In this paper, in-situ Raman technique was used to research the photochemistry in nitrate-glycine mixed particles at various RHs. The apparent nitrate photolysis rate constants and percentages GC decay were obtained. The phase transition behaviors of nitrate-glycine were obtained, which showed the role of molecular interaction in determining the physicochemical properties and chemical reactivity of particles. In AN+GC mixed particles, the glycine photochemistry is negligible, and nitrate photochemistry is weak. But in SN+GC mixed particles, products of nitrate and glycine photochemistry, HNO₂/NO₂^-, amide, ammonia or/and amine are detected, and the apparent nitrate photolysis rate constant is 4.5-folds higher than that of AN+GC particles.

Questions and comments:

1. In line 57, the word “behaviors” should be deleted.
2. In line 134, the number “1/2” in equation (6) should be removed.
3. In line 136, “Equation 9” does not exist, it should be changed to “Equation 7”.
4. In line 139, “/” in equation (7) is misleading, it is better to change it to “or”.
5. In this paper, the mole ratio of 1:1 for glycine and nitrate is used in all the experiments, but the photochemistry of pure glycine solution and pure nitrate solution are not studied. If the photochemistry of pure species is missing, how can we conclude that there exists an interaction between glycine and nitrate in the mixed particles affecting their photolysis?
6. In line 232, the viewpoint “The apparent nitrate photolysis rate constant J shows good correlation with the percentage GC decay (R²=0.99, Figure4b), which may suggest that nitrate photolysis is the key driver for the glycine decay” is proposed. But photolysis rate constant J is determined by illumination intensity, and according to Eq.2, J is independent on solute concentration. In Figure 4b, various apparent nitrate photolysis rate constants are displayed and the illuminant of 300nm LED lamp is used in the experiments, so what is the definition of apparent nitrate photolysis rate constant in this paper? Is the x-axis label wrong in Figure 4b? And should it be change to nitrate photolysis rate instead?

Review of “Distinct Photochemistry in Glycine Particles Mixed with Different Atmospheric Nitrate Salts” by Liang et al.

General:

The authors presented experimental results of (1) phase transition behaviors and (2) photochemical degradation of mixed particles with glycine (GC) and ammonium nitrate (AN) or sodium nitrate (SN). Experiments were conducted in a custom-built flow cell reactor with deposited particles, aided by in-situ Raman characterization and off-line chemical analysis. As relative humidity (RH) varied, the mixed particles underwent phase transitions in line with observations in literature. The photochemical behaviors (300-nm light illumination) of AN + GC particles and SN + GC particles, however, are distinctly different, with the latter being much more reactive than the former. The authors proposed that the availability of more abundant “free” nitrate ions in the SN + GC system (where nitrate is not strongly bonded with GC, and binding between carboxylic group of GC and sodium ion “frees up” the nitrate ions) might be the reason behind, as nitrate photolysis can generate an array of oxidants to oxidize GC. This statement was supported by Raman observation that SN + GC particles showed higher signal intensity of “free” nitrate ions. It was implied from this observation that GC, or more generally free amino acids, might decay faster in Na-rich particles (e.g., coarse sea spray aerosols) than in ammonium-rich particles (e.g., urban aerosols). The experiments were well designed and conducted, and arguments in the manuscript are also well articulated. I therefore recommend Minor Revision before publication.

Specific:

1. P7, reason(s) for faster photo-degradation of SN + GC particles. The authors presented at least three possible reasons to explain the observed faster decay of GC in the SN + GC particles than in the AN + GC particles: (1) more abundant “free” nitrate ions (the first paragraph in P7), (2) ionic form of GC (anion, zwitterion, or cation) (the second paragraph in P7), and (3) water-to-glycine molar ratio (the third paragraph in P7). It is not clear how these potential reasons are related to each other. That is, are they in parallel, being all possible, with the first one the most important (as the authors stated), and they might even be competing? Or are they intertwining to result in the observed result, i.e., the zwitterion ion (reason 2) form promotes the formation of “free” nitrate ions (reason 1)? Clarification of this might be helpful in understanding what other inorganic cations (e.g., potassium, magnesium etc.) and other free amino acids (what ion form is prevailing in relevant pH range) will behave in similar photochemical processes. In addition, the last sentence in L222 reads ambiguous. Not sure whether it is referring to SN + GC particles or AN + GC particles. If the former, it is contradictory to previous statements; if the latter, please specify.
2. P9, implications. What causes the stronger binding between sodium ion with the carboxylic group compared to that between ammonium ion with the carboxylic group. Is it because sodium is a stronger alkaline species than ammonium? Or due to some sort of indirect effect (e.g., how much solvated the carboxylic group is) from the low water-to-glycine ratio in the SN + GC system? In ambient aerosol particles, there might be other alkaline species too (potassium, magnesium, amines etc.). It would be good to comment on whether alkalinity of degree of solvation might lead to such increased photodegradation of free amino acids, if possible.

Technical:

1. P1/L13: add “of” before “glycine”.
2. P2/L49: remove “(Wen et al., 2022)” at the end of the sentence.
3. P2/L52: remove “behavior.”.
4. P4/L109: revise citation format of “Matsumoto et al….”.
5. P5/L135: I do not see Equation 9.
6. P6/L171: should NH3 be -NH2 or -NH3+? Are you referring to the amino group of GC?

The paper is concerned with a problem which is certainly of environmental interest, but which nevertheless touches problems in concentrated solutions, which are beyond the reach of classical thermodynamic models. Under conditions of such low solvent availability, the activity coefficients can explode to values ​​>1000, making the reactivity analysis very complex. I am personally in favor of making experiments other than the classic ones, such as in this paper. However, precisely because of this, interpretation can be difficult.

Amino acids in real conditions (in aqueous solutions and in crystalline form) are separate charge systems (NH+3–CH2–COO–). Several possible zwitterionic conformers of glycine have been calculated with the addition of 1-3 water molecules [Opt. Spectrosc. 128, 1598–1601 (2020). https://doi.org/10.1134/S0030400X20100161]. In both low water content and crystalline form GLY is ZW (NH+3–CH2–COO–), as in water, always (and pH only changes the fractional amount of charges from positive to negative). In several parts of the article GLY is referred to as neutral/non-ionized/less zwitterionic. This is a confusion, because the form NH2–CH2–COOH does not exist. Thus, the paper's key conclusion given on line 220 is inconsistent. Also, on the same lines, it is stated that “The -NH2 was unprotonated…and more susceptible to oxidation”. This is only possible at pH above 10, so the observed reactivity must have other explanations. In such a concentrated solution the pH is also difficult to define, and no idea of ​​the actual protonation constants is reported. Thus, if it is true that the reactivity of OH is greater for anionic GLY, the "formal" pH would be basic. Are there any hypotheses about it?

From the GLY ZW nature it follows that the addition of SN or AN could change:

1) The availability of free water, as part of it can be solvated by sodium or ammonium cations. This was deduced at line 219, as water to GLY ratio of 6 for AN and 2 for SN. Although it is reasonable as sodium is solvated more than ammonium, these ratios must be further supported by the authors.

2) the configuration of the GLY dimer or trimer as some possible complexation or charge interaction is possible both with the anion carboxylate, and the cationic protonated -NH3+ group. This last would change the photoreactivity due to the nitrate, the only absorbing species in the system (the absorption of GLY is below 260 nm, and not involved in the experiments). In Figure 3a the complexation of sodium is depicted, leaving free nitrate supposed to form reactive species. But also a weaker bond with ammonium would lead to the same configuration of free nitrate. Then, what is the ultimate explanation?

Overall the paper need a strong revision.