Regional Haze Episodes Research Articles

Experimental study of smouldering combustion and transient emissions from forest duff with dual layers

Smouldering combustion in wildfires is responsible for large amounts of carbon emissions and regional haze episodes. In the wildland, there are dual or multiple layers of forest duff with varying particle sizes, forming a non-homogeneous fuel system. However, understanding of the effects of the fuel heterogeneity on smouldering combustion is limited. In this study, we experimentally investigated the smouldering behaviour and gas emissions of forest duff consisting of dual layers with particle diameters ranging in three sizes (1 mm ≤ d1 < 2 mm, 0.425 mm ≤ d2 < 1 mm, and d3 < 0.425 mm). A comprehensive smouldering combustion rig was designed and built to simultaneously measure the mass loss, temperature profile, time-resolved gas concentration for 20 species, and visual and infrared imaging for 6 dual-layered duff samples (d1 - d1, d1 - d2, d1 - d3, d2 - d2, d2 - d3, d3 - d3) under a controlled environment. A terrace smouldering front structure was observed for dual-layered fuel beds with coarse duff particle (d1 and d2) staying in the upper layer (50% of the volume of an open-top reactor with dimensions of 20 × 20 × 10 cm), while a surface cracking phenomenon was observed for sample consisting of fine duff particles d3 - d3. Increases in the particle size at the lower-layer with a fixed d1 upper-layer particles were found to have a greater impact on the oxygen supply, leading to a higher lateral spread rate, a higher modified combustion efficiency (MCE), and a lower fire-averaged CO/CO2. In contrast, changes in the upper-layer particle size with a fixed d3 lower-layer particles have a greater impact on the heat transfer. These findings advance the scientific understanding of smouldering fires spreading in dual forest fuel system, ultimately contributing to the development of wildfire mitigating technologies.

Evolution of atmospheric age of particles and its implications for the formation of a severe haze event in eastern China

Abstract. Atmospheric age reflects how long particles have been suspended in the atmosphere, which is closely associated with the evolution of air pollutants. Severe regional haze events occur frequently in China, influencing air quality, human health, and regional climate. Previous studies have explored the characteristics of mass concentrations and compositions of fine particulate matter (PM2.5) during haze events, but the evolution of atmospheric age remains unclear. In this study, the age-resolved University of California, Davis/California Institute of Technology (UCD/CIT) model was developed and applied to simulate the concentration and age distribution of PM2.5 during a severe regional haze episode in eastern China. The results indicated that PM2.5 concentrations in the North China Plain (NCP) gradually accumulated due to stagnant weather conditions during the beginning stage of the haze event. Accordingly, the atmospheric age of elemental carbon (EC), primary organic aerosol (POA), sulfate (SO42-), and secondary organic aerosol (SOA) gradually increased. The subsequent PM2.5 concentration growth was driven by the local chemical formation of nitrate (NO3-) under high relative humidity. The newly formed NO3- particles led to a decrease in the mean atmospheric age of NO3- particles. During the regional transport stage, aged particles from the NCP moved to the downwind Yangtze River Delta (YRD) region, leading to a sharp increase in PM2.5 concentrations and the average age of EC, POA, SO42-, and SOA in YRD. In contrast, the average age of NO3- and ammonium remained unchanged or even slightly decreased due to continuous local formation in the YRD region. Different evolution of the atmospheric age among these components provides a unique perspective on the formation of PM2.5 components during the regional haze event. The information can also be used for designing effective control strategies for different components of PM2.5.

Enhanced Production of Organosulfur Species during a Severe Winter Haze Episode in the Guanzhong Basin of Northwest China.

The molecular composition of organic aerosols in ambient PM2.5 was investigated in an urban area in the Guanzhong basin of northwest China during a severe regional haze episode in the winter of 2018/2019. Organic matter, accounting for 20-35% of PM2.5 mass concentration, was characterized using direct infusion and electrospray ionization coupled with high-resolution Orbitrap mass spectrometry. The number of organic molecular formula assignments was primarily dominated by organosulfur species (OrgS, including CHOS and CHONS) in negative ion mode. The number and peak signal intensity of OrgS distinctly increased during the severe haze episode. Organosulfates and nitrooxy-organosulfates constituted the majority number (72-94%) of OrgS over the entire period. Although the OrgS were mostly present in aliphatic molecular structures, an increase in the number of polycyclic aromatic OrgS on haze days revealed the enhanced contribution from anthropogenic sources. The number of OrgS strongly correlated with ambient relative humidity and the oxidation ratios of sulfur and nitrogen, suggesting the important roles of aqueous phase chemistry and atmospheric oxidation in the formation of OrgS. A thorough understanding of the significance of OrgS will be essential to assess and mitigate the adverse impacts of haze pollution.

Characteristics of chemical composition and source apportionment of PM2.5 during a regional haze episode in the yangtze river delta, china

The source of PM2.5 varies at different stages of urban haze pollution. In addition, there is obvious regional transport of pollutants between urban agglomerations. PM2.5 and its major chemical compositions in a regional haze episode were measured continuously from 16 to 27 November 2018 in Nanjing, China. The types of primary sources resolved by principal component analysis (PCA) and positive matrix factorization (PMF) were similar, and the result of PMF was more refined. The average contribution of each source by PMF was: secondary nitrate (64.01%), secondary sulfate (11.62%), incomplete combustion (4.49%), sea salt (8.61%), biomass burning (6.90%), and crustal dust (4.37%). In different haze stages, the distribution characteristics of air pollutants differed. The concentrations of SO42-, NO3−, NH4+, and black carbon were the highest in the haze developing stage, which was 2.0, 3.1, 3.0, and 2.4 times, respectively, higher than that under clean conditions. The increment of NO3− dominated the development of haze, and the proportion of NO3− from haze generation to development increased by 4.05%. The concentration contributions of secondary nitrate, sea salt, and biomass burning were highest in haze development, secondary sulfate was highest in haze generation, and incomplete combustion was highest in haze dissipation, which was 3.5, 1.8, 3.3, 1.7, and 9.5 times higher than the clean stage, respectively. In the haze episode, the contribution of crustal dust was lower than in the clean stage. Potential source contribution function (PSCF) and concentration weighted trajectory (CWT) revealed that the major source area of air pollutants in Nanjing came from the southeast, and the northwest was the major impact area.

Formation, radiative forcing, and climatic effects of severe regional haze

Abstract. Severe regional haze events, which are characterized by exceedingly high levels of fine particulate matter (PM), occur frequently in many developing countries (such as China and India), with profound implications for human health, weather, and climate. The occurrence of the haze extremes involves a complex interplay between primary emissions, secondary formation, and conducive meteorological conditions, and the relative contributions of the various processes remain unclear. Here we investigated severe regional haze episodes in 2013 over the Northern China Plain (NCP), by evaluating the PM production and the interactions between elevated PM and the planetary boundary layer (PBL). Analysis of the ground-based measurements and satellite observations of PM properties shows nearly synchronized temporal PM variations among the three megacities (Beijing, Baoding, and Shijiazhuang) in this region and a coincidence of the aerosol optical depth (AOD) hotspots with the three megacities during the polluted period. During the clean-to-hazy transition, the measured oxygenated organic aerosol concentration ([OOA]) well correlates with the odd-oxygen concentration ([Ox]=[O3]+[NO2]), and the mean [OOA] / [Ox] ratio in Beijing is much larger than those in other megacities (such as Mexico City and Houston), indicating highly efficient photochemical activity. Simulations using the Weather Research and Forecasting (WRF) model coupled with an explicit aerosol radiative module reveal that strong aerosol–PBL interaction during the polluted period results in a suppressed and stabilized PBL and elevated humidity, triggering a positive feedback to amplify the haze severity at the ground level. Model sensitivity study illustrates the importance of black carbon (BC) in the haze–PBL interaction and the aerosol regional climatic effect, contributing to more than 30 % of the PBL collapse and about half of the positive radiative forcing on the top of the atmosphere. Overall, severe regional haze exhibits strong negative radiative forcing (cooling) of −63 to −88 W m−2 at the surface and strong positive radiative forcing (warming) of 57 to 82 W m−2 in the atmosphere, with a slightly negative net radiative forcing of about −6 W m−2 on the top of the atmosphere. Our work establishes a synthetic view for the dominant regional features during severe haze events, unraveling rapid in situ PM production and inefficient transport, both of which are amplified by atmospheric stagnation. On the other hand, regional transport sufficiently disperses gaseous aerosol precursors (e.g., sulfur dioxide, nitrogen oxides, volatile organic compounds, and ammonia) during the clean period, which subsequently result in rapid in situ PM production via photochemistry during the transition period and via multiphase chemistry during the polluted period. Our findings highlight the co-benefits for reduction in BC emissions, which not only improve local and regional air quality by minimizing air stagnation but also mitigate the global warming by alleviating the positive direct radiative forcing.

Development of gas signatures of smouldering peat wildfire from emission factors

Smouldering peat fires are responsible for regional haze episodes and cause environmental, social and health crises. Owing to the unique burning characteristics of smouldering peat, identifying and detecting this kind of fire remains a challenge. This work explores smouldering peat gas signatures using emission factor (EF) data from literature. Systematic comparisons and statistical analyses were carried out to investigate 28 forms of EF combinations created from the four most abundant gas species: carbon dioxide (CO2), methane (CH4), carbon monoxide (CO) and ammonia, from smouldering peat, flaming savanna and grassland, agricultural residue and forest fires. Among the candidate gas signatures, the ratio of EF(CO2) to EF(CH4) for smouldering peat showed a significant improvement with statistically different ranges of values (134.6) compared to those from flaming savanna and grassland fire (940.2), agricultural residue fire (434.4 ), forest fire (368.8) and mixed burning peat fires (207.7). Additionally, we found that EF(CO2)/EF(CH4) is independent from fuel composition and could differentiate early ignition from the subsequent spread, making it the best gas signature among those analysed, including CO/CO2 ratio and the Modified Combustion Efficiency. This work presents the first scientific endeavour developing smouldering gas signatures, contributing to the scientific understanding and remote sensing and early detection of smouldering peat wildfires.

Quantification of different processes in the rapid formation of a regional haze episode in north China using an integrated analysis tool coupling source apportionment with process analysis

North China Plain (NCP) is one of the most of heavily polluted regions in the world, air pollution associated with haze threatens human health. A high PM2.5 concentration event in the NCP from 30 November to 10 December 2017 was simulated and analyzed by using the Weather Research and Forecasting (WRF) model and the Nested Air Quality Prediction Modeling System (NAQPMS) with an integrated analysis tool coupling source apportionment with process analysis. The weather field simulated by the WRF model and the PM2.5 concentration simulated by NAQPMS agreed well with observations, and the correlation coefficient of PM2.5 between the simulation and observation data remained >0.8 during the study period. We found that this high PM2.5 event can be divided into three phases in NCP: The accumulation of PM2.5 in Phase I was slow and dominated by south or weak winds in the stable boundary layer. In Phase II, PM2.5 concentrations kept a high value in the short term under northern winds resulting from the edge of a cold front. The sustainable high value of PM2.5 concentration in the middle and south of the NCP under northly wind has been less reported. An integrated process contribution analysis and source apportionment technology coupled with NAQPMS showed that the PM2.5 in the north and middle of the NCP were transported to the southern area by the horizontal transmission process. This transport kept PM2.5 high in the south of the NCP and even exceeded the contribution of local emissions. In the Phase III, the strong northern winds from main body of the cold front brought clear air masses and caused the PM2.5 decrease in the whole NCP. The study shows that high dense emissions in the middle of NCP are the main cause for the high PM2.5 events in the whole region, and the north wind could transport the pollutants form upstream region to downstream region, causing the continued high PM2.5 in the middle and south of NCP.

Chemical composition, water content and size distribution of aerosols during different development stages of regional haze episodes over the North China Plain

Aerosol size distribution and chemical composition are found to have significant effects on its hygroscopicity, acidity/alkalinity and light extinction. Heavy haze pollution occurred frequently in the wintertime of NCP, with long duration time and large impact area, which had important influences on air quality and human health. However, the study concerning the formation mechanism and physicochemical characteristics of aerosols in different haze stages have been rarely carried out. In this study, aerosol size distribution in the range of 10 nm - 10 μm, water soluble inorganic ions (WSIIs), PM (PM2.5, PM10), trace gases, organic carbon (OC), elemental carbon (EC) and meteorological elements were derived during a regional haze pollution episode from Nov. 9 to 16, 2018. The aerosol water content and pH were further calculated using the ISORROPIA model. We divided the whole observation period into segments of clean days, fog processes, and haze processes, including three stages of I: accumulation, II: growth, and III: explosion, and a dry haze (D-haze) process based on the PM2.5 concentration, visibility and RH. Given the shift of particle size to larger segment ascribed to the ageing process during fog/haze process, the size distribution peak in aerosol number concentration was located at 100 nm in the fog/haze episode and was 70 nm larger than that on clean days. The concentration descended significantly in stage III, peaking at 160 nm, suggesting a strong aging process of aerosols. The PM2.5 increased sharply under high RH, static weather conditions and strengthening oxidation that favored liquid and heterogeneous reactions. The nitrate concentration was found to account for 20.5% (D-haze) - 29.0% (fog) of the total water soluble inorganic ions (WSIIs) during the fog/haze process, while sulfate constituted only 13.8% (stage III) - 21.3% (stage I), revealing dominant nitrate pollution. Hence, nitrate was considered to originate mainly from photochemical and heterogeneous reactions in haze episodes and was generated only by heterogeneous reactions. Higher (lower) aerosol water content made aerosols more acidic (alkaline) under similar chemical compositions. Therefore, the chemical reactions may differ under dry and wet haze pollution. Sources of carbonaceous aerosols changed in different haze stages with descending (enhancing) contributions from coal combustion (vehicle exhaust).

Source apportionment of PM2.5 and its optical properties during a regional haze episode over north China plain

Based on the data of air pollutants in a typical regional haze pollution episode over North China Plain (NCP) in November 2018, Positive matrix factorization (PMF), potential source contribution function (PSCF) and concentration-weighted trajectory (CWT) models were used to analyze the sources and affecting areas of PM2.5 under different development stages. The whole observation period was divided into 6 segments (clean days, fog process, haze process including three stages of I: accumulation, II: growth, III: explosion, and dry haze process) according to PM2.5 concentration, visibility and relative humidity (RH). The dominant source in clean days was soil dust, with contribution percentage of 30.0%. The contributions of coal combustion and secondary nitrate increased with the development of haze pollution, while those of photochemical product, soil dust and secondary sulfate decreased. Coal combustion contributed the most in stage III (35.8%), indicating that the local coal combustion emission was the main reason for the rapid explosion of pollutants. The contribution of vehicular emission was 16.7%, 30.1% and 12.8% in stage I, II, and III, respectively, which was 2.1, 3.8, and 1.6 times that on clean days, presenting significant regional distribution characteristics. The significant local contribution and short-range transport were the major sources of haze pollution episodes in NCP, and Xianghe will bring pollutants to its northeast and southwest directions in the future. (NH4)2SO4, NH4NO3, and organic matter (OM) were the major components of aerosol extinction coefficient (bext) under all development stages, together accounting for 59.5%–78.6%. With the development of haze process, the contribution of (NH4)2SO4 to bext decreased, while that of NH4NO3 increased.

Real-time physiochemistry of urban aerosols during a regional haze episode by a single-particle aerosol mass spectrometer: Mixing state, size distribution and source apportionment

Regional haze pollution can normally be divided into four stages comprising pre-accumulation, explosion, duration and dissipation, under which the physiochemical characteristics of aerosols were clearly distinct. In this study, the chemical compositions and mixing state of aerosols and their source apportionments during a regional haze pollution process in the North China Plain (NCP) were comparably discussed for the different haze evolution stages. Six main particle groups were identified, including carbon-rich particles (53.17%), K-rich particles (27.41%), heavy-metal particles (10.9%), sodium particles (5.61%), dust particles (1.62%) and ammonium particles (1.29%). Subsequently, the chemical components and size distributions of aerosols were found to vary during the different haze stages. Contrary to the OCEC, K–CN, Cu and Pb particles, the EC-nitrate, EC-secondary and Fe particle groups had increasing proportions with the development of the haze events. The fraction of K particles was higher during the pre-accumulation stage and descended rapidly during the subsequent haze stages. In contrast to the OCEC particles, the EC-nitrate and EC-secondary particles from 0.6 to 1.2 μm increased significantly with haze deterioration, implying the enhancement (decline) of aged (primary) carbonaceous particles during the haze episode. The source contributions of PM2.5 changed with the evolution of the haze events, with the largest percentage up to 52.23% from residual and industry emissions during the explosion stage. In contrast to industry and direct combustion emissions, the aging process and traffic emission contributed increasingly with the aggravation of the haze episodes, with higher fractions of 33.17% and 15.57%, respectively, throughout the duration stage.

Size-distribution-based assessment of human inhalation and dermal exposure to airborne parent, oxygenated and chlorinated PAHs during a regional heavy haze episode

The adverse health effects of haze and particle-bound contaminants in China have recently caused increasing concern, and particle size plays a significant role in affecting human exposure to haze-correlated pollutants. To this background, size-segregated particulate samples (nine size fractions (<0.4, 0.4–0.7, 0.7–1.1, 1.1–2.1, 2.1–3.3, 3.3–4.7, 4.7–5.8, 5.8–9.0 and > 9.0 μm) were collected in three scale-gradient cities in northern China and analysed for a series of parent, oxygenated and chlorinated polycyclic aromatic hydrocarbons (PAHs, O-PAHs and Cl-PAHs). The total geometric mean concentrations of PAHs and O-PAHs for Beijing, Zhengzhou and Xinxiang were 98.1 and 27.2, 77.9 and 77.5, 41.0 and 30.7 ng m−3, respectively, which were 50–200 times higher than those for Cl-PAHs (0.5, 0.7 and 0.4 ng m−3). Though unimodal size-distribution patterns were found for all these contaminants for these three cities, PAHs represented distinctly higher concentration levels around the peak fraction (0.7–2.1 μm) than O-PAHs and Cl-PAHs. With 4–6 ring PAHs as dominant components in all samples, the percentage proportion of 2–3 ring PAHs (ranging from 1% to 26%) generally increased with particle size increasing, implying the sources of these compounds varied little among the 9 size fractions in all three cities. The International Commission on Radiological Protection (ICRP) model and permeability coefficient method were synchronously applied to the size-segregated data for inhalation and dermal exposure assessment to intensively estimate the human exposure doses to airborne PAHs. Further, the incremental lifetime cancer risk (ILCR) was calculated and it’s found that ILCR from inhalation was higher than that from dermal uptake for children and adults in Beijing and Zhengzhou, while the ILCR for Xinxiang presented a contrary pattern, revealing dermal uptake to be an equally significant exposure pathway to airborne PAHs compared to inhalation.

Глубина прогорания торфа и потери углерода при лесном подземном пожаре

Глубина прогорания торфа и потери углерода при лесном подземном пожаре

A novel computational solution to the health risk assessment of air pollution via joint toxicity prediction: A case study on selected PAH binary mixtures in particulate matters

Regional haze episode has already caused overwhelming public concern. Unraveling the health effects of the representative composition mixtures of atmospheric fine particulate matters (PM2.5) becomes a top priority. In this study, a novel computational solution integrating chemical-induced genomic residual effect prediction with in vitro-based risk assessment is proposed to obtain the cumulative health risk of typical chemical mixtures of particulate matters (PM). The joint toxicity of binary mixtures is estimated by analyzing both genomic similarity and dose-response curve of relevant pollutants for the chemical-induced genomic residual effect. Specifically, the modified relative potency factor (mRPF) of mixtures is introduced for this purpose, and the ratio of activation (RA) value is defined to assess the corresponding health risks of the mixtures. As a methodology demonstration, the health risk of typical binary polycyclic aromatic hydrocarbon (PAH) mixtures in PM, containing Benzo[a]pyrene (BaP) as a component, is assessed using the proposed solution. Our results indicate that the combined effect of pairwise PAHs of BaP with Benzo[b]fluoranthene (BbF) and Benz[a]anthracene (BaA) is synergistic on p53 pathway, and that the health risk of the such mixtures increases compared to that of the individual ones. Obviously, the cumulative health risk of environmental mixtures will be underestimated when the synergistic effect is wrongly assumed to be additive. To our knowledge, this is the first study ever report on a computational solution to the health risk assessment of environmental pollution via joint toxicity prediction. The novel methodology proposed here makes full use of the open-access in vitro assay data and transcriptomic information in literatures and provides a successful demonstration of the concept of systems biology and translational science.

Review of emissions from smouldering peat fires and their contribution to regional haze episodes

Smouldering peat fires, the largest fires on Earth in terms of fuel consumption, are reported in six continents and are responsible for regional haze episodes. Haze is the large-scale accumulation of smoke at low altitudes in the atmosphere. It decreases air quality, disrupts transportation and causes health emergencies. Research on peat emissions and haze is modest at best and many key aspects remain poorly understood. Here, we compile an up-to-date inter-study of peat fire emission factors (EFs) found in the literature both from laboratory and from field studies. Tropical peat fires yield larger EFs for the prominent organic compounds than boreal and temperate peat fires, possibly due to the higher fuel carbon content (56.0 vs 44.2%). In contrast, tropical peat fires present slightly lower EFs for particulate matter with diameter ≤2.5 μm (PM2.5) for unknown reasons but are probably related to combustion dynamics. An analysis of the modified combustion efficiency, a parameter widely used for determining the combustion regime of wildfires, shows it is partially misunderstood and highly sensitive to unknown field variables. This is the first review of the literature on smouldering peat emissions. Our integration of the existing literature allows the identification of existing gaps in knowledge and is expected to accelerate progress towards mitigation strategies.

Regional transport, source apportionment and health impact of PM10 bound polycyclic aromatic hydrocarbons in Singapore's atmosphere

A study of 16 United States Environmental Protection Agency (USEPA) priority listed PAHs associated with particulate matter ≤ 10 μm (PM10) was conducted in Singapore during the period 29th May 2015 to 28th May 2016. The sampling period coincided with an extensive, regional smoke haze episode (5th September to 25th October) that occurred as a result of forest and peat fires in neighboring Indonesia. Throughout this study, 54 atmospheric PM10 samples were collected in 24 h periods using a high volume sampler (HVS) and quarts fiber filters (QFF) as the collection medium. Hysplit software for computing 3-D backward air mass trajectories, diagnostic ratio analysis and ring number distribution calculations were used to examine the sources of PAHs in the atmosphere in Singapore. Under normal conditions the total PAH concentrations were in a range from 0.68 ng m−3 to 3.07 ng m−3, while for the high haze period the results showed approximately double the concentrations with a maximum value of 5.97 ng m−3. Diagnostic ratio (DR) and principal component analysis (PCA) were conducted and indicated the contribution of the traffic as a dominant pyrogenic source of PAHs during normal periods, while results from the haze dataset showed relatively strong influence of smoke from peat and forest fires in Indonesia. Environmental and health risk from PAHs were assessed for both regular and hazy days.

Modelling Seasonal GWR of Daily PM2.5 with Proper Auxiliary Variables for the Yangtze River Delta

Over the past decades, regional haze episodes have frequently occurred in eastern China, especially in the Yangtze River Delta (YRD). Satellite derived Aerosol Optical Depth (AOD) has been used to retrieve the spatial coverage of PM2.5 concentrations. To improve the retrieval accuracy of the daily AOD-PM2.5 model, various auxiliary variables like meteorological or geographical factors have been adopted into the Geographically Weighted Regression (GWR) model. However, these variables are always arbitrarily selected without deep consideration of their potentially varying temporal or spatial contributions in the model performance. In this manuscript, we put forward an automatic procedure to select proper auxiliary variables from meteorological and geographical factors and obtain their optimal combinations to construct four seasonal GWR models. We employ two different schemes to comprehensively test the performance of our proposed GWR models: (1) comparison with other regular GWR models by varying the number of auxiliary variables; and (2) comparison with observed ground-level PM2.5 concentrations. The result shows that our GWR models of “AOD + 3” with three common meteorological variables generally perform better than all the other GWR models involved. Our models also show powerful prediction capabilities in PM2.5 concentrations with only slight overfitting. The determination coefficients R2 of our seasonal models are 0.8259 in spring, 0.7818 in summer, 0.8407 in autumn, and 0.7689 in winter. Also, the seasonal models in summer and autumn behave better than those in spring and winter. The comparison between seasonal and yearly models further validates the specific seasonal pattern of auxiliary variables of the GWR model in the YRD. We also stress the importance of key variables and propose a selection process in the AOD-PM2.5 model. Our work validates the significance of proper auxiliary variables in modelling the AOD-PM2.5 relationships and provides a good alternative in retrieving daily PM2.5 concentrations from remote sensing images in the YRD.

The contribution of residential coal combustion to PM2.5 pollution over China's Beijing-Tianjin-Hebei region in winter

The Beijing-Tianjin-Hebei (BTH) region experiences severe haze episodes throughout the year, and especially during the winter heating season. Residential combustion of coal has increasing been cited as a possible source for the PM2.5 pollution that causes the haze episodes. To investigate these claims, a WRF-CMAQ system is used to reproduce the regional haze episodes observed during December 2015. The contribution of residential coal combustion to PM2.5 concentrations in the BTH region is quantified using the Brute Force approach. Across the region, residential coal combustion contributed 46% of the monthly averaged PM2.5 concentration (3% each from Beijing and Tianjin and 40% from Hebei Province). During the haze episodes, the contribution varied between 30 and 57%. At the city scale, the contribution ranged from 22 to 58% averaged across the month and 15–65% during the haze episodes. Langfang was the city that was the most affected by residential coal combustion in the BTH region. The large contribution to air pollution in Tianjin and Beijing from households in Hebei Province suggests that regional control measures are required.

Characteristics and formation mechanism of regional haze episodes in the Pearl River Delta of China

To investigate the characteristics and the specific mechanism of continuous haze, comprehensive measurements were conducted from 15 October to 19 November in the Atmospheric Environment Monitoring Super-Station in Heshan of Guangdong province. Five haze episodes occurred in October and November 2014 in the Pearl River Delta (PRD) region. The meteorological parameters, gas data, chemical compositions, and optical parameters of the aerosols were obtained. Among these events, the second haze episode, with the highest concentration of PM2.5 of 187.51μg/m3, was the most severe. NO3− was always higher than SO42−, which indicated that motor vehicles played an important role in the haze, even though the oxidation rate from SO2 to SO42− was faster than that of NOX to NO3−. The difference between the hourly averages of Na+ and K+ during the haze episode and clean days was small, implying that straw combustion and sea salt had no significant effect on the occurrence of haze, and the backward trajectories of the air masses also conformed with this result. The air pollutants were difficult to disperse because of the significant decrease in the planetary boundary layer (PBL) height. Relative humidity played a crucial role in the formation of haze by leading to hygroscopic growth of the diameter of aerosols.

Morphology, Composition, and Mixing State of Individual Aerosol Particles in Northeast China during Wintertime

Northeast China is located in a high latitude area of the world and undergoes a cold season that lasts six months each year. Recently, regional haze episodes with high concentrations of fine particles (PM2.5) have frequently been occurring in Northeast China during the heating period, but little information has been available. Aerosol particles were collected in winter at a site in a suburban county town (T1) and a site in a background rural area (T2). Morphology, size, elemental composition, and mixing state of individual aerosol particles were characterized by transmission electron microscopy (TEM). Aerosol particles were mainly composed of organic matter (OM) and S-rich and certain amounts of soot and K-rich. OM represented the most abundant particles, accounting for 60.7% and 53.5% at the T1 and T2 sites, respectively. Abundant spherical OM particles were likely emitted directly from coal-burning stoves. Soot decreased from 16.9% at the T1 site to 4.6% at the T2 site and sulfate particles decrease from 35.9% at the T2 site to 15.7% at the T1 site, suggesting that long-range transport air masses experienced more aging processes and produced more secondary particles. Based on our investigations, we proposed that emissions from coal-burning stoves in most rural areas of the west part of Northeast China can induce regional haze episodes.

Rapid formation of a severe regional winter haze episode over a mega-city cluster on the North China Plain

The Nested Air Quality Prediction Model System (NAQPMS) was used to investigate an extreme regional haze episode persisting over the Beijing-Tianjin-Hebei megacity cluster from November 26 to December 1, 2015. During this extreme haze event, the regional daily mean PM2.5 exceeded 500 μg/m3. We found that local emissions were the main source of haze over Beijing and Hebei in the early formational stage of this episode. The accumulation of regionally transported, highly aged secondary inorganic aerosols (SIA) along the foot of the mountains was responsible (60%) for the rapid increase of surface PM2.5 in Beijing between November 30 and December 1, although PM2.5 concentrations in the source regions of Hebei province were lower. The height of regional transport ranged from 200 to 700 m above ground level, with a slow increase with increasing distance of the source regions from Beijing. This indicates that more attention should be given to point sources at heights of 200–500 m in order to reduce the contribution of transport. The contribution of local emissions to haze in Beijing was mostly concentrated below 300 m above ground level, and was more significant for black carbon (BC) and organic matter (OM) than SIA. Tagging of pollutants by emission time showed that PM2.5 had been aged before it arrived at Beijing, and PM2.5 formed one or more days prior to arrival was twice that formed on the arrival day. This suggests that control measures would be more effective if they were implemented two days prior to haze episodes. In contrast to Beijing, haze in Tianjin was governed by transport from outside sources, whereas in cities located in Hebei province this episode resulted from local emissions.