Chemical Characterization of Submicron Aerosol and Particle Growth 1 Events at a National Background Site (3295 m a.s.l.) on the Tibetan 2 Plateau 3

19 Atmospheric aerosols exert highly uncertain impacts on radiative forcing and also 20 have detrimental effects on human health. While aerosol particles are widely 21 characterized in megacities in China, aerosol composition, sources and particle growth in rural areas in the Tibetan Plateau remain less understood. Here we present 23 the results from an autumn study that was conducted from 5 September to 15 October 24 2013 at a national background monitoring station (3295 m a.s.l.) in the Tibetan 25 Plateau. The submicron aerosol composition and particle number size distributions 26 were measured in situ with an Aerodyne Aerosol Chemical Speciation Monitor 27 (ACSM) and a Scanning Mobility Particle Sizer (SMPS). The average mass 28 concentration of submicron aerosol (PM 1 ) is 11.4 μ g m -3 (range: 1.0 - 78.4 μ g m -3 ) for 29 the entire study, which is much lower than those observed at urban and rural sites in 30 eastern China. Organics dominated PM 1 on average accounting for 43%, followed by 31 sulfate (28%) and ammonium (11%). Positive matrix factorization analysis of ACSM 32 organic aerosol (OA) mass spectra identified an oxygenated OA (OOA) and a biomass 33 burning OA (BBOA). The OOA dominated OA composition accounting for 85% on 34 average, 17% of which was inferred from aged BBOA. The BBOA contributed a 35 considerable fraction of OA (15%) due to the burning of cow dung and straws in 36 September. New particle formation and growth events were frequently observed (80% 37 of time) throughout the study. The average particle growth rate is 2.0 nm hr -1 (range: 38 0.8 – 3.2 nm hr -1 ). By linking the evolution of particle number size distribution to 39 aerosol composition, we found an elevated contribution of organics during particle 40 growth periods and also a positive relationship between the growth rate and the 41 fraction of OOA in OA, which potentially indicates an important role of organics in 42 particle growth in the Tibetan Plateau. continuation to stage associated with synchronous increases of both organics and sulfate. The results indicate that both organics and sulfate contribute to the particle growth after mixed with anthropogenic sources from ~18:00 in the


Response to Reviewer #1
The manuscript entitled, "Chemical characterization of submicron aerosol and particle growth events at a National Background Site (3295 m a. s. l.) on the Tibetan Plateau" by W. Du et al., presents non-refractory plus black carbon (BC) aerosol chemical composition and particle size distribution data from a remote location on the northeastern region of the Tibetan Plateau. This paper is of interest to many readers of Atmospheric Chemistry and Physics and it is important that it is published there. The authors put much effort and care into responding to the first round of reviewers' comments. The manuscript has improved with this revision. Thank you for making many changes and thank you to the other reviewer for focusing on details that I did not address.
It is apparent that my suggestion to provide an analysis of air mass histories at the site is beyond the scope of this manuscript. The assertion that the air was regionally-transported to the site was not backed by more information on how air was actually getting there on average. Hopefully, data from this paper will be analyzed further by someone who can study the large scale modeling and meteorology. Overall, the manuscript is pretty close to being ready to publish in ACP.
We appreciate the reviewer's constructive comments that help improve the manuscript, and we also thank the reviewer's understanding. We agree with the reviewer that the current analysis is not adequate enough to have a full understanding of the aerosol chemistry at the Menyuan national background site. We are also looking forward to further analysis by the people who are interested in this work.
To help put these results in a broader context and make it easier for the global atmospheric community to use, I strongly recommend some simple changes be incorporated into the present manuscript. 1) In lieu of an analysis of air mass histories in the present paper, it would be extremely helpful if there is a paper discussing the meteorology at the site that could be referenced in section 2.1 or perhaps references to other measurements that have been taken at the site in the past, if available.
We thank the reviewer's suggestions. This is the first time to report the measurements and the results at the Menyuan national background site. There are no references to discuss the meteorology and measurements at this site yet.
2) The maps in Figs. 1 and S3 do not show scales -for example, how many km are represented in each of them? Please add that information.
The scale of the map was added in Figure 1 and Figure S3 in the revised manuscript.
3) For Figure 1, please add to the caption text that the pie charts show data from AMS (non-refractory composition only) plus black carbon. Also, please add to the caption an explanation for the dotted blue lines. Along the same lines, please indicate in the caption for table S1 that all the measurements were using an AMS and in some cases with black carbon.
Thank the reviewer's comments. Figure 1 and Table S1 was revised according to the reviewer's suggestions.
" Figure 1. Map of the sampling site (Menyuan, Qinghai). Also shown is the chemical composition of submicron aerosols (NR-PM 1 + BC if it was available) measured at selected rural/remote sites in East Asia except Lanzhou, an urban site in northwest China. The two dotted blue lines are used to guide eyes for the three rural/remote regions from the west to the east. The detailed information of the sampling sites is presented in Table S1." " Table S1. A summary of mass concentration and composition of NR-PM 1 species measured by the AMS and in some cases with black carbon (BC) at different locations in East Asia." 4) Please put the two altitude profiles of the back trajectories back into the SI, along with an indication of where Xining is on the "Clean 1" trajectory of altitude as function of time back. Please add a note in the caption that the black line is ground level, if that is what it means.
We thank the reviewer's comments. The altitude profiles of the back trajectories were added in Figure S3 in the revised manuscript.
5) The diurnal plots of the meteorological data in the SI are very important, especially when combined with the aerosol and gas phase measurements. The winds appear to be changing from the west to from the south, just at the time that the new particles are detected. If possible, it would be worthwhile to classify these air mass histories. This could be important in further interpreting the new particle formation events.
We thank the reviewer's comments. We also noticed such a change. However, the diurnal cycle of wind direction showed a large variation (25%-75% percentile, > 100 o ) during this period of time. In addition, we didn't observe a clear wind direction change between 8:00 -10:00 based on the time series of meteorological variables in Figure 3. For these reasons, it is difficult to link new particle formation events to the change of wind direction.
6) The building was at a higher temperature than ambient and the air sampled by the instrument was in the inlet for about 5 seconds. Please add a comment in the text on the how much ammonium nitrate (and other semi-volatile species) could have evaporated before sampling. Could this be another reason why nitrate levels were lower for the present study compared to sites in eastern China?
The loss of ammonium nitrate and other semi-volatile species at 23 o C were small based on the thermodenuder measurements (Huffman et al., 2009). In addition, most of previous AMS measurements in China were conducted under the similar room conditions, i.e., 23-25 o C air conditioned room or trailers. Therefore, the lower nitrate concentration level at Menyuan site was not likely due to the evaporative loss of ammonium nitrate. 7) I respectfully disagree with the authors' choice of units for the aerosol mass and number concentrations in the present paper. I understand that previous data from the region was reported for ambient conditions and that the present results are reported for ambient conditions for consistency. It is unfortunate that the prior data from this region were not reported for standard conditions, especially since some sites are very high in altitude. Also, I realize that converting the current data into standard units is somewhat trivial if the ambient conditions are also reported (here sampling temperature was about 23 degrees C and the ambient pressure was not reported and is not readily apparent). However, when working with data sets at pressures significantly different from standard conditions (such as high altitude or aircraft data or global/regional modeling results and comparing the current results to sites at sea level like most listed in Figure 1 and Table S1), it is significantly easier to compare data if it is all reported using standard conditions. Because the site was relatively high in altitude (3295 m a. s. l.), the ambient pressure is roughly 0.67 times sea level pressure, resulting in a correction of a factor of 1.5. Correcting for sampling temperature to standard temperature of 273.15 K will be another factor of 1.08. Please clearly add a sentence (or a two) in the text that mentions the conversion to standard units is roughly a factor of 1.6 (or the average for the ambient conditions), so that the data reported for standard conditions can be quickly estimated if needed.
We thank the reviewer's comments. The average pressure during this study period is 695.4 hPa, and the average ambient air temperature is 4.9 o C (-8.7 -17.9 o C), so the conversion factor to standard units is roughly 1.5. Following the reviewer's suggestion, a sentence was added in Section 2.2.
"All the data are reported at ambient conditions in Beijing Standard Time. Note that the concentrations would be a factor of approximately 1.5 of the current values if the data are converted to mass loadings at standard temperature and pressure (STP, 273K and 1013.25 hPa)." 8) I also respectfully disagree with the authors' choice of units for the gas phase species. While it may be convenient for aerosol scientists to report gas phase data in micrograms per cubic meter, these units for gas phase species are not commonly used in atmospheric science and gas phase data are virtually always reported as volume mixing ratios (and what does micrograms per cubic meter mean for NOx?). Again, it is difficult to compare the data reported in this paper without recalculating the gas phase data into units of mixing ratio -for the species here "ppbv" is the most convenient. Please convert all reported gas phase data into mixing ratios of ppbv and change them in the figures and tables.
We thank the reviewer's comments. Following the reviewer's suggestions, all gas phase data were converted to volume mixing ratios using Eq. (1).
Where C is the volume mixing ratio, X is the mass concentration in µg m -3 , M is the molecular weight of gas species, and P and T refer to the pressure and temperature, respectively.

Response to Reviewer #2
General comments: I found the resubmitted paper much easier to read, and several of my comments were addressed in sufficient detail. Many thanks for your revised manuscript, which I am happy to accept for publication subject to a few minor revisions as detailed below. growth in rural areas in the Tibetan Plateau remain less understood. Here we present 23 the results from an autumn study that was conducted from 5 September to 15 October September. New particle formation and growth events were frequently observed (80% 37 of time) throughout the study. The average particle growth rate is 2.0 nm hr -1 (range: 38 0.8 -3.2 nm hr -1 ). By linking the evolution of particle number size distribution to 39 aerosol composition, we found an elevated contribution of organics during particle Beijing-Tianjin-Hebei, Pearl River Delta and Yangtze River Delta.

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The Tibetan Plateau (~ 2,000,000 square kilometers) is the highest plateau in the 68 world with an average altitude of over 4000 meters above sea level. The Tibetan

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Plateau is an ideal location for charactering rural and regional background aerosol due  In 248 particular, sulfate shows ~60% higher contribution, yet BC is more than twice lower 249 than that observed at the urban site (Fig. 1) sulfate is dominantly from regional sources and transport, nitrate is more likely 264 influenced by anthropogenic NO x emissions over a smaller regional areas.

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Aerosol species also varied dramatically throughout the study. For example, the reasons were likely due to that the air masses during clean periods were either from a 288 longer transport when ammonium nitrate was deposited or evaporated due to dilution 289 processes, or from less anthropogenic influenced regions with low NO x emissions.

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The aerosol particle acidity was evaluated using the ratio of measured  sulfate during the transport while gaseous ammonia is not enough to neutralize the 302 newly formed sulfate. This is supported by the overall higher contribution of sulfate at 303 rural/remote sites than that at urban sites. Also note that the newly formed sulfate 304 particles during the frequent NPF events might also have played a role.

Diurnal variations 306
The diurnal cycles of aerosol species and PM 1 are shown in Fig. 4a. The PM 1 307 shows a pronounced diurnal cycle with the concentration ranging from 7.9 to 13.4 μg 308 m -3 . The PM 1 shows a visible peak at noon time and then has a gradual decrease 309 reaching the minimum approximately at 16:00. After that, the PM 1 starts to build up 310 and reaches the highest level at midnight. Such a diurnal cycle is similar to those of 311 SO 2 and CO (Fig. 4d), which likely indicates that the major source of PM 1 at the 312 Menyuan NBS is from regional transport. All aerosol species present similarly The sulfate contributes more than 25% to PM 1 with the highest contribution as much 320 as 33% between 12:00 -14:00. Nitrate and chloride shows relatively stable 321 concentrations before 11:00 and then gradually decreased to low ambient levels 322 during daytime. Such diurnal variations still exist after considering the dilution effects 323 of boundary layer height using the conserved tracer CO as a reference (Fig. 4c). This 324 indicates that gas-particle partitioning affected by temperature and humidity has 325 played an important role in driving the diurnal variations of nitrate and chloride.

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Consistently, the nitrate contribution to PM 1 during late afternoon is ~7-8% which is       for ~70%. In contrast, the average size distribution during non-NPE was characterized 440 by a bi-modal distribution with the GMD peaking at 59 nm and 146 nm, respectively.

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The peak diameters were shifted to the larger sizes compared to those during NPE.

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Such a size shift from clean days to polluted days was also observed previously in particles are more likely from regional transport.
459 Figure 8 shows the diurnal evolution of particle number size distributions, aerosol 460 composition, and gaseous species during NPE and non-NPE days. The particle species however showed decreases during the particle growth period between 12:00 -469 17:00, and the gaseous CO and SO 2 showed similar variations as aerosol species. By 470 excluding the dilution effect of PBL using CO as a tracer, we found that organics was 471 the only species showing a gradual increase during the particle growth period (Fig. 8a) 472 while other species remained minor changes or even slightly decreased. The growth from ~50 nm to 60 nm during the first 6 hours, which is likely a continuation 491 of previous NPE. Compared to the early stage of particle growth during NPE, the 492 particle growth during non-NPE is associated with synchronous increases of both 493 organics and sulfate. The results indicate that both organics and sulfate contribute to 494 the particle growth after mixed with anthropogenic sources from ~18:00 in the 495 previous day. 496 We further calculated the particle growth rates (GR) for NPE events without 497 interruptions due to meteorological changes using Eq. (2).
Wwhere D m is the geometric mean diameter from the log-normal fitting, ∆D m is the 500 difference of diameter during the growth period and ∆t is the duration of growth time.

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The calculated GR and the corresponding average chemical composition and fraction 502 of OOA during the growth period are shown in Fig. 9a. The GR ranges from 0.8 nm is 11.4 (± 8.5) μg m -3 for the entire study, which is lower than those observed at urban 521 and rural sites in eastern China. Organics constituted the major fraction of PM 1 , on 522 average accounting for 43% followed by sulfate (28%) and ammonium (11%).

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Several periods with the contribution of organics as much as 70% due to biomass 524 burning impacts were also observed. All aerosol species presented similar diurnal 525 cycles that were mainly driven by the dynamics of planetary boundary layer and for 17% of OA. New particle formation and particle growth events were frequently 531 observed during this study. The particle growth rates varied from 0.8 to 3.2 nm hr -1 532 with an average growth rate of 2.0 nm hr -1 . Organics was found to be the only species 533 with gradually increased contribution to PM 1 during NPE. Also, higher contribution 534 of organics during NPE than non-NPE days was observed. These results potentially 535 illustrate the important role of organics in particle growth. Further analysis showed a 536 positive correlation of particle growth rate with the fraction of OOA suggesting that 537 oxidized OA plays a critical role contributing to the particle growth.    Table S1.