites ranged from 50% -90%; the highest value was at Mt. Bachelor which was frequently influenced by transported biomass burning plume, while the relative low OA contribution (38%) at sub-Antarctic was due to the dominant source from marine emission containing higher sulfate.The mass concentration of PM 1 varied dynamically during the study with distinct difference between pre-monsoon and monsoon periods.
The average mass concentrations of PM 1 for these two periods were 2.6 and 1.2 µg m -3 , respectively.For comparison, the chemical species for these two periods were side-by-side displayed in Figure 4a.The ratios between pre-monsoon and monsoon for all the species were higher than 1 with the maxima for ammonium (3.1) and sulfate (2.8), suggesting high efficiency of wet scavenging for ammonium sulfate during monsoon.
The contribution of OA was thus slightly higher during the monsoon period than during the pre-monsoon (71% vs. 64%).
Based on the mass concentration and temporal variations of PM 1 species and weather conditions, the premonsoon period could be further divided into two, i.e., period 1 (P1: May 30 to June 7) and period 2 (P2: June 8 to June 13).The P1 was characterized by higher sulfate concentration (0.46 vs. 0.41 µg m -3 ) and sunny days, while P2 was characterized by higher PM 1 concentration (2.9 vs. 2.1 µg m -3 ), higher nitrate contribution, and wet and cold days (Figure S4).The air masses for P2 had higher contribution from west and north than those during P1 (Figure 3). Figure 4b shows the comparisons of mass concentration of different species between these two periods.For chemical species, from P1 to P2, nitrate and OA increased dramatically by a factor of 1.6 and 2.2, respectively, and BC and ammonium were also increased, suggesting the influence of transported polluted air mass.However, sulfate and chloride decreased slightly (the ratios between P1 and P2 were ~0.95).The variations of sulfate and nitrate during these two periods could be related to the photochemical conditions and origination of air mass (see section 3.3).
The particles were generally chemical neutralized as illustrated from the scatter plot between predicted and measured ammonium (slope = 0.91) (Figure S5a).The neutralized PM 1 were likely due to the high availability of ammonia from agriculture emission in North of India (Clarisse et al., 2009;Van Damme et al., 2015).The slope was ~0.75 during P2 (Figure S5a), which suggested that there was over ammonium to neutralize sulfate and nitrate.We check whether the high concentration of ammonium was from fragmentation of organic nitrogen such as amine, however, the ratios of NH + and NH 2 + were consistently during the study (Figure S5b), suggesting there was no amine compounds.This high contribution from ammonium could be related to the presence of significant amounts of organic anions in aerosol (such as carboxylic acids) or variation of RIE for ammonium.

Diurnal variation and chemical process of different species
The diurnal variation of OA, sulfate, nitrate, ammonium, and BC during different periods are shown in Figure 5.All these species unexpected present significant diurnal patterns, especially during P2.The diurnal variation of aerosol are usually related to the local source emission (e.g., Xu et al., 2016).The diurnal variation at Nam Co may be related to the diurnal variations of long-range transport air mass, photochemistry, and the variation of plenary boundary layer (PBL).OA, sulfate, ammonium, and BC showed a similar pattern with low values during nighttime to early morning and high values during afternoon.We check the diurnal variation of the origination of air masses and found that there were increased air masses from south during nighttime and from west during afternoon (Figure S6), which could be related to the plateau monsoon during summer (Tang and Reiter, 1984).The enhanced air mass from west during afternoon could be more favorable for transport polluted air mass.The diurnal variations of chemical species during monsoon period were relatively flat comparing with those during P1 and P2.
Nitrate presented a significant different diurnal variation with high values during nighttime to early morning and low values during afternoon.These features were highly correlated with that of RH and air temperature (Figure 5) suggesting the important chemical processing of particle/gas partitioning of ammonium nitrate and hydrolysis of N 2 O 5 .In addition, during early morning time (6:00 -8:00), there were a peak for most of species, which were corresponding with the lowest air temperature and the highest RH, then lower PBL which could concentrate all the polluted air in a smaller space (Yanai and Li, 1994).
Although the diurnal patterns for each species were similar during the three periods, the relationships of chemical species with wind conditions were significant different based on bivariate polar plot analysis (Figure 6).Sulfate and ammonium during P1 had hotspots from all of the directions at a big range of wind speed (0 -10 m s −1 ) suggesting the local/regional transport and formation of ammonium sulfate from different air mass; while OA were concentrated depending to the prevailing wind direction from southeast and northwest suggesting important transport routes for OA from these two directions; nitrate had hotspots at the lowest wind speed (0 -8 m s −1 ) only from southwest suggesting the nearby sources and/or local formation; while BC had hotspots from south, west and northwest at high wind speed (4 -12 m s −1 ) during P1 suggesting long-range transport from the concentrated polluted regions in northern India and northwestern India.During P2, all species except nitrate had hotspots from south at high wind speed (6 -14 m s −1 ), suggesting the different synoptic process comparing with that during P1.Nitrate still had hotspots at a relative low wind speed from southwest during P2.These results suggested that the sources and chemical formation for P1 and P2 were similar, albeit the wind conditions were different.During monsoon period, all species had more similar distribution of hotspots and strongly from northwest and weakly form southeast.In order to further check on the chemical evolutions of these two species, we dig out the high concentration periods based on nitrate concentration (> average + 2σ). Figure 7 showed all periods during the high nitrate period of the study and the corresponding meteorological conditions.It is easy to find that most of the high loading periods occurred during the nighttime.The wind speed and wind direction were varied dynamically and most of them were from southwest.Higher wind speed from this wind direction could transport more polluted air mass to Nam Co as illustrated from the event 1 (E1) during which all the species (OA, sulfate, ammonium, and BC) except nitrate increased; the weather during this type event was accompanied with warm and dry air conditions.While when the wind direction was from southwest with lower wind speed (E2), the RH increased to higher than 90% accompanying deceased air temperature, and the mass concentrations of nitrate, sulfate and OA increased significantly and BC decreased in contrast with the increase of nitrate being the most significant.The decreased mass concentration of BC indicated no primary aerosol transportation, thus the increased species were mainly secondary formation.

The average chemical feature of organic aerosol
The average mass spectrum of OA was shown in Figure 8a.The organic mass was on average composed of 51% oxygen, 44% carbon, and 5% hydrogen with an average nominal formula being C 1 H  8c).
The diurnal variations of both O : C and OM : OC ratios presented higher values during late morning to afternoon and lower values during early morning (Figure 9a).H : C presented an opposite trend.These patterns suggested that OA was more oxidized during daytime which could be due to photo oxidation and/or transport of highly oxidized OA during daytime.We check the variation of elemental ratios at the function of OA mass and found that the O : C increased significantly accompanied with the increase of OA mass concentration (Figure 9b).This result likely suggested the importance of transportation on the oxidized OA.However it was not possible that the polluted air mass arrived at Nam Co within several hours due to the long distance from source regions (more than 1000 km from India).Previous studies have reported the presence of an aerosol layer between 6 -18 km a.s.l altitude over the Tibet Plateau during summer (Tobo et al., 2007;Vernier et al., 2011) and modeling studies have also shown that deep convection over the Tibet Plateau during daytime which is one of the important routes for tropospheric and stratospheric exchange (Cristofanelli et al., 2009;Cristofanelli et al., 2010;Lin et al., 2016).Thus the enhanced aerosol during afternoon could possible from the mixed downward of aerosol layer at 16 -18 km altitude during the growth of boundary layer.This type of transportation could not be captured by the re-analysis data used in the back-trajectory analysis likely due to the low time and spatial resolution.The need further validation in the future in this region.

The PMF decomposition on the OA
The two OA factors determined by PMF have distinctly different mass spectra and diurnal patterns (Figure 10), including a less oxidized OOA (LO-OOA; O : C = 0.49) and a more oxidized OOA (MO-OOA; O : C = 0.96).Both factors appeared to be secondary in nature.Our inability to separate an HOA factor is consistent with the fact that C 4 H 9 + was a minor peak in the OA spectra (0.6% of the total signal) and the average organic-equivalent concentration of C 4 H 9 + was only 0.008 μg m -3 during this study (Figure 5a).Collier et al. (2015) reported that the average fC 4 H 9 + (fraction of total organic signal accounted for by C 4 H 9 + ) was 8.1% in the mass spectrum of POA from vehicle emissions.Based on this relationship, even assuming C 4 H 9 + was completely contributed by primary OA (POA) from vehicle emissions, the average HOA concentration would be only ~ 0.1 ug m -3 , or 7% of total OA mass, during this study.Note that in this study, C 4 H 9 + was fit well in the PMF analysis with very small residual, indicating that the signal of this ion was properly apportioned between the two OOA factors.
The mass spectra of MO-OOA and LO-OOA were characterized by high peaks at m/z 44 (mostly CO 2 + ) and LO-OOA had a large peak at m/z 43 (mostly C 2 H 3 O + ) as well (Figure 10a).The contributions of the C x H y O 2 + ion category in these two mass spectra were 15.1% and 28.6%, respectively, while the contributions of C x H y O 1 + were 37.8% and 41.5%, respectively.The time series of LO-OOA correlated well with C x H y O 1 + and C x H y + ions, while MO-OOA correlated well with C x H y O 2 + ions (Figure S7).In addition, the time series of MO-OOA correlated well with sulfate (R 2 = 0.55), BC (R 2 = 0.54) and less well with nitrate (R 2 = 0.33), while LO-OOA correlated poorly with sulfate, BC and nitrate (Figure 10b and S8).accounted for 41% and 59% on average, respectively, of total OA mass during this study (Figure 11a), and their relative contributions varied across the study (Figure 2e).For example, LO-OOA accounted for 41% of the total OA mass during P1 of the pre-monsoon period, 24% during P2, and 67% during the monsoon period (Figure 11b).The diurnal pattern of MO-OOA was characterized by higher concentrations during afternoon similar to those of sulfate and BC.While the diurnal pattern of LO-OOA peaked at early evening time (Figure 10c).The polar plot showed concentrated hotspots to the northwest of the sampling site for MO-OOA, and southeast for LO-OOA (Figure 6), indicating that the sources of these two components were totally different.MO-OOA was likely closely related to long-range transport of air mass from South Asia, while LO-OOA could from relative shorter distance transport such as marine air mass and regional background aerosol during the nighttime.Shen et al. (2015) reported that there were significant aerosol source from biogenic emission during summer near the Nam Co Station.As shown in Figure 12a and b, MO-OOA, which was highly oxidized (O : C = 0.96), appeared on the up-left corner of the triangle plot while LO-OOA was in the middle part with an O:C ratio of 0.49.The high oxidation degree of MO-OOA was likely related to extensive aging processes occurred during long-range transport.The slope of linear fitting of all the data points in V-K diaphragm is -0.76 suggesting the evolution of OA as carboxylic acid functionalization (Figure 12c).
Biomass burning emission is an important source in South Asia and could be transported to Himalayas and TP during pre-monsoon (Engling et al., 2011;Kumar et al., 2011;Sang et al., 2013;Cong et al., 2015), however there are not evidently signals of biomass emitted aerosol in our AMS results such as signals at m/z 60 and 73 in mass spectrum of OA which were found to be associated with levoglucosan formed from the pyrolysis of cellulose (Alfarra et al., 2007).The contribution of f60 (fraction of total organic signal accounted for by m/z 60) for LO-OOA and MO-OOA were 0.2% and 0.3%, respectively, which were similar with the global background level (less than 0.3%) suggested by Cubison et al. (2011).These results suggest that OA, if ever partly originated from biomass burning emission, could have been highly oxidized during transport.This behavior had been observed in a few studies that levoglucosan could be quickly (within a few hours) oxidized after being emitted (Ortega et al., 2013).In addition, Zhou et al. (2017) recently reported the observation of a highly aged BBOA factor with f60 < 0.3% in its mass spectrum, in aged wildfire plumes that had gone through extensive photochemical oxidation.

Sensitivity of the chemical characteristics of PM 1 to synoptic process
Synoptic process is an important factor determining if the regional emissions can be transported to the TP.
It is interesting to know what kind of synoptic process is favorable for transporting the polluted air mass to the Himalayas and the TP.A 3-D Regional chemical transport Model (REAM) coupled with the Weather Research and Forecasting model (WRF) was used to examine the chemical evolution and regional transport  2017) and supplementary material.REAM could capture some synoptic processes which cannot be simulated by the normally used reanalysis data due to their low-resolution and the complexity of terrain in the Tibet Plateau (Zhang et al., 2017).Figure 13 shows the distribution of simulated daily surface wind, 300 hPa geopotential height fields and concentrations of reactive aromatics over the Tibetan Plateau during 30 May -13 June, 2015.During 30 May to 7 June corresponding to P1 in this study, there was a trough over the north propagated from west to east and this low pressure induced increasingly stronger surface wind from India to the TP, which could lead to transport of polluted air mass as illustrated by the results of model and AMS.The simulated concentration of reactive aromatics showed a peak during this period (Figure S9).During 8-11 June, there was a weak ridge system over the north.Intensified wind from north was observed as illustrated by HYSPLIT results (Figure 5b) and the simulated concentrations of reactive aromatics were sharply decreased (Figure S9).After that, a weak low-pressure trough system was observed again.The increased concentrations of reactive aromatics were also observed accompanying with intensified southern wind.
These trends were basically consistent with that AMS results.The potential reason for this difference was that the weak trough during P2 intensified the wind from west other than south where a lot of biomass burning emission sources were located (Figure 5b).Meanwhile the interaction between cold-dry air mass from north and warm-wet air mass from south increased the RH at Nam Co which was very favorable for aqueous formation.Zhang et al., (2017) suggested that a low-cut system from stratosphere could be an important driver for pollution transport into the Tibet Plateau.However, this effect tends to be weaker in summer than in the other seasons because the tropopause is higher and stratospheric wave activity is weaker in summer.In our study, the trough/ridge system could be an important factor affecting on the transport of air pollution from south and west.

Conclusion
The average PM 1 loading measured at Nam Co during June 2015 was 2.0 µg m -3 with organics accounting for 68%, followed by sulfate (15%), black carbon (8%), ammonium (7%), and nitrate (2%).This mass concentration was comparable to some AMS observations from mountain-top sites.The mass concentration of PM 1 varied during different weather conditions with higher concentration during pre-monsoon and lower concentration during monsoon.The pre-monsoon period could also be divided into two periods (P1 and P2) based on meteorological conditions and aerosol chemical composition.During P1, PM 1 was characterized with high contribution from OA and sulfate, while increased contribution of nitrate was observed during P2 with wet and cold weather conditions.All PM 1 species had clear diurnal variations with OA, sulfate, BC, and ammonium peaking during afternoon due to photochemical production of these species coupled with transport of polluted air mass.Nitrate, however, peaked during the nighttime and early morning, which was related to the higher RH condition.The formation of nitrate was highly correlated with transport of air masses from southwest under very low wind speeds, while the mass concentrations of sulfate, OA, and BC were highly correlated with air masses from northwest and southeast under higher wind speed conditions.
OA was overall highly oxidized during the entire study with higher O/C ratios during the pre-monsoon period.Based on PMF analysis, the OA was found to be composed of a LO-OOA and a MO-OOA.LO-OOA was mainly associated with air masses originated from south, while MO-OOA was mainly from northwest.MO-OOA dominated OA during the pre-monsoon period, while LO-OOA dominated during the monsoon period.The transport mechanism of polluted air plume was further checked using the REAM chemical model coupled with the WRF model.The polluted air plume was found to be more easily transported to the TP and Himalayas during low pressure trough weather.
and b).The average air temperature was 8.4 ± 3.6 °C with a diurnal hourly average ranging from 4.8 to 12.3 °C; the diurnal average wind speed (WS) ranged from 4.5 to 6.5 m s -1 .The dominant wind direction was south and southwest, although it varied during different periods (Figure2b and S2).The wind direction (WD) had distinct diurnal variation with air mass from south during night to morning and west during afternoon.
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-587Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 1 September 2017 c Author(s) 2017.CC BY 4.0 License.The significant difference of diurnal variation between sulfate and nitrate suggested the different chemical evolution of these two species.Nitrate and sulfate during three periods all had weak correlations.During P1, the ratios of sulfate versus nitrate had peaks (> 20) during afternoon time, while during P2, the ratios all kept at low values (< 20) (not shown).

Figure 1 .
Figure 1.Location map for (a) the Tibetan Plateau and (b) Nam Co Station colored by the 690 elevation.(c) The elevation profile from costal area to Nam Co Station (vertical dash line).The 691 red arrow in the map represent the possible wind direction.692

Figure 2 .
Figure 2. The combo plot of the data of the Nam Co study including (a) the meteorological conditions (T: air temperature; RH: relative humidity; Precip.: precipitation), (b) the variation of WS (wind speed) colored by WD (wind direction), (c) the temporal variation of mass concentration of PM 1 species and the average contribution each species (pie chart), (d) the mass contribution of each PM 1 species and the total mass concentration of PM 1 , and (e) the mass contribution of PMF results (section 3.5).Three periods based on the meteorological conditions were marked.

Figure 10 .
Figure 10.PMF results of (a) the mass spectra, (b) the temporal variation and (c) diurnal variations of two factors.The temporal and diurnal variations of sulfate and nitrate are also shown for comparisons.
Figure 12.Scatter plots of (a) f44 vs. f43, (b) fCO 2 + vs. fC 2 H 3 O + , and (c) H:C vs. O:C for the OA.The Figure 13.Daily distribution of WRF-simulated surface wind and REAM-simulated 752 concentrations of reactive aromatics over the Tibet Plateau during 30 May to 13 June, 2015.753 . He et al. (2014) examined the vertical profiles of aerosol extinction coefficients measured with a Micro Pulse Lidar at Naqu, about 100 km east of Nam Co Station, and observed a maximum in aerosol extinction coefficient between 18 -19 km a.s.l during summer 2011.