Significant contrasts in aerosol acidity between China and the United States

Aerosol acidity governs several key processes in aerosol physics and chemistry, thus affecting aerosol mass and composition and ultimately climate and human health. Previous studies have reported aerosol pH values separately in China and the United States (USA), implying different aerosol acidity between these two countries. However, there is debate about whether mass concentration or chemical composition is the more important driver of differences in aerosol acidity. A full picture of the pH difference and the underlying mechanisms responsible is hindered by the scarcity of simultaneous measurements of particle composition and gaseous species, especially in China. Here we conduct a comprehensive assessment of aerosol acidity in China and the USA using extended ground-level measurements and regional chemical transport model simulations. We show that aerosols in China are significantly less acidic than in the USA, with pH values 1–2 units higher. Based on a proposed multivariable Taylor series method and a series of sensitivity tests, we identify major factors leading to the pH difference. Compared to the USA, China has much higher aerosol mass concentrations (gas + particle, by a factor of 8.4 on average) and a higher fraction of total ammonia (gas + particle) in the aerosol composition. Our assessment shows that the differences in mass concentrations and chemical composition play equally important roles in driving the aerosol pH difference between China and the USA – increasing the aerosol mass concentrations (by a factor of 8.4) but keeping the relative component contributions the same in the USA as the level in China increases the aerosol pH by ~1.0 units and further shifting the chemical composition from US conditions to China’s that are richer in ammonia increases the aerosol pH by ~0.9 units. Therefore, China being both more polluted than the USA and richer in ammonia explains the aerosol pH difference. The difference in aerosol acidity highlighted in the present study implies potential differences in formation mechanisms, physicochemical properties, and toxicity of aerosol particles in these two countries.


Supplementary Information
Text. S1 Effects of ammonium on aerosol pH The result of MTSM indicates that the difference in TNH3 is one of the predominant reasons causing the pH difference. In order to study the effect of TNH3, we conduct sensitivity tests for China and the US separately to investigate the responses of aerosol pH to changing TNH3. We change the TNH3 concentrations from 0.1 to 1000 µg m -3 while keep all other components constant at their annual average levels based on observation data (Table 2). We also use simulation data with population as the weight to study the effects, which consider other areas in China where TNH3 concentration is not as high as in NCP. The results are shown in Fig. S10. It is clearly illustrated that, over a large range of TNH3 concentrations, aerosol pH increases with the increase in TNH3 because the production process of NH4 + from NH3 consumes aqueous H + . The local sensitivity of pH to TNH3, expressed as the pH increase per tenfold increase in TNH3 at current TNH3 level, is higher in the US (3.0 based on observational data and 1.6 based on simulation data) than in China (0.4 based on observational data and 1.2 based on simulation data), indicated a higher sensitivity of aerosol pH to TNH3 in the US than in China. Besides, we find that the responses of pH to TNH3 are nonlinear and anisotropic. With all others equal, pH in the US could be closer to the level in China if the TNH3 increases to the level in China. On the other hand, the pH in China would be lower than the US if the TNH3 decreases to the US level because of the relative higher abundances of acidic components (SO4, TNO3, TCl) than basic ions (TNH3, NVCs) (Fig. S10a). In both countries, the sensitivities would quickly diverge from the original values toward higher values as TNH3 decreases, with the sensitivities in China changing at a faster pace. As TNH3 increases, however, the sensitivities in these two countries would gradually become constant, stabilizing at comparable levels (0.002 pH unit per TNH3 increase in both two countries). Results based on simulation data are similar with results based on observational data, especially the sensitivity of aerosol pH at high level of TNH3 (represented by similar slope). Higher pH values in China based on simulation data at low TNH3 level (1-10 μg m -3 ) could be caused by lower SO4 2concentrations. However, lower value of aerosol pH at high level of TNH3 (> 50 μg m -3 ) based on simulation data even with lower SO4 2concentrations indicates the limit effect of TNH3 at this level and potential effect of other components.
The effects of TNH3 on the gas-particle partitioning of NH3-NH4 + and HNO3-NO3are illustrated in Fig. S10b and S10c, showing a decreasing trend of ε(NH4 + ) and an increasing trend of ε(NO3 -) as TNH3 increases. In the range of observation cases the value of ε(NH4 + ) in China is smaller than in the US, suggesting excess presence of TNH3 compared to other aerosol components (e.g., TNO3 and SO4 2-). The gas to particle partitioning of NH3 produces inorganic ammonium salt of ammonium bisulfate (NH4HSO4) and ammonium sulfate ((NH4)2SO4) first because the affinity of sulfuric acid for NH3 is much larger than that of nitric and hydrochloric acid for NH3, especially when TNH3 concentration is relatively low (Behera et al., 2013); The excess TNH3 may also react with nitric acid and hydrochloric acid to form salt of NH4NO3 and NH4Cl which will dissolve in the aerosol liquid water (Zhao et al., 2016). That is why the increase of ε(NO3 -) is small at the beginning and gradually become faster later. At the same level of TNH3, ε(NH4 + ) in China is higher than ε(NH4 + ) in the US, causing by formation of NH4NO3 due to higher level of TNO3. Both the lower ε(NH4 + ) and higher ε(NO3 -) in China estimated by the sensitivity curves are consistent with observations. is enough to balance particle phase anions. The distribution of pH values in three groups in two countries is shown in Figure   S11 by boxplots. Note that no data in the US fall in Group C, making up only two groups in Figure S11 (b). Overall, cases in groups B and C with higher relative abundance of ammonia are more likely to have higher aerosol pH than in group A, though the relationship is not linear and does not happen in all the cases. Note that although the relative abundance of NH4 + in group B is smaller than in group C, the transition from group B to group C due to TNH3 increase does not always happen.
and more TNO3 and TCl would shift into the particle phase, leading to the increase of WSI concentration. However, the average WSI concentration in group B is 55.03±46.79 µg m -3 in China, significantly higher than that in group C in China (31.60±20.29 µg m -3 ). LWC in group B (22.90±7.38 µg m -3 ) is also higher than that in group C (14.37±16.85 µg m -3 ). We find that most of the cases in group B could be identified as highly polluted cases where large amount of NH4NO3 is formed and dissolves in the aerosol water.
Throughout the observed cases, 85% in China are in Group C (i.e., aerosol systems with excess NH4 + ), and 55% in the US are in Group A (i.e., aerosol systems with insufficient NH4 + ). Overall, the positive sensitivity of pH to TNH3 and the different dominant groups in these two countries (Group C in China, Group A in the US) suggest that the high abundance of TNH3 in China increases the aerosol pH and is one of the major reasons for the pH difference between the two countries.

Text. S2 The relationship between sulfate/nitrate and aerosol pH
Besides the effect of TNH3 discussed in Text. S1, other species, especially the acidic species which mainly include SO4 and TNO3, could also affect aerosol pH because of their effects on H + air concentration as well as on LWC (Ding et al., 2019). This effect is investigated in a sensitivity test by changing TNO3 or SO4 concentration while keeping other inputs constant as the average levels (Fig. S12). Similar to the MSTM results as shown in Fig. 6, elevated SO4 2significantly increases aerosol acidity by increasing H + air. On the other hand, elevated TNO3 only slightly increases H + air, indicating a weaker acidity than that of SO4 2-, in line with the result in a previous study (Guo et al., 2017b). This is partially due to the semi-volatile property of TNO3 (Ding et al., 2019). Notably, even in China where ε(NO3 -) are mostly close to 1, the variation of aerosol pH with TNO3 (roughly equals to NO3in this case) is also subtle. Therefore, for two systems with different moles of SO4 2and NO3neutralized by same moles of NH4 + , the system with more SO4 2will likely have a lower pH. This result indicates that higher aerosol acidity is associated with higher availability of SO4 2rather than TNO3, which can be confirmed by observed data in Based on observation data, 74.5% of the cases in China have NO3 -/SO4 2molar ratio larger than one, while only 22.3% in the United States. The different NO3 -/SO4 2ratios, could subsequently affect other aerosol properties, such as aerosol water uptake ability, which is one of the important reasons causing haze events in China during wintertime (Xie et al., 2019;Wang et al., 2020). Although nitrate aerosol and sulfate aerosol absorb similar amounts of water per mass, heavy haze events in China are usually associated with increased LWC with enhanced RH levels under nitrate-dominate condition .
In order to study this effect, we categorize the observation data into a nitrate-rich group (Group N, and a sulfate-rich group (Group S, where [NO3 -]/[SO4 2-] < 1) and compare these two groups under different RH conditions. The ratio 3 in group N is mentioned in lab studies and is a more typical value of nitrate-rich conditions in field observations (Ge et al., 1998;Xie et al., 2020).
The results in Fig. S14 show that aerosol pH values in the same groups in China and the US have similar responses to the changes in RH. In both countries, as RH increases, the pH in group N decreases, and the pH in group S increases (Fig. S14a).
Both the values and the increasing rate of LWC in group N is larger than in group S, suggesting a higher water uptake ability in nitrate-rich condition, which is likely due to higher aerosol mass compared with group S as shown in Fig. S14f. The nearly two times aerosol mass in group N as in group S indicates the co-condensation effect of nitrate aerosol and LWC (Guo et al., 2017a), which suggests that NO3formed in aerosol leads to a higher LWC due to the increase in aerosol mass, while higher LWC dilutes H + air and increases pH, which is favorable for more HNO3 shifting from gas phase to particle phase and thus continually increases particle NO3concentration. This effect will reach a balance when most of the gas phase HNO3 is in the particle phase with enough NH4 + , and, therefore, ε(NO3 -) is close to 100% in group N in the two countries (Fig. S14e).
Besides, water uptake by hygroscopic aerosols increases the aerosol surface area and volume, enhancing the hydrolysis of N2O5 across particles and forming NO3 - (Tian et al., 2018;Wang et al., 2020).
The condition in group N usually has a higher LWC and aerosol mass, due to the mutual promotion between LWC and particle nitrate. And such a condition in group N occurs more often in China than in the United States, which is probably one of the reasons leading to high particle concentrations on hazy days in China.
The nitrate/sulfate ratio depends on the emission ratio of NOx/SO2, the availability of cations due to the dependency of ε(NO3 -) on TNH3 (Fig. S10c), and other factors such as the atmospheric oxidizing capacity. Further investigation into the total emissions shows that the emission molar ratios of [NOx]/[SO2] are close to 3:1 in both countries (2.92 In China in 2017 and 3.12 in the US in 2011 when assuming the emission NOx is in the form of NO2), indicating that the emission difference is not the major factor leading to the nitrate/sulfate ratio difference. On the other hand, the emission molar ratio of [NH3]/([NOx]+2×[SO2]) in China (0.75) is 1.6 times higher than that in the US (0.46), which is consistent with the measured high relative abundance of TNH3 in China and confirms that high availability of cations (mainly NH4 + caused by high NH3 emission) is one of the causes for the high nitrate/sulfate ratio in China.

Fig. S1 Comparison of results in pH calculation when using different methods to estimate HCl concentration in the United States.
Group "Cl only" means using particle Clconcentration as total Cl and ignore gas phase HCl; group "5Cl" means assuming total Cl equals to 5 times particle Clconcentration therefore HCl concentration equals to 4 times particle Clconcentration; group "Cl ratio" means using measured particle Clconcentration divided by CMAQ simulation partitioning ratio to estimate the amount of total Cl. The result showed the three methods will lead to essentially the same pH at most of the monitoring sites.    (panels b, d, f, h, j). The error bars represent the standard deviation of all the cases in each month.

Fig. S7
Step-specific contributions of individual factors to the pH difference between China and the US.  Table. S4, sensitivity test.   (Zheng et al., 2020). For individual factors, the sum of the contributions through the two pathways yields the net contribution of this factor to aerosol pH. The case in the United States is chosen as the starting point, and China as the ending point.       Table S4. Summary of the inputs of Multivariable Taylor Series Method (MTSM) calculation. The unit of concentrations is μg m -3 , the RH is a relative number with no unit, and the unite of temperature is K. The values in "observation" group are the average values based on observation data, the values in "simulation group" are the average values based on CMAQ simulation data and the "Simulation, population-weighted" group is the population-weighted values based on CMAQ simulation data.