Elucidating Decade-Long Trends and Diurnal Patterns in Aerosol Acidity in Shanghai

: Aerosol acidity is a critical factor affecting atmospheric chemistry. Here, we present a study on annual, monthly, and daily variations in PM 2.5 pH in Shanghai during 2010–2020. With the effective control of SO 2 emissions, the NO 2 /SO 2 ratio increased from 1.26 in 2010 to 5.07 in 2020 and the NO 3 − /SO 4 2 − ratio increased from 0.68 to 1.49. Aerosol pH decreased from 3.27 in 2010 to 2.93 in 2020, regardless of great achievement in reducing industrial SO 2 and NOx emissions. These findings suggest that aerosol acidity might not be significantly reduced in response to the control of SO 2 and NOx emissions. The monthly variation in pH values exhibited a V-shape trend, mainly attributable to aerosol compositions and temperature. Atmospheric NH 3 plays the decisive role in buffering particle acidity, whereas Ca 2+ and K + are important acidity buffers, and the distinct pH decline during 2010–2016 was associated with the reduction of Ca 2+ and K + while both temperature and SO 42 − were important drivers in winter. Sensitivity tests show that pH increases with the increasing relative humidity in summer while it is not sensitive to relative humidity in winter due to proportional increases in H + air and aerosol liquid water content (ALWC). Our results suggest that reducing NOx emissions in Shanghai will not significantly affect PM 2.5 acidity in winter.


Introduction
Aerosol acidity is a critically important indicator affecting human health and atmospheric chemistry.Aerosol acidity can affect chemical reactions; for example, the main oxidation pathway of SO 2 is related to the acidity of the aerosol [1,2].With the increase in acidity, the solubility of transition metal ions (such as Fe 2+ and Mn 2+ ) increases in atmosphere aerosol, the number of cations involved in the oxidation reaction increases, and then the ability to catalyze the oxidation of SO 2 is enhanced [3].On the contrary, in neutral and alkaline atmospheres, the solubility of metal ions is low, the ability of metal ions to catalyze the oxidation of SO 2 decreases, and the oxidation of SO 2 by O 3 and NOx becomes the main way [4].In addition, when the solubility of metal ions increases with the acidity of aerosols, the toxicity of aerosols is enhanced, which then affects the ecosystem [5][6][7].Recently, significant correlations were confirmed between the pH value, water-soluble Fe, the concentration causing 20% inhibition of cell viability (IC20), and the concentration of exposure substance corresponding to a 1.5-fold increase in reactive oxygen species generation relative to control (EC1.5),indicating the strong impact of acidity on aerosol toxicity by increasing toxic equivalent concentrations of metals [8].In addition, aerosol acidity can influence the acid-catalyzed heterogeneous reactions in the atmosphere, leading to a potential multiplication of secondary organic aerosol (SOA) mass [9,10].Furthermore, aerosol acidity controls the phase distribution of semi-volatile compounds, such as HNO 3 /NH 4 NO 3 and HCOOH/HCOONH 4 systems [11,12].
Over the past decade, the Chinese government has implemented strict air pollution control strategies, such as the Air Pollution Prevention and Control Action Plan (2013-2017) and the three-year action plan for winning the blue-sky defense battle (2018-2020), leading to a significant reduction in PM 2.5 and great changes in aerosol chemical compositions.In the Yangtze River Delta (YRD), the proportion of sulfate in PM 2.5 was reduced from 18% to 14% between 2011 and 2018, while the proportion of nitrate increased from 24% to 29% [13,14].Similarly, the concentration of PM 2.5 in Beijing significantly decreased from 88 µg m −3 in 2011 to 26 µg m −3 in 2020, and the mass ratio of NO 3 − /SO 4 2− in PM 2.5 increased from 0.88 to 1.70 during the same period [15].
The acidity of atmospheric particles has been extensively studied before.Based on model predictions that the aerosols were moderately acidic, Liu et al. (2017) questioned the role of the aqueous oxidization of SO 2 by NO 2 in sulfate productions in haze events in China [16].For the highly acidic aerosols in Canada, aerosol pH has different responses to the changes in chemical composition in different seasons [17].An observation in a northern city in China showed that aerosol pH values ranged between 0.33 and 13.6, and were highly dependent on the source contributions of water-soluble ions such as coal combustion, mineral dust, and vehicle exhaust [18].Similarly, Sharma et al. (2022) highlighted the important role of SO 4 2− , NH 3 , and K + in determining aerosol pH [19].Fu et al. (2022) suggested that seasonal pH changes were mainly determined by aerosol compositions in Shanghai and reducing NH 3 emissions by 20% could not effectively mitigate winter PM 2.5 pollution but significantly increased particle acidity [20].
Changes in the aerosol chemical composition may change aerosol acidity.A model simulation showed that PM 2.5 pH increased from 4.4 to 5.4 during haze episodes in Beijing when the molar ratio of NO 3 − /SO 4 2− increased from 1 to 5 [21].In contrast, another observation in Beijing showed that PM 2.5 pH in winter and autumn decreased significantly with elevated TNO 3 [22].Model simulations showed that aerosol acidity strongly decreased over Europe and North America in recent decades while it increased over Asia [1].This assumption was supported by observations in Guangzhou [23] and Beijing [24].However, Zhou et al. (2022) reported that aerosol acidity increased in Shanghai in recent decades [25].
An in-depth analysis of aerosol pH changes is crucial for understanding long-term trends in aerosol pH and dominant drivers of pH variations, helping to predict pH in the future and formulate air pollution control strategies.This study investigated annual, monthly, and daily variations in PM 2.5 pH in Shanghai during 2010-2020.The main objectives of this study are (1) to elucidate the long-term trends in aerosol pH in highly urbanized areas over ten years; and (2) to explore the role of meteorological conditions and chemical compositions in driving pH changes.

Measurement Site and Instrumentation
The observation in this study was carried out in the downtown area of Shanghai during 2010-2021.The samples from 2010 to 2013 were collected on the roof of the No. 4 Teaching Building in the main campus of Fudan University (121.50N), less than 10 km away from Fudan University (Figure S1).The two sites are surrounded by commercial and residential buildings, representing Shanghai's densely populated urban areas.There may be a small difference in aerosol composition between the two measurement sites.
A Monitor for Aerosols and Gases in Air (MARGA, ADI 2080, Metrohm Applikon B.V., Barendrecht, The Netherlands) with a time resolution of 1 h was used to determine aerosol SO 4 2− , NO 3 − , Cl − , NH 4 + , Na + , K + , Ca 2+ , and Mg 2+ , and their gas precursors NH 3 , HCl, and HNO 3 .Due to the lack of MARGA data in 2012-2013, filter-based sampling with a time resolution of 24 h was used instead.Data were not available for 2011.Detailed information on MARGA was reported previously [20].In brief, the water-soluble gases were absorbed by a wet rotating denuder while the water-soluble ions in the aerosols were extracted using a steam jet aerosol collector.The collected gas and aerosol samples were analyzed online using a dual-channel ion chromatograph.Technical specifications for QA/QC followed local standards in the YRD region (DB31/T 310006-2021), which regulate that the correlation coefficients (r) for all targeted ions are larger than 0.995 [20].The method detection limit of all components was 0.10 µg per cubic meter of air or better, except for K + (0.16 µg m −3 ), Mg 2+ (0.12 µg m −3 ), and Ca 2+ (0.21 µg m −3 ) [26].For filter-based sampling, QA/QC parameters for the determination of inorganic ions were previously reported [27].Hourly meteorological data (temperature, relative humidity (RH), wind speed, and wind direction) were released by the China Meteorological Administration website.Hourly concentrations of PM 2.5 , PM 10 , SO 2 , NO 2 , CO, and O 3 were provided by Pudong Environmental Monitoring Center.

ISORROPIA II
ISORROPIA II is a computational and efficient aerosol thermodynamic equilibrium model, which has been widely used to calculate the thermodynamic equilibrium in the aerosol NH 4 + -SO 4 2− -NO 3 − -Cl − -Na + -Ca 2+ -K + -Mg 2+ -H 2 O system and the corresponding gas precursor [28].ISORROPIA II operates in two modes: forward mode and reverse mode.In the forward mode, the input variables are temperature, RH, and the concentrations of TNH 3 (NH 3 and NH 4 + ), TNO 3 (HNO 3 and NO 3 − ), TCl (HCl and Cl − ), SO 4 2− , Na + , Ca 2+ , K + , and Mg 2+ .In the reverse mode, the input variables are ambient temperature, RH, and the concentrations of SO 4 2− , NO 3 − , Cl − , NH 4 + , Na + , Ca 2+ , K + , and Mg 2+ .The output of both modes is aerosol liquid water content (ALWC), hydrogen ion content, and the concentration of species in the gas and aerosol phases [29].The forward model predicts aerosol composition and gas-particle partitioning much more accurately than the reverse model, because the forward model uses gas and aerosol inputs, effectively limiting the impact of measurement errors [30].Therefore, our study chose to run the forward mode and assumed that the aerosol was metastable with no solid precipitation.Aerosol pH is defined as follows: where γ H + refers to the activity coefficient of hydrogen ion (assumed to be 1).H + aq (mol L −1 ) and H + air (ng m −3 ) are the concentrations of H + in aqueous particles and ambient air, respectively.ALWC (µg m −3 ) is the water uptake by inorganic species.
Overall, nitrate in the particle phase was underestimated when RH was lower than 40% since urban particles possibly dehydrate at RH below 40% [20].When RH is above 95%, ALWC may grow exponentially with RH, resulting in large uncertainties of pH [31].Therefore, samples collected at RH > 95% and RH < 40% were discarded in the study.

Long-Term Variations in the Nitrate-to-Sulfate Ratio
Figure 1 shows the decade changes in SNA (sulfate, nitrate, and ammonium) and corresponding gas precursors from 2010 to 2020.The annual variation in atmospheric SO 2 showed a distinct downward trend, from an average of 28.8 ± 10.8 µg m −3 in 2010 to 7.1 ± 2.8 µg m −3 in 2020.In contrast, the long-term trend in atmospheric NO 2 was relatively flat, with an average of 38.7 µg m −3 in 2010 and 36.1 µg m −3 in 2020.As a result, the atmospheric ratio of NO 2 /SO 2 increased from 1.26 in 2010 to 5.07 in 2020.Like their gas precursors, the sum concentration of sulfate plus nitrate decreased from approximately 20 µg m −3 to less than 12 µg m −3 , a decline of 40%.The lower SNA concentration in 2010 relative to the following years might be attributed to different sampling locations.The mass ratio of nitrate to sulfate (NO 3 − /SO 4 2− ) increased from 0.68 to 1.49, with a ratio below 1.0 before 2015.Ye et al. (2021) reported that NO 3 − /SO 4 2− in rainwater increased from approximately 0.3 to above 1.0 during the same period [32], consistent with the aerosol NO 3 − /SO 4 2− ratio.The lower ratio of NO 3 − /SO 4 2− in rainwater might be associated with the in-cloud reactions of SO 2 .It is worth noting that the ratio of NO 3 − /SO 4 2− was always less than that of NO 2 /SO 2 due to the longer atmospheric life of SO 2 .These variations can be attributable to the non-proportional reduction of SO 2 and NO x emissions.As illustrated in Figure S2, industrial SO 2 emissions in Shanghai were reduced from 2.2 × 10 5 tons in 2010 to 5.2 × 10 3 tons in 2020, indicating that the industrial desulfurization and clean energy substitution strategies have achieved great success.Since the reduction in industrial NOx emissions was almost offset by the increasing number of on-road vehicles, NOx pollution shifted from industry-dominated to vehicle-dominated in 2016.Generally, the total emissions of acidic pollutants were significantly reduced.Meanwhile, the concentration of Ca 2+ dropped by 76% during the observation period, indicating the significant achievement in the control of soil dust and the weakened capacity to neutralize atmospheric acidity.These findings suggest that aerosol acidity might not be significantly reduced in response to the control of SO 2 and NO x emissions.
following years might be attributed to different sampling locations.The mass ratio of nitrate to sulfate (NO3 − /SO4 2− ) increased from 0.68 to 1.49, with a ratio below 1.0 before 2015.Ye et al. (2021) reported that NO3 − /SO4 2− in rainwater increased from approximately 0.3 to above 1.0 during the same period [32], consistent with the aerosol NO3 − /SO4 2− ratio.The lower ratio of NO3 − /SO4 2− in rainwater might be associated with the in-cloud reactions of SO2.It is worth noting that the ratio of NO3 − /SO4 2− was always less than that of NO2/SO2 due to the longer atmospheric life of SO2.These variations can be attributable to the nonproportional reduction of SO2 and NOx emissions.As illustrated in Figure S2, industrial SO2 emissions in Shanghai were reduced from 2.2 × 10 5 tons in 2010 to 5.2 × 10 3 tons in 2020, indicating that the industrial desulfurization and clean energy substitution strategies have achieved great success.Since the reduction in industrial NOx emissions was almost offset by the increasing number of on-road vehicles, NOx pollution shifted from industry-dominated to vehicle-dominated in 2016.Generally, the total emissions of acidic pollutants were significantly reduced.Meanwhile, the concentration of Ca 2+ dropped by 76% during the observation period, indicating the significant achievement in the control of soil dust and the weakened capacity to neutralize atmospheric acidity.These findings suggest that aerosol acidity might not be significantly reduced in response to the control of SO2 and NOx emissions.

Annual Variations in Aerosol pH
Figure 2 shows the long-term trend in aerosol pH in Shanghai over the decade from 2010 to 2020.The aerosol acidity showed an overall enhanced trend, with the annual average pH decreasing from 3.27 in 2010 to 2.93 in 2020.We should point out that the potential contributions of secondary organic aerosols to water uptake and organic acids to H + are not considered in this study, contributing to uncertainty in aerosol pH assessment.In contrast to the atmospheric sulfate and nitrate decrease by 40% over the

Annual Variations in Aerosol pH
Figure 2 shows the long-term trend in aerosol pH in Shanghai over the decade from 2010 to 2020.The aerosol acidity showed an overall enhanced trend, with the annual average pH decreasing from 3.27 in 2010 to 2.93 in 2020.We should point out that the potential contributions of secondary organic aerosols to water uptake and organic acids to H + are not considered in this study, contributing to uncertainty in aerosol pH assessment.In contrast to the atmospheric sulfate and nitrate decrease by 40% over the decade, aerosol acidity increased by 0.34 units pH, supporting the idea that aerosol acidity responded nonlinearly to SO 2 and NO x emission control.Similar to our results, an annual pH decline rate of around 0.04 units was reported previously [25].The small discrepancies between the two studies could be attributed to different sampling locations and statistical methods.The pH of rainwater increased by 0.8 units during the same period [32], possibly because rain droplets can scavenge alkaline coarse particles.The median pH for each year was generally higher than the mean, indicating that the distribution of hourly pH was skewed to the lower pH direction.The changing trend in aerosol pH displays a two-stage shape, with a distinct decrease during 2010-2016 and a weaker variation from 2016 to 2020.Meanwhile, the concentration of Ca 2+ decreased from 0.85 µg m −3 to 0.13 µg m −3 from 2010 to 2016 while it remained around 0.1 µg m −3 from 2016 to 2020, indicating that the nonvolatile cations were important drivers affecting the decade aerosol pH variations.
decade, aerosol acidity increased by 0.34 units pH, supporting the idea that aerosol acidity responded nonlinearly to SO2 and NOx emission control.Similar to our results, an annual pH decline rate of around 0.04 units was reported previously [25].The small discrepancies between the two studies could be attributed to different sampling locations and statistical methods.The pH of rainwater increased by 0.8 units during the same period [32], possibly because rain droplets can scavenge alkaline coarse particles.The median pH for each year was generally higher than the mean, indicating that the distribution of hourly pH was skewed to the lower pH direction.The changing trend in aerosol pH displays a two-stage shape, with a distinct decrease during 2010-2016 and a weaker variation from 2016 to 2020.Meanwhile, the concentration of Ca 2+ decreased from 0.85 µg m −3 to 0.13 µg m −3 from 2010 to 2016 while it remained around 0.1 µg m −3 from 2016 to 2020, indicating that the nonvolatile cations were important drivers affecting the decade aerosol pH variations.Table 1 summarizes aerosol pH in different measurement sites over the world.The overall aerosol pH in Shanghai (3.1 ± 0.6) was comparable to Nanjing (3.3 ± 0.1) and Wuhan (3.0 ± 1.0) in the middle and lower reaches of the Yangtze River, but significantly lower than that of Beijing (4.2 ± 0.4) and Tianjin (4.9 ± 1.4) in North China.Intriguingly, these aerosols with high loadings of sulfate and nitrate were weakly acidic while the aerosols were highly acidic (pH ≈ 1.0) in less-polluted Alabama and Crete where sulfate and nitrate loadings were much lower, indicating that aerosol acidity was not necessarily consistent with the mass loadings of sulfate and nitrate.As shown in Table 1, aerosol pH was positively correlated with the concentration of atmospheric NH3, suggesting that ammonia was the major driving factor affecting global particle acidity distributions.Wang et al. (2016) attributed the difference between highly acidic London smog in 1952 and less acidic Beijing aerosols in 2012 to NH3 levels [33].The lower pH in Guangzhou than in Table 1 summarizes aerosol pH in different measurement sites over the world.The overall aerosol pH in Shanghai (3.1 ± 0.6) was comparable to Nanjing (3.3 ± 0.1) and Wuhan (3.0 ± 1.0) in the middle and lower reaches of the Yangtze River, but significantly lower than that of Beijing (4.2 ± 0.4) and Tianjin (4.9 ± 1.4) in North China.Intriguingly, these aerosols with high loadings of sulfate and nitrate were weakly acidic while the aerosols were highly acidic (pH ≈ 1.0) in less-polluted Alabama and Crete where sulfate and nitrate loadings were much lower, indicating that aerosol acidity was not necessarily consistent with the mass loadings of sulfate and nitrate.As shown in Table 1, aerosol pH was positively correlated with the concentration of atmospheric NH 3 , suggesting that ammonia was the major driving factor affecting global particle acidity distributions.Wang et al. (2016) attributed the difference between highly acidic London smog in 1952 and less acidic Beijing aerosols in 2012 to NH 3 levels [33].The lower pH in Guangzhou than in Shanghai might be attributed to higher temperatures.These findings indicated that aerosol pH was much more sensitive to the amount of atmospheric NH 3 available for neutralizing acidic sulfate and nitrate.In addition, nonvolatile cations such as Na + , K + , Ca 2+ , and Mg 2+ could affect aerosol acidity when their concentrations were significant relative to anions [34].To validate the central role of NH 3 , aerosol pH in Pasadena and Beijing was re-calculated by inputting the average NH 3 concentration in Shanghai (5.4 µg m −3 ).The re-calculated pH increased from 2.7 to 4.0 in Pasadena as the NH 3 concentration increased from 0.8 to 5.4 µg m −3 and decreased from 4.2 to 3.5 in Beijing as the NH 3 concentration decreased from 18.3 to 5.4 µg m −3 , confirming that both ammonia and alkaline nonvolatile cations were important buffers of aerosol acidity.

Seasonal Variations in Aerosol pH
Figure 3 illustrates the long-trend in seasonal pH values from 2010 to 2020.The monthly variation in pH values exhibited a V-shape trend, decreasing from February and increasing from September.Aerosol pH in August was the lowest (with an average of less than 2.5) and the fluctuation range was the largest.From a seasonal perspective, aerosol pH followed the order of winter > spring > autumn > summer, indicating that ambient temperature was an important driver for pH variation.In this study, the seasons from spring to winter are defined as the period from March to May, June to August, September to November, and December to February, respectively.Similar seasonal pH trends were reported previously [17,22,36,38,43].As indicated by Equation (1), aerosol pH depends on H + air and ALWC, which are functions of pollutant concentrations, temperature, and RH.Elevated ALWC by increasing RH can dilute H + aq and increase pH as well as promoting nitrate formation via nighttime N 2 O 5 hydrolysis.The approximately one-unit difference in pH between winter and summer was mainly attributed to aerosol compositions because a doubling ALWC in winter increases aerosol pH by only 0.3 units [20].Although temperature is not a parameter in Equation (1), it greatly affects the photooxidation rate of SO 2 and NOx and the gas-particle partitioning of semi-volatile NH 4 NO 3 and NH 4 Cl, with an indirect effect on aerosol acidity.During 2010-2015, the seasonal average pH showed a steady downward trend in winter while remaining stable in summer and autumn, resulting in a decline in the seasonal pH difference from 1.14 to 0.72.Interestingly, the seasonal difference in pH returned to approximately 1.0 in 2016-2020.In addition, the seasonal average pH was unexpectedly high in the summer and autumn of 2015.The mechanisms driving seasonal pH variations will be discussed in the next sections.

Diurnal Variation in Aerosol pH
Figure 4 illustrates the diurnal variation in aerosol pH along with ALWC and the mass ratio of NO 3 − /SO 4 2− .Overall, aerosol pH began to decrease in the early morning, reaching the minimum values at noon, followed by a continuous increase in the afternoon and high values at night, further supporting the temperature dependence on aerosol acidity.In summer, the average pH values at night were approximately 0.4-0.7 units larger than those at noon.With the strongest acidity, the aerosol noon minimum dropped to nearly 2.1 in 2016.Similar to summer, aerosol pH in winter showed a V-shaped diurnal trend but the diurnal range was much narrower.In 2020, the daily variation in aerosol pH narrowed to below 0.3 units.Contrary to our results, the diurnal variation in aerosol pH over the southeastern United States was approximately one unit larger in winter than in summer [38], indicating that the diurnal pH trends are highly related to the regional pollution background.

Diurnal Variation in Aerosol pH
Figure 4 illustrates the diurnal variation in aerosol pH along with ALWC and mass ratio of NO3 − /SO4 2− .Overall, aerosol pH began to decrease in the early morn reaching the minimum values at noon, followed by a continuous increase in the aftern and high values at night, further supporting the temperature dependence on aer acidity.In summer, the average pH values at night were approximately 0.4-0.7 u larger than those at noon.With the strongest acidity, the aerosol noon minimum drop to nearly 2.1 in 2016.Similar to summer, aerosol pH in winter showed a V-shaped diu trend but the diurnal range was much narrower.In 2020, the daily variation in aerosol narrowed to below 0.3 units.Contrary to our results, the diurnal variation in aerosol over the southeastern United States was approximately one unit larger in winter tha summer [38], indicating that the diurnal pH trends are highly related to the regio pollution background.Both ALWC and NO3 − /SO4 2− followed a similar diurnal pattern as pH, indicating that they provided an additive effect on pH variation.As ALWC is a function of RH and the total amount of hygroscopic aerosols, ALWC generally peaked in the early morning and rapidly decreased after sunrise.ALWC at noon was reduced by 50-70% relative to the early morning, corresponding to a pH increase of 0.3-0.5 units, indicating that ALWC was Both ALWC and NO 3 − /SO 4 2− followed a similar diurnal pattern as pH, indicating that they provided an additive effect on pH variation.As ALWC is a function of RH and the total amount of hygroscopic aerosols, ALWC generally peaked in the early morning and rapidly decreased after sunrise.ALWC at noon was reduced by 50-70% relative to the early morning, corresponding to a pH increase of 0.3-0.5 units, indicating that ALWC was the main driver for the strong diurnal pH variation.The flat diurnal pattern of the NO 3 − /SO 4 2− ratio in 2010 indicates an insignificant effect of chemical composition on the diurnal trend in pH, providing a reasonable explanation for the narrowest diurnal variation in pH.
From the perspective of aerosol composition, sulfate and nitrate are regarded as the major driving factors of diurnal patterns in pH [38].When sulfate and nitrate are neutralized primarily by ammonium, the concentration of H + is controlled primarily by the thermodynamic equilibrium between particle-phase NH 4 + and gaseous NH 3 .As (NH 4 ) 2 SO 4 is less volatile than NH 4 NO 3 , the elevated sulfate leads to a much greater increase in H + air than that of TNO 3 .NH 4 NO 3 tended to evaporate with increasing temperature and decreasing ALWC during the daytime.In contrast, uptake on humid particles was favored at night due to its dissociation being highly sensitive to temperature and RH changes, resulting in a V-shaped diurnal pattern.The NO 3 − /SO 4 2− ratio in winter was significantly larger than in summer, which can be partly responsible for the weaker acidity of winter aerosols.

Effect of Alkaline Buffers on Interannual pH Variations
As aforementioned, ammonia is the major alkaline buffer for aerosol pH globally while nonvolatile cations exhibit a non-negligible impact on aerosol pH when their concentrations become significant.In this study, temperature, relative humidity, and concentrations of chemical composition were averaged for each summer and winter (Table S1). Figure 5 illustrates the aerosol pH predicted by the average concentrations and the buffer effect of NH 3 and nonvolatile cations on summer and winter aerosols.The aerosol pH values predicted by the averaged concentrations of chemical composition and meteorological parameters are close to those averaged by hourly pH values, indicating that they can represent the typical pH trends.The inserted images represent the predicted aerosol pH values by setting the concentration of NH 3 to zero while the other variables are fixed at their average values.It is worth noting that aerosol pH would be below zero in the absence of NH 3 , emphasizing the decisive role of NH 3 in buffering particle acidity.Similar to NH 3 , aerosol pH is predicted by inputting the concentrations of Ca 2+ and K + as 0 while other variables are fixed (Table S2).The buffering effects of Ca 2+ and K + , ∆pH, are obtained by comparing the pH decline when the concentrations of Ca 2+ and K + are set to 0. Ca 2+ and K + are tracers of crustal dust and biomass burning, respectively.Coal combustion is another important source of K + in many Chinese cities [20].The buffing effect of Ca 2+ was more significant than K + before 2013, but K + remained higher in 2014-2015.The combined buffing effect of Ca 2+ and K + decreased by 0.4 units of pH during the winter of 2010-2016, corresponding to the aerosol pH decline during this period, indicating that the reduction in acidic pollutant emissions was largely offset by the reduction in alkaline buffers.During 2016-2020, the buffing effect of nonvolatile cations could be ignored in summer while the buffing effect of K + remained at approximately 0.4 units pH in winter.Long-range pollutant transport from north China contributes greatly to the increase in K + concentration and PM 2.5 pollution in Shanghai because the prevailing wind direction in Shanghai varies from northwest to northeast in the winter [26,44].The increase in K + in winter was possibly related to the transportation of biomass burning and coal combustion pollutants under the regime of prevailing winds from the north.In contrast to aerosol pH below zero in the absence of NH 3 , the largest buffering effect of nonvolatile cations was less than 0.6 units pH, emphasizing the decisive role of NH 3 in buffering particle acidity.
wind direction in Shanghai varies from northwest to northeast in the winter [26,44].Th increase in K + in winter was possibly related to the transportation of biomass burning an coal combustion pollutants under the regime of prevailing winds from the north.contrast to aerosol pH below zero in the absence of NH3, the largest buffering effect nonvolatile cations was less than 0.6 units pH, emphasizing the decisive role of NH3 buffering particle acidity.

Effects of Meteorological Parameters and Chemical Composition on Diurnal pH Variations
The contributions of individual driving factors to diurnal pH variations are estimate in Figure 6 and Tables S4-S7.Similar to the above section, the effect of a driving factor o aerosol pH is evaluated by replacing this factor at 10:00, 14:00, 20:00, and 24:00 with th value at 5:00 in the morning.The reference point is set at 5:00 when aerosol pH w generally the highest.In both summer and winter, the temperature during the day w higher than that at night, while the humidity was just the opposite.The increase 2 0 1 3 2 0 1 4 2 0 1 5 2 0 1 6 2 0 1 7 2 0 1 8 2 0 1 9 2 0 2

Effects of Meteorological Parameters and Chemical Composition on Diurnal pH Variations
The contributions of individual driving factors to diurnal pH variations are estimated in Figure 6 and Tables S4-S7.Similar to the above section, the effect of a driving factor on aerosol pH is evaluated by replacing this factor at 10:00, 14:00, 20:00, and 24:00 with the value at 5:00 in the morning.The reference point is set at 5:00 when aerosol pH was generally the highest.In both summer and winter, the temperature during the day was higher than that at night, while the humidity was just the opposite.The increase in temperature can reduce aerosol pH by partitioning aerosol NH 4 NO 3 and NH 4 Cl into the gas phase, leading to decreases in NO 3 − /SO 4 2− and pH [41].The effect of RH on aerosol pH is more complicated.On the one hand, elevated RH can enhance ALWC which dilutes the ionic concentration.On the other hand, the increase in ALWC favors more gaseous NH 3 and HNO 3 partitioning into the particle phase.The diurnal pH variation in summer was mainly driven by temperature and RH.Only a 7 • C increase in temperature is required for a 0.5 unit drop in pH, which is lower than that in Canada [17], possibly due to the higher mass loading of secondary inorganic aerosols in our study.As shown in Tables S4 and S6, the narrow fluctuations of dew point temperature (T d ) indicate that the diurnal RH variations were mainly driven by temperature, further highlighting the important role of temperature in the diurnal variations in aerosol pH.The effects of SO 4 2− , TNO 3, and TNH 3 were much weaker than those of temperature and RH in summer.It is worth noting that both elevated SO 4 2− and decreased NO 3 − in the daytime contributed to the decreasing aerosol pH.In contrast, the main drivers of diurnal pH variation in winter were temperature and SO 4 2− .Another significant difference from summer was that the decreasing RH could enhance aerosol pH in winter.It is worth noting that RH played a minor role in diurnal pH patterns in winter.In summary, the diurnal variation in aerosol pH in summer is mainly affected by temperature and RH, while the diurnal variation in aerosol pH in winter is sensitive to both meteorological parameters and aerosol chemical composition.
For an in-depth understanding of the effect of meteorological parameters and chemical composition on aerosol pH, sensitivity tests were performed based on two cases of summer and winter aerosols (Figure 7).The concentration of NH 4 + decreased nonlinearly with the increase in temperature because the concentration product of NH 3 and HNO 3 is an exponential function of temperature [4].Aerosol pH almost linearly decreased with the increase in temperature since the concentration of H + in ambient air H + air ) almost exponentially increases with the increasing temperature.Although ALWC always increased with the increase in RH, H + air displayed different trends between summer and winter.In summer, H + air first decreased with the increase in RH which favors more gaseous NH 3 partitioning into the aerosol water.In contrast, H + air in winter increased with the increase in RH, leading to a slight pH decrease at RH < 80%.As shown in Figure 7e,f, the concentration of NH 4 + increased linearly with the increase of SO 4 2− due to the higher affinity of H 2 SO 4 to NH 3 .However, the hygroscopic growth of (NH 4 ) 2 SO 4 was lower than that of NH 4 NO 3 , indicating that aerosol pH decreased with the decrease of NO 3 − /SO 4 2− .In contrast, the concentration of aerosol NH 4 + only slightly increased with the increasing TNO 3 in summer due to most of the TNO 3 partition to the gas phase.Aerosol pH increased with the increase in TNO 3 due to the increase in NO 3 − /SO 4 2− .In winter, aerosol pH increased slightly with the increase in TNO 3 for NO 3 − /SO 4 2− < 1.5.However, aerosol pH slightly decreased with the increase in TNO 3 for higher NO 3 − /SO 4 2− .An observation in Beijing showed that PM 2.5 pH increased with increasing NO 3 − /SO 4 2− [21], while another observation found that PM 2.5 pH decreased with increasing TNO 3 [22].This finding suggests that the impact of NO 3 − /SO 4 2− on pH depends on the pollution background, providing a reasonable explanation for the different trends observed in Beijing.Our results suggest that reducing NOx emissions in Shanghai will not significantly affect PM 2.5 acidity in winter.
gas phase, leading to decreases in NO3 − /SO4 2− and pH [41].The effect of RH on aerosol pH is more complicated.On the one hand, elevated RH can enhance ALWC which dilutes the ionic concentration.On the other hand, the increase in ALWC favors more gaseous NH3 and HNO3 partitioning into the particle phase.The diurnal pH variation in summer was mainly driven by temperature and RH.Only a 7 °C increase in temperature is required for a 0.5 unit drop in pH, which is lower than that in Canada [17], possibly due to the higher mass loading of secondary inorganic aerosols in our study.As shown in Tables S4  and S6, the narrow fluctuations of dew point temperature (Td) indicate that the diurnal RH variations were mainly driven by temperature, further highlighting the important role of temperature in the diurnal variations in aerosol pH.The effects of SO4 2− , TNO3, and TNH3 were much weaker than those of temperature and RH in summer.It is worth noting that both elevated SO4 2− and decreased NO3 − in the daytime contributed to the decreasing aerosol pH.In contrast, the main drivers of diurnal pH variation in winter were temperature and SO4 2− .Another significant difference from summer was that the decreasing RH could enhance aerosol pH in winter.It is worth noting that RH played a minor role in diurnal pH patterns in winter.In summary, the diurnal variation in aerosol pH in summer is mainly affected by temperature and RH, while the diurnal variation in aerosol pH in winter is sensitive to both meteorological parameters and aerosol chemical composition.), SO4 2− (10.17 µg m −3 ), NH4 + (13.32 µg m −3 ), NO3 − (6.77 µg m −3 ), Cl − (1.59 µg m −3 ), Ca 2+ (0.81µg m −3 ), K + (0.31µg m −3 ), Mg 2+ (0.09 µg m −3 ), RH (0.64), T (303.2K).The winter sample: Na + (0.3 µg m −3 ), SO4 2− (5.72 µg m −3 ), NH4 + (9.89 µg m −3 ), NO3 − (11.39 µg m −3 ), Cl − (1.19 µg m −3 ), Ca 2+ (0.14 µg m −3 ), K + (0.22 µg m −3 ), Mg 2+ (0.05 µg m −3 ), RH (0.65), T (284.7 K).

Conclusions
This study investigated the long-term trends in PM2.5 pH in response to emission control in Shanghai.The annual average ratio of NO3 − /SO4 2− increased from 0.68 in 2010 to 1.49 in 2020, attributable to the significant reduction in SO2 emissions and the less effective control of NOx emissions.PM2.5 acidity showed a slightly increasing trend since the reduction in acidic emissions was partly offset by the decrease in alkaline nonvolatile

Conclusions
This study investigated the long-term trends in PM 2.5 pH in response to emission control in Shanghai.The annual average ratio of NO 3 − /SO 4 2− increased from 0.68 in 2010 to 1.49 in 2020, attributable to the significant reduction in SO 2 emissions and the less effective control of NOx emissions.PM 2.5 acidity showed a slightly increasing trend since the reduction in acidic emissions was partly offset by the decrease in alkaline nonvolatile cations.The monthly variation in pH values exhibited a V-shape trend, decreasing from February and increasing from September, mainly attributed to aerosol compositions and temperature which controls the partitioning of HNO 3 /NH 4 NO 3 .The diurnal pH pattern showed a V-shaped trend with stronger fluctuation in summer than in winter due to diurnal variations in ALWC and NO 3 − /SO 4 2− .Atmospheric NH 3 plays a decisive role in buffering particle acidity, providing a plausible explanation on moderately acidic aerosols in the Yangtze River Delta, highly polluted with NOx emissions.Ca 2+ and K + were important buffers of particle acidity and the reduction in Ca 2+ and K + was responsible for the pH decline during 2010-2016.
The diurnal pH variations in summer were mainly affected by temperature and RH.The diurnal RH variations were mainly driven by temperature, underlying the decisive role of temperature in the diurnal variations in aerosol pH.In contrast, the dominant drivers of

Figure 1 .
Figure 1.Annual average concentrations of major ions and acidic gas pollutants during 2010-2020.

2 Figure 1 .
Figure 1.Annual average concentrations of major ions and acidic gas pollutants during 2010-2020.

Figure 2 .
Figure 2. Annual variation in aerosol pH during 2010-2020 in Shanghai.The shaded area shows the range of 25% to 75% of the hourly observational data.

Figure 2 .
Figure 2. Annual variation in aerosol pH during 2010-2020 in Shanghai.The shaded area shows the range of 25% to 75% of the hourly observational data.

Figure 3 .
Figure 3. Seasonal variation in aerosol pH during 2010-2020 in Shanghai.(a) The box spans range from the 25th to the 75th percentiles and the whiskers denote the 5th and 95th percen The red circle in the box represents the average.(b) The error bar represents one standard devia

Figure 3 .
Figure 3. Seasonal variation in aerosol pH during 2010-2020 in Shanghai.(a) The box spans the range from the 25th to the 75th percentiles and the whiskers denote the 5th and 95th percentiles.The red circle in the box represents the average.(b) The error bar represents one standard deviation.Atmosphere 2024, 15, 1004 8 of 15

Figure 5 .
Figure 5. Contribution of alkaline buffers to aerosol acidity.The black line represents the predicte aerosol pH based on the annual average parameters.The column chart illustrates the buffer effe from Ca 2+ and K + .The inserted image represents the predicted pH by setting the concentration NH3 to zero.

Figure 5 .
Figure 5. Contribution of alkaline buffers to aerosol acidity.The black line represents the predicted aerosol pH based on the annual average parameters.The column chart illustrates the buffer effect from Ca 2+ and K + .The inserted image represents the predicted pH by setting the concentration of NH 3 to zero.

Figure 6 .Figure 6 .Figure 7 .
Figure 6.Contributions of chemical composition and meteorological parameters to the diurnal pH variation.The blue scatter line represents the diurnal pH variation with pH at 5:00 am as the baseline.The stacked column chart represents the contribution of each driving factor to ΔpH.
• E, 31.30• N), and the samples from 2014 to 2021 were collected in the Pudong New Area Environmental Monitoring Station (121.54 • E, 31.

Table 1 .
Concentrations of major inorganic ions and aerosol pH in different measurement sites worldwide (unit: µg m −3 ).