Aerosol acidity in a megacity with high ambient temperature and 1 relative humidity of Central China : temporal variation , determining 2 factors and pollution transition effect 3

1. Department of Environmental Science and Technology, School of Environmental Studies, China University of 7 Geosciences (Wuhan), 430074, Wuhan, China 8 2. Department of Atmospheric Sciences, School of Environmental Studies, China University of Geosciences 9 (Wuhan), 430074, Wuhan, China 10 3. Hubei Environment Monitoring Center, Wuhan, 430072, China 11 4. School of Earth, Atmospheric & Environmental Sciences, University of Manchester, UK 12 5. Beijing Weather Modification Office, Beijing 100089, China 13 6. Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, 14 Nanjing University of Information Science and Technology, Nanjing 210044, China 15

The aerosol acidity exhibited spatiotemporal discrepancy, owing to the diversities of source emission and meteorological conditions.Currently, the research of aerosol acidity in China is becoming a hotspot, with studies mainly concentrated in northern cities like Beijing (He et al., 2012;Yang et al., 2015;Cheng et al., 2016;Wang et al., 2016a), eastern coastal cities like Shanghai (Pathak et al., 2009), southern coastal cities including Guangzhou (Huang et al., 2011) and Hongkong (Pathak et al., 2003;Pathak et al., 2004a;Yao et al., 2006;Yao et al., 2007), and western cities of Chongqing (He et al., 2012) and Lanzhou (Pathak et al., 2009).One study has been conducted at a background mountainous site (Zhou et al., 2012).In general, aerosol acidity in China was lower than those in Europe (Bougiatioti et al., 2016) and United States (Weber et al., 2016).He et al. (2012) indicated that the aerosol acidity in North China was lower than that in the south.Even in the same city, the aerosol acidity was different.He et al. (2012) observed that fine particles (PM2.5) in Beijing was strong acidic with pH ranging in 0-3; Liu et al. (2017) calculated the pH value of PM2.5 in Beijing as about 4.2; Wang et al. (2016a) indicated that PM2.5 in Beijing was completely neutralized with pH as about 7. One of the key factors for these diversities is that these studies were done at a given period with different pollution levels.Liu et al. (2017) found that along with the pollution levels increased, the aerosol pH values decreased for a typical haze formation processes in Beijing.Former researchers highlighted the importance to obtain the aerosol acidity under different time scales and different pollution episodes, which is important to better identify the formation mechanism of air pollution at a certain region.Till now, few studies were reported to analyze the aerosol acidity for a long-time period, owing to the absence of online high-resolution dataset for gaseous pollutants and ionic components synchronously.It may limit the understanding of how the aerosol acidity affects the atmospheric chemistry, in view of a pollution transition event, monthly variation and diversities for different air mass trajectories.
AWC exerts a driving role in the variation of aerosol acidity (Liu et al., 2017).Zhou et al. (2012) indicated high water content can cause HSO4 -dissociating to form free H + when RH > 0.65.
In addition, AWC serves as a medium for aqueous phase reaction of SO2 oxidation (Pilinis et al., 1989;Ervens et al., 2011;Cheng et al., 2016), which can also lead to the increase of aerosol acidity (Seinfeld et al., 2006).However, high water content of aerosols in return may dilute the hydrogen ion concentrations and increase pH value.Guo et al. (2015) showed that the diurnal variation of pH was mainly driven by AWC dilution.It was observed that the temporal trends of particle pH were in accordance with AWC rather than H + , and the aerosol acidity added with the decrease of AWC in Beijing, despite the decreasing of H + (Liu et al., 2017).AWC was not only closely associated with atmospheric relative humidity (RH) and temperature (Temp) (Guo et al., 2015), but also well correlated with secondary inorganic salt (Cheng et al., 2016) which will again assimilate more water because of hygroscopicity (Engelhart et al., 2011;Bian et al., 2014).So is there an inflection point of AWC between the promoting role through the aqueous phase formation and the inhibition role by the dilution effect on the acidic inorganic ions under a specific circumstance?Till now, as obvious studies are always conducted in a limited season or pollution episodes (Zhou et al., 2012;Bian et al., 2014;Guo et al., 2015;Cheng et al., 2016;Wang et al., 2016a;Liu et al., 2017;Wang et al., 2018), no studies have listed observed evidence directly to answer this question.
Except for AWC, ammonia and ammonium were also regarded as determining factors on aerosol acidity (Liu et al., 2017), which were the main species to neutralize acid (Sun and 1998;Wang et al., 2015).The ammonium-to-sulfate ratio was used to describe the ammonium-rich or ammonium-poor conditions (Pathak et al., 2004a;Huang et al., 2011;Kumar and Sunder Raman, 2016), with the ratio higher than 1.5 implying an ammonia rich atmosphere (Kumar and Sunder Raman, 2016).Pathak (2004) indicated that pH increased with increasing ammonium-to-sulfate ratio.The correlation of NH4 + with SO4 2-and NO3 -also can be used to judge the ammonia rich or poor status (Kumar and Sunder Raman, 2016).Recently, Liu et al. (2017) concluded that the excess ammonia resulted in the pH increasing by logarithm growth form in north China.Under ammoniarich air, positive correlation between excess NH4 + and NO3 -was found (Pathak et al., 2009).
Therefore, for the regions with abundant agricultural activities which are the main emission sources for ammonia, the relationship of ammonia, aerosol acidity and air pollution needs a clear illustration.
Wuhan and its surrounding cities (WSC) belong to the key regions in the State Council's Action Plan on Prevention and Control of Atmospheric Pollution (Figure 1a).WSC has become the most polluted region with high PM2.5 concentrations except for the regions of Beijing-Tianjin-Hebei (Qi et al., 2017;Gao et al., 2018), Yangtze River Delta (Shen et al., 2015;Chen et al., 2017) and Sichuan Basin (Wang et al., 2017a;Ning et al., 2018) (Figure 1b).Meanwhile, as WSC is located in the center of China, the air masses from other different polluted regions all can impact the aerosol concentrations and compositions.At the west of WSC, there locates JiangHan Plain (JHP) which is a key food-cotton base of China.The WSC and surrounding JHP hold high ammonium emission intensity (Huang et al., 2012;Xu et al., 2015b;Wang et al., 2018).At the northeast and northwest of WSC, there locates TongBai Mountain and DaBie Mountain (Figure 1c), which directly decided the transportation routes of air masses in cold period from North China Plain when the north winds dominated (Figure 1b).Wuhan belongs to subtropical monsoon climate, and owns hundreds of lakes (Figure 1d), which lead to its higher ambient temperature and atmospheric relative humidity (RH).
The unique geographical, emission and meteorological conditions implied that the aerosol acidity and its role in air pollution transition should be quite different from other regions, especially for the recently studied cities in North China.Meanwhile, the high RH (annual mean 0.74, in this research, all the PM2.5 compositions and meteorological data for rainy day were excluded), temperature (annual mean 292.2K) and serious air pollution in Wuhan provide an ideal site to identify how the RH, AWC and precursor gases affect aerosol acidity.
Therefore, based on one-year continuous observation of hourly water-soluble inorganic ions in

Sampling and instrumentation
The observation site (30.53ºE, 114.36ºN) located in the city center (Figure 1d).It is about 20 m above ground and surrounded by commercial/residential mixed area with no obvious industrial sources.Water-soluble ions (WSI) including SO4 2-, NO3 -, Cl -, NH4 + , Na + , K + , Ca 2+ and Mg 2+ in PM2.5 and atmospheric NH3, HCl, HNO3, and HNO2 were synchronously measured by an online ion chromatography analyzer with one-hour resolution by Marga ADI 2080 (Monitor for Aerosols and Gases in Ambient Air).The detailed description of the equipment can be found in previous studies (Rumsey et al., 2014).One-year continuous monitoring was done from September 2015 to August 2016, except data missing due to equipment maintain in February.Concurrently, hourly PM2.5, O2, SO2, NO2, and O3 were observed by automatic on-line monitoring instrument, with the methods of β-ray, ultraviolet fluorescence, chemiluminescence and ultraviolet absorption, respectively.The synchronous meteorological data including relative humidity (RH) and temperature (Temp) were obtained from local meteorological observatory.Ionic equivalent ratio was employed to a preliminary qualitative estimation of the particle neutralization and acidity.The ratio of total cations and anions were calculated as following (Zhang et al., 2007;Tanner et al., 2009;Hennigan et al., 2015): as temperature and relative humidity.In this study, the forward mode with metastable state was adopted as its better performance (Fountoukis and Nenes, 2007;Guo et al., 2015;Hennigan et al., 2015;Guo et al., 2016;Weber et al., 2016).Well correlations between the predicted and observation data were found in Figure 2.

Trajectory clustering
Backward trajectory analysis by HYSPLIT model (Version 4) was used to explore the influence of air masses originated from different directions on the aerosol acidity of Wuhan.72 h backward trajectories were calculated for 4 times each day (00:00, 06:00, 12:00 and 18:00), with the starting height at 200 m above ground.The input meteorological data was in 6-h hourly resolution acquired from NOAA/ARL.The trajectories were clustered by the clustering analysis procedure provided in the HYSPLIT user's guide.

General characteristic of WSI in Wuhan
As listed in Table 1, the annual average mass concentration of PM2.5 in Wuhan was 63.4 ± 35.2 μg m -3 , exceeding the annual average secondary standard of China Ambient Air Quality (35 μg m - 3 ).PM2.5 showed the maximum concentration in winter (92.6 ± 45.6 μg m -3 ) and the minimum value in summer (37.3 ± 12.0 μg m -3 ), which was consistent with former researches.Total WSI concentrations accounted for 70% of PM2.5 mass concentration on average, also higher in winter and lower in summer.
The meteorological condition and precursor emissions, can explain the seasonal variation pattern ( Fu et al., 2016).Although there was higher sulfate formation rate in summer in Wuhan, the SO2 in summer was low to 4.7 μg m -3 (Table 2).The low SNA concentrations in summer were mainly related to the lower nitrate and ammonium formation rate (Table 2) and high atmospheric boundary layer height (Figure 4a).
NO3 -concentrations exhibited the highest concentration in winter (25.3 μg m -3 ) and the lowest in summer (5.6 μg m -3 ), with winter/summer ratio of 4.5.SO4 2-concentrations also exhibited the same seasonal variation.Lower sulfate concentration in summer was contrary to previous studies (Tai et al., 2010;Yang et al., 2011;Hand et al., 2012), which indicated that strong atmospheric oxidation (higher O3 concentration in summer), higher temperature and relative humidity were beneficial to sulfate formation from SO2.While in this study with lower precursor concentration and higher boundary layer height, relatively low concentrations of sulfate and nitrate in summer were found in Wuhan.
High intensity discharge of K + from fireworks and crackers in Spring Festival periods could not be ignored, which usually occurred in January or February (Kong et al., 2015).

Aerosol acidity in Wuhan
Figure 3a shows the daily variation of PM2.5 aerosol acidity in Wuhan.The daily pH values ranged between 2.21 -4.19, with an average of 3.30, indicating moderate acidic in Wuhan.The lowest pH value occurred in summer, averaged as about 2.84.The pH values in winter were higher as about 3.71.The AWC in autumn, winter, spring, and summer were 66.9, 118.5, 66.5 and 36.2 μg m -3 , respectively, which can explain the seasonal variation of pH values primarily.There is a double effect of AWC on aerosol pH, including the enhancement of bisulfate dissociation to form free H + through the hydrolysis process and dilution effects of proton concentrations in droplets (Pathak et al., 2004a;Zhou et al., 2012).At higher RH, the dilution effect on the molarity of acidic species was more important than H + releasing from bisulfate (Pathak et al., 2004a).Previous studies showed that aerosol pH varied in consistency with AWC (Liu et al., 2017).From September to January, the aerosol pH gradually increased from 2.75 to 3.77 (Figure 5) along with the increasing of AWC, which buffered the aerosol acidity.From May to August, when AWC decreased from 80.8 to 32.4 µg m -3 , the pH decreased 0.65 units from 3.44 to 2.79.The H + concentrations generally exhibited an opposite variation of pH value except for November when more H + did not result in lower pH value (3.53) because of higher AWC (151.7 μg m -3 ).
The ratios of cations to anions were also listed in Figure 3(c).It could be found that there was no obvious relationship between pH and cations/anions ratios.The annual mean cations/anions ratio was near unity (1.10), reflecting completely neutralized, which was not in accordance with the prediction by thermodynamic models.It might be resulted from the negligence in considering the particle liquid water and dissociation state of individual ion (Guo et al., 2015).Thus, ionic molar ratios could not be the proxy for discussing aerosol acidity.
Table 3 compared the aerosol acidity of Wuhan with other cities.Aerosols mostly exhibited acidic around the world.Particle pH in Wuhan was higher than most of the other cities.As discussed above, the specific climate, geography, and emissions may explain it.Besides, aerosol acidity was dynamically changing.The particle pH increased in last decade of Beijing as Table 3 shown, which might be due to the increase of atmospheric NH3 and decreasing of SO2 (Meng et al., 2011;Liu et al., 2013).Detailed discussion about the key impacting factors on aerosol acidity is listed below.

Key driving factors for aerosol acidity variation
As mentioned above, AWC exerted an important role in aerosol acidity variation.Figure 6a exhibited the relationship between pH with AWC.From the fitting curve, the particle pH firstly showed a decreasing trend with AWC increased under much lower AWC level (less than about 15 μg m -3 ).Then, with the AWC increasing from ~15 to ~380 μg m -3 , the dilution effect on aerosol acidity gradually dominated, which offset or even surpass the liquid-phase reaction and formation of secondary inorganic ions, causing the obvious and quick increase of particle pH.The SNA contents exhibited its peak value when the AWC increased to about 380 μg m -3 , then with the AWC increasing, SNA contents decreased (Figure 6d).When AWC > 380 μg m -3 , along with the increasing of AWC, a slowly increasing and gradually no significant growth of pH was found.
It was favored by the SNA-RH correlation coefficient variations with RH increasing (Figure 7b) and relationship of AWC-RH (Figure 7c), SNA-AWC (Figure 7d) and pH-RH (Figure 7e).Lower RH corresponded to lower AWC (Figure 7c).Under lower RH (< ~0., with corresponding RH as about 0.75-0.90,reflecting active liquid phase reaction, and resulting in buffering effect on pH increase.That might be the reason for the pH slowly growth when AWC was at ~380 μg m -3 .Then when RH was higher than 0.95, with the corresponding AWC was greater than 380 μg m -3 , obviously negative correlation between RH and SNA was obtained (R 2 =0.59, p < 0.05), and it implied the dominated role of buffering effect of AWC, which lead to the continuously increasing of pH and gradually slowdown.
AWC is closely related to atmosphere temperature and relative humidity (RH) (Bian et al., 2014).High RH can facilitate the production of secondary inorganic salt, which will again assimilate more water because of hygroscopicity (Engelhart et al., 2011;Bian et al., 2014).However, in this study the RH was higher in July, August and September, while corresponding AWC were nearly the lowest (Figure 5), which may be resulted from the higher water evaporation from aerosol (Bougiatioti et al., 2016) due to the high temperature in summer period.It could be seen there was an exponential relationship between AWC and RH (R 2 = 0.71, p < 0.05) (Figure 6c).The correlation coefficient of atmospheric RH and fine particle pH in Wuhan was 0.39 (p < 0.05) by polynomial fitting (Figure 6e), showing that pH decreased with RH when RH was less than about ~0.48, and then pH increased with RH increasing when RH was higher than about 0.48.Atmospheric temperature can also pose impacts on AWC and pH (Figure 6f).Generally, lower temperature was corresponded to the higher AWC and pH values.When the temperature was higher than about 303 K (about 30 ℃), the AWC was all less than ~380 μg m -3 , and particles were more acidic.Figure 6e also showed that there were no obvious relations between SNA/PM2.5 ratio with pH.It illustrated that the particle acidity or pH could be affected by other components except for inorganics, such as organics whose role in aerosol acidity can not be ignored (Wang et al., 2018).
Except for the AWC, RH and temperature, the excess ammonia or ammonium was also proved to affect aerosol acidity (Liu et al., 2017).Different from southeastern US (Weber et al., 2016) where the aerosols usually presented strong acid, the higher pH in this study was also likely related with abundant ammonia and ammonium.As discussed above, the ammonium in Wuhan was at a higher level in China.
Ammonium was the primary basic species to neutralize acid (Seinfeld and Pandis, 2006) and it preferred to neutralize sulfate first, then reacted with nitrate when redundant, owing to the higher salting out efficiency of SO4 2-than NO3 - (Pathak et al., 2004a).A critical molar ratio of NH4 + to SO4 2-was suggested as 1.5, implying SO4 2-was completely neutralized and NO3 -was stabilized by NH4 + (Pathak et al., 2004b;Huang et al., 2011).When the NH4 + /SO4 2-ratio was higher than 1.5, it indicated an ammonia rich atmosphere, and the excess NH4 + could be calculated as: In Figure 7a, the molar ratio of NH4 + to SO4 2-in Wuhan mainly varied from 2:1 to 6:1, indicating that SO4 2-was completely neutralized and mainly presented in the form of (NH4)2SO4.Along with the increase of NH4 + /SO4 2-ratio, NO3 -production increased, and it preponderated SO4 2-formation The correlation coefficient between NH4 + /SO4 2-and NO3 -/SO4 2-in Wuhan was 0.85.In adequate ammonium condition, excess NH4 + was well correlated with NO3 -(R 2 = 0.95) (Figure 7b), which was in consistency with previous studies (Pathak et al., 2009).The correlation of NH4 + with SO4 2- and NO3 -can also be used to judge the rich or poor status of NH4 + , which was not obvious in poor NH4 + condition (Kumar and Sunder Raman, 2016).In Wuhan, the positive significant correlation between NH4 + vs SO4 2-and NH4 + vs NO3 -verified the NH4 + rich conditions.The excess ammonium may be in the form of NH4Cl, with the high correlation between NH4 + and Cl -(R 2 = 0.71).
The NHx concentrations of Wuhan were in the range of 2.27 -48.3 μg m -3 , averaged as 19.2 μg m -3 , which still located in the range for the rapid growth stage of pH (Figure 8b).It was verified by the positive relationship between △pH and the excess NH4 + (R 2 = 0.45, p < 0.05).However, it was different from the curve in Beijing which performed a logarithmic relationship (Liu et al., 2017).
The same excess NH4 + concentrations in Beijing and Wuhan exhibited different stage of pH change, and higher excess NH4 + was afforded in Wuhan than Beijing for reaching the inflection point when the excess NH4 + increasing held little influence on pH change.Thus, it again confirmed that the aerosol acidity in Wuhan of Central China was impacted more by excess NH4 + than that of Beijing in North China.Wang et al. (2016a) indicated that the increase of pH value will promote precursor solubility and increase aqueous reaction rates, which will accelerate the formation of secondary inorganic salts and aggravate air pollution, resulting in more severe haze.Therefore, it should be emphasized that if the excess NH4 + in Wuhan was not controlled, the aerosol pH value will increase, which would worsen the air quality.

Aerosol acidity transition from clean to polluted periods
The visibility less than 5 km, 5-10 km and higher than 10 km were defined as polluted period, transition and clean period, respectively.The averaged visibility in Wuhan for the three episodes were 3.78 ± 0.96 km, 7.56 ± 1.59 km and 17.7 ± 5.71 km, with averaged PM2.5 concentrations as 136.4,85.0 and 45.9 μg m -3 , respectively.The average pH value at the clean stage was 3.07 ± 0.45, accompanied with low AWC (33.2 ± 42.9 μg m -3 ), low excess NH4 + (3.70 ± 2.15 μg m -3 ), high temperature (296.1 ± 8.02 K) and relative low RH (0.70 ± 0.12).The averaged pH value at transition period was 3.63 ± 0.27, and the AWC, excess NH4 + , temperature, and RH were 115.1 ± 79.4 μg m - 3 , 9.04 ± 3.67 μg m -3 , 286.2 ± 6.45 K and 0.80 ± 0.12, respectively.From clean to transition and polluted periods, the aerosol pH values gradually increased with the increasing of AWC, excess NH4 + , RH and the decreasing of temperature (Figure 9).From transition to polluted periods, the aerosol pH value increased by 0.21 units, which was less than that from the clean to transition periods (by 0.56 units).However, the increasing amounts of AWC from the transition to polluted periods were higher than those from the clean to transition periods.It further verified that the effect of AWC on aerosol acidity was not linear as mentioned above.
The O3 concentrations were the lowest in polluted periods, indicating weak photochemical activity (Wang et al., 2016a).Moreover, accompanied with high RH, aqueous reaction was the dominated atmospheric reaction in polluted periods, resulting in the increasing of SOR (sulfur oxidation rate), NOR (nitrogen oxidation rate) and AOR (ammonia conversion rate).High pH can further accelerate the SOR, NOR and AOR rate.During the transition processes, no significant correlations were found between mineral components (Ca 2+ , Mg 2+ , K + , and Na + ) and pH, implying the impact of mineral components on aerosol acidity was negligible in Wuhan.
A conceptual schematic was proposed as Figure 10 shown, along with the air quality worsened from clean to transition and to polluted situation, the fraction of second inorganic salts in PM2.5, AWC, excess NH4 + and the pH of fine particles all added.Excess NH4 + and SNA formation showed steady growth from clean-transition-polluted. However, the AWC increase was more significant at higher polluted periods and the temperature decreasing was more obvious at lower polluted periods.
The aerosol pH increasing unit from clean to transition episodes were larger than that for the pollution transformation from the transition to polluted episodes.The conceptual model comprehensively explained the role of ambient RH and temperature on the formation of SNA and AWC calculating, and furthermore the pH variation during different pollution periods.

Aerosol acidity for different origination of air masses
In the study period, there were four main directions for the air masses transported to Wuhan, namely northeast (C1), northwest (C2), south (C3) and west (C4), with the proportion of 24%, 16%, 57% and 4%, respectively (Figure 11).The aerosol acidity for C1 and C2 air masses were higher than those of C3 and C4, and associated with lower values of AWC and excess NH4 + .The RH in southern China was always higher than those in northern China for each season (Figure 4b).Air masses from northwest mainly in winter were usually dry and saturated with small amounts of water, resulting in low pH of aerosols.More moisture air from south mainly in summer elevated aerosol pH value.Although there were minor air masses from the west to Wuhan, the pH for C4 was higher, which may be resulted from the higher AWC for C4 (Figure 11) with higher RH (Figure 4b).The amounts of excess NH4 + was in the order as C4 > C3 > C2≈C1, and interestingly, the pH value exhibited the similar rule, which further reflected the effect of excess NH4 + on aerosol acidity in Central China.Cluster analysis in this section highlighted that the current conclusions drawn from the research on aerosol acidity and its role in a typical haze formation event in North China could be not suitable for that in Wuhan, which needs a further detailed and comprehensive study, along with more chemical components investigated.

Conclusion
This study observed the hourly water-soluble inorganic ions from September 2015 to August The seasonal and monthly aerosol acidity variation and its role in air pollution transition were discussed, and the possible impacting factors were identified.
The mass fraction of SNA in total water-soluble inorganic ions and PM2.5 were 92% and 64% on average, respectively.Moderate acidic of fine particles in Wuhan was found, with the average of pH as 3.30.The aerosol acidity was higher in July, August and September and lower in January and March.The higher RH (averaged as 0.74 ± 0.13), excess ammonium (averaged as 6.06 ± 4.51 μg m -3 ) and abundant aerosol water content (averaged as 71.0 ± 82.8 μg m -3 ) were the key reasons for the lower aerosol acidity in Wuhan compared with other cities.At much lower AWC level (about lower than 15 μg m -3 ), the particle pH showed a decreasing trend with AWC increased.Along with the AWC continuous increasing (> ~15 μg m -3 ), a logarithmic correlation was found between AWC and aerosol pH.When AWC was higher than about 380 μg m -3 , along with the further increasing of AWC, a slowly increasing and gradually no obvious growth of pH was found.pH firstly decreased with atmospheric RH increasing at lower RH values and then increased with RH increasing, with the turning point of RH as about 0.48.Lower temperature was corresponded with higher AWC and pH, and when the temperature was higher than 303 K (about 30 ℃), more acidic aerosols were found.
There also was a logarithmic growth of particle pH with total NHx (NH3 + NH4 + ) increasing.
Aerosol in Wuhan belonged to the stage of pH rapid growth with ammonia or ammonium increasing.
It can be predicted that fine particles pH in Wuhan will continuously increase if the ammonia or ammonium was not controlled effectively, and the increasing extent will be more obvious than that in northern China.Aerosol acidity gradually decreased from clean to transition and to polluted periods.Air masses originated from northeast and northwest directions with lower AWC and excess NH4 + contributed to the higher acidic aerosol of Wuhan, while air masses from south and west regions exhibited lower aerosol acidity, with more AWC and excess NH4 + carried.
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-367Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 May 2018 c Author(s) 2018.CC BY 4.0 License.PM2.5 and precursor gases from September 2015 to August 2016, PM2.5 acidity was calculated by thermodynamic model ISOROPPIA-Ⅱ.The variation of acidity in different months, pollution episodes and for different air mass transportation directions were discussed and possible reasons were investigated.The results are urgently and helpful to understand the relationship of chemical composition-aerosol acidity-air pollution in different time scales and to enrich the formation mechanism of serious air pollution out of North China Plain.
48) and AWC (< ~15 μg m -3 ) Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-367Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 May 2018 c Author(s) 2018.CC BY 4.0 License.values, the AWC dilution effect on aerosol acidity was negligible, and the dry particle quickly deliquesced and release H + , resulting in the decrease of pH.The weaker or even negative correlation coefficient of SNA with RH was found when RH varied in 0.48-0.75,accompanied with AWC and pH quick increasing, which reflected that the dominated role of dilution effects of AWC.As discussed above, SNA content reached its peak values when the AWC increased to about 380 μg m - 3 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2018-367Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 May 2018 c Author(s) 2018.CC BY 4.0 License.

Figure 1 Figure 2
Figure 1 (a) Wuhan and its surrounding cities (WSC) belong to the key regions in the State Council's Action Plan on Prevention and Control of Atmospheric Pollution; (b) spatial distribution of the 15-year (2001-2015) mean PM2.5 concentrations at a resolution of 1 km (Lin et al., 2018) (BTH-Beijing-Tianjin-Hebei region; YRD-Yangtze River Delta region; SCB-Sichuan Basin; PRD-Pearl River Delta region; BJ-Beijing; WH-Wuhan; SH-Shanghai; CQ-Chongqing; GZ-Guangzhou); (c) the topography of North China Plain (NCP) and surroundings of Wuhan (TBM-TongBai Mountain, DBM-Dabie Mountain).The two red lines indicate the typical transportation route of air masses in autumn and winter from NCP to Wuhan; (d) the topography of Wuhan, with the Yangtze River flowing from southwest to the northeast and abundant of lakes.The sampling site locates in the city center.

Figure 3
Figure 3 Daily variation of aerosol pH and AWC (a), RH and temperature (b), [H + ] and cation/anion (c), PM2.5 concentrations and visibility (d) in Wuhan from September 2015 to August 2016.

Figure 4 Figure 9
Figure 4 Atmospheric boundary layer height (a), relative humidity (b) and wind field in different seasons of Wuhan.

Figure 10 A
Figure 10 A conceptual schematic for the variations of fine particle mass concentrations and the driving factors impacted on aerosol pH along with the pollution aggravation.

Figure 11
Figure 11 Backward trajectories for each cluster and corresponding pH, water content and excess NH4 + .

Table 2 .
Statistic of meteorological parameters and main gaseous pollutant concentrations (average ± standard deviation, μg m -3 ) in Wuhan during September 2015 to August 2016.