This paper investigates the trends in aerosol liquid water content (ALWC) and pH at a rural site in Germany over a 10-year period. Factors controlling pH and an assessment on how to lower NH4NO3 concentrations through NH3 or HNO3 control are discussed. The data set is unique. The paper is of great interest and very well organized and easily to read.  There are major issues, however. The aerosol composition data is PM10, which greatly limits assessing fine particle pH, which is much more important than pH of PM10, but this is never discussed.  The ALWC is calculated only from inorganic species, it is not clear if OA data was available, which could be used to assess relative contribution of OA particle water. This could be important as the inorganic species concentrations decrease, as noted.  The analysis is interesting, but the assessment of factors that affect pH is hard to interpret.  Greater physical/mechanistic analysis would help. For example, how is one to interpret the claim of an overarching effect of T on pH variability – why does this happen? Overall, an interesting and important paper.

Specific Comments.

The title nor abstract state if this is PM2.5 or PM10, which matters greatly for pH. It is first clarified in line 105 in the methods.  This is a critical issue that must be addressed up front.  What is the meaning of a pH calculation that covers (mixes) fine and coarse modes that likely have highly different pHs. This is noted in lines 342 to 346 discussing changes in pH with size at this site (pH~3 for PM1 and pH~4 for PM1-2.5 ).  What is the pH of Dp 2.5 to 10 um?  I guess the authors are assuming that the PM chemical species included in the pH calculation are much higher in PM2.5 or PM1 than PM2.5-10 and so pH fine particles is same as pH PM10. This needs substantial clarification.

In the abstract it states: “Aerosol liquid water content (ALWC) decreased by 50% during the analyzed time period in response to decreasing sulfate and nitrate. Aerosol pH exhibited an increase of 0.06 units per year, a trend that was distinct from other regions”.   Be consistent?  Ie give each change per year or overall change during the measurement period or both.  Stating that this is a change in pH of 0.6 units in 10 years (line 219), or give a % change would provide a direct comparison.  Also, the 50% decrease in ALWC does not include contribution of organic species, this is a calculated quantity, and it should be clearly noted this number is based only inorganic species – a major, and possibly very impactful limitation since it may not be the actual change in ALWC, especially since inorganic species are decreasing.  Finally, how does ALWX associated with PM10 compare to PM fine, ie, how to interpret this data.

In the abstract: “Seasonal analysis showed strong variability in factors controlling aerosol pH….”   Is this referring to variability between seasons or within a season.  Does the term variability overall mean for the complete measurement period?

Last part of Abstract regarding NOx controls to lower PM NH4NO3, why NOx control, what about factors that affect OH or O3, ie VOCs, since oxidants may have more control over HNO3 than NOx). A number of modeling papers have looked into this.  Isn’t it more correct to call it factors that control HNO3?

Line 55, the role of T is critical, (as noted many times further on in the paper), I suggest this be expanded more with a few sentences. Might what to look at DOI: 10.1126/sciadv.ado4373 (or https://www.science.org/doi/10.1126/sciadv.ado4373)

Eg, if most pH buffering is by NH3/NH4+, NH3 volatility (and water vapor volatility) is very important. More on this below regarding how to interpret the importance of T.

Line 70. See the following recent paper for more pH estimated over years. https://doi.org/10.1038/s41561-024-01455-9

Line 145 and on. NH3/NH4+ partitioning measured vs compared looks good, as is practically always the case in other studies. What about HNO3/NO3-, HCl/Cl-, which often does not look so good? If HNO3/NO3- is not so good, what are the implications for the findings of this paper, such as, does it affect predictions on HNO3 vs NH3 control?

In Equation 1, what is the variable on the bottom left in within the bracket, X(NH4+), mole faction?

It would be helpful (one could even say critical) to give a physical explanation for all the factors that are derived in Eq 3.  Example, how does the NH3 factor alter pH, how does the T factor alter pH?  Please do this for all factors so that the results can be interpreted later in the paper.  Are the underlying mechanism affecting these factors mutually exclusive, eg, does T have an effect on many of them?  If so, what does the T factor mean and how is one to interpret the results of this analysis?  Overall, I find this approach very confusing, possibly overly simplistic, due to lack of mechanistic explanations.

Lines 207 to 210 states (SNA) are the most abundant inorganic aerosol components in Melpitz (Spindler et al., 2013), and thus, they control ALWC abundance. True, if only inorganic species are included in the calculation of ALWC. Consider OA water, as noted before.

Line 230 to 236.  Please explain how (mechanistically why) pH of aerosol particles and cloud/fog water trends would be related in an environment of changing PM chemical species concentrations.  As one example, does NH3/NH4+ buffering apply to cloud/fogs as it does to fine PM (or I guess PM10 in this case)?

Lines 347 re Fig 5 and buffering capacity.  If Melpitz has a higher buffer capacity that explains dampened diurnal pH variability why does the same site have a larger trend in pH interannually (ie, the trend over the 10 years of the study)?  Can the authors give a physical explanation of why NH3/NH4+ has the highest buffering capacity, ie higher than other chemical species.  Could it be that the gas-particle partitioning for NH3/NH4+ is closer to 50:50 compared to the other semi-volatile components. In fact, maybe this explains why the pH is in the predicted range.

Regarding section 3.4 Drivers of pH variability.  This section goes into detail on what factors can be attributed to most of the model-predicted pH variability. It is noted that T is the largest overall driver since it has effects in multiple ways.  Why not interpret all the trends by looking at the effect of T in each season and see if it provides a more unifying explanation.  For example, how much of what is observed is because of T effect on volatility of the semi-volatile species (NH3, HNO3, H2O). When T is lower, more NH3 partitions to the particle due to less volatile, same for H2O, both may move the NH3/NH4+ to more or less buffering (need to know NH4 epsilon), and is that consistent with observed greater NH3 buffering in winter.  Looking at S curves one may get a physical sense of what is going on. This is essentially stated in lines 426 to 428 at the start of section 3.5 (although could add H2O to this list).  Unless some in-depth explanations are provided, the method of Tao and Murphy may obscures insight.

Line 373 to 374. Explain this line, lower T produced increase in pH, higher T a lower pH. Why does that happen?

As noted in line 387, the factors are not independent., even within the thermodynamics of the process (eg, excluding other external factors such as effect of T on ambient NH3 concentrations).  Take the stated variables, T and NH3 concentration. Mechanistically, how does T and NH3 concentration change pH. (This question was asked above as part of the section where the equations are presented). Are they related, eg, T affects volatility of NH3, lower T results in more NH3 to NH4+ which raises pH which then affects other things that go back and influence pH and NH3 concentration.  Eg, T affects the epsilon (NH3), which for a constant total ammonia (NH3+NH4+) leads to change in NH3 concentration.  So, what is the meaning of the T effect vs the NH3 concentration effect?  Is it even meaningful to try and separate these two effects?

Can a physical explanation for line 401 to 402 be provided, which states:  During winter, NH3 was a more important driver of pH variability than temperature (38% and 32%, respectively), but temperature was far more important during summer (51% to 15%). If one does not know what the T and NH3 factors are influenced by, this is hard to interpret.

Again, questions about OA water. Section 3.5, specifically discussing how ALWC, eg lines 450 to 455, what if organic species, such as WSOC, significantly affect ALWC, which is not considered here. What is the implication? This is another useful aspect of the Nenes formulation since you could make similar plots with ALWC that includes OA. Or since OA water does not greatly affect pH, add OA ALWC with appropriate Kappa values and see how this shifts the data relative to the regimes.  Or run ISORROPIA Lite.

As noted before, support the assertion that NOx is the key emission driving HNO3 concentration, lines 460.

Comment regarding Figure 7.  The location of the lines (blue and red) delineating the regimes appear exactly the same in all four seasons, (all four plots), despite substantial temperature differences (ie,  line 434-435: 275 K in winter, 281 K in spring, 291 K in summer, and 282 K in fall).  Is that correct?

The authors investigate diurnal, seasonal, and annual trends of aerosol pH, liquid water content, inorganic species, and their controlling factors at a continental background site in Germany over a 10-year period. They found that temperature and ammonia was the most important factors in controlling the aerosol pH, and that NH4NO3 formation was most frequently HNO3 limited, suggesting that NOx controls would be more effective that NH3 controls in reducing particulate mass concentrations. The findings are scientifically sound, the manuscript is very well written and interesting. I recommend publishing it after the authors address the following comments.

Comments

1) In section 2.2, it is mentioned that HCl and Cl- were measured, but these species were not discussed or shown in any Figures. Was chloride always below detection limit? Please clarify.

2) While the seasonal and annual trends of total inorganic species can be readily understood as shown in Figure 1, the roles of temperature and relative humidity on gas/particle partitioning of semivolatile species (HNO3 and NH3) and pH are a bit unclear. Temperature directly modulates the Henry’s Law constant and other equilibrium constants that govern the partitioning of HNO3, HCl, and NH3 as well as bisulfate ion dissociation. At the same time, temperature together with absolute humidity govern the relative humidity, which in turn governs the equilibrium aerosol liquid water content. Thus, the dependence of pH and PM mass on temperature and RH is rather complex and intertwined. For instance, pH and ALWC show inverse diurnal behavior with respect to temperature, but diurnal variation of RH would also have a strong influence on ALWC and thus also on pH. I suggest adding a panel in Figure 4 for the diurnal profiles of RH for each season to show a more complete picture of the dependence of pH and ALWC on T and RH.