the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Simultaneous Formation of Sulfate and Nitrate via Co-uptake of SO2 and NO2 by Aqueous NaCl Droplets: Combined Effect of Nitrate Photolysis and Chlorine Chemistry
Ruifeng Zhang
Abstract. SO2 and NO2 are the critical precursors in forming sulfate and nitrate in ambient particles. We studied the mechanism of sulfate and nitrate formation during the co-uptake of NO2 and SO2 into NaCl droplets at different RHs under irradiation and dark conditions. A significant formation of nitrate attributable to NO2 hydrolysis was observed during the NO2 uptake under all conditions, and its formation rate increases with decreasing RH. The averaged NO2 uptake coefficient, γNO2, from the unary uptake of NO2 into NaCl droplets under dark are 1.6 × 10-5, 1.9 × 10-5, and 3.0 × 10-5 at 80 %, 70 %, and 60 % RH, respectively. Chloride photolysis and nitrate photolysis play a crucial role in sulfate formation during the co-uptake. Nitrate photolysis generates reactive species (e.g., OH radicals, NO2, and N(III)) that directly react with S(IV) to produce sulfate. The generated OH radicals from nitrate photolysis can also react with chloride ions to form reactive chlorine species and then sulfate. To parameterize the role of nitrate photolysis and chloride photolysis in forming sulfate, the SO2 uptake coefficient, γSO2, as a function of nitrate photolysis rate, PNO3- (= jNO3- × [NO3-]), and chloride photolysis rate, PCl- (= jCl- × [Cl-]), as γSO2 = 0.41 × PNO3- + 0.34 × PCl- was derived. Our findings open up new perspectives on the formation of secondary aerosol from the combined effect of nitrate photolysis and chlorine chemistry.
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Ruifeng Zhang and Chak Keung Chan
Status: open (until 28 Mar 2023)
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RC1: 'Comment on egusphere-2023-223', Anonymous Referee #1, 12 Mar 2023
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The study by Zhang et al. reported sulfate and nitrate formation during the co-uptake of NO2 and SO2 into NaCl droplets with chamber experiments and kinetic modelling. In general, the study is well designed and presented, and I'd recommend the publishment if the following issues can be well addressed.
1. Figure 1: What does the red and black arrows mean? In the OH+NO2 to NO3- under hv reaction, why there're arrows on both ends? The arrow on the NO3- side should be removed if it's indicating SR1.
2. Section 3.2: The concentrations of which oxidants increase at low RH? And does the rate constant change?
3. Line 243-244: If the derived expression is "constrained to the conditions in the presence of nitrate and chloride", I'd expect a range of the corresponding [NO3-]/[Cl-] ratio, not only an upper limit (i.e.< 3). Moreover, is there any constraint on the relative ratio to SO2/NO2? A systematic modelling study would help if the models have been validated by the experimental results.
4. Line 299: How can the reaction of NO2 with S(IV) form nitrate in droplets under dark conditions? Isn't it nitrite?
5. While the aqueous-phase sulfate production rate increase with decreasing RH, how about the total sulfate production rate? That is, will the increased rate overcome the decreased droplet water and result in an overall more sulfate production rate with decreasing RH?
6. Section 4: I feel this part is extending too much. For urban aerosols, the major composition is usually ammonium sulfate and barely NaCl, and the whole pH range would differ. I'd suggest revise this part and focus mainly on the implications on sea salt chemistry. In addtion, with the parameterization given (i.e., γSO2= 0.41 × PNO3- + 0.34 × PCl-), how much sulfate can be produced in the atmosphere under typical coastal conditions?
7. There're some typos and misuse of sysbols, and the authors should check carefully. For example, there's a blank line in the end of Table S1. Is there any reaction missing or simply a typo?Citation: https://doi.org/10.5194/egusphere-2023-223-RC1 -
RC2: 'Comment on egusphere-2023-223', Anonymous Referee #2, 18 Mar 2023
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The manuscript by Zhang and Chan reports an experimental result for reactive uptake of SO2 and NO2 by aqueous NaCl droplets using the micro Raman spectroscopy. The experiment was conducted both under dark and irradiation by a Xe lamp. Nitrate formation was observed for droplets both under dark and light-irradiated conditions. Sulfate was observed only when light was available. The experimental result was parameterized using a chemical kinetic model.
Both the experimental technique and result sound. The major conclusion is well supported by the experimental data. The topic is within the scope of the interest of the readers of the journal, although the reviewer wonders if the concentration ranges of chemical species and light intensities are atmospheric relevant. The manuscript is reasonably well written. Revision will be required prior to the publication of this manuscript.
Major comments
Atmospheric relevance of the experimental conditions
The concentration ranges for both SO2 (~6.5 ppm) and NO2 (~10 ppm) were a few orders higher than that for typical atmospheric conditions. As the authors constructed a chemical kinetic model for reproducing the experimental result, it might be interesting to simulate if the finding of the present study would be important for atmospheric relevant concentration and light intensity ranges.
Descriptions about the pH measurement
Figure S5 shows change of droplet pH during the experiment. The reviewer wonders how the measurement method of pH was validated. As described in the manuscript, pH is very important for sulfate formation. It would be beneficial to have a detailed description about pH measurement. If it were to be estimated by a thermodynamic model, it should clearly be mentioned with detailed information on how it was calculated.
Minor comments
Phase state of NaCl
Some experiments were conducted at 60%RH, which is lower than the deliquescence RH for NaCl. Did droplets form supersaturated aqueous solution, or did they crystalize?
L136: ‘However, our open system experiments with a continuous airflow (~ 0.5 L/min) would have removed the formed O3 efficiently.’
Do the authors have data for O3 concentration in the gas phase to support this statement?
Citation: https://doi.org/10.5194/egusphere-2023-223-RC2
Ruifeng Zhang and Chak Keung Chan
Ruifeng Zhang and Chak Keung Chan
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