The simultaneous observation and analysis of atmospheric fine particles (PM_2.5) on a regional scale is an important approach to develop control strategies for haze pollution. In this study, samples of filtered PM_2.5 were collected simultaneously at three urban sites (Beijing, Tianjin, and Shijiazhuang) and at a regional background site (Xinglong) in the Beijing–Tianjin–Hebei (BTH) region from June 2014 to April 2015. The PM_2.5 at the four sites was mainly comprised of organic matter, secondary inorganic ions, and mineral dust. Positive matrix factorization (PMF) demonstrated that, on an annual basis, secondary inorganic aerosol was the largest PM_2.5 source in this region, accounting for 29.2–40.5 % of the PM_2.5 mass at the urban sites; the second-largest PM_2.5 source was motor vehicle exhaust, particularly in Beijing (24.9 %), whereas coal combustion was also a large source in Tianjin (12.4 %) and Shijiazhuang (15.5 %), with particular dominance in winter. Secondary inorganic aerosol plays a vital role in the haze process, with the exception of the spring haze in Shijiazhuang and Tianjin, for which the dust source was crucial. In addition to secondary transformations, local direct emissions (coal combustion and motor vehicle exhaust) significantly contribute to the winter haze at the urban sites. Moreover, with the aggravation of haze pollution, the OC ∕ EC mass ratio of PM_2.5 decreased considerably and the nitrate-rich secondary aerosol increased during all four seasons in Beijing, both of which indicate that local motor vehicle emissions significantly contribute to the severe haze episodes in Beijing. To assess the impacts of regional transport on haze pollution, the PMF results were further processed with backward-trajectory cluster analysis, revealing that haze pollution usually occurred when air masses originating from polluted industrial regions in the south prevailed and is characterized by high PM_2.5 loadings with considerable contributions from secondary aerosols. This study suggests that control strategies to mitigate haze pollution in the BTH region should focus on the reduction of gaseous precursor emissions from fossil fuel combustion (motor vehicle emissions in Beijing and coal combustion in Tianjin, Hebei, and nearby provinces).

The Campaign on Atmospheric Aerosol Research network of China (CARE-China) is a long-term project for the study of the spatio-temporal distributions of physical aerosol characteristics as well as the chemical components and optical properties of aerosols over China. This study presents the first long-term data sets from this project, including 3 years of observations of online PM_2.5 mass concentrations (2012–2014) and 1 year of observations of PM_2.5 compositions (2012–2013) from the CARE-China network. The average PM_2.5 concentration at 20 urban sites is 73.2 µg m⁻³ (16.8–126.9 µg m⁻³), which was 3 times higher than the average value from the 12 background sites (11.2–46.5 µg m⁻³). The PM_2.5 concentrations are generally higher in east-central China than in the other parts of the country due to their relatively large particulate matter (PM) emissions and the unfavourable meteorological conditions for pollution dispersion. A distinct seasonal variability in PM_2.5 is observed, with highs in the winter and lows during the summer at urban sites. Inconsistent seasonal trends were observed at the background sites. Bimodal and unimodal diurnal variation patterns were identified at both urban and background sites. The chemical compositions of PM_2.5 were analysed at six paired urban and background sites located within the most polluted urban agglomerations – North China Plain (NCP), Yangtze River delta (YRD), Pearl River delta (PRD), North-east China region (NECR), South-west China region (SWCR) – and the cleanest region of China – the Tibetan Autonomous Region (TAR). The major PM_2.5 constituents across all the urban sites are organic matter (OM, 26.0 %), SO₄²⁻ (17.7 %), mineral dust (11.8 %), NO₃⁻ (9.8 %), NH₄⁺ (6.6 %), elemental carbon (EC) (6.0 %), Cl⁻ (1.2 %) at 45 % RH and unaccounted matter (20.7 %). Similar chemical compositions of PM_2.5 were observed at background sites but were associated with higher fractions of OM (33.2 %) and lower fractions of NO₃⁻ (8.6 %) and EC (4.1 %). Significant variations of the chemical species were observed among the sites. At the urban sites, the OM ranged from 12.6 µg m⁻³ (Lhasa) to 23.3 µg m⁻³ (Shenyang), the SO₄²⁻ ranged from 0.8 µg m⁻³ (Lhasa) to 19.7 µg m⁻³ (Chongqing), the NO₃⁻ ranged from 0.5 µg m⁻³ (Lhasa) to 11.9 µg m⁻³ (Shanghai) and the EC ranged from 1.4 µg m⁻³ (Lhasa) to 7.1 µg m⁻³ (Guangzhou). The PM_2.5 chemical species at the background sites exhibited larger spatial heterogeneities than those at urban sites, suggesting different contributions from regional anthropogenic or natural emissions and from long-range transport to background areas. Notable seasonal variations of PM_2.5-polluted days were observed, especially for the megacities in east-central China, resulting in frequent heavy pollution episodes occurring during the winter. The evolution of the PM_2.5 chemical compositions on polluted days was consistent for the urban and nearby background sites, where the sum of sulfate, nitrate and ammonia typically constituted much higher fractions (31–57 %) of PM_2.5 mass, suggesting fine-particle pollution in the most polluted areas of China assumes a regional tendency, and the importance of addressing the emission reduction of secondary aerosol precursors including SO₂ and NO_x. Furthermore, distinct differences in the evolution of [NO₃⁻]∕[SO₄²⁻] ratio and OC∕EC ratio on polluted days imply that mobile sources and stationary (coal combustion) sources are likely more important in Guangzhou and Shenyang, respectively, whereas in Beijing it is mobile emission and residential sources. As for Chongqing, the higher oxidation capacity than the other three cities suggested it should pay more attention to the emission reduction of secondary aerosol precursors. This analysis reveals the spatial and seasonal variabilities of the urban and background aerosol concentrations on a national scale and provides insights into their sources, processes and lifetimes.

The characteristics of ozone variations and the impacts of synoptic and local meteorological factors in northern China were quantitatively analyzed during the warm season from 2013 to 2017 based on multi-city in situ ozone and meteorological data as well as meteorological reanalysis. The domain-averaged maximum daily 8 h running average O₃ (MDA8 O₃) concentration was 122±11 µg m⁻³, with an increase rate of 7.88 µg m⁻³ yr⁻¹, and the three most polluted months were closely related to the variations in the synoptic circulation patterns, which occurred in June (149 µg m⁻³), May (138 µg m⁻³) and July (132 µg m⁻³). A total of 26 weather types (merged into five weather categories) were objectively identified using the Lamb–Jenkinson method. The highly polluted weather categories included the S–W–N directions (geostrophic wind direction diverts from south to north), low-pressure-related weather types (LP) and cyclone type, which the study area controlled by a low-pressure center (C), and the corresponding domain-averaged MDA8 O₃ concentrations were 122, 126 and 128 µg m⁻³, respectively. Based on the frequency and intensity changes of the synoptic circulation patterns, 39.2 % of the interannual increase in the domain-averaged O₃ from 2013 to 2017 was attributed to synoptic changes, and the intensity of the synoptic circulation patterns was the dominant factor. Using synoptic classification and local meteorological factors, the segmented synoptic-regression approach was established to evaluate and forecast daily ozone variability on an urban scale. The results showed that this method is practical in most cities, and the dominant factors are the maximum temperature, southerly winds, relative humidity on the previous day and on the same day, and total cloud cover. Overall, 41 %–63 % of the day-to-day variability in the MDA8 O₃ concentrations was due to local meteorological variations in most cities over northern China, except for two cities: QHD (Qinhuangdao) at 34 % and ZZ (Zhengzhou) at 20 %. Our quantitative exploration of the influence of both synoptic and local meteorological factors on interannual and day-to-day ozone variability will provide a scientific basis for evaluating emission reduction measures that have been implemented by the national and local governments to mitigate air pollution in northern China.

To investigate the spatiotemporal variability of the mixing layer height (MLH) on the North China Plain (NCP), multi-site and long-term observations of the MLH with ceilometers at three inland stations (Beijing, BJ; Shijiazhuang, SJZ; Tianjin, TJ) and one coastal site (Qinhuangdao) were conducted from 16 October 2013 to 15 July 2015. The MLH of the inland stations in the NCP were highest in summer and lowest in winter, while the MLH on the coastal area of Bohai was lowest in summer and highest in spring. As a typical site in southern Hebei, the annual mean of the MLH at SJZ was 464 ± 183 m, which was 15.0 and 21.9 % lower than that at the BJ (594 ± 183 m) and TJ (546 ± 197 m) stations, respectively. Investigation of the shear term and buoyancy term in the NCP revealed that these two parameters in southern Hebei were 2.8 times lower and 1.5 times higher than that in northern NCP within 0–1200 m in winter, respectively, leading to a 1.9-fold higher frequency of the gradient Richardson number > 1 in southern Hebei compared to the northern NCP. Furthermore, combined with aerosol optical depth and PM_2.5 observations, we found that the pollutant column concentration contrast (1.2 times) between these two areas was far less than the near-ground PM_2.5 concentration contrast (1.5 times). Through analysis of the ventilation coefficient in the NCP, the near-ground heavy pollution in southern Hebei mainly resulted from the lower MLH and wind speed. Therefore, due to the importance of unfavorable weather conditions, heavily polluting enterprises should be relocated and strong emission reduction measures should be introduced to improve the air quality in southern Hebei.

Previous studies have emphasized that the decrease in photolysis rate at the surface induced by the light extinction of aerosols could weaken ozone photochemistry and then reduce surface ozone. However, quantitative studies have shown that weakened photochemistry leads to a much greater reduction in the net chemical production of ozone, which does not match the reduction in surface ozone. This suggests that in addition to photochemistry, some other physical processes related to the variation of ozone should also be considered. In this study, the Weather Research and Forecasting with Chemistry (WRF-Chem) model coupled with the ozone source apportionment method was applied to determine the mechanism of ozone reduction induced by aerosols over central East China (CEC). Our results showed that weakened ozone photochemistry led to a significant reduction in ozone net chemical production, which occurred not only at the surface but also within the lowest several hundred meters in the planetary boundary layer (PBL). Meanwhile, a larger ozone gradient was formed in the vertical direction, which led to the high concentrations of ozone aloft being entrained by turbulence from the top of the PBL to the surface and partly counteracting the reduction in surface ozone. In addition, contribution from dry deposition was weakened due to the decrease in surface ozone concentration. The reduction in the ozone's sink also slowed down the rate of the decrease in surface ozone. Ozone in the upper layer of the PBL was also reduced, which was induced by much ozone aloft being entrained downward. Therefore, by affecting the photolysis rate, the impact of aerosols was a reduction in ozone not only at the surface but also throughout the entire PBL during the daytime over CEC in this study. The ozone source apportionment results showed that 41.4 %–66.3 % of the reduction in surface ozone came from local and adjacent source regions, which suggested that the impact of aerosols on ozone from local and adjacent regions was more significant than that from long-distance regions. The results also suggested that while controlling the concentration of aerosols, simultaneously controlling ozone precursors from local and adjacent source regions is an effective way to suppress the increase in surface ozone over CEC at present.

The ozone weekend effect (OWE) was first investigated in the metropolitan area of Beijing–Tianjin–Hebei (BTH), China, using in situ measurements from the Atmospheric Environment Monitoring Network from July 2009 to August 2011. The results indicate that there is an obvious weekly periodical variation in the surface ozone concentration. There is a lower ozone concentration from Wednesday to Friday (weekday) and a higher concentration from Saturday to Monday (weekend) at all the locations of the study. NO_x also displays a weekly cycle, with the maximum level occurring on weekdays and the minimum level on weekends, especially later on Sunday night and early Monday morning. This pattern may be responsible for the higher concentration of ozone on weekends. Additionally, the vertical variations in O₃ and NO_x from the 8 m, 47 m, 120 m and 280 m observation platforms on the 325 m Beijing meteorological tower displayed obvious weekly cycles that corresponded to the surface results. A smaller decrease in volatile organic compounds (VOCs; using CO as a proxy) and much lower NO_x concentrations on the weekend may lead to higher VOC / NO_x ratio, which can enhance the ozone production efficiency in VOC-limited regime areas. Additionally, a clear weekly cycle in the fine aerosol concentration was observed, with maximum values occurring on weekdays and minimum values occurring on weekends. Higher concentrations of aerosol on weekdays can reduce the UV radiation flux by scattering or absorbing, which leads to a decrease in the ozone production efficiency. A significant weekly cycle in UV radiation, consistent with the aerosol concentration, was discovered at the Beijing meteorological tower site (BJT), validating the assumption. A comprehensive understanding of the ozone weekend effect in the BTH area can provide deep insights into controlling photochemical pollution.