Case study of spring haze in Beijing: Characteristics, formation processes, secondary transition, and regional transportation

Analysis of Pollution Characteristics and Contributions of Firework Burnings in Nanchang during the Spring Festival

This study investigates the impact of fireworks on air quality during the Spring Festival in Nanchang City, utilizing high-resolution monitoring data from February 7th to 15th, 2021. Significant variations in K+ concentrations were observed, indicating severe air quality impacts. During the most intense discharge event A, K+ concentrations were 20.7 times higher than background levels, with PM2.5 and PM10 levels rising to 3.63 and 3.32 times above the background, respectively. The contribution of fireworks to PM2.5 was determined to be 72.5 ± 25.6%. Sulfate (SO4 2-) and nitrate (NO3 -) concentrations also increased significantly, with Δ[SO4 2- ] and Δ[NO3 - ] accounting for 15.4 ± 18.7% and 10.9 ± 12.3% of PM2.5, respectively. The study highlights the necessity for effective emission control strategies to mitigate the adverse effects of fireworks on urban air quality and public health. Future research should focus on the detailed chemical pathways and long-term impacts of these episodic emissions.

Aerosol liquid water in PM2.5 and its roles in secondary aerosol formation at a regional site of Yangtze River Delta

Aerosol liquid water content (ALWC) plays an important role in secondary aerosol formation. In this study, a whole year field campaign was conducted at Shanxi in north Zhejiang Province during 2021. ALWC estimated by ISORROPIA-II was then investigated to explore its characteristics and relationship with secondary aerosols. ALWC exhibited a highest value in spring (66.38 µg/m3), followed by winter (45.08 µg/m3), summer (41.64 µg/m3), and autumn (35.01 µg/m3), respectively. It was supposed that the secondary inorganic aerosols (SIA) were facilitated under higher ALWC conditions (RH > 80%), while the secondary organic species tended to form under lower ALWC levels. Higher RH (> 80%) promoted the NO3- formation via gas-particle partitioning, while SO42- was generated at a relative lower RH (> 50%). The ALWC was more sensitive to NO3- (R = 0.94) than SO42- (R = 0.90). Thus, the self-amplifying processes between the ALWC and SIA enhanced the particle mass growth. The sensitivity of ALWC and OX (NO2 + O3) to secondary organic carbon (SOC) varied in different seasons at Shanxi, more sensitive to aqueous-phase reactions (daytime R = 0.84; nighttime R = 0.54) than photochemical oxidation (daytime R = 0.23; nighttime R = 0.41) in wintertime with a high level of OX (daytime: 130-140 µg/m3; nighttime: 100-140 µg/m3). The self-amplifying process of ALWC and SIA and the aqueous-phase formation of SOC will enhance aerosol formation, contributing to air pollution and reduction of visibility.

PM2.5 and water-soluble inorganic ion concentrations decreased faster in urban than rural areas in China

We investigated variations of PM_2.5 and water-soluble inorganic ions chemical characteristics at nine urban and rural sites in China using ground-based observations. From 2015 to 2019, mean PM_2.5 concentration across all sites decreased by 41.9 µg/m³ with a decline of 46% at urban sites and 28% at rural sites, where secondary inorganic aerosol (SIAs) contributed to 21% (urban sites) and 17% (rural sites) of the decreased PM_2.5. SIAs concentrations underwent a decline at urban locations, while sulfate (SO₄^2-), nitrate (NO₃^-), and ammonium (NH₄⁺) decreased by 49.5%, 31.3% and 31.6%, respectively. However, only SO₄^2- decreased at rural sites, NO₃^- increased by 21% and NH₄⁺ decreased slightly. Those changes contributed to an overall SIAs increase in 2019. Higher molar ratios of NO₃^- to SO₄^2- and NH₄⁺ to SO₄^2- were observed at urban sites than rural sites, being highest in the heavily polluted days. Mean molar ratios of NH₃/NH_x were higher in 2019 than 2015 at both urban and rural sites, implying increasing NH_x remained as free NH₃. Our observations indicated a slower transition from sulfate-driven to nitrate-driven aerosol pollution and less efficient control of NO_x than SO₂ related aerosol formation in rural regions than urban regions. Moreover, the common factor at urban and rural sites appears to be a combination of lower SO₄^2- levels and an increasing fraction of NO₃^- to PM_2.5 under NH₄⁺-rich conditions. Our findings imply that synchronous reduction in NO_x and NH₃ emissions especially rural areas would be effective to mitigate NO₃^--driven aerosol pollution.

Unbalanced emission reductions and adverse meteorological conditions facilitate the formation of secondary pollutants during the COVID-19 lockdown in Beijing

During the coronavirus disease 2019 (COVID-19) lockdown in 2020, severe haze pollution occurred in the North China Plain despite the significant reduction in anthropogenic emissions, providing a natural experiment to explore the response of haze pollution to the reduction of human activities. Here, we study the characteristics and causes of haze pollution during the COVID-19 outbreak based on comprehensive field measurements in Beijing during January and February 2020. After excluding the Spring Festival period affected by fireworks activities, we found the ozone concentrations and the proportion of sulfate and nitrate in PM_2.5 increased during the COVID-19 lockdown compared with the period before the lockdown, and sulfate played a more important role. Heterogeneous chemistry and photochemistry dominate the formation of sulfate and nitrate during the whole campaign, respectively, and the heterogeneous formation of nitrate at night was enhanced during the lockdown. The coeffects of more reductions in NO_x than VOCs, weakened titration of NO, and increased temperature during the lockdown led to the increase in ozone concentrations, thereby promoting atmospheric oxidation capacity and photochemistry. In addition, the increase in relative humidity during the lockdown facilitated heterogeneous chemistry. Our results indicate that unbalanced emission reductions and adverse meteorological conditions induce the formation of secondary pollutants during the COVID-19 lockdown haze, and the formulation of effective coordinated emission-reduction control measures for PM_2.5 and ozone pollution is needed in the future, especially the balanced control of NO_x and VOCs.

Exploring sources and health risks of metals in Beijing PM2.5: Insights from long-term online measurements

To gain a comprehensive understanding of sources, health risks, and regional transport of PM_2.5-bound metals in Beijing, one-year continuous measurement (K, Fe, Ca, Zn, Pb, Mn, Ba, Cu, As, Se, Cr, and Ni) was conducted from December 2016 to November 2017 and Positive Matrix Factorization analysis (PMF) was applied for source apportionment. It was found that the seasonal variation of sources could vary significantly among metals. Sources of Ca, Ba, As, Se, and Cr did not show much seasonal variations, with the contribution of its predominant source higher than 35% in each season. However, the major sources of K, Fe, Zn, Pb, Mn, Cu, and Ni exhibited obvious seasonal variations. In addition, the characteristics of metals in haze episodes were comprehensively investigated. Haze episodes in Beijing were characterized by higher metal concentrations and health risks, which were about 2- 6 times higher than non-haze periods. Moreover, the types of haze episode were different in winter and spring. Haze episodes in winter were mostly influenced by coal combustion, the contribution of which increased greatly and accounted for about 30% of PM_2.5. The metals such as K, Zn, Pb, As, and Se significantly increased, which were mainly transported from south of Beijing. During haze episodes in spring, dust was an important source, which contributed to higher concentrations of crustal metals that transported from northwest of Beijing. To quickly and effectively identify source regions of metals in Beijing during haze episodes, a new diagnostic ratio method using Ca as a reference was developed. The ratios of some anthropogenic metals to Ca significantly increased when air mass was mainly from south of Beijing during haze episodes while the ratios remained constantly low in non-haze periods, when local emissions dominated. This method could be useful for rapid identification and control of metal pollution in Beijing.

Spatio-Temporal Characteristics of Air Quality Index (AQI) over Northwest China

In recent years, air pollution has become a serious threat, causing adverse health effects and millions of premature deaths in China. This study examines the spatial-temporal characteristics of ambient air quality in five provinces (Shaanxi (SN), Xinjiang (XJ), Gansu (GS), Ningxia (NX), and Qinghai (QH)) of northwest China (NWC) from January 2015 to December 2018. For this purpose, surface-level aerosol pollutants, including particulate matter (PMx, x = 2.5 and 10) and gaseous pollutants (sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), and ozone (O3)) were obtained from China National Environmental Monitoring Center (CNEMC). The results showed that fine particulate matter (PM2.5), coarse particulate matter (PM10), SO2, NO2, and CO decreased by 28.2%, 32.7%, 41.9%, 6.2%, and 27.3%, respectively, while O3 increased by 3.96% in NWC during 2018 as compared with 2015. The particulate matter (PM2.5 and PM10) levels exceeded the Chinese Ambient Air Quality Standards (CAAQS) Grade II standards as well as the WHO recommended Air Quality Guidelines, while SO2 and NO2 complied with the CAAQS Grade II standards in NWC. In addition, the average air quality index (AQI), calculated from ground-based data, improved by 21.3%, the proportion of air quality Class I (0–50) improved by 114.1%, and the number of pollution days decreased by 61.8% in NWC. All the pollutants’ (except ozone) AQI and PM2.5/PM10 ratios showed the highest pollution levels in winter and lowest in summer. AQI was strongly positively correlated with PM2.5, PM10, SO2, NO2, and CO, while negatively correlated with O3. PM10 was the primary pollutant, followed by O3, PM2.5, NO2, CO, and SO2, with different spatial and temporal variations. The proportion of days with PM2.5, PM10, SO2, and CO as the primary pollutants decreased but increased for NO2 and O3. This study provides useful information and a valuable reference for future research on air quality in northwest China.

Characterization of haze pollution in Zibo, China: Temporal series, secondary species formation, and PMx distribution

An online field observation was conducted in Zibo, China from September 1, 2018 to February 28, 2019, covering autumn and winter. Within the investigation period, the mean mass concentrations of PM₁, PM_2.5, and PM₁₀ were 49.3, 86.1, and 136.5 μg m^-3, respectively. OA (organic aerosol) was the most dominant species in PM_2.5 (39.7 %), followed by NO₃^- (26.3 %) and SO₄^2- (17.0 %), indicating the importance of secondary species on PM_2.5. Increase of particles were always accompanied increasing relative humidity (RH), slow wind, and increasing precursors, contributing the secondary transition. SO₄^2- was more susceptible to RH, indicating the dominant role of heterogeneous processes in its secondary formation. As RH increased, its strengthening effect on SO₄^2- increased as well. Photochemistry was the main contributor to the secondary formation of NO₃^-. The morning and evening rush hours determined the peak of absolute NO₃^- throughout the day. By classifying particles into three bins, we found that smaller particles were the biggest contributors (larger PM₁/PM_2.5) of slight pollution (35 < PM_2.5<115 μg m^-3). When severe haze occurred, PM_2.5 contributed more than particles of other sizes (PM₁ or PM₁₀). Secondary species contributed more to particles within 2.5 μm but less to larger particles. PM₁/PM_2.5 was high from 9:00 to 15:00, indicating the strong effect of photochemistry on smaller particles. In comparison, larger particles favored more humid conditions. NO₃^- preferentially existed in larger particles because the hygroscopicity of preexisting species (SO₄^2- and NO₃^-) promoted partitioning. SO₄^2- appeared a stable diurnal variation, replying its stable contribution to particles of different sizes.

Profiling of Dust and Urban Haze Mass Concentrations during the 2019 National Day Parade in Beijing by Polarization Raman Lidar

The polarization–Raman Lidar combined sun photometer is a powerful method for separating dust and urban haze backscatter, extinction, and mass concentrations. The observation was performed in Beijing during the 2019 National Day parade, the particle depolarization ratio at 532 nm and Lidar ratio at 355 nm are 0.13 ± 0.05 and 52 ± 9 sr, respectively. It is the typical value of a mixture of dust and urban haze. Here we quantify the contributions of cross-regional transported natural dust and urban haze mass concentrations to Beijing’s air quality. There is a significant correlation between urban haze mass concentrations and surface PM2.5 (R = 0.74, p < 0.01). The contributions of local emissions to air pollution during the 2019 National Day parade were insignificant, mainly affected by regional transport, including urban haze in North China plain and Guanzhong Plain (Hebei, Tianjin, Shandong, and Shanxi), and dust aerosol in Mongolia regions and Xinjiang. Moreover, the trans-regional transmission of natural dust dominated the air pollution during the 2019 National Day parade, with a relative contribution to particulate matter mass concentrations exceeding 74% below 4 km. Our results highlight that controlling anthropogenic emissions over regional scales and focusing on the effects of natural dust is crucial and effective to improve Beijing’s air quality.

Evolution of secondary inorganic aerosols amidst improving PM2.5 air quality in the North China plain

The Clean Air Action implemented by the Chinese government in 2013 has greatly improved air quality in the North China Plain (NCP). In this work, we report changes in the chemical components of atmospheric fine particulate matter (PM_2.5) at four NCP sampling sites from 2012/2013 to 2017 to investigate the impacts and drivers of the Clean Air Action on aerosol chemistry, especially for secondary inorganic aerosols (SIA). During the observation period, the concentrations of PM_2.5 and its chemical components (especially SIA, organic carbon (OC), and elemental carbon (EC)) and the frequency of polluted days (daily PM_2.5 concentration ≥ 75 μg m^-3) in the NCP, declined significantly at all four sites. Asynchronized reduction in SIA components (large decreases in SO₄^2- with stable or even increased NO₃^- and NH₄⁺) was observed in urban Beijing, revealing a shift of the primary form of SIA, which suggested the fractions of NO₃^- increased more rapidly than SO₄^2- during PM_2.5 pollution episodes, especially in 2016 and 2017. In addition, unexpected increases in the sulfur oxidation ratio (SOR) and the nitrogen oxidation ratio (NOR) were observed among sites and across years in the substantially decreased PM_2.5 levels. They were largely determined by secondary aerosol precursors (i.e. decreased SO₂ and NO₂), photochemical oxidants (e.g. increased O₃), temperature, and relative humidity via gas-phase and heterogeneous reactions. Our results not only highlight the effectiveness of the Action Plan for improving air quality in the NCP, but also suggest an increasing importance of SIA in determining PM_2.5 concentration and composition.

Stronger secondary pollution processes despite decrease in gaseous precursors: A comparative analysis of summer 2020 and 2019 in Beijing

To control the spread of COVID-19, China implemented a series of lockdowns, limiting various offline interactions. This provided an opportunity to study the response of air quality to emissions control. By comparing the characteristics of pollution in the summers of 2019 and 2020, we found a significant decrease in gaseous pollutants in 2020. However, particle pollution in the summer of 2020 was more severe; PM_2.5 levels increased from 35.8 to 44.7 μg m^-3, and PM₁₀ increased from 51.4 to 69.0 μg m^-3 from 2019 to 2020. The higher PM₁₀ was caused by two sandstorm events on May 11 and June 3, 2020, while the higher PM_2.5 was the result of enhanced secondary formation processes indicated by the higher sulfate oxidation rate (SOR) and nitrate oxidation rate (NOR) in 2020. Higher SOR and NOR were attributed mainly to higher relative humidity and stronger oxidizing capacity. Analysis of PM_x distribution showed that severe haze occurred when particles within Bin2 (size ranging 1-2.5 μm) dominated. SO₄^2-_(1/2.5) and SO₄^2-_(2.5/10) remained stable under different periods at 0.5 and 0.8, respectively, indicating that SO₄^2- existed mainly in smaller particles. Decreases in NO₃^-_(1/2.5) and increases in NO₃^-_(2.5/10) from clean to polluted conditions, similar to the variations in PM_x distribution, suggest that NO₃^- played a role in the worsening of pollution. O₃ concentrations were higher in 2020 (108.6 μg m^-3) than in 2019 (96.8 μg m^-3). Marked decreases in fresh NO alleviated the titration of O₃. Furthermore, the oxidation reaction of NO₂ that produces NO₃^- was dominant over the photochemical reaction of NO₂ that produces O₃, making NO₂ less important for O₃ pollution. In comparison, a lower VOC/NO_x ratio (less than 10) meant that Beijing is a VOC-limited area; this indicates that in order to alleviate O₃ pollution in Beijing, emissions of VOCs should be controlled.

Air pollutants and outpatient visits for influenza-like illness in Beijing, China

Background

Air pollution leads to many adverse health conditions, mainly manifested by respiratory or cardiac symptoms. Previous studies are limited as to whether air pollutants were associated to influenza-like illness (ILI). This study aimed to explore the association between air pollutants and outpatient visits for ILI, especially during an outbreak of influenza.

Methods

Daily counts of hospital visits for ILI were obtained from Peking University Third Hospital between January 1, 2015, and March 31, 2018. A generalized additive Poisson model was applied to examine the associations between air pollutants concentrations and daily outpatient visits for ILI when adjusted for the meteorological parameters.

Results

There were 35862 outpatient visits at the fever clinic for ILI cases. Air quality index (AQI), PM_2.5, PM₁₀, CO and O₃ on lag0 days, as well as nitrogen dioxide (NO₂) and sulfur dioxide (SO₂) on lag1 days, were significantly associated with an increased risk of outpatient visits for ILI from January 2015 to November 2017. From December 2017 to March 2018, on lag0 days, air pollutants PM_2.5 [risk ratio (RR) = 0.971, 95% CI: 0.963-0.979], SO₂ (RR = 0.892, 95% CI: 0.840–0.948) and CO (RR = 0.306, 95% CI: 0.153–0.612) were significantly associated with a decreased risk of outpatient visits for ILI. Interestingly, on the lag2 days, all the pollutants were significantly associated with a reduced risk of outpatient visits for ILI except for O₃. We did not observe the linear correlations between the outpatient visits for ILI and any of air pollutants, which were instead associated via a curvilinear relationship.

Conclusions

We found that the air pollutants may be associated with an increased risk of outpatient visits for ILI during the non-outbreak period and with a decreased risk during the outbreak period, which may be linked with the use of disposable face masks and the change of outdoor activities. These findings expand the current knowledge of ILI outpatient visits correlated with air pollutants during an influenza pandemic.

Quantify the Contribution of Dust and Anthropogenic Sources to Aerosols in North China by Lidar and Validated with CALIPSO

Persistent heavy haze episodes have repeatedly shrouded North China in recent years. Besides anthropogenic emissions, natural dust also contributes to the aerosols in this region. Through continuous observation by a dual-wavelength Raman lidar, the primary aerosol types and their contributions to air pollution in North China were determined. The following three aerosol types can be classified: natural dust, anthropogenic aerosols, and the mixture of anthropogenic aerosols and dust (polluted dust). The classification results are basically consistent with the classification results from the cloud–aerosol lidar and infrared pathfinder satellite observations (CALIPSO) satellite measurements. The relative bias of the lidar ratio between the Raman lidar and CALIPSO is less than 25% over 90% of the cases, indicating that the CALIPSO lidar ratio selection algorithm is reasonable. The classification results show that approximately 45% of aerosols below 1.8 km are contributed by polluted dust during our one year observations. The contribution of dust increased with height, from 6% at 500 m to 28% at 1,800 m, while the contribution of anthropogenic aerosols decreased from 49% to 25%. In addition, polluted dust is the major aerosol subtype below 1.0 km in spring (over 60%) and autumn (over 70%). Anthropogenic aerosols contribute more than 75% of air pollution in summer. In winter, anthropogenic aerosols prevailed (over 80%) in the lower layer, while polluted dust (around 60%) dominated the upper layer. Our results identified the primarily aerosol types to assess the contributions of anthropogenic and natural sources to air pollution in North China, and highlight that natural dust plays a crucial role in lower-layer air pollution in spring and autumn, while controlling anthropogenic aerosols will significantly improve air quality in winter.

Atmospheric boundary layer turbulence structure for severe foggy haze episodes in north China in December 2016

This paper aims to identify the atmospheric boundary layer turbulence structure and its effect on severe foggy haze events frequently occurring in Northern China. We use data collected from a ground eddy covariance system, meteorology tower, and a PM_2.5 collector in Baoding, China during December 2016. The data shows that 73.5% of PM_2.5 concentration is greater than 100 μg m^-3 with a maximum of 522 μg m^-3. Analyses on vertical turbulence spectrum also reveal that 1) during the pollution period, lower wind can suppress large-scale turbulence eddies, which are more likely inhomogeneous, breaking into small-scale eddies, and 2) the air pollutant scattering effect for radiation could decrease the air temperature near the ground and generate weak vertical turbulence during the daytime. At night, air pollutants suppress the land surface cooling and decrease the air temperature difference as well as the vertical turbulence intensity difference. The vertical turbulence impact analysis reveals that the percentage of large-scale turbulence eddies can also change the atmospheric vertical mixing capacity. During the daytime, the air pollution evolution is controlled by the wind speed and vertical turbulence intensity. While at night, the vertical turbulence is weak and the atmospheric vertical mixing capacity is mainly controlled by the large-scale eddies' percentage. The increased number of large-scale turbulence eddies led by low wind at night could increase the vertical mixing of air pollutants and decrease its concentration near the ground.

Spatial–Temporal Heterogeneous Evolution of Haze Pollution in China as Deduced with the Use of Spatial Econometrics

Haze Pollution, consisting essentially of PM2.5 and PM10, has been arousing wide public concern home and abroad. It has become a universal urgency for atmospheric researchers, governments, organizations, institutions, and the general public to conduct corresponding actions. Therefore, this paper aims to explore the institutional distribution and the regional evolution trend of path characteristics of haze pollution in China under the spatial–temporal heterogeneity on the basis of spatial econometrics, by incorporating the spatial element into the framework of the Multiple Influencing Factors mechanism. The results show that it has been abating under the governance year by year, though with a decreasing intensity; the major polluted regions have been moving from the East to the central and western area; there is significant spatial autocorrelation among the highly polluted area, but the effective local regulations of les- polluted regions do not impact the neighboring regions correspondingly; among the impacting factors, industrial structure, energy intensity, and traffic pollution have a significant Positive Impact on haze pollution, and the level of urbanization has a Negative Impact, while economic growth and innovation performance have no significant Positive Impact and are both weak in promotion. This research, theoretically and practically, offers reference for the Chinese government to integrate regional effective systems into multiregional diversified environmental governance, so as to realize its Green Ecology Transformation Development Strategy.

Nitrate dominates the chemical composition of PM2.5 during haze event in Beijing, China

Water-soluble inorganic ions (WSI), a major component of PM_2.5, often increased rapidly during the haze event in Beijing. Sulfate (SO₄^2-), Nitrate (NO₃^-), and Ammonium (NH₄⁺) are three main components of WSI. Since year 2015, sulfate concentrations in PM_2.5 has gradually decreased owing to the effective control of SO₂ emissions. However, the contribution of nitrate to PM_2.5 has significantly increased during haze events in Beijing at the same time. In this study, a highly time-resolved online analyzer (Monitor for Aerosols and Gases, MARGA) was employed to measure the WSI in PM_2.5 in Beijing from 5 February to 15 November 2017. Three typical haze events during this sampling period were investigated. During heavy pollution episodes in winter, nitrate concentrations increased from 7.5 μg/m³ to 45.6 μg/m³ (45.0% of WSI), while sulfate increased from 4.2 μg/m³ to 20.1 μg/m³ (19.8% of WSI). This indicated that nitrate is more important than sulfate as a driver for the growth of PM_2.5 during the period of heavy air pollution in winter. Nitrate also dominates the increase of WSI in the pollution episodes in autumn, with an average concentration of 52.5 μg/m³, and contributed up to 67% of WSI. The average concentration ratio of NH₄⁺ to SO₄^2- was higher in autumn (1.02) than that in summer (0.74) and close to that in winter (1.00). This is mainly because the emission control of coal combustion in Beijing and surrounding areas results in an NH₃-rich and SO₂-lean atmosphere, which promoted the formation of ammonium nitrate. Our study indicates that nitrate has become the most important component of WSI in PM_2.5 and is driving the rapid growth of PM_2.5 concentrations during heavy pollution episodes in Beijing. Therefore, more efforts shall be made to reduce the nitrogen oxide and ammonia emissions in Beijing and surrounding areas.

Characteristics and formation mechanisms of winter haze in Changzhou, a highly polluted industrial city in the Yangtze River Delta, China

Changzhou, an industrial city in the Yangtze River Delta, has been experiencing serious haze pollution, particularly in winter. However, studies pertaining to the haze in Changzhou are very limited, which makes it difficult to understand the characteristics and formation of winter haze in this area, and develop effective control measures. In this study, we carried out continuous online observation of particulate matter, chemical components, and meteorology in Changzhou in February 2017. Our results showed that haze pollution occurred frequently in Changzhou winter and exhibited two patterns: dry haze with low relative humidity (RH) and wet haze with high RH. Water-soluble inorganic ions (SO₄^2-, NO₃^-, and NH₄⁺) accounted for ∼52.2% of the PM_2.5 mass, of which sulfate was dominant in wet haze periods while nitrate was dominant in other periods. With the deterioration of haze pollution, the proportion of nitrate in PM_2.5 increased, while sulfate proportion increased under wet haze and decreased under dry haze. Dry haze and wet haze appeared under slow north wind and south wind, respectively, and strong north wind or sea breeze scavenged pollution. We found that formation of nitrate occurred rapidly in daytime with high concentrations of odd oxygen (O_x = O₃ + NO₂), whereas formation of sulfate occurred rapidly during nighttime with high RH, indicating that photochemistry and heterogeneous reaction were the major formation mechanisms for nitrate and sulfate, respectively. Through the cluster analysis of 36-h backward trajectories, five sources of air masses from three directions were identified. High PM_2.5 concentrations (84.1 μg m^-3 on average) usually occurred under the influence of two clusters (46%) from the northwest, indicating that regional transport from northern China aggravated the winter haze pollution in Changzhou. Emission reduction, particularly the mobile sources, and regional joint prevention and control can help to mitigate the winter haze in Changzhou.

Sources of humic-like substances (HULIS) in PM2.5 in Beijing: Receptor modeling approach

Recent work has identified the presence of humic-like substances (HULIS) in ambient fine particulate matter (PM_2.5) in Beijing, China and that residential coal combustion as well as biomass burning are significant contributors to its presence. These results were based on the characterization of emissions from representative stoves and modeling of the aerosol with the Community Multiscale Air Quality (CMAQ) chemical transport model. The CMAQ source apportionment estimated that residential coal and biofuel burning and secondary aerosol formation were important annual sources of ambient HULIS, contributing 47.1%, 15.1%, and 38.9%, respectively. In this study, chemical composition data including concentrations of water-soluble organic carbon and HULIS across four seasons during 2012-2013 were analyzed with positive matrix factorization (PMF) to provide a complementary source apportionment. The PMF results indicate that the identified sources were Traffic, Biomass Burning, Nitrate/Sulfate, Incineration, Sulfate, Coal Combustion/Ammonium Chloride, Residential Coal/Biofuel Combustion, and Road Dust/Soil with mass contributions (fractions) to PM_2.5 of 12.35 (10.4%), 8.70 (8.9%), 24.51 (22.4%), 5.64 (7.2%), 25.14 (24.5%), 7.10 (6.2%), 14.18 (15.4%), and 5.33 μg/m³ (5.0%), respectively. The contributions to the observed HULIS concentrations were 0.63 (10.9%), 0.38 (6.4%), 0.07 (1.7%), 0.00 (0%), 1.12 (28.8%), 0.00 (0%), 1.50 (52.2%), and 0.01 μg/m³ (0.3%), respectively. These PMF modeling results were in reasonable agreement with the CMAQ values supporting the attribution of significant amounts of primary HULIS to residential coal and biofuel combustion. Currently, efforts are underway in China to replace solid fuel combustion for heating and cooking with natural gas and electricity by 2020. Thus, future studies should be able to see substantial reductions in both PM_2.5 and HULIS in the near term future.