With the intensification of global warming and economic development in China, the near-surface ozone (O₃) concentration has been increasing recently, especially in the Beijing-Tianjin-Hebei (BTH) region, which is the political and economic center of China. However, O₃ has been measured in real time only over the past few years, and the observational records are discontinuous. Therefore, we propose a new method (WRFC-XGB) to establish a near-surface O₃ concentration dataset in the BTH region by integrating the Weather Research and Forecasting with Chemistry (WRF-Chem) model with the extreme gradient boosting (XGBoost) algorithm. Based on this method, the 8-h maximum daily average (MDA8) O₃ concentrations are obtained with full spatiotemporal coverage at a spatial resolution of 0.1° × 0.1° across the BTH region in 2018. Two evaluation methods, sample- and station-based 10-fold cross-validation (10-CV), are used to assess our method. The sample-based (station-based) 10-CV evaluation results indicate that WRFC-XGB can achieve excellent accuracy with a high coefficient of determination (R²) of 0.95 (0.91), low root mean square error (RMSE) of 13.50 (17.70) µg m⁻³, and mean absolute error (MAE) of 9.60 (12.89) µg m⁻³. In addition, superb spatiotemporal consistencies are confirmed for this model, including the estimation of high O₃ concentrations, and our WRFC-XGB model outperforms traditional models and previous studies in data mining. In addition, the proposed model can be applied to estimate the O₃ concentration when it has not been measured. Furthermore, the spatial distribution analysis of the MDA8 O₃ in 2018 reveals that O₃ pollution in the BTH region exhibits significant seasonality. Heavy O₃ pollution episodes mainly occur in summer, and the high O₃ loading is distributed mainly in the southern BTH areas, which will pose challenges to atmospheric environmental governance for local governments.

Carbonaceous aerosols have great influence on the air quality, human health and climate change. Except for organic carbon (OC) and elemental carbon (EC), brown carbon (BrC) mainly originates from biomass burning as a group of OC, with strong absorption from the visible to near-ultraviolet wavelengths, and makes a considerable contribution to global warming. Large numbers of studies have reported long-term observation of OC and EC concentrations throughout the world, but studies of BrC based on long-term observations are rather limited. In this study, we established a two-wavelength method (658 and 405 nm) applied in the Sunset thermal–optical carbon analyzer. Based on a 1-year observation, we firstly investigated the characteristics, meteorological impact and transport process of OC and EC. Since BrC absorbs light at 405 nm more effectively than 658 nm, we defined the enhanced concentrations (dEC = EC_405 nm − EC_658 nm) and gave the possibility of providing an indicator of BrC. The receptor model and MODIS fire information were used to identify the presence of BrC aerosols. Our results showed that the carbonaceous aerosol concentrations were the highest in winter and lowest in summer. Traffic emission was an important source of carbonaceous aerosols in Nanjing. Receptor model results showed that strong local emissions were found for OC and EC; however, dEC was significantly affected by regional or long-range transport. The $\mathrm{dEC}/\mathrm{OC}$ and $\mathrm{OC}/\mathrm{EC}$ ratios showed similar diurnal patterns, and the $\mathrm{dEC}/\mathrm{OC}$ increased when the $\mathrm{OC}/\mathrm{EC}$ ratios increased, indicating strong secondary sources or biomass burning contributions to dEC. A total of two biomass burning events both in summer and winter were analyzed, and the results showed that the dEC concentrations were obviously higher on biomass burning days; however, no similar levels of the OC and EC concentrations were found both in biomass burning days and normal days in summer, suggesting that biomass burning emissions made a great contribution to dEC, and the sources of OC and EC were more complicated. Large number of open fire counts from the northwestern and southwestern areas of the study site were observed in winter and significantly contributed to OC, EC and dEC. In addition, the nearby Yangtze River Delta area was one of the main potential source areas of dEC, suggesting that anthropogenic emissions could also be important sources of dEC. The results proved that dEC can be an indicator of BrC on biomass burning days. Our modified two-wavelength instrument provided more information than the traditional single-wavelength thermal–optical carbon analyzer and gave a new idea about the measurement of BrC; the application of dEC data needs to be further investigated.

Compared with other regions in China, air pollution on the North China Plain (NCP) is serious. Fine particle pollution has been studied in-depth, but there is less research on long-term troposphere ozone (O₃) variation. This study focuses on the summer interannual tropospheric O₃ variation on the NCP and its influential factors. Our analysis relies on satellite observations (O₃, nitrogen dioxide (NO₂), sulfur dioxide (SO₂), carbon monoxide (CO), and formaldehyde (HCHO), determined as vertical column density of the troposphere) and dynamical processes (El Niño-Southern Oscillation (ENSO), potential vorticity (PV), the quasibiennial oscillation (QBO), and East Asian summer monsoon index (EASMI)). Our results show the vertical column density of tropospheric O₃ has a transition from the increasing trend to decreasing trend during the summer of 2005–2016. The summer series of tropospheric O₃ show two distinct phases: the first phase (2005–2011), with an average growth rate of 0.55 ± 0.20 DU/yr, and a second phase (2012–2016), with an average reduction rate of 0.16 ± 0.23 DU/yr. The tropospheric NO₂ column in the NCP also has a transition from the increasing trend to decreasing trend during the summer of 2005–2016. Tropospheric NO₂ and CO column concentrations obtained from satellite observations indicate that emission reductions might be the main cause of the tropospheric O₃ decrease. Particularly, the reduction of nitrogen oxides (NO_x) is more significant, and NO₂ decreased by (0.45 ± 0.11) × 10¹⁵ molec·cm⁻² per year in summer since 2012. However, tropospheric column HCHO shows an increase of 0.05 × 10¹⁵ molec·cm⁻² per year during the whole period of 2005 to 2016. An O₃-NO_x-VOC sensitivity experiment in the NCP showed that the O₃ is still in a NO_x-saturated state in some heavily polluted cities, although the NO_x emissions are decreasing overall. In addition to the chemical reactions, atmospheric dynamic processes also have an effect on tropospheric O₃. Finally, we built a model to analyze the contributions of chemical processes and dynamic processes to the tropospheric O₃ column in the NCP. For the chemical process variables, 69.73% of the observed trend of tropospheric O₃ could be explained by the NO₂ tropospheric column. Therefore, the reduction of tropospheric O₃ since 2012 is associated with the reduction of NOx. For the dynamical process variables, ENSO, PV, and EASMI can explain 60.64% of the observed trend of tropospheric O₃. This result indicates that the atmospheric circulation of the western Pacific Ocean in summer has a significant impact on the interannual trends of tropospheric O₃ in the NCP. It is also found that chemical processes had a more important impact on interannual tropospheric O₃ than dynamic processes, although the dynamic processes cannot be neglected.

Based on satellite observations, the effects of emission reduction measures on air pollutants (NO₂ (nitrogen dioxide), SO₂ (sulfur dioxide), and aerosols) were estimated in Beijing and its surroundings during the 2015 World Championships and Parade (WCP) held in Beijing. Compared with the same period in the previous three years (2012–2014), the tropospheric vertical column densities of NO₂ and SO₂ in 2015 were significantly reduced in Beijing and its surroundings. During the WCP, aerosol optical depth was reduced most significantly in Beijing, while NO₂ and SO₂ were reduced most significantly in Hebei and Tianjin, respectively. The meteorological conditions were also favorable during this period. During the WCP, the North China Plain was located under high pressure from the north of China. A southward pressure gradient force and weak northerly and northeasterly winds blocked the transport of pollution from southern and western parts of the industrial area (Hebei, Shanxi and Shandong provinces). Cold air masses pushed southwards weakly. These conditions were beneficial to the removal of air pollutants in Beijing.

Atmospheric fine particulate matter (PM_2.5) pollutions are of particular concern because of their direct and indirect harm to humans and organisms. China has suffered from severe air pollution for the past ten years, related to heavy pollution emissions and compounded by the effects of atmospheric circulation. This study applied statistical methods, observational data of ground pollutants, and meteorological data to analyze the impact of large-scale atmospheric circulations on PM_2.5 pollution over China. Empirical orthogonal function (EOF) analysis was used to evaluate the main PM_2.5 patterns and total contributions of the leading four EOFs. The results indicate that the total contributions of the leading four EOFs accounted for 50.5% of the total variance, reflecting four main types of PM_2.5 pollution, namely, overall pollution phase, north–south phase, east–west phase and north–center–south phase, with contributions of 28.4%, 9.7%, 6.5% and 5.9%, respectively. We selected indices of the Asian Polar Vortex (APV) to analyze the impact of large-scale atmospheric circulations on PM_2.5 pollution over China. The most pronounced APV control occurred in Beijing and its surroundings, specifically, along the Bohai Sea and the Northeast Plain.

Carbonaceous aerosols have great influence on the air quality, human health and climate change. Except for organic carbon (OC) and elemental carbon (EC), brown carbon (BrC), mainly originates from biomass burning, as a group of OC with strong absorption from the visible to near-ultraviolet wavelengths, makes a considerable contribution to global warming. Large amounts of studies have reported long-term observation of OC and EC concentrations throughout the word, but studies of BrC based on long-term observations are rather limited. In this study, we established a two-wavelength method (658 nm and 405 nm) applied in the Sunset thermo/optical carbon analyzer. Based on one-year observation, we firstly investigated the characteristics, meteorological impact and transport process of OC and EC. Due to BrC absorbs light at 405 nm more effectively than 658 nm, we defined the enhanced concentrations (dEC = EC_405 nm−EC_658 nm) and gave the possibility to provide an indicator of BrC. The receptor model and MODIS fire information were used to identify the presence of BrC aerosols. Our results showed that the carbonaceous aerosols concentrations were highest in winter and lowest in summer. Traffic emission was an important source of carbonaceous aerosols in Nanjing. Receptor model results showed that strong local emissions were found in OC and EC aerosols, however dEC aerosols were significantly affected by regional or long-range transport. The dEC / OC and OC / EC ratios showed similar diurnal patterns and the dEC / OC increased when the OC / EC ratios increased, indicating strong secondary sources or biomass burning contributions to dEC. Two biomass burning events both in summer and winter were analyzed and the results showed that the dEC concentrations were obvious higher in biomass burning days, however, no similar levels of the OC and EC concentrations were found both in biomass burning days and normal days in summer, suggesting that biomass burning emission made a great contribution to dEC and the sources of OC and EC were more complicated. Large number of open fire counts from the northwest and southwest areas of the study site were monitored in winter, significantly contributed to OC, EC and dEC. In addition, the near-by YRD area was one of the main potential source areas of dEC, suggesting that anthropogenic emissions could also be important sources of dEC. The results proved that dEC can be an indicator of BrC in biomass burning days. Our modified two-wavelength instrument provided more information than traditional single-wavelength thermo/optical carbon analyzer and gave a new idea about the measurement of BrC, the application of dEC data need to be further investigated.