Heavy Pollution Episodes Research Articles

Contribution of Regional PM2.5 Transport to Air Pollution Enhanced by Sub-Basin Topography: A Modeling Case over Central China

The Twain-Hu basin (THB), covering the lower plain of Hubei and Hunan provinces in Central China, has experienced severe air pollution in recent years. However, the terrain effects of such sub-basin on air quality over the THB have been incomprehensibly understood. A heavy PM2.5 pollution event occurred over the THB during 4–10 January 2019. By using the observations and WRF-Chem simulations, we investigated the underlying mechanisms of sub-basin effects on the air pollution with several sensitivity experiments. Observationally, air pollution in the western THB urban area with an average PM2.5 concentration of 189.8 μg m−3, which was more serious than the eastern urban area with the average PM2.5 concentration of 106.3 μg m−3, reflecting a different influence of topography on air pollution over the THB. Simulation results revealed that the terrain effect can contribute 12.0% to increasing the PM2.5 concentrations in the western THB, but slightly mitigate the pollution extent in the eastern THB with the contribution of −4.6% to PM2.5 during the heavy pollution episode. In particular, the sub-basin terrain was conducive to the accumulation of PM2.5 by regional transport with the contribution of 39.1 %, and contrarily lowered its local pollution by −57.0% via the enhanced atmospheric boundary layer height and ventilation coefficients. Given a heavy air pollution episode occurring over the THB, such inverse contribution of terrain effects reflected a unique importance of sub-basin topography in regional transport of air pollutants for air pollution in central China.

The dominant mechanism of the explosive rise of PM2.5 after significant pollution emissions reduction in Beijing from 2017 to the COVID-19 pandemic in 2020

The Chinese government implemented strict emission reduction measures of air pollution between 2013 and 2017. However, from the winter of 2017 until February 2020, during the COVID-19 pandemic, the twenty explosive rise (ER) events of PM2.5 mass in twelve heavy aerosol pollution episodes (HPEs) still appeared in Beijing and its vicinity (BIV). To explore the controlling mechanism for the ER under the condition of drastically reduced emissions, the vertical structure of meteorological elements by L-band second-level sounding and aerosol properties by Lidar were investigated associating with the analysis of surface concentration in PM2.5 mass, its main precursor gases, as well as black carbon (BC) by seven-wavelength Aethalometer. The planetary boundary layer height (BLH) was also estimated together with an analysis of the unfavorable meteorological index (PLAM) that can quantify the impact of unfavorable meteorological conditions to cause the change of PM2.5 concentration. The results suggested that the ER reoccurrence's fundamental cause is that the emissions have not yet fallen sufficiently to a level to decouple HPEs from unfavorable meteorological conditions. During the ER period, the BLH dropped significantly. The fact that PM2.5, its precursor gases, and black carbon increased almost in a similar proportion, indicating that the boundary layer structure change caused by aerosol accumulation is the dominant reason for the ER phenomenon compared to the chemical conversion factor. The two-way feedback effect between the further worsened meteorological conditions and the accumulation of PM2.5 typically interpreted 54%–93% of the ER. An HPE starting 8 Feb. 2020 during the COVID-19 period underwent one of the worst meteorological conditions, quantified by PLAM, in BIV since 2013. However, with a similar level of unfavorable meteorological conditions, the average PM2.5 concentration during the HPE in 2020 was only about 66% of that of a similar HPE in 2016. It shows that the substantial reduction of emissions reduces the PM2.5 pollution level primarily as before when facing an equivalent level of unfavorable meteorological conditions. These results combined suggest that China's continuous efforts to reduce emissions proceed in the right direction and have achieved the desired results.

The effects of meteorological conditions and long-range transport on PM2.5 levels in Hanoi revealed from multi-site measurement using compact sensors and machine learning approach

Hanoi, the capital of Vietnam, frequently experiences heavy air pollution episodes in the winter, causing health concerns for the 7.5 million people living there. Spatial-temporal variations in PM2.5 levels can provide useful information about the sources and transportation of PM2.5. However, the published spatial-temporal data in the area are limited. In this research, PM2.5 concentrations at two sites in Hanoi and a site in Thai Nguyen (60 km north of Hanoi) were observed from October 2017 to April 2018, using newly available low-cost sensors. Hourly concentrations of PM2.5 at the three sites were similar on average (57.5, 54.9, and 53.6 μg m−3) and clearly co-varied, suggesting remarkable large-scale effects. The contribution of long-range transport and meteorological factors on PM2.5 levels were investigated with a machine learning technique based on a random forest (RF) algorithm and concentration weight trajectory (CWT). The results showed that the contribution of long-range transport from the north and northeast to local PM2.5 levels was significant. Moreover, weather normalized PM2.5 concentrations and partial plots of meteorological factors on the levels of PM2.5 showed that meteorological conditions play a significant role in the formation of winter haze events.

Atmospheric oxidation of 1-butene initiated by OH radical: Implications for ozone and nitrous acid formations

The present study combines (i) field measurements of 1-butene over Xi'an and surrounding areas during a heavy ozone pollution episode with (ii) quantum chemical calculations on the photochemical transformation of 1-butene initiated by OH radical. The measurements show that the diurnal variation of both 1-butene and ozone exhibits a negative relationship during the sampling campaign. The concentrations of 1-butene vary from 0.04 to 0.67 ppbv, and the lowest and highest values emerge in the noon time (1 p.m.–3 p.m.) and in the night time (1 a.m.–3 a.m.). The concentrations of O3 range from 32.36 to 88.82 ppbv, and the high values appear in the noon time. Quantum chemical calculations show that the OH-addition pathways are more energetically preferable than H-abstraction channels, and the rate coefficient exhibits a negative T-dependence in the temperature range of 273–400 K. The autoxidation of peroxy radicals (RO2) formed from the addition of molecular oxygen to OH-addition products S2 and S3 leads to the formation of highly oxygenated molecules (HOMs), C4H9O5, in which the 1,5-H shift reaction is favoured. In the low-NOx regions, RO2 radicals mainly react with HO2 radical leading to hydroperoxide ROOH on both the singlet and triplet PESs, in which the triplet PES is preferable. In the high-NOx regions, RO2 radicals mainly react with NO leading to the formation of RO· + NO2, aldehyde + HONO and organic nitrate, in which the HONO formation appears to be previously unconsidered in the photochemical transformation of alkenes. The branching ratio of RO· + NO2 increases slowly with increasing temperature (from 51.3 to 52.7%), whereas aldehyde + HONO gradually decreases with the temperature rising (from 48.7 to 47.3%). The formation of organic nitrate is negligible over the temperature range of 273–400 K. These findings are expected to deepen our understanding the photochemistry oxidation of alkene under realistic atmospheric conditions.

Assessing the pollutant evolution mechanisms of heavy pollution episodes in the Yangtze-Huaihe valley: A multiscale perspective

The Yangtze-Huaihe (YH) region experiences heavy aerosol pollution, characterized by high PM2.5 concentration. To unravel the pollutant evolution mechanism during the heavy pollution episodes (HPEs), this study combined observational data analysis and three-dimensional WRF-Chem simulations. From December 2, 2016 to January 15, 2017, YH region experienced 4 HPEs under the control by synoptic system, normally associated with a transport stage (TS) and a cumulative stage (CS). During the TS, pollutants are transported to the north of YH region through the near-surface, and then transported to the “mountain corridor” through the residual layer (RL) under the influence of prevailing wind. For the RL transport mechanism, the change of pollutant concentration cannot only consider the net flux in the horizontal direction, but also the role of the vertical movement is extremely important and cannot be ignored. By analyzing the mass conservation equation of pollutant, the results show that the advection transport and turbulent diffusion have a synergistic effect on the change of pollutant in the CS of three HPEs. The change of turbulence characteristics also affected by topography. For the “mountain corridors”, which is accompanied by variable wind direction and turbulence diffusion is easily affected by wind shear. In addition, the turbulence characteristics are different during the TS and CS, especially the strong stable conditions in the CS at nighttime. The turbulence is intermittent, and the model has insufficient performance for turbulence, which will lead to differences for the simulation of pollutant concentration. In short, as the PM2.5 concentration linearly increases, the friction velocity (turbulent diffusion coefficient) decreases 63% (80%), 61% (78%) and 45% (68%), respectively. Therefore, the change of pollutants is less sensitive to the change of turbulence during the HPEs. The contribution of regional transport (local emissions) reaches 43% (47%), thus we need pay attention to the contribution of each part during the HPEs, which will help us to build a certain foundation for the emission reduction work in the future.

Assessment of Emergency Emission Reduction Effect During the Heavy Air Pollution Episodes in Beijing, Tianjin, Hebei, and Its Surrounding Area("2+26" Cities) from October to December 2019

To evaluate the effect of emergency emission reduction measures during the heavy air pollution episodes in Beijing, Tianjin, Hebei, and its surrounding areas, a scenario simulation method was used. The concentrations of PM2.5, PM10, SO2, NO2, CO, and O3-8h, air quality index (AQI), characteristics of heavy air pollution, and climate and meteorological factors were analyzed using the observation data available from October to December 2019. The 24 h, 72 h, and 144 h prediction results of NAQPMS model were analyzed. The uncertainties of the assessment and model prediction were discussed. The results showed that the average PM2.5 concentration in Beijing, Tianjin, and its surrounding 26 cities ("2+26" cities) from October to December 2019 was 64 μg ·m-3, indicating a decrease of 10 μg ·m-3 as compared with that during the same period in 2018. There were 4 occurrences of regional heavy air pollution episodes, with the average PM2.5 concentration of 156 μg ·m-3 of affected cities. The value of evaluation on meteorological condition index of PM2.5 pollution (EMI) of "2+26" cities ranged from -15.6%-16.8%. The meteorological conditions of 12 cities, including Beijing, Tianjin, and Shijiazhuang, deteriorated as compared with those during the same period in 2018, and the changes ranged from 3.2%-16.8%. However, the emergency emission reduction measures effectively reduced the occurrence of regional heavy air pollution episodes, the peak concentration of PM2.5 was decreased significantly, and no severe regional pollution episode occurred. The daily PM2.5 concentrations reduced by 2% to 9% in Beijing, Shijiazhuang, Baoding, Tangshan, and other cities during a typical heavy air pollution period. The quarterly average concentrations of PM2.5 in the "2+26" cities reduced by 1 to 3 μg ·m-3. The regional emergency emission reduction measures have played an active role in protecting the health of the people and improving the quality of ambient air.

Changes in ammonia and its effects on PM2.5 chemical property in three winter seasons in Beijing, China

NH3, SO2, NOx and the inorganic ions of PM2.5 in winter 2009, 2014 and 2016 were examined to investigate the change in NH3 and aerosol chemistry in Beijing, China. NH3 concentrations showed an increase by 59% on average, in contrast to the decrease of SO2 by 63% from winter 2009 to 2016. The mean mass ratio of NH3/NHx was 0.83 ± 0.12 in 2016, which is higher than those obtained in 2009 and 2014, implying more NHx remaining as free NH3 in 2016 winter. Our findings suggest that vehicles exhaust emissions are an important NH3 source in urban central atmosphere of Beijing in winter. Despite the observed NOx presenting declining trends from 2014 to 2016, nitrate concentrations even exhibited a significant increasing trend, which may be largely attributable to high NH3 levels. An in-depth analysis of measured NH3 and aerosol species in a heavy pollution episode in December 2016, combined with the acidity predicted by ISORROPIA II model demonstrated abundant NH3 most of the time in air, where NH3 is not only a precursor for NH4+ but also effect the neutralization of SO42− and NO3− in PM2.5. With high RH and low photochemical activity, elevated NO3− concentration was attributed to an enhanced heterogeneous conversion of NOx to HNO3 to form NH4NO3 in pollution transport stage. The decrease in NOx from high level and the increase in NH3, with peaks of SO42− occurring were observed in pollution cumulative stage. The aqueous-phase oxidation of SO2 by NO2 to sulfate might play an important role with high pH values. Our results suggested that the simultaneous control of NH3 emissions in conjunction with SO2 and NOx emissions would be more effective in reducing particulate matter PM2.5 formation.

Four-Month Changes in Air Quality during and after the COVID-19 Lockdown in Six Megacities in China.

The pandemic of coronavirus disease 2019 (COVID-19) resulted in a stringent lockdown in China to reduce the infection rate. We adopted a machine learning technique to analyze the air quality impacts of the COVID-19 lockdown from January to April 2020 for six megacities with different lockdown durations. Compared with the scenario without lockdowns, we estimated that the lockdown reduced ambient NO2 concentrations by 36–53% during the most restrictive periods, which involved Level-1 public health emergency response control actions. Several cities lifted the Level-1 control actions during February and March, and the avoided NO2 concentrations subsequently dropped below 10% in late April. Traffic analysis during the same periods in Beijing and Chengdu confirmed that traffic emission changes were a major factor in the substantial NO2 reduction, but they were also associated with increased O3 concentrations. The lockdown also reduced PM2.5 concentrations, although heavy pollution episodes occurred on certain days due to the enhanced formation of secondary aerosols in association with the increased atmospheric oxidizing capacity. We also observed that the changes in air pollution levels decreased as the lockdown was gradually eased in various cities.

Characterization of Two Heavy Pollution Episodes in Tianjin in 2020

High-resolution online monitoring data from January to February in 2020 was used to study the characterization of two heavy pollution episodes in Tianjin in 2020; the heavy pollution episode that lasted from January 16 to 18, 2020 (referred to as episode Ⅰ) and that from February 9 to 10, 2020 (referred to as episode Ⅱ) were analyzed. The results showed that two heavy pollution episodes were influenced by regional transportation in the early stage and local adverse meteorological conditions in the later stage. During these episodes, the average wind speed was low, the average relative humidity was close to 70%, and relative humidity approached the saturated, the boundary layer heights were below 300 m, and the horizontal and vertical diffusion conditions were poor. Compared to episode Ⅰ, the concentration of pollutants decreased during episode Ⅱ, especially for the concentration of NO2. During the episode Ⅱ, the concentrations of PM2.5 and CO were higher in the north of Tianjin. The chemical component concentrations and their mass ratios to PM2.5 changed significantly in both episodes; the concentrations of secondary inorganic ions (NO3-, SO42-, and NH4+), elemental carbon (EC) and Ca2+were higher in episode Ⅰ, the concentrations of organic carbon (OC) and Cl- slightly increased in episode Ⅱ; and the concentrations of K+were higher in episode Ⅱ. Compared to episode Ⅰ, because of the increase in the combustion sources and significant reductions in the number of vehicles, the mass ratios of SO42-, OC, and K+ to PM2.5 increased while the mass ratios of NO3- and EC to PM2.5 decreased in episode Ⅱ; the mass ratios of NH4+ and Cl- to PM2.5 were relatively higher due to the continuity of the industrial production processes; the mass ratios of Ca2+ to PM2.5 were lower in two heavy pollution episodes because construction activities were halted. Source apportionment of PM2.5 was performed using the positive matrix factorization (PMF) model. In episode Ⅰ, the major sources of PM2.5 in Tianjin were secondary sources, industrial and coal combustion, vehicle exhaust, crustal dust, fireworks and biomass burning, with contributions of 53.8%, 20.2%, 18.6%, 6.3%, and 1.1%, respectively. In episode Ⅱ, the same sources were identified in the PMF analysis with contributions of 48.3%, 28.2%, 8.7%, 2.6%, and 12.2%, respectively. Compared to episode Ⅰ, the contributions of industrial and coal combustion, fireworks and biomass burning increased, and the contributions of secondary sources, vehicle exhaust, and crustal dust decreased in episode Ⅱ; contributions of vehicle exhaust and crustal dust decreased by 53.2% and 58.7%, respectively.

Characteristics of Secondary Organic Particles and the Potential Formation of SOA from VOCs During Wintertime Heavy Pollution Episodes in Tianjin

The characteristics of secondary organic reactions were studied based on supersite monitoring data from January to March, 2019, in Tianjin. During heavy pollution episodes, SOC (secondary organic carbon) accounted for between 3.1% and 3.8% of PM2.5, and the growth rate of SOC was obviously higher than that of PM2.5, thus indicating that secondary organic reactions had a considerable effect on PM2.5. The growth rate of VOCs (volatile organic compounds) was lower than that of PM2.5, which was probably due to the fact that VOCs were consumed as precursors to secondary particles. The ratio of ethane to acetylene was higher than 2.0 during heavy pollution episodes indicating that air masses were old, and the ratio was lower than clean air days showing that the reaction activities were higher than before. During the heavy pollution episodes, the potential formation of SOA (secondary organic aerosol) from VOCs ranged from 0.49 to 1.21 μg·m-3. Among the species, aromatic hydrocarbons contributed the most, whereby the highest contribution exceeded 90%, and their growth rates were also the highest; hence, aromatic hydrocarbons were the VOCs species that had the greatest effect on SOA.

Understanding the formation of high-ozone episodes at Raoyang, a rural site in the north China plain

Abstract An observational field campaign was carried out from 20 June to July 31, 2016 at Raoyang, a rural site in the North China Plain (NCP). In-situ measurements of O3 and related gases were obtained and analyzed in this work. Heavy photochemical pollution episodes lasting for a few days were observed in the campaign. The episodes could be divided into two categories, with one being influenced by plume from northeast (NE) via Beijing-Tianjin cities and the other from southeast (SE) via Shandong province. The SE episodes were more frequent with higher O3 concentrations and lagged daily peaks of O3 despite of lower concentrations of precursors. Such episodes could last for several days or even a week. In contrast, the NE episode was associated with relatively lower O3 levels and earlier daily O3 peaks though the levels of precursors were higher. The WRF-Chem model was used to study the differences between typical SE and NE episodes in regional distributions of O3 concentrations, chemical regimes of O3 formation, ozone production efficiency (OPE), photochemical age of plume, and evolution of O3 concentration within the boundary layer. Our results suggest that the O3 production regimes were different for the Raoyang site, with the NE episode being VOCs-sensitive and the SE episode in the VOC-sensitive to NOx-sensitive transition regime. In both episodes, a major part of the NCP was under the VOCs-sensitive regime. Plumes in the SE episodes were more aged than those in the NE episode. The OPE values were higher in more aged SE plumes, resulting in higher O3 concentrations despite of lower precursor concentrations. Our results also indicate that downward mixing of O3 in the nocturnal residual layer played an important role in rapid buildup of surface O3 in the next morning in both NE and SE episodes. The formation of photochemical smog seems to be particularly effective in the SE episode since O3 is rapidly produced during daytime, stored aloft at night, and mixed downward to the ground-level, and the process can circulate for several consecutive days. Our study highlights the complicated role of chemical reaction, transport and boundary layer processes in the formation of severe photochemical smog in the region.

Temporal and spatial variations of haze and fog and the characteristics of PM2.5 during heavy pollution episodes in China from 2013 to 2018

In recent years, the frequent occurrence of haze and fog events in East and Central China has caused a serious reduction in atmospheric visibility and had a huge impact on human health. Based on the relative humidity, visibility and PM2.5 concentration, the occurrence frequencies of haze as well as the spatial and temporal variations of fog, haze and mist in China from 2013 to 2018 were analyzed. The occurrence frequency of the three weather phenomena increased from 2013 until 2016, and then decreased. Regarding seasonal distribution, haze days are concentrated in winter, while mist often appears in spring and autumn. Clean fog mainly occurs in high-mountain stations and clean areas with high relative humidity, while polluted fog is more common in densely populated cities. In Beijing, the proportion of clean weather increases year by year, PM2.5 in haze weather in Beijing mainly consists of organic carbon, nitrate, sulfate and other secondary aerosols. Most fog events are accompanied by air pollution. The dominant haze is humid haze, which means that its occurrence and aggravation are usually related to aerosol hygroscopic growth.

Characterization and Source Apportionment of Fine Particles during a Heavy Pollution Episode over the Yangtze River Delta, China

Regional-scale field observations of fine particles (PM2.5) were carried out at urban, suburban and regional background sites across the Yangtze River Delta (YRD) from 15–30 January 2015. The coefficients of divergence (CD) values reveal the similarity of dataset at the three sites. The PM2.5 concentrations and meteorological data exhibit temporal synchronization. From January 15 to 26, the YRD experienced severe PM2.5 pollution resulting from a cold front moving through and high-pressure control. Then, a 4-day intermittent rain event from 27–30 January significantly scavenged PM2.5. For the chemical components in PM2.5, secondary inorganic ions were dominant, and they accounted for larger proportions at the urban and suburban sites than at the regional background site. The OC/EC ratios were higher in daytime than at night, and were lower on polluted days than on clean (rainy) days. The principal sources of PM2.5 were secondary nitrate (38%) and sulfate (23%) formation, biomass burning (14%), and marine source (8%). Marine (16%) and sulfate (30%) sources were enhanced on clean (rainy) days, indicating the notable effect of marine air masses on PM2.5 chemical components. The open burning source contribution at the regional site was the largest during the polluted period because more air masses arrived from combustion zones.

Impact of “blocking” structure in the troposphere on the wintertime persistent heavy air pollution in northern China

In the winters of 2012–2018, a total of 25 ‘ultra-long’ (≥6 days), wide-ranging pollution events occurred in northern China. The results showed that the atmospheric circulation pattern corresponding to 62.5% of the persistent ‘most serious’ and ‘more serious’ air pollution events in northern China were the blocking structures, and that 43.75% of the 500-hPa atmospheric circulation anomalies in the middle and high latitudes of Eurasia were ‘dual-blocking’, 18.75% of them were ‘single-blocking’. The abnormally stable blocking situation provided a special circulation background for the occurrence and maintenance of persistent heavy air pollution in northern China. The Okhotsk blocking is significantly positively correlated with the persistent ‘most serious’ air pollution events. ‘Stagnation’ of the blocking system and its dynamic effect play an important role in regulating atmospheric environmental capacity and accelerating the accumulation of aerosols during the persistent heavy pollution episodes. Due to the synergy between the weak wind effect of the leeward slope on the eastern side of the Loess Plateau in this region and the downward airflow of the large-scale blocking system, the effect of sustained suppression of atmospheric pollutant diffusion in northern China is more significant. The downward air flow along the eastern leeward slope of the Loess Plateau is very important for accumulation of air pollutants, which is controlled by the tropospheric blocking high. In addition, the ‘subsidence (temperature) inversion’ effect produced by the synergy between the downward airflow of the eastern leeward slope of the Loess Plateau and the large-scale blocking system creates a continuous and stable ‘warm-cover’ structure in the middle of the troposphere on the eastern of the Loess Plateau; this effect strengthens the radiation effect of aerosols in the atmospheric pollutants, as well as the ‘two-way feedback’ mechanism between adverse meteorological conditions in the boundary layer and atmospheric pollutants.

The Impact of Air Pollution on Movie Theater Admissions

We provide new evidence of pollution avoidance in the movie theater market. Our analysis is based on a unique data set of high-frequency movie ticket sales in China at the movie- and city-level during 2012-2014. We estimate that one pollution day reduces the market share of a movie by 2.26%, other things being equal. On average, the popular movies are inelastic to air pollution. However, as air quality deteriorates, even the blockbuster movies start to suffer a box-office loss during the heavy pollution episodes. Our results suggest that air pollution suppresses demand for a consumer sector in a largely indoor environment.

Impact of emission reduction on aerosol-radiation interaction during heavy pollution periods over Beijing-Tianjin-Hebei region in China

In December 2015, the Beijing-Tianjin-Hebei (BTH) region experienced several episodes of heavy air pollution. The government immediately issued emergency control measures to reduce the pollution, which provided a good opportunity to explore the impact of emission reduction on aerosol-radiation interaction. In this study, four tests were conducted, including the base1 simulation with emission reduction and aerosol-radiation interaction on, the base2 simulation with emission reduction and aerosol-radiation interaction off, the scenario1 simulation without emission reduction and aerosol-radiation interaction on and the scenario2 simulation without emission reduction and aerosol-radiation interaction off. We find that the aerosol-radiation interaction decreased the downward shortwave radiation and the temperature at 2 m, reduced the planetary boundary layer height (PBLH) in the region, and increased the relative humidity at 2 m, which is favorable for pollution accumulation. Our results revealed that the interaction effect due to emission reductions increased downward shortwave radiation by an average of 0–5 W/m2, leading to increase in surface temperature of 0–0.05°C, increase in the daytime mean PBL high of 0–8 m, and decrease in daytime mean relative humidity at 2 m of 0.5%. We found that if there were aerosol-radiation interaction, it would enhance the effectiveness of emission control measures on air pollution control. The enhance of PM2.5 (particulate matter less than 2.5 µm), PM10 (particulate matter less than 10 µm), and NO2 (nitrogen dioxide) emission reduction effects reached 7.62%, 6.90%, 11.62%, respectively, over this region.

Evolution of source contributions during heavy fine particulate matter (PM2.5) pollution episodes in eastern China through online measurements

Abstract Ambient heavy fine particulate matter (PM2.5) pollution events occur frequently during winter seasons in eastern China. Investigating the evolution of source contributions during heavy pollution episodes is critical for strategies of pollution relief. In this study, a two-month field campaign was conducted in the winters of 2015 and 2016 at a regional supersite in eastern China and over one thousand hourly online measurements for twenty PM2.5 species were obtained. Hourly source apportionment for the total mass of PM2.5, and its major species (primary organic carbon, elemental carbon, nitrate, sulfate and ammonium), were then performed by Positive Matrix Factorization (PMF) and Multilinear Engine-2 (ME2) models. Three PM2.5 heavy pollution episodes including ten high concentration peaks were identified for further analysis. Results showed that ME2 performed better than PMF by fixing source profiles of secondary nitrate, secondary sulfate and sea salt, although they have dominant consistencies. Two types of pollution sources were identified from high PM2.5 mass peaks: coal combustion-oriented responsible for four peaks and secondary inorganic aerosol-oriented responsible for the remaining six peaks. Low wind speed and planetary boundary layer favorited the coal combustion-oriented peaks, but also weakened the secondary inorganic formation due to low temperature and accumulated nitric oxide. Primary emissions from coal combustion can contribute 18% to ammonium in addition to 72% from secondary inorganic aerosol. Secondary organic aerosol would contribute 20% of PM2.5 mass during the heaviest episodes. Findings in this study provide insights into the causes of heavy pollution episodes and support implementing effective control strategies to mitigate heavy pollution events in eastern China.

High resolution temporal profiles in the Emissions Database for Global Atmospheric Research

Emissions into the atmosphere from human activities show marked temporal variations, from inter-annual to hourly levels. The consolidated practice of calculating yearly emissions follows the same temporal allocation of the underlying annual statistics. However, yearly emissions might not reflect heavy pollution episodes, seasonal trends, or any time-dependant atmospheric process. This study develops high-time resolution profiles for air pollutants and greenhouse gases co- emitted by anthropogenic sources in support of atmospheric modelling, Earth observation communities and decision makers. The key novelties of the Emissions Database for Global Atmospheric Research (EDGAR) temporal profiles are the development of (i) country/region- and sector- specific yearly profiles for all sources, (ii) time dependent yearly profiles for sources with inter-annual variability of their seasonal pattern, (iii) country- specific weekly and daily profiles to represent hourly emissions, (iv) a flexible system to compute hourly emissions including input from different users. This work creates a harmonized emission temporal distribution to be applied to any emission database as input for atmospheric models, thus promoting homogeneity in inter-comparison exercises.

Meteorological Variables and Synoptic Patterns Associated with Air Pollutions in Eastern China during 2013-2018.

Steady meteorological conditions are important external factors affecting air pollution. In order to analyze how adverse meteorological variables affect air pollution, surface synoptic situation patterns and meteorological conditions during heavy pollution episodes are discussed. The results showed that there were 78 RPHPDs (regional PM2.5 pollution days) in Jiangsu, with a decreasing trend year by year. Winter had the most stable meteorological conditions, thus most RPHPDs appeared in winter, followed by autumn and summer, with the least days in spring. RPHPDs were classified into three patterns, respectively, as equalized pressure (EQP), advancing edge of a cold front (ACF) and inverted trough of low pressure (INT) according to the SLP (sea level pressure). RPHPDs under EQP were the most (51%), followed by ACF (37%); INT was the minimum (12%). Using statistical methods and meteorological condition data on RPHPDs from 2013 to 2017 to deduce the thresholds and 2018 as an independent dataset to validate the proposed thresholds, the threshold values of meteorological elements are summarized as follows. The probability of RPHPDs without rain was above 92% with the daily and hourly precipitation of all RPHPDs below 2.1 mm and 0.8 mm. Wind speed, RHs, inversion intensity(ITI), height difference in the temperature inversion(ITK), the lower height of temperature inversion (LHTI) and mixed-layer height (MLH) in terms of 25%–75% high probability range were respectively within 0.5–3.6 m s−1, 55%–92%, 0.7–4.0 °C 100 m −1, 42–576 m, 3–570 m, 200–1200 m. Two conditions should be considered: whether the pattern was EQP, ACF or INT and whether the eight meteorological elements are within the thresholds. If both criteria are met, PM2.5 particles tend to accumulate and air pollution diffusion conditions are poor. Unfavorable meteorological conditions are the necessary, but not sufficient condition for RPHPDs.

Effects of aerosol pollution on PM2.5-associated bacteria in typical inland and coastal cities of northern China during the winter heating season

Frequent heavy aerosol pollution occurs during the winter heating season in northern China. Here, we characterized the airborne bacterial community structure and concentration, during typical pollution episodes that occurred during the winter heating season of 2017–2018 in Jinan and Weihai. During this heating season, five and four heavy pollution episodes were observed in Jinan and Weihai, respectively. Compared with December and January, pollution episodes in March were significantly affected by sand dust events. Higher Bacillales were identified in the March samples from Jinan, indicating that sand dust influences bacterial communities. During similar pollution episodes, air pollution in the coastal city of Weihai was lower than the inland city of Jinan. The predominant bacteria included Staphylococcus, Cyanobacteria, Lactobacillus, Deinococcus, Enbydrobacter, Ralstonia, Bacillus, Comamonas, and Sphingomonas. These predominant bacteria are mainly from Proteobacteria, Firmicutes, Cyanobacteria, Actinobacteria, and Bacteroidetes phyla. Bacterial concentration showed significant variation with increased airborne pollutants. The highest concentration appeared during moderate pollution (up to 106 cells/m3), whereas bacterial concentration decreased during heavy and severe pollution (105 cells/m3), which may be related to toxic effects of high pollutant concentrations during heavy or severe pollution. Community structure variation indicated that Cyanobacterial genera were dominant in clean or slight pollution. With increased PM2.5, Staphylococcus increased and became the most abundant bacteria in moderate pollution (up to 40%). During heavy or severe pollution, bacteria that are adaptable to harsh or extreme environments predominate, such as Deinococcus and Bacillus. In the assessment of health risks from air pollution, the bioaerosols risks must consider. Additionally, although most microbial genera are similar between the two cities, there are important differences associated with pollution level. During air pollution regulation in different regions with varied geographical and climatic conditions, bioaerosol pollution difference is an unignored factor.