This manuscript presents measurements and model predictions of aerosol composition at 6 sites in Greece. Standard thermodynamic modeling techniques are used to estimate the pH and aerosol liquid water based on measured aerosol composition and levels. Gas-phase NH3 and HNO3 constraints are used—in most cases, the concentrations were not directly measured coincident with the aerosol and were estimated. This study adds to the community’s knowledge of atmospheric aerosol acidity levels which is inherently limited due to a lack of operational measurement techniques. Some recommendations for revision are provided below.

Main comments:

1. The abstract and conclusions (line 481) emphasize K+ levels drove the higher pH/less acidic nature of aerosols at IOA. How was that effect identified? Is that based on previous knowledge or a specific calculation/test? The K+ as a driver of seasonality seems to be a separate conclusion supported by Figure 6 (line 490).
2. How do organics affect inorganic species in ISORROPIA-lite? Section 3.2.3: What interaction cause the unexpected decrease in pH when organics were removed? Are any of the organics dissociating acids? Does the organic water combine with the inorganic water for calculation of NHx and TNO3 partitioning?
3. Line 448, line 508, and elsewhere regarding NOx emissions and nitrate: Womack et al. (2019) show that nitrate formation can be NOx or VOC limited. Consider that VOC emissions may govern total nitrate abundance. Do you know if nitrate in the airshed(s) is more sensitive to NOx vs VOC controls?
4. If a figure/analysis can be added to address the uncertainty introduced by the lack of complete gas-phase NH3 and HNO3 measurements, that would be useful. That could inform future work by letting the community know how precise they likely need to with estimated NH3 and HNO3 concentrations if they need to fill in that data due to lack of measurements. Did the authors consider an iterative technique in which an initial guess of gas-phase NH3 and HNO3 was used to construct total NHx and TNO3 input to predict a new NH3 and HNO3 gas estimate and run until predicted aerosol NH4 and NO3 converged to measurements? Zheng et al. (2020) showed that the lack of NH3 gas values can lead to an incorrect direction in the pH trend over time. It is unclear what size error in NH3 is acceptable.

Minor comments:

1. Line 86: Define “neutral levels”.
2. Section 2.3: What time averaging was used for the input conditions to ISORROPIA-lite?
3. Line 360-361: Order the variable impacts from largest to smallest impact.
4. Line 431: Change “most insensitive” to “relatively insensitive” or “least sensitive”.
5. Line 464: check wording.
6. In several figures (Fig 3, 6, 8), subplot labels on the figure would be useful.
7. Does the inclusion of organics and their associated water substantially affect Figures 9-10?

References:

Womack, C. C., McDuffie, E. E., Edwards, P. M., Bares, R., de Gouw, J. A., Docherty, K. S., et al. (2019). An odd oxygen framework for wintertime ammonium nitrate aerosol pollution in urban areas: NO_x and VOC control as mitigation strategies. Geophysical Research Letters, 46, 4971–4979. https://doi.org/10.1029/2019GL082028

Guangjie Zheng et al. ,Multiphase buffer theory explains contrasts in atmospheric aerosol acidity.Science369,1374-1377(2020).DOI:10.1126/science.aba3719