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Brown carbon (BrC) represents an important target for the "win-win" strategy of mitigating climate change and improving air quality. However, estimating co-benefits of BrC control remains difficult for China, partially because current measurement results are insufficient to represent the highly variable emission sources and meteorological conditions across different regions. In this study, we investigated, for the first time, the diurnal variations of BrC during two distinct seasons in a megacity in northeast China. The winter campaign conducted in January 2021 was characterized by low temperatures rarely seen in other Chinese megacities (down to about -20 ∘ C). The mass absorption efficiencies of BrC at 365 nm (MAE 365) were found to be ∼ 10 % higher at night. The variations of MAE 365 could not be explained by the influence of residential biomass burning emissions or secondary aerosol formation but were strongly associated with the changes of a diagnostic ratio for the relative importance of coal combustion and vehicle emissions (RS/N). Given that most coal combustion activities were uninterruptible, the higher nighttime MAE 365 in winter were attributed primarily to increased emissions from heavy-duty diesel trucks. The spring campaign conducted in April 2021 was characterized by frequent occurrences of agricultural fires, as supported by the intensive fire hotspots detected around Harbin and the more-than-doubled ratios of levoglucosan to organic carbon (LG/OC) compared to the winter campaign. In spring, the MAE 365 depended little on RS/N but exhibited a strong positive correlation with LG/OC , suggesting open burning emissions as the dominant influencing factor for BrC's light absorption capacity. MAE 365 were ∼ 70 % higher at night for the spring campaign, pointing to the prevalence of nighttime agricultural fires, which were presumably in response to local bans on open burning. It is noteworthy that the agricultural-fire emissions resulted in a distinct peak at ∼ 365 nm for the light absorption spectra of BrC, and candidates for the compounds at play were inferred to be aromatic species with nitro-functional groups. The presence of the ∼ 365 nm peak complicated the determination of absorption Ångström exponents for the agricultural-fire-impacted samples. In addition, the ∼ 365 nm peak became much less significant during the day, likely due to photobleaching of the relevant chromophores. [ABSTRACT FROM AUTHOR]

Fireworks are banned in many Chinese cities but never eliminated, reflecting strong public demand on this traditional activity for festival celebrations. On the other hand, the assessment of firework contributions to air pollution remains vague, raising concerns on the necessity of the mandatory bans. Here we investigated the characteristics of firework episodes in a megacity in Northeast China, based on field campaigns conducted in four successive winters during 2018–2022. Although prohibited, the firework influences remained evident during the Chinese New Year periods, as suggested by the enhancements of water-soluble potassium (K+). In addition, significant annual variations were identified for the firework episodes, with the following features observed. First, the firework-induced enrichment ratios of K+ and chloride exhibited increasing trends across years, climbing from 4.4 to 8.6 and from 1.7 to 2.9, respectively. Second, the enrichment ratio of sulfate dropped from 2.8 to 1.6, indicating that the firework contributions to sulfate decreased but remained considerable. Third, fireworks turned into an unimportant source for organic carbon and nitrate in the most recent winter of 2021–2022, with enrichment ratios of ∼1 for both species. Fourth, the firework-driven increases in fine particle concentration were as high as ∼100% for the two winters during 2019–2021, whereas the increase dropped sharply to ∼30% for 2021–2022. These variations were in line with the promotion of environmentally friendly fireworks. Our results indicated that the air pollution caused by fireworks could be reduced substantially by advanced manufacturing technologies and thus it is time to rethink the firework bans. [Display omitted] • Firework-emitted aerosols changed dramatically during recent years in Harbin. • Firework-induced enrichments in K+ and Cl− increased across years. • Firework contributions to organic matter and nitrate became negligible in 2022. • The relative importance of fireworks compared to other PM 2.5 sources decreased. • Green fireworks provide solution to balance air quality concerns and public demand. [ABSTRACT FROM AUTHOR]

Despite the growing interest in understanding haze formation in Chinese megacities, air pollution has been largely overlooked for the Harbin–Changchun (HC) metropolitan area, located in the severe cold climate region in northeast China. In this study, we unfolded significant variations of fine particulate matter (PM 2.5) in HC's central city (Harbin) during two sequential heating seasons of 2018–2019 and 2019–2020, and we explored major drivers for the observed variations. The two campaigns showed comparable organic carbon (OC) levels but quite different OC sources. The biomass burning (BB) to OC contribution decreased substantially for 2019–2020, which was attributed primarily to the transition of local policies on agricultural fires, i.e., from the "legitimate burning" policy released in 2018 to the "strict prohibition" policy in 2019. Meanwhile, the contribution of secondary OC (OC sec) increased significantly, associated with the much more frequent occurrences of high relative humidity (RH) conditions during the 2019–2020 measurement period. Similar to OC sec , the major secondary inorganic ions, i.e., sulfate, nitrate and ammonium (SNA), also exhibited RH-dependent increases. Given the considerable aerosol water contents predicted for the high-RH conditions, heterogeneous reactions were likely at play in secondary aerosol formation even in the frigid atmosphere in Harbin (e.g., with daily average temperatures down to below - 20 ∘ C). In brief, compared to 2018–2019, the 2019–2020 measurement period was characterized by a policy-driven decrease of biomass burning OC, a RH-related increase of OC sec and a RH-related increase of SNA, with the first two factors generally offsetting each other. In addition, we found that open burning activities were actually not eliminated by the strict prohibition policy released in 2019, based on a synthesis of air quality data and fire count results. Although not occurring during the 2019–2020 measurement period, agricultural fires broke out within a short period before crop planting in spring of 2020, and this resulted in off-the-charts air pollution for Harbin, with 1 h and 24 h PM 2.5 concentrations peaking at ∼ 2350 and 900 µ g/m 3 , respectively. This study indicates that sustainable use of crop residues remains a difficult challenge for the massive agricultural sector in northeast China. [ABSTRACT FROM AUTHOR]

Cities in Northeast China, e.g., Harbin, were brought to the forefront of air pollution control by a national-level policy promulgated in 2021, i.e., the Circular on Further Promoting the Pollution Prevention and Control Battle (the FP3CB Circular) which aimed at eliminating heavy or severe air pollution events. In this study, we explored the response of Harbin aerosol to the FP3CB Circular, based on observational results from two campaigns conducted during 2020–2021 and 2021–2022. A clear decreasing trend was identified for the impact of domestic biomass burning between the two winters, presumably driven by the clean heating actions. The 2021–2022 winter was also characterized by reduced formation of secondary organic aerosol but enhanced production of nitrate, which could be attributed to the less humid conditions but higher temperatures, respectively, compared to the 2020–2021 winter. The overall effect of these changes was a decrease in the contribution of organic species to wintertime aerosol in Harbin. In addition, the number of heavy or severe pollution days rebounded in the 2021–2022 winter compared to 2020–2021 (5 vs. 3), indicating that the emissions of primary particles and gaseous precursors must be further reduced to achieve the ambitious goals of the FP3CB Circular. [Display omitted] ● Biomass burning impacts decreased in the 2021–2022 winter compared to 2020–2021. ● SOA formation was reduced in the 2021–2022 winter, accompanied with decreased RH. ● NOR was enhanced in the 2021–2022 winter, driven by increased temperatures. ● OA contribution to aerosol mass decreased in the 2021–2022 winter. ● Heavy pollution days rebounded in the 2021–2022 winter despite clean air actions. [ABSTRACT FROM AUTHOR]

Fine particulate matter (PM 2.5) in Northeast China was targeted by national-level clean air policy for the first time in 2022, with the release of Action Plan to eliminate heavy air pollution events. In this study, we investigated sources of PM 2.5 during three successive winters in Harbin, a megacity in Northeast China, based on observational results from several recent campaigns in 2018–2021. During the 2020–2021 campaign, daytime and nighttime samples were collected in specific months in addition to 24-h integrated measurements, and the two sets of samples were combined in different ways to run a positive matrix factorization model. The source apportionment results suggested that the resolved secondary organic carbon (SOC PMF) had an uncertainty of ∼12%. Secondary aerosols were found to show the following features for the typical winters without agricultural fires. First, SOC PMF could be properly constrained by results from another widely-used approach for SOC estimation, the elemental carbon-tracer method. Second, secondary PM 2.5 calculated using SOC PMF and secondary inorganic ions were generally in line with the independent estimations based on air quality data. Third, secondary components accounted for more than 50% of PM 2.5 on average and contributed even more significantly during severe haze episodes, which were the focus of the latest Action Plan. This study also found that the wintertime PM 2.5 decreased more slowly during 2017–2021 compared to 2013–2017, by ∼1 and 10 μg/m3 per year, respectively, for the metropolitan area where Harbin is located at. Our results highlighted the importance of secondary aerosols for further improving air quality in Northeast China, and for avoiding heavy pollution as required by the latest Action Plan. [Display omitted] • By integrating multi-methods, reliable secondary aerosol estimation was achieved. • Secondary aerosols dominated wintertime PM 2.5 , more significantly in haze events. • Heterogeneous chemistry was at play despite the frigid winter. • PM 2.5 flattened during recent winters for typical city cluster in Northeast China. [ABSTRACT FROM AUTHOR]

Fine particulate matter (PM 2.5) harms human health and hinders normal human life. Considering the serious complexity and obvious regional characteristics of PM 2.5 pollution, it is urgent to fill in the comprehensive overview of regional characteristics and interannual evolution of PM 2.5. This review studied the PM 2.5 pollution in six typical areas between 2014 and 2022 based on the data published by the Chinese government and nearly 120 relevant literature. We analyzed and compared the characteristics of interannual and quarterly changes of PM 2.5 concentration. The Beijing-Tianjin-Hebei region (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD) made remarkable progress in improving PM 2.5 pollution, while Fenwei Plain (FWP), Sichuan Basin (SCB) and Northeast Plain (NEP) were slightly inferior mainly due to the relatively lower level of economic development. It was found that the annual average PM 2.5 concentration change versus year curves in the three areas with better pollution control conditions can be merged into a smooth curve. Importantly, this can be fitted for the accurate evaluation of each area and provide reliable prediction of its future evolution. In addition, we analyzed the factors affecting the PM 2.5 in each area and summarize the causes of air pollution in China. They included primary emission, secondary generation, regional transmission, as well as unfavorable air dispersion conditions. We also suggested that the PM 2.5 pollution control should target specific industries and periods, and further research need to be carried out on the process of secondary production. The results provided useful assistance such as effect prediction and strategy guidance for PM 2.5 pollution control in Chinese backward areas. [Display omitted] [ABSTRACT FROM AUTHOR]

COVID-19 rebounded in China in January 2021, with Heilongjiang as one of the worst-affected provinces. This resulted in a new round of lockdown in Harbin, the capital city of Heilongjiang, from 20 January to 22 February of 2021. A field campaign was conducted to explore the responses of haze pollution in Harbin to the lockdown. Levoglucosan was used to reflect biomass burning emissions, while the molar ratio of sulfur (the sum of sulfur dioxide and sulfate) to nitrogen (the sum of nitrogen dioxide and nitrate), i.e., R S/N , was used as an indicator for the relative importance of coal combustion and vehicle emissions. Based on a synthesis of the levoglucosan and R S/N results, reference period was selected with minimal influences of non-lockdown-related emission variations. As indicated by the almost unchanged sulfur dioxide concentrations, coal combustion emissions were relatively stable throughout the lockdown and reference periods, presumably because the associated activities, e.g., heating supply, power generation, etc., were usually uninterruptible. On the other hand, as suggested by the increase of R S/N , vehicle emissions were considerably reduced during lockdown, likely due to the stay-at-home orders. Compared to results from the reference samples, the lockdown period exhibited higher levels of ozone and various indicators for secondary aerosol formation, pointing to an enhancement of secondary pollution. In addition, photochemistry-related reactions in aqueous phase appeared to be present during the lockdown period, which have not been reported in the frigid atmosphere over Northeast China., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The massive agricultural sector in the Northeast Plain, which is of great importance for the food security in China, results in a huge amount of crop residues and thus substantial concern on haze pollution due to biomass burning (BB). To seek for effective control measures on BB emissions, a dramatic transition of open burning policy occurred in Heilongjiang Province, from the "legitimate burning" policy released in 2018 to the "strict prohibition" policy implemented in 2019 and beyond. Here we explored the BB aerosols during 2020-2021 in Harbin, the capital city of Heilongjiang. Although open burning was strictly prohibited by mandatory bans, agricultural fires were not actually eliminated, as indicated by the levoglucosan levels and fire count results. In general, the BB aerosols in Harbin were attributed to the overlaying of household burning and agricultural fire emissions. The former factor laid the foundation of biomass burning impacts, with BB contributions to organic carbon and elemental carbon (f BBOC and f BBEC ) of 35 and 47%, respectively. The latter further enhanced the BB impacts during specific episodes breaking out in the spring of 2021 as well as the fall of 2020, when f BBOC and f BBEC increased to 64 and 57%, respectively. In addition, comparing to the fires of 2018-2019 which occurred in winter (in response to the "legitimate burning" policy), the agricultural fires were shifted to spring and fall in the 2020-2021 campaign, accompanied with an increase of combustion efficiency. This study illustrated how the agricultural fire emissions were influenced by the transition of open burning policy., 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.)