We investigated the spatial-temporal characteristics of the correlations between observed PM_2.5 and O₃ over China at a national-scale level, and examined the underlying reasons for the varying PM_2.5–O₃ correlations by using a chemical transport model. The PM_2.5 concentrations were positively correlated with O₃ concentrations for most regions and seasons over China, while negative correlations were mainly observed in northern China during winter. The strongest positive PM_2.5–O₃ correlations with correlation coefficients (r) larger than +0.7 existed in southern China during July, and the strongest negative correlations (r < −0.5) were observed in northern China during January. It was a very interesting phenomenon that the positive PM_2.5–O₃ correlations prevailed for high air temperature samples, while the negative correlations were generally found in cold environments. Together, the effective inhibitory effect of PM_2.5 on O₃ generation by reducing photolysis rates and the strong titration effect of freshly emitted NO with O₃ contributed to the strongest negative PM_2.5–O₃ correlations in northern China during January (i.e., in cold environments). The strongest positive correlations in southern China during July (i.e., at high temperature), however, were mainly attributed to the promoting effect of high O₃ concentration and active photochemical activity on secondary particle formation.

A total of 14 chemical transport models (CTMs) participated in the first topic of the Model Inter-Comparison Study for Asia (MICS-Asia) phase III. These model results are compared with each other and an extensive set of measurements, aiming to evaluate the current CTMs' ability in simulating aerosol concentrations, to document the similarities and differences among model performance, and to reveal the characteristics of aerosol components in large cities over East Asia. In general, these CTMs can well reproduce the spatial–temporal distributions of aerosols in East Asia during the year 2010. The multi-model ensemble mean (MMEM) shows better performance than most single-model predictions, with correlation coefficients (between MMEM and measurements) ranging from 0.65 (nitrate, ${\mathrm{NO}}_{\mathrm{3}}^{-}$ ) to 0.83 (PM_2.5). The concentrations of black carbon (BC), sulfate (${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ ), and PM₁₀ are underestimated by MMEM, with normalized mean biases (NMBs) of −17.0 %, −19.1 %, and −32.6 %, respectively. Positive biases are simulated for ${\mathrm{NO}}_{\mathrm{3}}^{-}$ (NMB = 4.9 %), ammonium (${\mathrm{NH}}_{\mathrm{4}}^{+}$ ) (NMB = 14.0 %), and PM_2.5 (NMB = 4.4 %). In comparison with the statistics calculated from MICS-Asia phase II, frequent updates of chemical mechanisms in CTMs during recent years make the intermodel variability of simulated aerosol concentrations smaller, and better performance can be found in reproducing the temporal variations of observations. However, a large variation (about a factor of 2) in the ratios of SNA (sulfate, nitrate, and ammonium) to PM_2.5 is calculated among participant models. A more intense secondary formation of ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ is simulated by Community Multi-scale Air Quality (CMAQ) models, because of the higher SOR (sulfur oxidation ratio) than other models (0.51 versus 0.39). The NOR (nitric oxidation ratio) calculated by all CTMs has larger values (∼0.20) than the observations, indicating that overmuch ${\mathrm{NO}}_{\mathrm{3}}^{-}$ is simulated by current models. NH₃-limited condition (the mole ratio of ammonium to sulfate and nitrate is smaller than 1) can be successfully reproduced by all participant models, which indicates that a small reduction in ammonia may improve the air quality. A large coefficient of variation (CV > 1.0) is calculated for simulated coarse particles, especially over arid and semi-arid regions, which means that current CTMs have difficulty producing similar dust emissions by using different dust schemes. According to the simulation results of MMEM in six large Asian cities, different air-pollution control plans should be taken due to their different major air pollutants in different seasons. The MICS-Asia project gives an opportunity to discuss the similarities and differences of simulation results among CTMs in East Asian applications. In order to acquire a better understanding of aerosol properties and their impacts, more experiments should be designed to reduce the diversities among air quality models.

Fine-particle pollution associated with haze threatens human health, especially in the North China Plain region, where extremely high PM_2.5 concentrations are frequently observed during winter. In this study, the Weather Research and Forecasting with Chemistry (WRF-Chem) model coupled with an improved integrated process analysis scheme was used to investigate the formation and evolution mechanisms of a haze event over the Beijing–Tianjin–Hebei (BTH) region in December 2015; this included an examination of the contributions of local emissions and regional transport to the PM_2.5 concentration in the BTH area, and the contributions of each detailed physical or chemical process to the variations in the PM_2.5 concentration. The mechanisms influencing aerosol radiative forcing (including aerosol direct and indirect effects) were also examined by using process analysis. During the aerosol accumulation stage (16–22 December, Stage 1), the near-surface PM_2.5 concentration in the BTH region increased from 24.2 to 289.8 µg m⁻³, with the contributions of regional transport increasing from 12 % to 40 %, while the contribution of local emissions decreased from 59 % to 38 %. During the aerosol dispersion stage (23–27 December, Stage 2), the average concentration of PM_2.5 was 107.9 µg m⁻³, which was contributed by local emissions (51 %) and regional transport (24 %). The 24 h change (23:00 minus 00:00 LST) in the near-surface PM_2.5 concentration was +43.9 µg m⁻³ during Stage 1 and −41.5 µg m⁻³ during Stage 2. The contributions of aerosol chemistry, advection, and vertical mixing to the 24 h change were +29.6 (+17.9) µg m⁻³, −71.8 (−103.6) µg m⁻³, and −177.3 (−221.6) µg m⁻³ during Stage 1 (Stage 2), respectively. Small differences in the contributions of other processes were found between Stage 1 and Stage 2. Therefore, the PM_2.5 increase over the BTH region during the haze formation stage was mainly attributed to strong production by the aerosol chemistry process and weak removal by the advection and vertical mixing processes. When aerosol radiative feedback was considered, the 24 h PM_2.5 increase was enhanced by 4.8 µg m⁻³ during Stage 1, which could be mainly attributed to the contributions of the vertical mixing process (+22.5 µg m⁻³), the advection process (−19.6 µg m⁻³), and the aerosol chemistry process (+1.2 µg m⁻³). The restrained vertical mixing was the primary reason for the enhancement in the near-surface PM_2.5 increase when aerosol radiative forcing was considered.

We examined the impacts of aerosol–radiation interactions, including the effects of aerosol–photolysis interaction (API) and aerosol–radiation feedback (ARF), on surface-layer ozone (O₃) concentrations during four multi-pollutant air pollution episodes characterized by high O₃ and PM_2.5 levels during 28 July to 3 August 2014 (Episode1), 8–13 July 2015 (Episode2), 5–11 June 2016 (Episode3), and 28 June to 3 July 2017 (Episode4) in North China, by using the Weather Research and Forecasting with Chemistry (WRF-Chem) model embedded with an integrated process analysis scheme. Our results show that API and ARF reduced the daytime shortwave radiative fluxes at the surface by 92.4–102.9 W m⁻² and increased daytime shortwave radiative fluxes in the atmosphere by 72.8–85.2 W m⁻², as the values were averaged over the complex air pollution areas (CAPAs) in each of the four episodes. As a result, the stabilized atmosphere decreased the daytime planetary boundary layer height and 10 m wind speed by 129.0–249.0 m and 0.05–0.15 m s⁻¹, respectively, in CAPAs in the four episodes. Aerosols were simulated to reduce the daytime near-surface photolysis rates of J[NO₂] and J[O¹D] by 1.8 × 10⁻³–2.0 × 10⁻³ and 5.7 × 10⁻⁶–6.4 × 10⁻⁶ s⁻¹, respectively, in CAPAs in the four episodes. All of the four episodes show the same conclusion, which is that the reduction in O₃ by API is larger than that by ARF. API (ARF) was simulated to change daytime surface-layer O₃ concentrations by −8.5 ppb (parts per billion; −2.9 ppb), −10.3 ppb (−1.0 ppb), −9.1 ppb (−0.9 ppb), and −11.4 ppb (+0.7 ppb) in CAPAs of the four episodes, respectively. Process analysis indicated that the weakened O₃ chemical production made the greatest contribution to API effect, while the reduced vertical mixing was the key process for ARF effect. Our conclusions suggest that future PM_2.5 reductions may lead to O₃ increases due to the weakened aerosol–radiation interactions, which should be considered in air quality planning.

The co-polluted days by ozone (O₃) and PM_2.5 (particulate matter with an aerodynamic equivalent diameter of 2.5 µm or less) (O₃–PM_2.5PDs) were frequently observed in the Beijing–Tianjin–Hebei (BTH) region in warm seasons (April–October) of 2013–2020. We applied the 3-D global chemical transport model (GEOS-Chem) to investigate the chemical and physical characteristics of O₃–PM_2.5PDs by composited analyses of such days that were captured by both the observations and the model. Model results showed that, when O₃–PM_2.5PDs occurred, the concentrations of hydroxyl radical and total oxidant, sulfur oxidation ratio, and nitrogen oxidation ratio were all high, and the concentrations of sulfate at the surface were the highest among all pollution types. We also found unique features in vertical distributions of aerosols during O₃–PM_2.5PDs; concentrations of PM_2.5 decreased with altitude near the surface but remained stable at 975–819 hPa. Process analyses showed that secondary aerosols (nitrate, ammonium, and sulfate) had strong chemical productions at 913–819 hPa, which were then transported downward, resulting in the quite uniform vertical profiles at 975–819 hPa on O₃–PM_2.5PDs. The weather patterns for O₃–PM_2.5PDs were characterized by anomalous high-pressure system at 500 hPa as well as strong southerlies and high RH at 850 hPa. The latter resulted in the strong chemical productions around 850 hPa on O₃–PM_2.5PDs. The physical and chemical characteristics of O₃–PM_2.5PDs are quite different from those of polluted days by either O₃ alone or PM_2.5 alone and have important implications for air quality management.

We apply an online‐coupled meteorology‐chemistry model (WRF‐Chem) embedded with an improved process analysis to examine aerosol‐radiation feedback (ARF) impacts on effectiveness of emission control due to Coronavirus Disease 2019 (COVID‐19) lockdown over North China Plain. Emission reduction alone induces PM_2.5 decrease by 16.3 μg m⁻³ and O₃ increase by 10.2 ppbv during COVID‐19 lockdown. The ARF enhances PM_2.5 decrease by 2.7 μg m⁻³ (16.6%) and O₃ increase by 0.8 ppbv (7.8%). The ARF‐induced enhancement of PM_2.5 decline is mostly attributed to aerosol chemistry process, while enhancement of O₃ rise is ascribed to physical advection and vertical mixing processes. A set of sensitivity experiments with emission reductions in different degrees indicate that the ARF‐induced enhancements of PM_2.5 declines (O₃ rises) follow a robust linear relationship with the emission‐reduction‐induced PM_2.5 decreases. The fitted relationship has an important implication for assessing the effectiveness of emission abatement at any extent.