Surface ozone (O₃) pollution during summer (June–August) over eastern China has become more severe in recent years, resulting in a co-occurrence of surface O₃ and PM_2.5 (particulate matter with aerodynamic diameters ≤ 2.5 µm in the air) pollution. However, the mechanisms regarding how the synoptic weather pattern (SWP) might influence this compound pollution remain unclear. In this study, we applied the T-mode principal component analysis (T-PCA) method to objectively classify the occurrence of four SWPs over eastern China, based on the geopotential heights at 500 hPa during summer (2015–2018). These four SWPs over eastern China were closely related to the western Pacific subtropical high (WPSH), exhibiting significant intra-seasonal and interannual variations. Based on ground-level air quality observations, remarkable spatial and temporal disparities of surface O₃ and PM_2.5 pollution were also found under the four SWPs. In particular, there were two SWPs that were sensitive to compound pollution (Type 1 and Type 2). Type 1 was characterized by a stable WPSH ridge with its axis at about 22^∘ N and the rain belt located south of the Yangtze River Delta (YRD); Type 2 also exhibited WPSH dominance (ridge axis at ∼ 25^∘ N) but with the rain belt (over the YRD) at a higher latitude compared to Type 1. In general, SWPs have played an important role as driving factors of surface O₃–PM_2.5 compound pollution in a regional context. Our findings demonstrate the important role played by SWPs in driving regional surface O₃–PM_2.5 compound pollution, in addition to the large quantities of emissions, and may also provide insights into the regional co-occurring high levels of both PM_2.5 and O₃ via the effects of certain meteorological factors.

Surface ozone (O₃) is an important air pollutant and greenhouse gas. Land use and land cover is one of the critical factors influencing ozone, in addition to anthropogenic emissions and climate. Land use and land cover change (LULCC) can on the one hand affect ozone “biogeochemically”, i.e., via dry deposition and biogenic emissions of volatile organic compounds (VOCs). LULCC can on the other hand alter regional- to large-scale climate through modifying albedo and evapotranspiration, which can lead to changes in surface temperature, hydrometeorology, and atmospheric circulation that can ultimately impact ozone “biogeophysically”. Such biogeophysical effects of LULCC on ozone are largely understudied. This study investigates the individual and combined biogeophysical and biogeochemical effects of LULCC on ozone and explicitly examines the critical pathway for how LULCC impacts ozone pollution. A global coupled atmosphere–chemistry–land model is driven by projected LULCC from the present day (2000) to the future (2050) under RCP4.5 and RCP8.5 scenarios, focusing on the boreal summer. Results reveal that when considering biogeochemical effects only, surface ozone is predicted to have slight changes by up to 2 ppbv maximum in some areas due to LULCC. It is primarily driven by changes in isoprene emission and dry deposition counteracting each other in shaping ozone. In contrast, when considering the combined effect of LULCC, ozone is more substantially altered by up to 5 ppbv over several regions in North America and Europe under RCP4.5, reflecting the importance of biogeophysical effects on ozone changes. In boreal and temperate mixed forests with intensive reforestation, enhanced net radiation and sensible heat induce a cascade of hydrometeorological feedbacks that generate warmer and drier conditions favorable for higher ozone levels. In contrast, reforestation in subtropical broadleaf forests has minimal impacts on boundary-layer meteorology and ozone air quality. Furthermore, significant ozone changes are also found in regions with only modest LULCC, which can only be explained by “remote” biogeophysical effects. A likely mechanism is that reforestation induces a circulation response, leading to reduced moisture transport and ultimately warmer and drier conditions in the surrounding regions with limited LULCC. We conclude that the biogeophysical effects of LULCC are important pathways through which LULCC influences ozone air quality both locally and in remote regions even without significant LULCC. Overlooking the effects of hydrometeorological changes on ozone air quality may cause underestimation of the impacts of LULCC on ozone pollution.

Coal-to-gas/electricity conversion (hereafter referred to as CTGC/CTEC) as the core project of a clean heating campaign has been widely adopted to replace and reduce the combustion of residential coal in Northern China since 2017. In this study, simulations based on the WRF-Chem model were carried out to quantitatively assess the impacts of the CTGC/CTEC project on air quality in the Beijing–Tianjin–Hebei (BTH) region. It was found that the CTGC/CTEC projects exert a remarkable effect on improving the air quality in the BTH region, especially in the plain area. The maximum decrease in the concentrations of PM_2.5 and PM₁₀ averaged during January can reach 30 and 40 μg/m³, respectively. In addition, the spillover effects due to CTGC/CTEC projects are rather small; that is, the local reduced emissions tend to provide more benefit to the local air quality but less for its surrounding regions. It is also noteworthy that the effects due to meteorological condition changes are comparable with, or even larger, than those due to CTGC/CTEC projects, which are not spatially uniform for the BTH region among various cities. Overall, these results not only demonstrate the effectiveness of CTGC/CTEC projects on air-quality improvement in the BTH region, but also indicate the importance of meteorological conditions in modulating the local air quality. To sustain better air quality in the future, residential coal replacement, all over China, can be further promoted. In addition, continued policy refinement can be essential for the nationwide implementation of clean heating projects.

The Intensive Supervision Mechanism (hereafter referred to as ISM) is one of the most important institutional management innovations for air pollution control in China, but there is currently no consensus on the effects of the ISM on air quality improvement. In this study, a reliable quantitative model based on the Difference-in-Differences (DID) analysis was designed to evaluate the impacts of ISM on air quality (as indicated by good air quality days (hereafter referred to as GAD) and the concentrations of six major air pollutants (i.e. PM_2.5, PM₁₀, O₃_8H, NO₂, SO₂, and CO)), in China with focuses on the implementation cities of Henan Province. To optimize the model design, six meteorological factors, five socio-economic indicators, and VIIRS (Visible Infrared Imaging Radiometer Suite) data were also considered as alternative control variables for more comprehensive and effective results. In addition, the redundancy analysis (RDA) and Monte Carlo simulation were conducted to determine the optimal combination of those control variables which can best reflect the changes in explanatory variables. The main findings are as follows: (1) the statistical model applied in this study can well evaluate the impacts of ISM; (2) the implementation of ISM can significantly reduce the concentrations of SO_2, CO, and NO₂, but the improvements for PM_2.5, PM₁₀, GAD and O₃_8H were not significant. (3) the potential for air quality improvement due to ISM tends to be reduced over time, and thus the positive effects of ISM at its second stage were not increased significantly compared with those observed during its first stage. In general, those results not only demonstrate the effectiveness of ISM on air quality improvement, but also provide insights into how the ISM can be optimized to gain a sustained improvement of the ambient air quality in the future.