Role of Carbon Dioxide, Ammonia, and Organic Acids in Buffering Atmospheric Acidity: The Distinct Contribution in Clouds and Aerosols

Acidity is one central parameter in atmospheric multiphase reactions, influencing aerosol formation and its effects on climate, health, and ecosystems. Weak acids and bases, mainly CO2, NH3, and organic acids, are long considered to play a role in regulating atmospheric acidity. However, unlike strong acids and bases, their importance and influencing mechanisms in a given aerosol or cloud droplet system remain to be clarified. Here, we investigate this issue with new insights provided by recent advances in the field, in particular, the multiphase buffer theory. We show that, in general, aerosol acidity is primarily buffered by NH3, with a negligible contribution from CO2 and a potential contribution from organic acids under certain conditions. For fogs, clouds, and rains, CO2, organic acids, and NH3 may all provide certain buffering under higher pH levels (pH > ∼4). Despite the 104to 107 lower abundance of NH3 and organic weak acids, their buffering effect can still be comparable to that of CO2. This is because the cloud pH is at the very far end of the CO2 multiphase buffering range. This Perspective highlights the need for more comprehensive field observations under different conditions and further studies in the interactions among organic acids, acidity, and cloud chemistry.


INTRODUCTION
−12 Understanding the key influencing factors is thus crucial for accurate predictions of the acidity and efficiency of multiphase reactions.
−19 Later studies, however, find that the acidity can vary much at given ratios of acids to bases, 20 due to the large variations in the efficiency of these species in influencing the acidity, depending on their properties and environmental conditions.Here, the efficiency refers to the fraction of dissociated aqueous-phase anions/cations in atmospheric water that one species can contribute at given total (gas + particle phase) concentrations.For species associated with nonvolatile strong monoacids or bases (e.g., Na + , K + ), the mechanism is the simplest, i.e., merely through neutralization reactions, and the efficiency can be considered as one.For nonvolatile weak acids or bases, they can influence the acidity through neutralization reactions and buffering effects.For semivolatile acids or bases, the mechanisms are more complex, where both gas−particle partitioning and aqueous-phase dissociation play a role in determining their efficiencies.Moreover, the nonideality, precipitation equilibrium, etc. would also influence the final acidity, especially in aerosol water.
Atmospheric weak acids and bases, mainly the CO 2 / HCO 3 − /CO 3 2− system, organic acids, and ammonia, are long considered to play a role in regulating atmospheric acidity.Their quantified importance and major influencing mechanisms, however, seem confusing at a glance.For example, CO 2 determines the pH of "pure" raindrops (∼5.6, namely, the pH when the water is in equilibrium with gas-phase CO 2 mixing ratios of ∼350 ppm; see detailed processes in ref 2), while its influence is often neglected in aerosol acidity calculations.In addition, while some studies 6,21−29 suggest that carbonates and organic weak acids (e.g., formic acid, acetic acid, and oxalic acid) can "buffer" the aerosol acidity, the recently proposed multiphase buffer theory 7 suggests that this effect is usually negligible compared with the ammonia multiphase buffering. 7,30In comparison, while the importance of ammonia in buffering the aerosols was well illustrated recently, 7 its role in fogs/clouds is less understood.How and to what extent these species influence atmospheric acidity need to be clarified.
Here, we explored this issue with the recent research advances, especially the multiphase buffer theory. 7The importance and mechanisms of CO 2 , organic acids, and ammonia are discussed for different types of atmospheric water (aerosol water, fogs, and cloud droplets), and key uncertainties and future studies needed are also discussed.

IDENTIFYING SIGNIFICANT CONTRIBUTORS TO THE SYSTEM BUFFERING EFFECT
The acidity of aerosols at various locations worldwide is buffered, and ammonia is proposed to be the dominant buffering species. 7,13As illustrated in Figure 1a, the aerosol pH shows little decrease until the added acid reaches a certain amount (molar ratios of equivalent added H + reaches ∼20% of the initial amount of anions), which is in sharp contrast with the behavior of nonbuffered system like the Na 2 SO 4 solutions.
The buffering effect is the process when the buffer agents, namely, the conjugate acid/base pairs that differ only by one proton, partially absorb the added H + or OH − through dissociation equilibrium.Take a weak acid HA with the acid dissociation constant K a for illustration, upon the addition of strong acids/bases, it can buffer the pH changes through which is a reversible reaction, and at equilibrium, the system should satisfy Note that the buffering effect is significant only within the buffering range, i.e., a certain pH range around pK a .Outside these buffer pH ranges, the buffer agent exists predominantly either as [A − ] or [HA], and the buffering effect is negligible, as detailed below.
2.1.Bulk Aqueous Solutions.The influence of buffering effects on acidity can be characterized by the buffering capacity β, which represents the resistance of pH changes upon addition of strong acids/bases, i.e., where n acid and n base refer to the amount of added strong acids or bases.For bulk aqueous systems, that is (see detailed derivation processes in refs 7 and 31) where K w is the water dissociation constant, and K a,i and [X i ] tot represent the acid dissociation constant and total molality of the buffering agent X i (i.e., [HA]+[A − ] for weak acids), respectively.Note that for nonbuffered aerosol systems, X i = 0, and β is where the remaining terms represent the water self-buffering effect. 7igure 1b shows the difference in the titration process between a buffered and a nonbuffered bulk aqueous system.For illustration, here we show the titration curve of adding a strong base (e.g., NaOH) into the solution with 1 mol kg −1 of (i) strong acid like HCl (the nonbuffered system, dashed gray line) or (ii) weak acid with a pK a of 4 (the buffered system, blue line).For the nonbuffered strong acid solution, the pH changes abruptly from ∼0 to ∼14 around the midpoint, i.e., when the added strong base equals the existing strong acid with the solution pH 7. For the buffered system with the weak acid, however, the pH changed slowly around the pK a of 4 upon the addition of strong base within the buffering pH ranges, indicating a strong buffering effect.Correspondingly, the β of the buffered system (β bulk ) differs much with that of the nonbuffered system (β nonbuf ) in this pH range (Figure 1c).When the added strong base is too much and the pH is elevated outside the buffering pH range (roughly pH above 6), the titration pH curve of the buffered system is roughly the same as the nonbuffered system (Figure 1b), indicating a negligible buffering effect.Correspondingly, β bulk and β nonbuf differ little in this pH range (β bulk − β nonbuf < 0.02 mol kg −1 ; Figure 1c), indicating the negligible buffering effect.
Importance of Buffering Effect of a Given Buffer Agent.Based on the analysis above, we can see that the contribution of a potential buffer agent to the system buffering effect is We propose that the buffering effect of a potential buffer agent at a given system pH can be treated as negligible when where ε and ε r are both arbitrarily selected small numbers close to zero and represent the minimum absolute and relative buffering capacity of interest, respectively.When either of the above 2 criteria is met, the buffering capacity provided by buffer agent i would be too small, so that the difference in pH responses upon addition of strong acids/bases with/without this buffer agent is hardly discernible.That is, the buffering effect of agent i is negligible.In the case shown in Figure 1, the criterion of ε of 0.02 mol kg −1 is applied.According to eq 4, the influencing factors of β i, bulk is the total amount of buffering agents [X i ] tot and the coefficient b i , and b i is determined by the difference between the system pH and the pK a,i (SI Text S1; Table S1).When pH and pK a,i differ too much (e.g., 6), the b i is so small (e.g., 1.0 × 10 −6 ) that the β i is significant only when [X i ] tot is extremely large (e.g., on the order of 10 5 mol kg −1 for the ε of 0.1 mol kg −1 ).Accordingly, the more abundant the buffering agent is, the larger buffer pH ranges it would influence.

Multiphase Systems.
The above analysis can be easily applied to multiphase systems like aerosols, when we replace β bulk in the bulk aqueous solutions (eq 2a) by β mp in the multiphase system 1,21 where K a,i * is the effective acid dissociation constant, and [X i ] tot * is total equivalent molality of X i including those that exist in the gas phase, as the gas−particle partitioning also plays a role.For a semivolatile acid HA and a semivolatile base BOH, the K a * are, respectively, where H X is Henry's constant (i.e., gas−particle partitioning constant) of species X in mol L −1 atm −1 , L w is the liquid water content in (g water)/(m 3 air), ρ w is the liquid water density (∼10 6 g m −3 ), R is the gas constant (8.205 × 10 −2 atm L mol −1 K −1 ), and T is the absolute temperature in K.
Similarly with that in the bulk aqueous solutions, , where and is determined by b i * and [X i ] tot *, where b i * depends on | pH−pK a,i *| (SI Text S1; Table S1), while pK a,i * depends further on K a,i , H i , L w and T (eq 6b).The L w of aerosols (i.e., aerosol water contents) typically varies between 10 −6 and 5 × 10 −4 g m −3 , while for clouds and fogs it can range between 0.05 and 3 g m −3 but is usually from 0.1 to 0.3 g m −3 (ref 2).Note that the L w values of typical raindrops are usually on the same order of precipitating clouds. 32,33Even for severe storms, the L w values are <14 g m −3 (ref 34).Therefore, here we consider the L w range of interest for atmospheric water as from 10 −6 to 14 g m −3 .Note that fogs, rains, and storms can all be viewed as a special type of activated water droplet.Similar to the bulk aqueous phase, the importance of the multiphase buffering effect can be judged by eq 5.In this study, we arbitrarily set ε r as 1%, and ε as the changes in particlephase anion/cation molality corresponding to 0.001 μmol m −3 of changes in atmospheric concentrations, i.e., where 10 −3 is the unit converter from (μmol g −1 ) to (mol kg −1 ).This is roughly the smallest measurement uncertainty of typical atmospheric species (e.g., 0.05 μg m −3 of sulfate or 0.02 μg m −3 of ammonium) and would represent the perturbation of interest for most studies.The ε mp thus represents the minimum buffer capacity that would provide this kind of minimum resistance of interest.Therefore, eq 5 can be rewritten as  7,35 with the pK a * increasing from ∼0.4 to ∼7.5 at 298 K in the L w range of interest (Figure 2a, orange line).This agrees well with the typical pH ranges of atmospheric water of <7 (ref 5).With the high abundances (i.e., high [X i ] tot *) and the general agreement between pK a * and pH (i.e., high b i *), the NH 3 /NH 4 + pair appears to be the dominant buffering species of aerosols for most populated continental areas (Figure 2b), where the aerosol pH usually varies around the pK a * of NH 3 .This has been well illustrated elsewhere (refs 7,  30, and 36).
For fogs and clouds, the abundances of ammonia are less studied, but are usually considered as lower than those in surface aerosols. 2Meanwhile, the pK a * of ammonia is elevated considering the higher L w range of fogs/clouds, which is 5.4− 7.5 at 298 K and even higher (6.7−8.9) at 273 K (Figure S1).Therefore, its buffering capacity is much lower than in aerosols, which gradually exceeds ε thr only at higher pH levels (> ∼4.5 for the "polluted fog" case and > ∼5.5 for the "cloud" case in Figure 2b).Nevertheless, it can still serve as the dominant buffering agent for the polluted conditions with high ammonia concentrations and high pH of 6−7, like the fogs observed in California's San Joaquin Valley [39][40][41]43,44 (i.e., "polluted fog" case in Figure 2b) and Italy's Po Valley.45−47 For acidified clouds/acid rains with lower pH of ∼4 (ref 5), the overall importance of ammonia buffering can be much lower or sometimes negligible (e.g., Figure 2b, "cloud" case). More obsrvations and further studies are needed to illustrate the frequency of occurrence and situations when it is important.
3.2.CO 2 /HCO 3 − Buffer Pair.−26 Especially, the pH of "pure" rainwater of ∼5.6 is determined by CO 2 , which just falls into the buffering pH ranges of H 2 CO 3 (6.4 ± 1), seemingly to support the strong buffering effect of CO 2 on rain.
Analysis based on multiphase buffer theory, however, indicates that the buffering effect of CO 2 is negligible for aerosols and limited for fog, cloud, or rain (Figure 2).In the L w range of aerosols (10 −6 to 5 × 10 −4 g m −3 ), the corresponding pK a * of CO 2 /HCO 3 − at 298 K is 15.8−18.4(Figure 2a, gray line) with little influence from temperature (Figure S1).This is much larger than the typical pH ranges of aerosols.Even if we  37 the more polluted winter aerosols in Beijing, 38 the polluted fog in San Joaquin Valley, California, 39−41 and a cloud case. 42See detailed scenario settings in Table S2.

Environmental Science & Technology
assume a fresh sea salt or dust aerosol with pH of 7−8, the | pH−pK a *| gap is still over ∼8, which corresponds to a b i * of <1 × 10 −8 and renders β mp,CO2 negligible (i.e., much smaller than ε mp ) even considering its high abundance (Figure 2b).The strong nonideality in aerosol water may influence the pH and pK a * by ∼1 unit, 36 which still corresponds to a small b i * of <1 × 10 −7 .
For fogs, clouds, and rains, the higher L w range decreases the pK a * of CO 2 /HCO 3 − to around 11−13 (Figure 2a, Figure S1).Although the |pH−pK a *| gap is still large (>4), the corresponding b i * of <1 × 10 −4 may be compensated by its high abundances when the cloud pH is higher.As shown in the example cloud case 42 (Figure 2b, "cloud" case), β mp,CO2 becomes important (i.e., exceeds ε mp ) when pH is over 5.This is consistent with the finding that for some fog samples in California's San Joaquin Valley, the measured internal buffering intensity can be nearly accounted for by the carbonate system, especially in the pH ranges of 5−6.5 (ref  22).Nevertheless, the buffering effect of CO 2 was only comparable to that of ammonia, despite the >10 5 higher abundances of total CO 2 than total ammonia (Table S2).
The "pure" raindrop pH of ∼5.6 is derived when the water is in equilibrium with gas-phase CO 2 mixing ratios of ∼350 ppm (see detailed processes in ref 2).The role of CO 2 during this process, however, is actually acidification, where the semivolatile carbonic acid acidified the pure water.This should not be confused with buffering, which is associated with the sensitivity of the system pH to the uptake of additional acids/ bases.The limited buffering effect of CO 2 can also explain the formation of acid rain and the rain pH in remote background areas.For example, the rain pH in remote background areas is typically 4−5 (refs 48 and 49), which is lower than the pH when the water is in equilibrium with gas phase CO 2 (i.e., the "pure" raindrop pH) of ∼5.6.This is usually attributed to the acidification by the naturally produced sulfate and weak organic acids. 48,49However, based on the traditional buffer theories of bulk aqueous solutions, the "pure" raindrop pH of ∼5.6 just falls in the pH range when the buffering effect of the CO 2 is the strongest (6.4 ± 1).In this case, it was hard to imagine that the high peak β associated with the abundant CO 2 (∼410 ppm) could be readily overcome by the trace amount of naturally produced acids so that the rain pH is acidified from ∼5.6 to 4−5 (refs 48 and 49).Similarly, the acid rain is usually attributed to the anthropogenic acid gases like SO 2 or NO x.However, these acid gases are typically smaller than several tens of parts per billion, which is from 10 4 to 10 5 lower than that of CO 2 and would hardly compete with the high peak β of CO 2 to acidify the water substantially.Based on the multiphase theory, however, we can see that the high abundance of CO 2 is largely undermined by the large |pH− pK a *| gap; thus, its β mp is only negligible to limited in the rain pH ranges and can be readily overcome by the acidification of trace acidic gases.See more detailed discussions in the illustrative case studies in SI Section S2 and Figure S2.

HCO 3
− /CO 3 2− Buffer Pair.The HCO 3 − /CO 3 2− buffer pair is nonvolatile, and its pK a is always kept at ∼10.Following the above analysis procedures, we can conclude that its buffering effect is negligible in all atmospheric water, from aerosol water to clouds or rains.In comparison, both the bicarbonate and the carbonate salts, widely existing in dusts, etc., can neutralize the acids and therefore decrease the acidity.−57 We call for the use of "neutralization" instead of "buffering" for this process to avoid confusion in the future.

Influencing Factors of the Contributions to
Buffering Effects.The organic acids, mostly carboxylic acids, are found to contribute significantly to both the free acidity (i.e., amount of dissociated acids) and total acidity (i.e., amount of acids in both dissociated and undissociated form) 58,59 of precipitations and therefore acid rains. 60,61specially in remote areas, their contributions can be dominant (up to 80%) 49,62−65 and are still increasing. 66These indicate important contributions of organic acids to acidity through neutralization reactions (i.e., acidification).While the acidity of samples collected in bulk solutions (collected fogs, rains, or water extracts of aerosols), as characterized by indicators like free acidity of precipitations, is of interest in terms of the acidification of ecosystems, it is the in situ acidity that matters during the atmospheric chemical processes.The importance and mechanisms of organic acids in influencing the in situ acidity of atmospheric water, however, are still under debate.Some studies suggested a large potential of organic acids to buffer the pH of aerosols and fogs and therefore influence the atmospheric processing, 22,27−29 while some others suggested negligible buffering effects. 67,68ere, we examined the potential contribution of organic acids to system buffering effects with the methods outlined in Section 2. One major concern is whether the organic acids can buffer in the typical pH ranges of atmospheric water, i.e., the influence of |pH−pK a,i *|.Based on eq 7b, we see the equivalent acid dissociation constant in multiphase system, K a *, differs from K a by which depends on the H i and L w at a given temperature (Figure S3).When ρ w (H i RTL w ) −1 ≪1, ΔpK a ≈ 0, and K a * is roughly the same as K a .At 298 K, this is roughly when the product of H i and L w is over 5 × 10 5 (mol kg −1 atm −1 )(g m −3 ).
That is, if one species is more soluble (with higher H i ), its K a * will approach K a and become insensitive to L w at lower L w levels.
Table S3 lists the thermodynamic properties of commonly observed water-soluble organic acids.For these organic acids, the pK a mostly ranges 3−5, and the H i mostly ranges from 10 3 to 10 12 mol kg −1 atm −1 (Table S3).Therefore, for most of these species, the ΔpK a will be <3 in clouds (L w > 0.1 g m −3 ),and thus would be buffering at the appropriate pH ranges of <7.For aerosol water, however, the potential contribution to system buffering would differ greatly with the pK a and H i .
Another concern is the influence of species abundances, i.e., the influence of [X i ] tot .However, full-spectrum measurements of all atmospheric organic acids in both gas and particle phases are unlikely considering their wide variety and low concentrations of some certain species.Therefore, equivalent concentrations of representative species may provide a good first-order estimate.As formic, acetic, and oxalic acids are the most abundant and most widely measured organic acids, 60,62,69−73 S3.At the typical L w range of aerosols, the pK a * of most n-alkanoic monocarboxylic acids are too high (> ∼8; Figure 3a), and their buffering effects are negligible due to the large pH−pK a,i * gap.Correspondingly, these acids are found to reside mostly in the gas phase in the absence of fogs/ clouds. 37,38,76,77In comparison, the C2−C9 aliphatic dicarboxylic acids and some other acids (Figure 3b, c) are with the pK a,i * values of 2−6 and may buffer the aerosols.These potential buffering species are flagged in Table S3 (see the column "potential aerosol buffers").
−80 For an upper-limit estimate, we assume that all of these potential buffering organic acids are buffering at the same pH range with a total abundance of 10 times that of total oxalates.Even so, the total concentrations of these organic acids are much lower than the inorganic buffering pairs like NH 4 + /NH 3 and are negligible in urban areas like Beijing (Figure 4b).Even in the organic-dominated areas like the agriculturally intensive rural southeastern U.S.A. site (Figure 4a), they may have a certain buffering effect only in the pH ranges when the contribution of NH 4 + /NH 3 is negligible (i.e., outside the ammonia-buffered pH ranges).

Contribution to Buffering
Effects in Fogs, Clouds, and Rains.For fogs and clouds, the aqueous-phase molalities of organic acids can be much lower than in aerosols due to the dilution of the much higher L w .Therefore, the buffering capacity of organic acids with a pK a,i * of <3 could hardly compete with the water self-buffering effect (Figure 4c) and can be negligible.This would exclude some species that have a potential contribution to buffering effects in aerosols (see Table S3, column "potential cloud buffers").In comparison, while most of the monocarboxylic acids cannot buffer in aerosols, they have pK a,i * values of 4−7 in the L w range of fogs and clouds (Figure 3a) and may contribute the system buffering (Figure 4c).
Measurements of chemical compositions including organic acids in both gas and particle phases of clouds/fogs are scarce.Figure 4(c) and (d) shows one polluted fog case in California's San Joaquin Valley 39−41 and one cloud event at the summit of the Puy de Dome, France, 42 while the situation may differ further in other places.As shown in Figure 4(c) and (d), the inorganic acids of HSO 4 − and HNO 3 are buffering at too low pH levels (<3), and their contributions to the buffering  S3. capacity are mostly below 1% that of a water self-buffering event (ε thr ; gray dotted line in Figure 4).The contribution of oxalate acid buffering can be negligible due to both the low concentrations and the low buffering pH ranges.In comparison, HCOOH and CH 3 COOH pairs can provide certain buffering effects at higher pH ranges of > ∼3.5.The HCOOH pair can even serve as the dominant buffering species in the pH ranges of 4−5 for the polluted fog case (Figure 4c), while the CH 3 COOH pair can dominate the buffering in the pH ranges of 5.1−5.9 for the cloud case (Figure 4d).In scenarios when the organic acids are more abundant (i.e., when [X i ] tot * is higher), their importance can be even higher.The spatiotemporal variations in the importance of HCOOH and CH 3 COOH buffering, as well as the buffering of other organic acids, need to be clarified with more observations.

SUMMARY AND FUTURE STUDIES
The carbon dioxide, ammonia, and organic acids show distinct contributions in buffering the acidity of aerosols and clouds.This is mainly due to the large shifts in their multiphase buffering pH ranges, considering the much higher liquid water contents of clouds than aerosols.For CO 2 /HCO 3 − , its pK a * for aerosols is about 15.8−18.4,which is too far away from the typical aerosol pH ranges of <7, and therefore, its buffering capacity is negligible.In comparison, for clouds, the pK a * of CO 2 /HCO 3 − would decrease to around 11−13, and the corresponding b i * can be compensated by its high abundances when the cloud pH is higher.For ammonia, its pK a * varied just in the right range (0−5) for aerosols and is usually the dominant buffering species for large parts of the continental urban areas.For clouds, the pK a * of ammonia increased to ∼7, and its contribution to the cloud buffering depends on the actual cloud pH.As for organic acids, most n-alkanoic monocarboxylic acids are unlikely to buffer the aerosols due to the too high pK a * values.While the C2−C9 aliphatic dicarboxylic acids and some other acids are with the right pK a,i * values, their contribution to aerosol buffering is often overwhelmed by that of ammonia.In clouds, however, most of the monocarboxylic acids have the proper pK a *.Combined with the typically higher abundances (especially HCOOH and CH 3 COOH), their contribution to cloud buffering can be important.Note that in clouds and rains, despite the 10 4 to 10 7 higher abundance of CO 2 , its buffering effect is only comparable with that of ammonia and organic acids due to the large pH−pK a * gaps of CO 2 .Therefore, the buffering effect of CO 2 can be readily overcome by the acidification of trace acidic gases such as SO 2 or NO x , which would result in acid rain.
Despite the progress made in the potential role and major influencing factors of atmospheric weak acids and bases in regulating the acidity of atmospheric water, substantial uncertainties remain in the quantified estimation of their importance.For a deeper and more quantified understanding, we propose that future studies should focus first on the following aspects.
Identifying Key Organic Acids and the Comprehensive Representation of Their Thermodynamic Properties.Currently, the representation of the fundamental thermodynamic properties of organic acids is insufficient.For example, the temperature dependences of K a and H i of many organic acids are lacking, 74,75 which can cause rather large estimation uncertainties for clouds or during winters of the temperate zone, where the temperature are usually below 0 °C.However, considering the wide varieties of organic acids, it can be quite time consuming and technically challenging to obtain all relevant thermodynamic properties experimentally for all species, even considering the advances in theoretical calculations (e.g., refs 81 and 82).Therefore, further studies are needed to identify the most important species and the major influencing factors of their properties under different conditions and thus to give the simplified and representative scenario-specific parametrizations.
As illustrated by the discussions above, the important organic acids in influencing the system buffering should meet the following criteria.First, they need to be with enough abundances (i.e., relatively high [X i ] tot ).Second, pK a * values need to be within the typical pH range of aerosols/clouds so that b i is not too small.Third, pK a * values should differ with that of ammonia at the given L w and temperature conditions in that region/periods; otherwise, the ammonia buffering would be totally overwhelming.Especially, for most continental aerosols, the ammonia buffering is so strong that the organic acids often play only a negligible or minor role (Section 4.2).Under such conditions, the influence of nonideality, etc. can be more important than that of organic acids.In clouds, however, the pK a * of ammonia is relatively high (Figure 4), and buffering of organic acids can be important.
More Sophisticated Chemical Spectrum Observations.Currently, the measurements of chemical compositions including both inorganic species and organic acids in both gas and particle phases are scarce, especially for fogs and clouds.In  42 See detailed scenario settings in Table S2.Note that in aerosol cases of (a) and (b) the concentration of total oxalate acid is enhanced by 10 times to provide an upper-limit estimate of all organic acids that would potentially buffer in aerosols.
addition, the pH of individual cloud drops can vary with drop sizes, etc. within a given cloud, 49,83−85 while the sizedependent measurements are also rare.As the contribution of organic acids to cloud acidity can be quite important, we encourage such campaigns in the future.
Influence of Nonideality in Aerosol Water.The deliquescent aerosols are highly nonideal, with high ionic strength up to ∼43 mol kg −1 in severe urban hazes. 8This kind of high nonideality can shift the pK a * of ammonia by up to ∼1 unit. 30,36Moreover, the influence of nonideality depends not only on the ionic strength, but also on the aerosol composition and the specific ion pairs. 36Different thermodynamic models disagree with each other even for the nonideality for inorganic species, 5,86,87 not to mention the organic acids.As discussed in Section 4.2, while the potentially buffering organic acids are usually with much lower abundances than that of ammonia, they may contribute certain buffering effects when their buffering ranges differ much with that of ammonia.At a given aerosol water content, the nonideality may either narrow or broaden the pK a * gaps between ammonia and organic acids and therefore enhance or weaken the contribution of organic acids in buffering the aerosol pH.
Interactions among Organic Acids, Cloud Acidity, and Cloud Chemistry.As discussed in Section 4.3, the organic acids, especially HCOOH and CH 3 COOH, can potentially exert strong buffering effects in fogs and clouds.On the other hand, in-cloud reactions are shown to be an important source of organic acids, 63,88,89 while the efficiency of which can be susceptible to acidity. 2 In addition, unlike aerosols, the gas−liquid equilibrium times for bigger droplets like clouds are longer, and the time scales may differ between the buffering effects and the in-cloud reactions.The feedback among these processes under different conditions needs further exploring.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.2c09851.S1: Additional discussion of the dependence of b i on |pH−pK a,i |.SI Text S2 and Figure S2: Case studies on the modification of cloud pH by atmospheric acids/bases.Figure S1: Additional information on the dependence of pK a * on temperature.Figure S3: Dependence of the difference between K a * and K a on liquid water content L w and Henry's constant H i .Table S2: Detailed scenario settings as shown in Figure 2b and Figure 3. Table S3: Acid dissociation constant K a and Henry's constant H i of some commonly observed low molecular weight organic acids in the atmosphere (PDF)

Figure 1 .
Figure 1.Influence of buffering effect on pH.(a) The buffering of aerosol pH observed worldwide.Taken from Figure S1 in ref 7 under the terms of AAAS Standard Author License to Publish.The x-axis is the molar ratio of sulfuric acid added to the anions initially present in the system.The "NCP", "NI", "WE", and "SE-US" scenarios refer to winter North China Plain, northern India, western Europe, and summer southeastern U.S.A., respectively, and the response of the 2.5 mol kg −1 Na 2 SO 4 aqueous solution is also shown for reference.See details in the Supporting Information of ref 7. (b, c) Comparison of the (b) pH and(c) buffering capacity during the titration process between buffered and nonbuffered bulk aqueous systems.Here, the titration process of adding a strong base (e.g., NaOH) into the solution with 1 mol kg −1 of strong acid like HCl (the nonbuffered system) or weak acid with a pK a of 4 (the buffered system) is shown.The yellow shaded area indicates the buffering range of the weak acid.

Figure 2 .
Figure 2. Importance of inorganic carbon systems in buffering the atmospheric water.Here, we assume a constant CO 2 of 410 ppm.(a) Variation of the pK a * of H 2 CO 3 /HCO 3 − , HCO 3 − /CO 3 2− in comparison with that of NH 4 + /NH 3 with liquid water content L w at 298 K. (b)The buffering capacity curves under four representative scenarios: the organic-rich clean southeastern U.S.A. aerosols in fall (SE-US Fall),37 the more polluted winter aerosols in Beijing,38 the polluted fog in San Joaquin Valley, California, 39−41 and a cloud case.42See detailed scenario settings in TableS2.

Figure 3 .
Figure 3. Variation of the equivalent multiphase acid dissociation constant K a * with liquid water content L w for commonly observed organic acids in the atmospheric at 298 K. (a) C1−C9 n-alkanoic monocarboxylic acids, (b) C2−C9 aliphatic dicarboxylic acids (dC2−dC9), and (c) other acids.See the explanations of the abbreviations in TableS3.

Figure 4 .
Figure 4. Buffering effects of the most abundant atmospheric organic acids of formic, acetic, and oxalic acids under (a) an agriculturally intensive region in the southeastern U.S.A. in fall 2016, which represents an organic-rich environment, 37 (b) a more polluted urban area in Beijing in winter 2002, which is less organic rich, (c) the polluted fog in San Joaquin Valley, California, 39−41 and (d) a cloud event (event #1) observed at the summit of the Puy de Dome, France, in winter 2001.42See detailed scenario settings in TableS2.Note that in aerosol cases of (a) and (b) the concentration of total oxalate acid is enhanced by 10 times to provide an upper-limit estimate of all organic acids that would potentially buffer in aerosols.
20 papers in Science, Nature, Cell, and their sister journals.He is a fellow of the American Association for the Advancement of Science (AAAS) and a Highly Cited Researcher (Web of Sciences and Clarivate).He currently serves as editor of AGU Advances and associate editors of Atmospheric Chemistry and Physics, Journal of Geophysical Research: Atmospheres, Atmospheric Measurement Techniques, and Advances in Atmospheric Sciences.He received his Ph.D. in atmospheric sciences from Peking University in 2008.He then worked as a postdoctoral fellow at the Max Planck Institute for Chemistry in Mainz, Germany, and was appointed as head of the Atmosphere−Biosphere−Cloud interactions group at the same institute.He received several prestigious awards and recognitions including the Arne Richter Award from the European Geosciences Union (EGU).Professor Yafang Cheng heads the Minerva Independent Research Group of Aerosols, Air Quality and Climate at the Max Planck Institute for Chemistry, Mainz, Germany.She is also a guest professor at Peking University, a guest professor at the University of Science and Technology of China, an elected Member of Academia Europaea, Fellow of the American Geophysical Union, and Editor-in-Chief of the Journal of Geophysical Research: Atmospheres.She has published over 160 articles, including a series of papers in interdisciplinary highlight journals in which she was the first or corresponding lead author: 3 in Science and over 10 in Science Advances, Proceedings of the National Academy of Sciences U.S.A., Nature Communications, Chem, and One Earth.She is a Highly Cited Researcher (Web of Science and Clarivate).Her research achievement has been recognized by prestigious awards and honors, including the Atmospheric Sciences Ascent Award and the Joanne Simpson Medal by the American Geophysical Union, the Top 10 Science Breakthroughs of the Year 2021 in Physical Sciences by the Falling Walls Foundation, and the Schmauss Award by the German Association for Aerosol Research.

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3. ROLE OF CO 2 AND NH3 SYSTEMS 3.1.NH 3 /NH 4 + Buffer Pair.While ammonia is a weak base, it works mostly like a weak acid in the atmospheric multiphase system,

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they can serve as good representative species, as detailed below.

■ AUTHOR INFORMATION Corresponding Authors Yafang
Cheng − Minerva Research Group, Max Planck Institute for Chemistry, Mainz 55128, Germany; orcid.org/0000-0003-4912-9879;Email: yafang.cheng@mpic.deGuangjie Zheng − Minerva Research Group, Max Planck Institute for Chemistry, Mainz 55128, Germany; State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China; orcid.org/0000-0002-8103-2594;Email: zgj123@mail.tsinghua.edu.cnProfessor Hang Su is Director of the State Key Laboratory of Atmospheric Environment and Extreme Meteorology at the Institute of Atmospheric Physics, CAS.He has published more than