the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Feb 2025
The impact of sea spray aerosol on photochemical ozone formation over eastern China: heterogeneous reaction of chlorine particles and radiative effect
Abstract. Eastern China has suffered from severe photochemical O3 (ozone) pollution in recent years. In this coastal region, atmospheric environment can be influenced by sea spray aerosol (SSA) from marine emissions. However, the extent and mechanisms by which SSA affects O3 formation remain incompletely understood. Here, using the WRF-CMAQ model, this study investigates the comprehensive effect of SSA on radical chemistry and O3 formation in the lower troposphere across four seasons. SSA (over 50 % are particulate chlorine) can reach further inland through an atmospheric “bridge” aloft, interacting with the nitrogen-containing gases from continental anthropogenic emissions to reduce NOx levels and release Cl radicals. The NOx reduction increases O3 in VOCs-limited region while decreasing them in NOx-limited zones. Elevated Cl radicals enhances VOCs degradation and O3 formation during morning hours. Meanwhile, the scattering properties of SSA reduces daytime O3 formation by diminishing photolysis rates. Due to the contrast effect of SSA via different mechanisms, the response of O3 vary seasonally and geographically. In winter, SSA increases O3 in eastern China due to the dominant effect of NOx reduction in VOCs-limited regions. In spring and autumn, similar effects occur in North China Plain, whereas southern China sees a decrease due to the NOx reduction in NOx-limited region and reduced photolysis rates. In summer, O3 increases are observed only around Bohai, with reductions elsewhere driven by NOx reductions in NOx-limited regions and decreased photolysis. This study highlights the important, varying, but previously unreported role of SSA in shaping tropospheric photochemistry over eastern China.
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Status: open (until 25 Mar 2025)
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RC1: 'Comment on egusphere-2024-4132', Anonymous Referee #1, 05 Mar 2025
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General comments:
"The impact of sea spray aerosol on photochemical ozone formation over eastern China: heterogeneous reaction of chlorine particles and radiative effect’"by Hong et al. is a well-written and well-motivated manuscript. It is of high interest and importance to detangle the influence of sea salt particles and their chloride-depleting reactions on ozone formation and the concentration of various species involved with the production of ozone. The manuscript is written clearly and concisely. I have a few questions/comments about the chemical reactions considered in the paper and the adjustment of photolysis rates due to increased scattering by aerosol particles. Those comments can be found below.
Specific scientific questions/comments:
Lines 124 – 126: Have the abilities of the CMAQ model to adjust photolysis rates based on the presence of aerosols been verified or evaluated in previous works? Adjustments to photolysis rates are a big discussion point for the paper and its results, so it would be great to understand how accurate the estimated adjustments are in the first place.
Lines 128-1137: I see that the model’s capability was not expanded to include reactions with SO2 (g) and sea salt particles, which can be another source of chlorine radicals via chloride depletion. Would you mind adding some discussion here as to why SO2 was not considered. I also wonder how it would affect the results and your comparisons to other studies if SO2 were considered… some discussion on this would be nice.
Lines 205-210: Just to confirm, when you are discussing particulate Cl- here, is that the chlorine remaining after chloride depleting reactions have already been processed in the model? Or are you just discussing the simple change in particulate chloride concentrations before and after including them in the model (BASE vs. NOSA)? I ask because at first, I thought you were just showing the change in particulate Cl- moving from NOSA to BASE, but before accounting for chloride depleting reactions. However, you mention that particulate chloride concentrations are higher aloft due to depletion reactions near the surface. This implies you have already run the model in full and accounted for depletion reactions when discussing these results. A bit of clarification would be very helpful here to know if 'particulate chloride' is referring to conditions before or after the depletion reactions have been accounted for by the model. The caption in figures S3 and S4 is not explicit in saying if these are particulate Cl- mass concentrations *before or after* accounting for chloride depleting reactions. I was a bit confused as it seemed the discussion of chloride depletion really began in the subsequent section (Sect. 3.2) and that Sect 3.1 was more centered on discussing the results of considering sea salt particles in the model.
Line 207-208: You mention that depletion reactions with HNO3 and H2SO4 may explain lower particulate Cl- concentrations near the surface. I didn’t think you were accounting for reactions with H2SO4 in the model, so how would reactions with H2SO4 be a partial explanation for the lower simulated Cl- mass concentrations near the surface?
Lines 223 – 228: Again, since you are providing specific numbers for changes in NOx, I think it would be great to reiterate that you are not accounting for reactions between SO2 and sea salt particles in your model simulations. If you have an idea of how significantly/insignificantly accounting for reactions with SO2 and sea salt particles would affect your results, that may be good to mention here.
Line 262: You mention increases in Cl radicals after sunrise are more pronounced at higher altitudes. Should there be something to direct readers to Fig. 5? I did not see Fig. 5 mentioned in the text, but it is possible I missed it. Can you be more clear by what you mean that ‘increases in Cl radicals are more pronounced at higher altitudes shortly after sunrise’? In Fig. 5, I see the changes in Cl concentrations after sunrise are pretty similar in the boundary layer compared to right above the boundary layer for the three cities. Changes in Cl radical concentrations above 2 km are often 0.
Lines 275 – 286: It’s interesting that J(NO2) was decreased considerably in the upper troposphere. That high, you would presumably have higher actinic flux and less scattering from sea salt than at the surface. Fig 1 shows that changes in sea salt particle mass (using Na+ as a proxy) in the upper troposphere are close to zero. Are these low mass concentrations enough to scatter enough radiation to reduce J(NO2) to the same degree as it is reduced at the surface, especially considering the higher actinic flux aloft? Perhaps some discussion here to elaborate on the result would be useful and of interest.
Lines 291 – 294: Correct me if I’m wrong but Fig. 7 shows differences between the NOSA and BASE simulations. Thus, wouldn’t the VOC concentrations be the same over the remote oceanic regions in both scenarios? If not, please explain why. If not, then perhaps the main explanation for lower HO2 concentrations over remote oceanic regions is reduced photolysis due to scattering by sea salt particles? It’s impressive the reduction in photolysis due to light scattering is enough to decrease production of HO2 when Cl-radicals from chloride depletion reactions are considered in the model. Fig. 4 shows that there were considerable increases in Cl- radicals over these remote oceanic regions, yet the competing effect of scattering by sea salt particles seems to have countered any increases in HO2 that would be caused by additionally available Cl- radicals. As mentioned earlier, it would be great to understand and/or mention the robustness and accuracy of the changes in photolysis rates due to increased SSA scattering in the model since this is such a prominent topic/result of the paper.
Lines 309-312: Similar comment as above. The decreases in OH concentrations in the troposphere are interesting considering that sea salt particle mass concentrations are presumably low at those altitudes and actinic fluxes are stronger. I know the transport of SSA is possible above the boundary layer as you mentioned, but changes in SSA above 3 km are mostly zero across all months. It is interesting that the scattering from such relatively low amounts of SSA is enough to offset any potential increases in OH due to increased presence of Cl- radicals (although the change in Cl- radicals above 3 km is also close to 0 for all months). There is not really an action item for this comment, I just found it interesting and I think the results would be strengthened by mentioning/citing the validity of the simulated changes in photolysis rates due to increased SSA scattering at least once somewhere in the paper.
Lines 335 – 356: The relationship between the sign of the change in O3 and whether or not the region is NOx- or VOC-limited is interesting. Previously, reductions in concentrations of various radicals and species involved with the NOx-VOC-O3 system of reactions were attributed primarily to changes in photolysis rates due to scattering by sea salt particles. I wonder now if whether those altitudes are NOx- or VOC-limited may be of interest to consider for explaining reductions in the concentrations of those species at higher altitudes? The regime is mentioned when considering the vertical profile in changes of O3 concentrations, so I wonder if a similar discussion of regime would be appropriate to at least mention for other species involved in reactions related to the production of O3?
Purely technical corrections:
Line 260: Typo? Says “Furthe es”
Citation: https://doi.org/10.5194/egusphere-2024-4132-RC1
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