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High loads of fine particulate matter (PM2.5) and ozone (O3) pollution occurred frequently since early spring and led to an increasing contribution of secondary organic aerosols (SOA) in organic aerosols. However, the characteristics of precursor volatile organic compounds (VOCs) have rarely been studied. In this study, the continuous observation of VOCs was performed by an offline VOC monitoring system and gas chromatography-mass/flame ionization detector from 1 April 2020 to 31 July 2020; the characterization of VOCs and their contribution to SOA was explored. The results showed that during the observation period, the average mixing ratio of TVOCs was 42.6 ± 11.2 ppbv, and the major VOCs species were OVOCs, followed by alkanes, halocarbons, aromatics, alkenes and acetylene. When the west circulation pattern functioned, the value of aromatics increased, and the relation between PM2.5, O3 and VOCs increased when the high-pressure system controlled by anticyclone functioned. In combination with the results of positive matrix factorization, the main emission sources of ambient VOCs were complex, and the fuel combustion, industry-related emission, vehicle emission, biogenic emission and solvent volatilization accounted for 27.1%, 24.4%, 24.3%, 12.1% and 12.0%, respectively. Moreover, the industry-related emission contributed the greatest to the generation of SOA. This result indicated that the restrictions on aromatics during the industrial process are vital to reducing SOA formation. [ABSTRACT FROM AUTHOR]

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 µ g m -3 , with an increase rate of 7.88 µ g 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 µ g m -3), May (138 µ g m -3) and July (132 µ g 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 µ g 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. [ABSTRACT FROM AUTHOR]

• The domain-averaged PM 2.5 in North China decreased significantly from 2013 to 2017. • Firstly established two automatic algorithms to identify regional pollution types and events. • The relationships of synoptic circulations and regional PM 2.5 levels were quantified. • Starting areas, synoptic and local meteorological mechanisms were analyzed based on persistent regional pollution events. • Quantifying the effects of weather changes on the interannual PM 2.5 variations. In the last decade, North China (NC) has been one of the most populated and polluted regions in the world. The regional air pollution has had a serious impact on people's health; thus, all levels of government have implemented various pollution prevention measures since 2013. Based on multi-city in situ environmental and meteorological data, as well as the meteorological reanalysis dataset from 2013 to 2017, regional pollution characteristics and meteorological formation mechanisms were analyzed to provide a more comprehensive understanding of the evolution of PM 2.5 in NC. The domain-averaged PM 2.5 was 79 ± 17 µg m−3 from 2013 to 2017, with a decreasing rate of 10 μg m−3 yr−1. Two automatic computer algorithms were established to identify 6 daily regional pollution types (DRPTs) and 48 persistent regional pollution events (PRPEs) over NC during 2014–2017. The average PM 2.5 concentration for the Large-Region-Pollution type (including the Large-Moderate-Region-Pollution and Large-Severe-Region-Pollution types) was 113 ± 40 µg m−3, and more than half of Large-Region-Pollution days and PRPEs occurred in winter. The PRPEs in NC mainly developed from the area south of Hebei. The number of Large-Region-Pollution days decreased notably from 2014 to 2017, the annual number of days varying between 194 and 97 days, whereas a slight decline was observed in winter. In addition, the averaged PM 2.5 concentrations and the numbers and durations of the PRPEs decreased. Lamb-Jenkinson weather typing was used to reveal the impact of synoptic circulations on PM 2.5 across NC. Generally, the contributions of the variations in circulation to the reduction in PM 2.5 levels over NC between 2013 and 2017 were 64% and 45% in summer and winter, respectively. The three most highly polluted weather types were types C, S and E, with an average PM 2.5 concentration of 137 ± 40 µg m−3 in winter. Furthermore, three typical circulation dynamics were categorized in the peak stage of the PRPEs, namely, the southerly airflow pattern, the northerly airflow pattern and anticyclone pattern; the averaged relative humidity, recirculation index, wind speed and boundary layer height were 63%, 0.33, 2.0 m s−1 and 493 m, respectively. Our results imply that additional emission reduction measures should be implemented under unfavorable meteorological situations to attain ambient air quality standards in the future. [ABSTRACT FROM AUTHOR]

An intensive and persistent regional ozone pollution event occurred over eastern China from 25 June to 5 July 2017. 73 out of 96 selected cities, most located in the Beijing-Tianjin-Hebei and the surrounding area (BTHS), suffered severe ozone pollution. A north-south contrast ozone distribution, with higher ozone (199 ± 33 μg/m3) in the BTHS and lower ozone (118 ± 25 μg/m3) in the Yangtze River Delta (YRD), was found to be dominated by the position of the West Pacific Subtropical High (WPSH) and mid-high latitude wave activities. In the BTHS, the positive anomalies of geopotential height at 500 hPa and temperature at the surface indicated favorable meteorological conditions for local ozone formation. Prevailing northwesterly winds in the mid-high troposphere and warm advection induced by weak southerly winds in the low troposphere resulted in low-moderate relative humidity (RH), less total cloud cover (TCC), strong solar radiation and high temperatures. Moreover, southerly winds prevailing over the BTHS aggravated the pollution due to regional transportation of O 3 and its precursors. On one hand, the deep sinking motion and inversion layer suppressed the dispersion of pollutants. On the other hand, O 3 -rich air in the upper layer was maintained at night due to temperature inversion, which facilitated O 3 vertical transport to the surface in the next-day morning due to elevated convection. Generally, temperature, UV radiation, and RH showed good correlations with O 3 in the BTHS, with rates of 8.51 (μg/m3)/°C (within the temperature range of 20–38°C), 59.54 (μg/m3)/(MJ/m2) and −1.93 (μg/m3)/%, respectively. Image 1 [ABSTRACT FROM AUTHOR]

The aerosol scattering coefficient is mainly affected by various factors, such as aerosol chemical composition, hygroscopicity and relative humidity. A comprehensive observation of the chemical and physical properties of aerosols was conducted in Xianghe, southeast of Beijing, from June 15 to 26, 2018. Five specific cases were analysed during this observation: (1) rainy (RA); (2) low visibility during the day (LD); (3) high visibility during the day (HD); (4) low visibility at night (LN); and (5) high visibility at night (HN). The mass percentages of a secondary inorganic aerosol (nitrate, sulfate and ammonium) measured with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) were 64.5% and 68.3% during LD and LN, which were higher than those (63.6% and 46.1%) observed during HD and HN, respectively. The mass percentages of secondary organic aerosol (SOA) and primary organic aerosol (POA) to organic aerosol were 76.6% and 23.4%, respectively, during LD. The hygroscopic parameters κ were 0.59 and 0.60 during LD and LN, which were slightly higher than those (0.57 and 0.58) observed during HD and HN, respectively, indicating that the variation of aerosol chemical composition had a limited impact on hygroscopicity. The average proportion, measured with a single particle aerosol mass spectrometer (SPAMS), of biomass burning, dust, heavy metal, soot-like, sea salt, secondary aerosols and other particles were 12.3%, 2.9%, 16.2%, 36.1%, 8.8%, 21.0% and 2.7%. The average proportions of soot-like and secondary aerosols were 42.4% and 19.2% from 0.2 to 1.0 μm. Although the variation and height of the mixing layer were similar between LD and HD, clouds reduced the solar radiation during LD, which caused a higher relative humidity (RH) (67.5%) than that (45.9%) observed during HD. A larger wind speed below 850 hpa during HN caused the mixing layer height (MLH) to reach 1910 m, which resulted in a lower RH (64.8%) than that (68.0%) observed during LN. The light scattering enhancement factors f(RH) during low visibility events were almost greater than those observed during high visibility events, regardless of they were measured during the day (0–9.7%) or night (0–13.1%). The aerosol hygroscopic growth factor (GF) maintained a similar trend with RH, indicating that RH was the main factor affecting the hygroscopic growth of the aerosol, resulting in the decrease in visibility. • The hygroscopic parameter κ is calculated by HR-ToF-AMS. • Soot-like and secondary aerosols are the main sources identified by SPAMS. • The hygroscopicity of aerosol is compared under different visibility conditions. [ABSTRACT FROM AUTHOR]

In recent decade, despite the implementation of strict emission reductions, PM 2.5 and surface ozone (O 3) in China are still at high pollution level. In this study, based on observations at air quality stations (∼1500 mainly in urban region), daily PM 2.5 and MDA8 (the daily maximum 8 h average) O 3 concentrations in high-density and uniform meteorological sites (∼2000) over China during 2017–2019 were retrieved using generalized additive models (GAMs) for different regions on a seasonal scale, in order to establish 0.1° gridded datasets and investigate their spatiotemporal characteristics, nonlinear effects of meteorology, and ambient air pollution-attributable health risks on a regional scale. This model can provide high accuracy in the estimation of daily PM 2.5 (O 3) in 2018 with overall cross-validation R2 and RMSE of 0.81 (0.80) and 12.73 (17.63) μg/m3 and predictive capability in 2017 and 2019 with R2 of 0.67 (0.70) for PM 2.5 (O 3). Temperature, relative humidity and visibility are three main meteorological factors driving PM 2.5 (O 3) response. PM 2.5 generally exhibits an exponential decrease with increasing visibility and an increase with increasing relative humidity, while is suppressed at the thresholds of 60% and 70%. For O 3 , in northern China, positive response of temperature is more important and O 3 suppression exists at 36 °C ∼ 41.5 °C, whereas negative response of relative humidity is dominating in southern China. Overall, annual averaged PM 2.5 (O 3) values across China reduced (increased) by 18.7% (11.2%) between 2017 and 2019, with the highest concentrations and variation in North China. These variations contributed to a 14.9% reduction in PM 2.5 attributed premature deaths, but a 10.4% increase in O 3 exposure related deaths. Generally, central-eastern China still exists greater health risk with per capita premature mortality ranging from 0.80‰ to 1.19‰ for PM 2.5 and 1.91‰ to 2.60‰ for O 3. Additionally, O 3 vegetation damages demonstrate a decline (increase) by 1.3% (21.7%) in North China with higher O 3 pollution (southern China with slight pollution) between 2017 and 2019. These findings highlight the essentiality for the synergistic control of PM 2.5 and O 3 based on regional differences in China. [Display omitted] • Daily PM 2.5 and O 3 at ∼2000 meteorological sites in China were constructed by GAM. • The PM 2.5 and O 3 models both shown a good performance and predictive capacity. • Northern China exhibits O 3 suppression, but more negative effect of humidity is in southern China. • PM 2.5 (O 3) levels and pollution- attributable premature deaths reduced (increased). • O 3 vegetation damages reduced (rose) in North (southern) China from 2017 to 2019. [ABSTRACT FROM AUTHOR]

Under weak weather systems, pollutants circulate on plains and hillsides by means of oscillatory mountain-plain breezes (MPB). To investigate the impact of MPB on ambient volatile organic compounds (VOCs) on the northern edge of the North China Plain (NCP) region, synchronous online VOCs observations were first performed in the southwestern (Beijing, urban area) and northeastern (Xinglong, background area) of the dominant wind directions from September 11, 2020, to October 13, 2020. The results revealed that during the observation period, the average mixing ratios of TVOCs in Beijing and Xinglong were 32.5 ± 21.6 and 14.2 ± 8.6 ppbv, respectively, and a coincident temporal evolution trend of VOCs was observed at the two sites. Combined with the results of the backward trajectory, southerly air transport led to a significant increase in VOCs concentration, and southerly airflow was usually accompanied by apparent MPB circulation. Furthermore, the occurrence of MPB led to a sharp increase (1.5 times) in VOCs concentration in both Beijing and Xinglong. This study indicates that the periodic oscillation of the MPB leads to the large-scale pollution of "garbage out and garbage in", and the VOCs discharged by industry are the key to future control., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

The vertical distribution of carbonaceous aerosol impacts climate change, air quality and human health, but there is a lack of in-situ vertical observations of black (BC) and brown carbon (BrC). Thus, the characteristic of vertical profiles of BC concentration, particle number concentration (PNC), O 3 concentration and optical absorption of BC and BrC were observed in a suburban site over North China Plain, where heavy pollution of PM 2.5 and O 3 always occurred in winter and summer, respectively. In winter, during a heavy pollution episode, the BC and PNC was near uniformly distributed within mixing layer (ML) (15.2 ± 6.7 μg m -3 and 678 ± 227 p cm -3 , respectively) and decreased with altitude above the ML. The BC heating rate reached about 0.13 K h -1 during the heaviest pollution day. In summer, the BC concentration (2.9 ± 1.3 μg m -3 ) in ML during the middle O 3 pollution events was higher than that (1.7 ± 0.6 μg m -3 ) during the light O 3 pollution. The light absorption coefficients of BC at 880 nm and BrC at 375 nm measured in the early morning were lower than that in the daytime, and the contribution of BrC to total light absorption of carbonaceous aerosols was in the range of 27-47%. In addition, BC was effectively transported to high altitude than BrC in the daytime. The light absorption of secondary BrC in the daytime was higher 10-20% than that in the early morning. Simultaneously, the contribution of secondary BrC to the total BrC light absorption at 375 nm was range from 32% to 68% within 1000 m., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Absorbing carbonaceous aerosols, i.e. black and brown carbon (BC and BrC), affected heavily on climate change, regional air quality and human health. The nationwide lockdown measures in 2020 were performed to against the COVID-19 outbreak, which could provide an important opportunity to understand their variations on light absorption, concentrations, sources and formation mechanism of carbonaceous aerosols. The BC concentration in Wuhan megacity (WH) was 1.9 μg m -3 during lockdown, which was 24% lower than those in the medium-sized cities and 26% higher than those in small city; in addition, 39% and 16-23% reductions occurred compared with the same periods in 2019 in WH and other cities, respectively. Fossil fuels from vehicles and industries were the major contributors to BC; and compared with other periods, minimum contribution (64-86%) mainly from fossil fuel to BC occurred during the lockdown in all cities. Secondary BrC (BrCsec) played a major role in the BrC light absorption, accounting for 65-77% in WH during different periods. BrCsec was promoted under high humidity, and decreased through the photobleaching of chromophores under higher Ox. Generally, the lockdown measures reduced the BC concentrations significantly; however, the variation of BrCsec was slight., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)