| 14 Jul 2026
Meteorological conditions drive divergent responses of co-occurring PM2.5 and O3 pollution to emission reductions and role of aerosol feedback in Beijing–Tianjin–Hebei-Shandong
Abstract. Co‑occurring PM2.5 and O3 pollution (Double High Pollution, DHP) presents a growing air quality concern, yet its response to emission controls under different meteorological conditions is poorly quantified. Previous studies have focused on synoptic‑scale or single meteorological variable with limited consideration of aerosol-meteorology feedback. This study investigated the DHP response to NOx–VOC emission reductions and the role of aerosol-meteorology feedback in Beijing–Tianjin–Hebei-Shandong in July 2020 using the WRF-Chem model. DHP occurred under warm (23–31 °C), moderately humid (45–80 %), and shallow boundary layer (0.5–1.1 km) conditions. These typical meteorological conditions were classified into five types: Convective, Stable, WarmHumid, DryHot, and Moderate based on boundary layer height, temperature and humidity. PM2.5 was highest under Stable and WarmHumid (~46 μg m-3) and lowest under DryHot (~40 μg m-3); O3 was highest under DryHot and Moderate (~83–84 ppb) and lowest under WarmHumid (~76 ppb). O3 responded most strongly to emission controls under WarmHumid (-25.2 % at 50 %/50 % NOx/VOC) and weakest under DryHot (-23.3 %) with stronger NOx sensitivity under WarmHumid and Convective; PM2.5 reductions were largest under Stable and WarmHumid (-20.9 % and -20.6 %) and smallest under DryHot (-15.2 %). Aerosol feedback enhanced PM2.5 reduction most under Stable and WarmHumid but weakened O3 reduction under Stable when NOx cuts were large without VOC co-reduction, driven by the most lasting PBLH increase and strong OH amplification. These results demonstrate that meteorological condition-specific strategies, particularly coordinated NOx–VOC control under stagnant and humid conditions, are essential for effective DHP mitigation.
This manuscript addresses a timely and relevant topic about how meteorological conditions modulate the response of co-occurring PM2.5 and O3 pollution to precursor emission reductions, including the role of online aerosol-meteorology coupling. The response matrices and the attempt to distinguish five local meteorological regimes are potentially useful. However, the current manuscript overstates the robustness and generality of several conclusions. The framing of DHP versus separate pollutant responses needs to be sharpened, the effective sample size and uncertainty are not addressed, and the paired ARI/ACI experiment does not isolate the individual feedback pathways. In addition, the model has substantial PBLH, wind, and O3 biases that directly affect the central analysis. I therefore recommend major revision. Comments to the authors
Major comments
1. At line 37, the abstract motivates the study through NOx-VOC emission reductions, but the manuscript is framed as a study of double-high pollution (DHP), which requires PM2.5 and O3 to be high simultaneously. Reducing NOx and VOCs primarily targets ozone chemistry; NOx also affects nitrate aerosol, but SO2, BC, OC, primary particulate matter, NH3, and other particle relevant emissions are not reduced in the stated scenarios. Thus, the current experiments do not represent general “emission reductions” or a complete DHP control strategy. Please clearly reframe the work as the response of DHP to NOx-VOC precursor perturbations. The abstract and conclusions should explain why a NOx-VOC-only matrix is scientifically sufficient for the question being asked, or narrow the question accordingly.
2. At line 46, “aerosol feedback” is ambiguous. It could mean aerosol-radiation interaction (ARI), aerosol-cloud interaction (ACI), the resulting changes in temperature, PBLH, humidity, and clouds, or the subsequent effect of those meteorological changes on DHP. Please define the term in the abstract and introduction using a clear causal chain. The paper should distinguish direct aerosol effects on radiation/cloud microphysics from the meteorological response and from the final change in PM2.5/O3 or joint DHP occurrence. It would also help to state whether the intended quantity is the feedback contribution to pollutant concentrations, the feedback contribution to the emission-control response, or the feedback contribution to the probability of DHP. At present, the abstract and Sect. 3.5 mainly discuss PM2.5 and O3 separately, so the reader cannot tell how the “aerosol feedback” changes the DHP.
3. The manuscript defines DHP using daily mean PM2.5 > 35 ug m-3 and MDA8 O3 > 100 ug m-3, while most of the results are reported in ppb. Please include a sensitivity test using alternative DHP thresholds or at least quantify how many grid-days change when the Class I thresholds are perturbed. Because DHP identification drives every subsequent result, the threshold choice should not be presented as self-evidently reliable.
4. EXP_W and EXP_WO differ simultaneously in aerosol-radiation interaction, aerosol-cloud interaction, cloud microphysics, radiation, and potentially the meteorological evolution. Setting cloud droplet number concentration to 10 cm-3 over continental BTHS is a strong intervention and is not equivalent to simply “turning off ACI.” The difference between the two experiments therefore represents a combined response to the chosen aerosol-coupling perturbation, not an isolated, uniquely attributable aerosol-meteorology feedback. The revised manuscript should also quantify changes in other cloud properties including cloud water, CDNC, and key meteorological fields between EXP_W and EXP_WO, and should explain whether the fixed-CDNC configuration changes wet removal or cloud lifetime.
5. The analysis covers 1-30 July 2020 only. July 2020 may contain unusual emissions, circulation, humidity, and post-lockdown conditions, and the five classes may not represent DHP in other seasons or years. The limitations section acknowledges this but the abstract, conclusions, and management implications remain broad.
Minor comments:
Use one regional abbreviation consistently. The manuscript alternates among BTHS, BTH, SD, and “Beijing-Tianjin-Hebei-Shandong”; define the spatial domain and the observational subregions once and use them consistently.
Correct the unit and wording errors in the text and captions, including “45% to 80%°C” in Sect. 3.2, “Figrure 7,” “effects” where “affects” is intended, and the double-minus notation “--0.12” in the discussion of Fig. 10.
Standardize O3 units throughout. Table 2 reports O3 in ug m-3, while the text and most figures use ppb.
The captions of Fig. S7 use labels such as Stable_Inv, Neutral_WarmHumid, Neutral_DryHot, and Neutral_Cool, which do not match the five classes used in the main text.
In Sect. 3.2, the statement that DHP O3 is “consistently above the all samples median” is inconsistent with the reported WarmHumid median of about 76 ppb versus an all-sample median near 79 ppb. Please revise the statement.
Please proofread the manuscript for grammar and spacing, for example “conditions types,” “which is attribute,” missing spaces around parentheses, and inconsistent use of hyphen/en dash in compound terms.