The Atmospheric Oxidizing Capacity in China: Part 1. Roles of different photochemical processes
Abstract. The atmospheric oxidation capacity (AOC) characterizes the ability of the atmosphere to scavenge air pollutants. However, it is not well understood in China, where anthropogenic emissions have changed dramatically in the past decade. A detailed analysis of different parameters that determine the AOC in China is presented on the basis of numerical simulations performed with the regional chemical-meteorological model WRF-Chem. The model results, with the aerosol effects of extinction and heterogeneous processes taken into account, show that the presence of aerosols leads to a decrease in surface ozone of approximately 8–10 ppbv in NOx-limited rural areas and an increase of 5–10 ppbv in VOC-limited urban areas. The ozone reduction in NOx-sensitive regions is due to the combined effect of nitrogen dioxide and peroxy radical uptake on particles and of the light extinction by aerosols, which affects the photodissociation rates. The ozone increase in VOC-sensitive areas is attributed to the uptake of NO2 by aerosols, which is offset by the reduced ozone formation associated with HO2 uptake and with the aerosol extinction. Our study concludes that more than 90 % of the daytime AOC is due to the reaction of the hydroxyl radical with VOCs and carbon monoxide. In urban areas, during summertime, the main contributions to daytime AOC are the reactions of OH with alkene (30–50 %), oxidized volatile organic compounds (OVOCs) (33–45 %), and carbon monoxide (20–45 %). In rural areas, the largest contribution results from the reaction of OH with alkenes (60 %). Nocturnal AOC is dominantly attributed to the nitrate radical (50–70 %). Our results shed light on the contribution of aerosol-related NOx loss and the high reactivity of alkenes for photochemical pollution. With the reduction of aerosols and anthropogenic ozone precursors, the chemistry of nitrogen and temperature-sensitive VOCs will become increasingly important. More attention needs to be paid to the role of photodegradable OVOCs and nocturnal oxidants in the formation of secondary pollutants.
Jianing Dai et al.
Status: open (until 20 Jun 2023)
- RC1: 'Comment on egusphere-2023-731', Anonymous Referee #1, 30 May 2023 reply
Jianing Dai et al.
Jianing Dai et al.
Viewed (geographical distribution)
The authors present a detailed narrative of WRF-Chem model outputs over China in one summer and one winter month during 2018. The main objective of this study is to characterize the current chemical conditions in China, particularly in light of the increasing ozone levels observed across the North China Plain since 2013. This manuscript provides a starting point for a companion paper that focuses on emissions changes.
While the manuscript does not necessarily present new science, the authors assess their model result with observations where possible, and provide many quantitative comparisons with prior studies. The topic is appropriate for ACP. The paper provides a useful and comprehensive quantitative assessment that the academic community will use as a useful point of comparison. I have a few comments below:
(1) I have some concerns about comparing outputs from a model at 36 km resolution to ground-based, urban observations. Does the coarse resolution cause any systematic biases?
(2) Model validation is lacking. The implications of model/observations discrepancies should be discussed. Specifically:
(3) The assessment of ozone production regimes through the use of formaldehyde to NOx ratios (FNRs) does not contribute to the discussion.
FNRs are arguably useful when they are known to reflect more direct, mechanistic metrics such as LROX/LNOx. If correlation is found/known/assumed, FNR observations can then be used to infer ozone production regimes. In this manuscript, no FNR observations are used, and direct metrics are already discussed. Therefore, the motivation for discussing FNRs is not well stated.
Furthermore, there are documented issues with the use of "threshold" FNR values (see Souri et al. (2020) and subsequent papers). The citation provided for the threshold on line 522 (Jing et al., 2021) is missing from the list of references.
Overall, I recommend that the authors either incorporate FNR observations, expand the discussion on what can be learned from this metric, or consider excluding the discussion leaving only the more mechanistic descriptors of ozone production regimes.
Figure S9: OH instead of HO on the y axis.
Souri, A. H., Nowlan, C. R., Wolfe, G. M., Lamsal, L. N., Chan Miller, C. E., Abad, G. G., Janz, S. J., Fried, A., Blake, D. R., Weinheimer, A. J., Diskin, G. S., Liu, X., and Chance, K.: Revisiting the effectiveness of HCHO/NO2 ratios for inferring ozone sensitivity to its precursors using high resolution airborne remote sensing observations in a high ozone episode during the KORUS-AQ campaign, Atmos. Environ., 224, 117341, https://doi.org/10.1016/j.atmosenv.2020.117341, 2020.