General comments

This manuscript describes measurements that aim to understand the SOA-forming capacity of green leaf volatiles (GLV) that react with the products of the aqueous phase photolysis of nitrate.    Specifically, the work describes measurements of the overall reaction rate constants (kobs) for 4 specific GLVs via high-resolution time-of-flight electrospray ionization mass spectrometer (HR-ToF-ESI) and separate experiments to determine the SOA yields.  In order to investigate both cloud/fog and aerosol-like conditions, both the ionic strength and pH of the solutions were varied.  Importantly, ammonium sulfate was used to control the ionic strength of the solutions, which led to complications in the interpretation of the results.  Under dilute cloud/fog-like conditions, the four GLVs had higher kobs at lower pH, which could be attributed to the pH-dependent formation of OH and other reactive species from nitrate photolysis. Ionic strength and sulfate had insignificant effects on kobs. In contrast, under concentrated aqueous aerosol-like conditions, the four GLVs had higher kobs at higher pH, as well as higher kobs values at higher ionic strength and sulfate concentration.  These effects are explained by the expected nitrate photolysis-initiated processes as well as the unexpected role of sulfate-related oxidation processes.  Higher SOA yields under both cloud/fog and aerosol-like conditions were observed at lower pH, which was attributed to acid-catalyzed accretion reactions.

Because of the importance of the study in helping to refine the formation mechanisms of SOA from such precursors as GLVs, this work will be of interest to general readers of EGUsphere.  The experiments are rationally designed, thoroughly analyzed, and the manuscript is generally well written.  However, the work is difficult to assess as it seems that it was designed as a careful study of the nitrate photolysis-initiated processes, but that design was compromised by the presence of unanticipated sulfate photolysis mechanisms.  The authors admit that the ionic strength dependence of the nitrate photolysis-initiated processes needs to be reinvestigated with a non-sulfate species.  The finding of sulfate photolysis-related processes is very important and worth reporting but is likewise complicated by the concurrent nitrate photolysis mechanism.  Therefore, it is quite obvious to the reader that new experiments should be designed that isolate the nitrate and sulfate photolysis processes.  Nonetheless, even though the work was not able to achieve its original goals of determining a rigorous quantitative understanding of the nitrate photolysis-related processes, it is still valuable as a qualitative outline of the combined importance of the nitrate and sulfate photolysis pathways.  

There are several items that should be addressed in a revised version of the manuscript:

Line 235:  It would have been relatively to test this hypothesis with a separate experiment that generated OH exclusively.  Is there a reason this was not done?

Line 246:  Why couldn’t the inorganic salts be separated before analysis?

Line 377:  This is a very out of date set of references for acid catalyzed SOA processes.  I suggest adding:

Epoxides:  Cooke et al. ES&T, 58, 10675-10684, 2024

Acetals:  Presberg et al. ACS Earth and Space Chem., 8, 1634-1645, 2024

Oligomers:  Maben et al., Environ Sci Process Impacts, 25, 214-228, 2023

Line 379:  Why would there be enhanced formation of organonitrates from RO2 + NO at high ionic strength?

Technical comments

Line 64:  typo “ideal”

Line 154:  extraneous “the” between “from” and “before”

Line 313:  typo in subscript for ionic strength “total”

General comments

The present manuscript explores the % secondary organic aerosol (SOA) contribution of green leaf volatiles (GLVs) resulting via nitrate mediated aqueous photooxidation pathway in presence of sulfate in varied reaction conditions. To do so, the authors determined the first order decay kinetics and SOA yields of the selected GLVs i.e., cis-3-hexen-1-ol (cHxO), trans-2-hexen-1-ol (tHxO), trans-2-penten-1-ol (tPtO), 2-methyl-3-buten-2-ol (MBO) resulting from the nitrate mediated photooxidation. They investigated the role of pH, ionic strength, and sulfate on these investigated reactions. The pseudo first order reaction rate constants (k_obs) for the selected GLVs were determined by following the GLV concentrations at different time intervals using ultrahigh-performance liquid chromatography coupled to a photodiode array detector (UPLC-PDA). While the %SOA yield was determined using the highly sophisticated Aerosol Chemical Speciation Monitor (ACSM). The authors applied different liquid aerosol conditions i.e., cloud/fog droplets-like and aqueous aerosol-like to determine the effect of pH, ionic strength and sulfate on the investigated reactions. The use of ammonium sulfate to control the ionic strength led clearly to the undetermined effect on these reactions. Briefly, presence of sulfate as a salt can first act as a precursor source of sulfate radicals in the system (being produced during nitrate photolysis); secondly as an additional pathway of reaction with nitrates resulting into nitrate radical and sulfate anion.  In cloud/fog-like conditions k_obs for the GLVs are relatively higher at lower pH=3 than at pH=5, while ionic strength had statistically insignificant effect. However, the %SOA yield was observed to be higher for the low pH=3 and low ionic strength (I=0.002 M). While with varying ionic strength the k_obs remains practically unchanged, the %yield does not. This is possibly due to the shift in increased side reaction of GLVs with increase in the concentrations of other radicals such as SO₄^-·anionradicals in the system at higher ionic strength. This shift does not significantly influence the overall k_obs while it completely changes the pathway of reaction resulting into more volatile fragments from GLV-sulfate reactions and hence the lower %SOA yield. In aqueous aerosol-like conditions the concentrations of nitrate and GLVs were set to be 100 times higher, while concentrations of salts i.e., ammonium sulfate to control ionic strength was 100 – 1000 times higher in different scenarios listed in Table 1. The k_obs here, increased with increasing ionic strength, while the pH had insignificant effect. The increased concentration of nitrates in the system results in exponential decrease in the nitrate photolysis rate and thereby decreased concentration of OH radicals (based on literature cited), resulting into little lower impact of pH on k_obsin this case. However, increasing the ionic strength may increase the presence of SO₄^-·anionradicals resulting into increased k_obs. The increased aqSOA at lower pH could be attributed to the formation of higher amount of low volatility product from acid catalyzed reactions. Additionally, the reactions at higher concentrations of GLVs in aqueous aerosol-like conditions results into higher amount of RO₂^· and RO^· combination reactions, possibly resulting into oligomers.

Since the presented work in the manuscript builds on to fill the existing gap in knowledge concerning the under investigated role of GLVs in the atmospheric reaction and SOA contribution, it will be highly valuable to the readers of EGUsphere and in general the atmospheric science community. The approach to determine the first order kinetics and %SOA yield is analytically sound and rational. The authors have tried to address the effect of pH, ionic strength and sulfate on the nitrate mediated aqueous-phase reactions of GLVs. But the gap in understanding remains, due to the additional experiments with ^·OH clearly required to resolve the same. The authors admits and state to investigate the role of sulfate in future studies using inert salts such as sodium perchlorate instead of ammonium sulfate. Additionally, aqueous phase reactions of these GLVs should be studied with ^·OH (H₂O₂) and HONO, respectively to evaluate the overall contribution of respective pathways of reactions with ^·OH, NO₃^·, NO₂^·, NO^· and SO₄^-·. It is understandable that these studies are deemed important however, are beyond the scope of the present work as it could make the study quite exhaustive. Despite the listed short comings, the manuscript thoroughly examines the kinetics and resulting SOA yield. The overall quality of the results and their interpretation within the study is of high scientific quality and significance.

Scientific comments

The manuscript holds potential for acceptance and would serve valuable for EGU Journal Atmospheric Chemistry and Physics in relatively-less investigated role of GLVs where very little to moderate is known in comparison to the other biogenic compounds such as isoprene, monoterpenes, and sesquiterpenes. The present listed issues need to be addressed before acceptance:

Line 133: The apparent or pseudo first order rate constants are usually determined in the conditions where one of the reactants is always in excess. However, the first order rate constants fit well according to equation 2 as stated. (Include one of these exemplary plots in SI).

Line 201: The decay of GLVs is governed by their reactions with ^·OH, however the experiments were never tested against ^·OH which is potentially a control experiment in the case. Can authors state the reason why it was not done in this case?

Line 101: In Table S1, although the sulfate photolysis and resulting side reactions seems to significantly influence the reactions, they are omitted from the presentation. It would be useful, if authors can present the list of the set of reactions in different reaction conditions within SI:

1. List of reactions possibly governing experimentally studied Cloud/fog like conditions and aqueous aerosol conditions
2. Control experiments (in presence and absence of sulfate/nitrate/OH, respectively)

Line 110: Which case of recorded molar absorptivity for NH₄NO₃ is presented in Figure S2? Was the molar absorptivity of NH₄NO₃ in presence of GLVs, (NH₄)₂SO₄, and H₂SO₄ within the standard deviation of the one in the absence?

Line 117: Sec 2.2 Photochemistry Experiments: The below mentioned comments should be addressed to increase the quality of the work and make easier for future studies to replicate the same.

• What does open cylindrical quartz tubes mean here? If the reactor is kept open aren’t GLVs susceptible to escape as they are moderately volatile? See Henry’s constant. And it is expected some GLVs (as stated for MBO) might be lost as well. Please, include in SI, the concentration time-series for the dark control vs the photo experiments for all GLVs.
• What was the total volume of reactor and reaction solution?
• What is the total reaction time in each case?
• Line 123: How much was the temperature deviation within the reactor from 30°C? As rate constants can be sensitive to the temperature change. Is it thermostatic with something other than cooling fan and how was it monitored through the reaction?
• Why was 30°C chosen as a temperature? What are the thoughts of author on temperature dependence of these reactions? Isn’t that necessary for future studies?
• Line 125: At different reaction time? How frequently was it sampled to determine the kinetics? Was it same for all experiments? If not, what was the number of aliquots taken out in each case respectively. In case of AqSOA yield aliquouts were extracted from the illuminated solutions at one GLV lifetime (Include these lifetime values in SI at least for readers reference)
• Line 135: “Only MBO showed some loss”. Was it quantified? Any specific reason why concentration-time series are not included within SI?
• Line 139: Sarang et al., 2023 highlights "HEXAL, in contrast, absorbs light in the range of 290–400 nm (ε = 51–0.6 M⁻¹ cm⁻¹, c.f. SI Section 1) and was therefore isomerized to the Z-isomer within irradiation experiments". It is unreasonable that no loss of this GLV occurred during illumination. Please provide the molar absorptivity of GLVs recorded at 311 nm, which is a peak of photon flux for the presently used reactor. Thus, concentration time-series of GLVs for all cases: experiments and control are recommended to be included in SI.

Line 199: “Other reactive species produced during nitrate photolysis (e.g., hydroperoxide radicals and superoxide ions are also expected to have lower reactivities compared to ·OH”

Mention sulfate radicals, as they are also present in the system.

 Line 295: The steady state concentrations of OH could not be determined experimentally in conditions of higher ionic strengths, however, is it possible to do the same using some kinetic modelling to provide approximate value. See (OH estimation model) in Otto et al., The Journal of Physical Chemistry A 2017 121 (34), 6460-6470

This would serve as a contrast to understand the role of OH even better between cloud/fog-like vs. aqueous aerosol-like conditions. Also, it will strengthen the statement in Line 305.

Line 301 – 305: The resulting decrease of the first order reaction rate constant can be attributed to the lower rate of reactions of GLVs with sulfate (10⁸ Lmol^-1s^-1) than OH (10⁹ L mol^-1 s^-1). The rate constants are mentioned in Line 347, however, is it possible that these differences in the second order rate constant values could also explain partially the decrease in k_obs under aqueous aerosol-like conditions with clearly higher sulfate present in the system.

The authors could list and compare the literature available rate constants for the GLVs against ^·OH, SO₄^-·and NO₃^· with in the SI and conclude in a sentence within the manuscript for the ease of understanding the concept and scientific clarity of the reader.

Technical and typographical comments

Line 29; The sentence should be corrected. Present form results into interpretation that other that Isoprene and monoterpenes, the rest/remaining comes solely from GLVs.

For better clarity state the approximate  %BVOC contribution of GLVs to the remaining half (citing literature)

Line 64: typo “non-deal” should be corrected.

Line 80: Figure 1 structure names: cis and trans could be in italics

Line 103: typo/grammatical error “to study pH the effects”.

Line 106: The pH (i.e., pH 3 vs. 5, Table 1)

Line 111: add (Table 1) in brackets to direct the reader for better clarity.

Line 142: Correct reference text format (U.S. EPA, 2024)

Line 154: Incorrect article/preposition usage “to remove inorganic salts from the before UPLC-MS analysis”