Role of secondary aerosols in haze formation in summer in the Megacity Beijing

Real-time, single-particle chemical composition, volatility and mixing state measurements of urban aerosol particles in southwest China

To investigate the volatility of atmospheric particulates and the evolution of other particulate properties (chemical composition, particle size distribution and mixing state) with temperature, a thermodenuder coupled with a single particle aerosol mass spectrometer was used to conduct continuous observations of atmospheric fine particles in Chengdu, southwest China. Because of their complex sources and secondary reaction processes, the average mass spectra of single particles contained a variety of chemical components (including organic, inorganic and metal species). When the temperature rose from room temperature to 280°C, the relative areas of volatile and semi-volatile components decreased, while the relative areas of less or non-volatile components increased. Most (> 80%) nitrate and sulfate existed in the form of NH4NO3 and (NH4)2SO4, and their volatilization temperatures were 50-100°C and 150-280°C, respectively. The contribution of biomass burning (BB) and vehicle emission (VE) particles increased significantly at 280°C, which emphasized the important role of regional biomass burning and local motor vehicle emissions to the core of particles. With the increase in temperature, the particle size of the particles coated with volatile or semi-volatile components was reduced, and their mixing with secondary inorganic components was significantly weakened. The formation of K-nitrate (KNO3) and K-sulfate (KSO4) particles was dominated by liquid-phase processes and photochemical reactions, respectively. Reducing KNO3 and BB particles is the key to improving visibility. These new results are helpful towards better understanding the initial sources, pollution formation mechanisms and climatic effects of fine particulate matter in this megacity in southwest China.

Assessing the chemical composition, potential toxicity and cancer risk of airborne fine particulate matter (PM2.5) near a petrochemical industrial area

In the vicinity of a petrochemical industrial region in São Paulo, Brazil, PM_2.5-bound organic carbon (OC), elemental carbon (EC), polycyclic aromatic hydrocarbons (PAHs), nitro-PAHs, oxy-PAHs, hopanes, and inorganic species were evaluated. Oxidative potential (OP), burden (OB), and Alivibrio fischeri bioluminescence inhibition (AFBIA) assays were conducted to determine the potential health effects of exposure to these compounds. The PM_2.5 mean concentration was 32.0±18.2µgm^-3, and benzo (a)pyrene was found to exceed recommended levels by at least four times. Secondary sources and vehicular emissions were indicated by nitro-PAHs, oxy-PAHs, and inorganic species. The OP and OB results revealed that secondary compounds favored antioxidant depletion. The AFBIA results showed that 64% of the samples were toxic. These findings emphasize the need to reduce the exposure risk and take measures to protect human health.

Critical contribution of moisture to the air quality deterioration in a warm and humid weather

The deterioration of air quality that threatens human health is recognized as focal compound hazard. Here, decisive thermodynamic conditions for activation of secondary aerosol formation have been investigated focused on Korea. In a dry environment with relative humidity < 60%, gas phase reaction to form fine particles depended largely on surface temperature. In a wet environment (relative humidity ≥ 60%), however, aqueous phase reaction of secondary inorganic aerosols, which are sulfate, nitrate, and ammonium, accounting for 67% of the total aerosol mass, was more activated. Thus, humidity is as important as temperature in the secondary production of aerosol; air quality often worsened when a low-pressure system was predominant over the Korean Peninsula. It is rather different from the general synoptic conditions of high concentrations of particulate matters characterized by high pressure and atmospheric stagnation. This study suggests additional favorable condition and responsible mechanism of air quality hazards that may be frequent in future.

Haze caused by NOx oxidation under restricted residential and industrial activities in a mega city in the south of North China Plain

The formation of secondary aerosol species, including nitrate and sulfate, induces severe haze in the North China Plain. However, despite substantial reductions in anthropogenic pollutants due to severe restriction of residential and industrial activities in 2020 to stop the spread of COVID-19, haze still formed in Zhengzhou. We compared ionic compositions of PM_2.5 during the period of the restriction with that immediately before the restriction and in the comparison period in 2019 to investigate the processes that caused the haze. The average concentration of PM_2.5 was 83.9 μg m^-3 in the restriction period, 241.8 μg m^-3 before the restriction, and 94.0 μg m^-3 in 2019. Nitrate was the largest contributor to the PM_2.5 in all periods, with an average mass fraction of 24%-30%. The average molar concentration of total nitrogen compounds (NO_x + nitrate) was 0.89 μmol m^-3 in the restriction period, which was much lower than that in the non-restriction periods (1.85-2.74 μmol m^-3). In contrast, the concentration of sulfur compounds (SO₂ + sulfate) was 0.34-0.39 μmol m^-3 in all periods. The conversion rate of NO_x to nitrate (NOR) was 0.35 in the restriction period, significantly higher than that before the restriction (0.26) and in 2019 (0.25). NOR was higher with relative humidity in 40-80% in the restriction period than in the other two periods, whereas the conversion rate of SO₂ to sulfate did not, indicating nitrate formation was more efficient during the restriction. When O₃ occupied more than half of the oxidants (O_x = O₃ + NO₂), NOR increased rapidly with the ratio of O₃ to O_x and was much higher in the daytime than nighttime. Therefore, haze in the restriction period was caused by increased NO_x-to-nitrate conversion driven by photochemical reactions.

Variation of Aerosol Optical Depth Measured by Sun Photometer at a Rural Site near Beijing during the 2017–2019 Period

In recent years, the Beijing–Tianjin–Hebei region has become one of the worst areas for haze pollution in China. Sun photometers are widely used for aerosol optical property monitoring due to the advantages of fully automatic acquisition, simple maintenance, standardization of data processing, and low uncertainty. Research sites are mostly concentrated in cities, while the long-term analysis of aerosol optical depth (AOD) for the pollution transmission channel in rural Beijing is still lacking. Here, we obtained an AOD monitoring dataset from August 2017 to March 2019 using the ground-based CE-318 sun photometer at the Gucheng meteorological observation site in southwest Beijing. These sun photometer AOD data were used for the ground-based validation of MODIS (Moderate Resolution Imaging Spectroradiometer) and AHI (Advanced Himawari Imager) AOD data. It was found that MODIS and AHI can reflect AOD variation trends by sun photometer on daily, monthly, and seasonal scales. The original AOD measurements of the sun photometer show good correlations with satellite observations by MODIS (R = 0.97), and AHI (R = 0.89), respectively, corresponding to their different optimal spatial and temporal windows for matching with collocated satellite ground pixels. However, MODIS is less stable for aerosols of different concentrations and particle sizes. Most of the linear regression intercepts between the satellite and the photometer are less than 0.1, indicating that the errors due to surface reflectance in the inversion are small, and the slope is least biased (AHI: slope = 0.91, MODIS: slope = 0.18) in the noon period (11 a.m.–2 p.m.) and most biased in summer (AHI: slope = 0.77, MODIS: slope = 1.31), probably due to errors in the aerosol model. The daily and seasonal variation trends between CE-318 AOD measurements in the Gucheng site and fine particulate observations from the national air quality site nearby were also compared and investigated. In addition, a typical haze–dust complex pollution event in North China was analyzed and the changes in AOD during the pollution event were quantified. In processing, we use sun photometer and satellite AOD data in combination with meteorological and PM data. Overall, this paper has implications for the study of AOD evolution patterns at different time scales, the association between PM2.5 concentrations and AOD changes, and pollution monitoring.

Haze Occurrence Caused by High Gas-to-Particle Conversion in Moisture Air under Low Pollutant Emission in a Megacity of China

Haze occurred in Zhengzhou, a megacity in the northern China, with the PM_2.5 as high as 254 μg m⁻³ on 25 December 2019, despite the emergency response measure of restriction on the emission of anthropogenic pollutants which was implemented on December 19 for suppressing local air pollution. Air pollutant concentrations, chemical compositions, and the origins of particulate matter with aerodynamic diameter smaller than 2.5 µm (PM_2.5) between 5–26 December were investigated to explore the reasons for the haze occurrence. Results show that the haze was caused by efficient SO₂-to-suflate and NO_x-to-nitrate conversions under high relative humidity (RH) condition. In comparison with the period before the restriction (5–18 December) when the PM_2.5 was low, the concentration of PM_2.5 during the haze (19–26 December) was 173 µg m⁻³ on average with 51% contributed by sulfate (31 µg m⁻³) and nitrate (57 µg m⁻³). The conversions of SO₂-to-sulfate and NO_x-to-nitrate efficiently produced sulfate and nitrate although the concentration of the two precursor gases SO₂ and NO_x was low. The high RH, which was more than 70% and the consequence of artificial water-vapor spreading in the urban air for reducing air pollutants, was the key factor causing the conversion rates to be enlarged in the constriction period. In addition, the last 48 h movement of the air parcels on 19–26 December was stagnant, and the air mass was from surrounding areas within 200 km, indicating weather conditions favoring the accumulation of locally-originated pollutants. Although emergency response measures were implemented, high gas-to-particle conversions in stagnant and moisture circumstances can still cause severe haze in urban air. Since the artificial water-vapor spreading in the urban air was one of the reasons for the high RH, it is likely that the spreading had unexpected side effects in some certain circumstances and needs to be taken into consideration in future studies.

Characteristics of fine particulate matter (PM2.5) at Jinsha Site Museum, Chengdu, China

Air pollution is a serious threat to ancient sites and cultural relicts. In this study, we collected indoor and outdoor PM_2.5 samples and individual particles at the Exhibition Hall of Jinsha Site Museum in June 2020, and then the chemical components, sources, morphology, and mixing state of the fine particulate matter were analyzed. Our results show that the indoor and outdoor PM_2.5 concentrations at the Exhibition Hall were 33.3±6.6 and 39.4±11.4 μg m^-3, respectively. Although the indoor and outdoor concentrations of OC and EC were close, the proportion of secondary organic carbon in OC outdoor (33%) was higher than that indoor (27%). The PM_2.5 was alkaline both indoors and outdoors, and the outdoor alkalinity was stronger than the indoor alkalinity. SNA (SO₄^2-, NO₃^-, and NH₄⁺) was the dominant component in the water-soluble inorganic ions; Na⁺, Mg²⁺, and Ca²⁺ were well correlated (R²> 0.9), and Cl^- and K⁺ were also highly correlated (R²> 0.8). Enrichment factor analysis showed that Cu (indoor) and Cd were the main anthropogenic elements and that Cd was heavily enriched. Principal components analysis showed that the main sources of PM_2.5 at Jinsha Site Museum were motor vehicles, dust, secondary sources, and combustion sources. The individual particles were classified as organic matter, S-rich, soot, mineral, and fly ash/metal particles, and most of these particles were internally mixed with each other. At last, we proposed pollution control measures to improve the air quality of museums and the preservation of cultural relicts.

Airborne Microplastic Concentrations in Five Megacities of Northern and Southeast China

Airborne microplastics (MPs) are receiving increasing attention due to their ubiquitous nature and the potential human health consequences resulting from inhalation. The limited data for airborne MP concentrations vary widely among studies (∼4 orders of magnitude), but comparisons are tenuous due to the inconsistent collection and detection/enumeration methodologies among studies. Herein, we used uniform methodologies to obtain comparable airborne MP concentration data to assess MP exposure intensity in five Chinese megacities. Airborne MP concentrations in northern cities (358 ± 132 items/m³) were higher than those in southeast cities (230 ± 94 items/m³) but of a similar order of magnitude, unlike previous studies. The majority (94.7%) of MPs found in air samples were smaller than 100 μm, and the main shape of airborne MPs was fragments (88.2%). Polyethylene, polyester, and polystyrene were the dominant polymers comprising airborne MPs. No consistent relationships were detected between airborne MP concentration and typical socioeconomic indices, and the spatial and diurnal patterns for airborne MPs were different from various components of air quality indices (PM_2.5, PM₁₀, etc.). These findings reflect the contrasting source/generation dynamics between airborne MPs and other airborne pollutants. Maximum annual exposure of humans to airborne MPs was estimated in the range of 1-2 million/year in these megacities, highlighting the need for additional research examining the human health risks from the inhalation of airborne MPs.

Spatio-Temporal Variability of Aerosol Components, Their Optical and Microphysical Properties over North China during Winter Haze in 2012, as Derived from POLDER/PARASOL Satellite Observations

Pollution haze is a frequent phenomenon in the North China Plain (NCP) appearing during winter when the aerosol is affected by various pollutant sources and has complex distribution of the aerosol properties, while different aerosol components may have various critical effects on air quality, human health and radiative balance. Therefore, large-scale and accurate aerosol components characterization is urgently and highly desirable but hardly achievable at the regional scale. In this respect, directional and polarimetric remote sensing observations have great potential for providing information about the aerosol components. In this study, a state-of-the-art GRASP/Component approach was employed for attempting to characterize aerosol components in the NCP using POLDER/PARASOL satellite observations. The analysis was done for January 2012 in Beijing (BJ) and Shanxi (SX). The results indicate a peak of the BC mass concentration in an atmospheric column of 82.8 mg/m2 in the SX region, with a mean of 29.2 mg/m2 that is about four times higher than one in BJ (8.9 mg/m2). The mean BrC mass concentrations are, however, higher in BJ (up to ca. 271 mg/m2) than that in SX, which can be attributed to a higher anthropogenic emission. The mean amount of fine ammonium sulfate-like particles observed in the BJ region was three times lower than in SX (131 mg/m2). The study also analyzes meteorological and air quality data for characterizing the pollution event in BJ. During the haze episode, the results suggest a rapid increase in the fine mode aerosol volume concentration associated with a decrease of a scale height of aerosol down to 1500 m. As expected, the values of aerosol optical depth (AOD), absorbing aerosol optical depth (AAOD) and fine mode aerosol optical depth (AODf) are much higher on hazy days. The mass fraction of ammonium sulfate-like aerosol increases from about 13% to 29% and mass concentration increases from 300 mg/m2 to 500 mg/m2. The daily mean PM2.5 concentration and RH independently measured during these reported pollution episodes reach up to 425 g/m3 and 80% correspondingly. The monthly mean mass concentrations of other aerosol components in the BJ are found to be in agreement with the results of previous research works. Finally, a preliminary comparison of these remote sensing derived results with literature and in situ PM2.5 measurements is also presented.

Variations in Levels and Sources of Atmospheric VOCs during the Continuous Haze and Non-Haze Episodes in the Urban Area of Beijing: A Case Study in Spring of 2019

To better evaluate the variations in concentration characteristics and source contributions of atmospheric volatile organic compounds (VOCs) during continuous haze days and non-haze days, hourly observations of atmospheric VOCs were conducted using a continuous on-line GC-FID (Airmo VOC GC-866) monitoring system during 1–15 March 2019, in urban areas of Beijing, China. The results showed that the total VOC concentrations during haze days and non-haze days were 59.13 ± 31.08 μg/m3 and 16.91 ± 7.19 μg/m3, respectively. However, the average O3 concentrations during the two haze days were lower than those of non-haze days due to the extremely low concentrations at night instead of the reported lower photochemical reaction in daytime. The ratio of OH radical concentration during haze and non-haze days indicating that the rate of photochemical reaction during haze days was higher than those of non-haze days from 13:00–19:00. The stable air conditions and the local diesel emission at night were the main reasons for the decreased O3 concentrations during haze days. Six major sources were identified by positive matrix factorization (PMF), namely, diesel exhaust, combustion, gasoline evaporation, solvent usage, gasoline exhaust, and the petrochemical industry, contributing 9.93%, 25.29%, 3.90%, 16.88%, 35.59% and 8.41%, respectively, during the whole observation period. The contributions of diesel exhaust and the petrochemical industry emissions decreased from 26.14% and 6.43% during non-haze days to 13.70% and 2.57%, respectively, during haze days. These reductions were mainly ascribed to the emergency measures that the government implemented during haze days. In contrast, the contributions of gasoline exhaust increased from 34.92% during non-haze days to 48.77% during haze days. The ratio of specific VOC species and PMF both showed that the contributions of gasoline exhaust emission increased during haze days. The backward trajectories, potential source contribution function (PSCF) and concentration weighted trajectory (CWT) showed that the air mass of VOCs during haze days was mainly affected by the short-distance transportation from the southwestern of Hebei province. However, the air mass of VOCs during non-haze days was mainly affected by the long-distance transportation from the northwest.

Associations between fine particulate matter constituents and daily cardiovascular mortality in Shanghai, China

Limited evidence is available for the associations between fine particulate matter (PM_2.5) constituents and daily cardiovascular disease (CVD) mortality in China. In present study, a time-series analysis was conducted to evaluate the associations of PM_2.5 constituents (two carbonaceous fractions, eight water-soluble inorganic ions and fifteen elements) with daily CVD mortality in Pudong New Area of Shanghai, China, from 2014 to 2016. Results showed that the effect estimates for the associations of PM_2.5 and its constituents with CVD mortality were generally strongest when using the exposures of the previous two day concentrations. The associations of organic carbon, sulfate, ammonia, potassium, copper, arsenic, and lead with daily CVD mortality were robust to the adjustment of PM_2.5 total mass, their collinearity with PM_2.5 total mass, and criteria gaseous air pollutants. An interquartile range increase in the previous two day concentrations of PM_2.5, organic carbon, sulfate, ammonia, potassium, copper, arsenic, and lead were associated with significant increments of 2.21% (95% confidence interval [95%CI]: 0.54%, 3.88%), 2.83% (95% CIs: 1.16%, 4.50%), 1.90% (95% CIs: 0.35%, 3.45%), 2.29% (95% CIs: 0.80%, 3.77%), 0.94% (95% CIs: 0.13%, 1.75%), 1.53% (95% CIs: 0.37%, 2.69%), 2.08% (95% CIs: 0.49%, 3.68%) and 1.98% (95% CIs: 0.49%, 3.47%) in daily CVD mortality, respectively, in single-pollutant models. In conclusion, this study suggested that organic carbon, sulfate, ammonia, potassium, copper, arsenic, and lead might be mainly responsible for the associations between short-term PM_2.5 exposures and increased CVD mortality in Shanghai, China.

Impacts of Sources on PM2.5 Oxidation Potential during and after the Asia-Pacific Economic Cooperation Conference in Huairou, Beijing

To illustrate the major sources responsible for the redox activity of ambient fine particles during the 2014 Asia-Pacific Economic Cooperation (APEC) conference in Beijing, 3 months of daytime (8:00-19:30 LST) and nighttime (20:00-7:30 LST) particulate kmatter (PM_2.5) was collected in Huairou, Beijing from November 3, 2014 through January 31, 2015. PM_2.5 compositions were analyzed, including elements, organic carbon, elemental carbon, water-soluble ions, organics, and redox activities measured by both the dithiothreitol and the macrophage reactive oxygen species (ROS) assays. The mass-normalized redox activity was approximately constant during the noncontrol period (NCP) and control period (CP). The absolute value of the volume-normalized redox activity was about 4 times higher during NCP than that during CP, indicating the effectiveness of the control measures. The statistical analysis results showed that an interquartile range increase in PM_2.5 mass, chemicals, and sources (μg/m³) was associated with the 1-3% increase in redox activity, indicating that the successful control did make a significant reduction in redox activity but did not elucidate that some source controls (i.e., vehicle emissions) could be more effective at reducing redox activity than other control programs (i.e., dust source). This study demonstrated that combustion particles from both solid fuels and liquid fuels could contribute to ROS generation. Furthermore, ROS could be formed in the atmosphere via photochemical reactions, which highlights the need to further research on their formation pathways. A better understanding of the relevant mechanistic pathways and different source contributors to ROS will help to guide strategies for targeted mitigation of the atmospheric oxidation potential and will also help to reduce the great disease stress caused by exposure to air pollution.

Molecular characteristics and stable carbon isotope compositions of dicarboxylic acids and related compounds in the urban atmosphere of the North China Plain: Implications for aqueous phase formation of SOA during the haze periods

In the past five years, Chinese government has promulgated stringent measures to mitigate air pollution. However, PM_2.5 levels in the China North Plain (NCP), which is one of the regions with the heaviest air pollution in the world, are still far beyond the World Health Organization (WHO) standard. To improve our understanding on the sources and formation mechanisms of haze in the NCP, PM_2.5 samples were collected during the winter of 2017 on a day/night basis at the urban site of Liaocheng, which is one of the most polluted cities in the NCP. The samples were determined for molecular distributions and stable carbon isotope compositions of dicarboxylic acids and their precursors (ketocarboxylic acids and α-dicarbonyls), levoglucosan, elemental carbon (EC), organic carbon (OC) and water-soluble organic carbon (WSOC). Our results showed that oxalic acid (C₂) is the dominant dicarboxylic acid, followed by succinic acid (C₄) and malonic acid (C₃), and glyoxylic acid (ωC₂) is the most abundant ketocarboxylic acids. Concentrations of C₂, glyoxal (Gly) and methylglyoxal (mGly) presented robust correlations with levoglucosan, suggesting that biomass burning is a significant source of PM_2.5 in the NCP. Moreover, C₂ and Gly and mGly linearly correlated with SO₄^2-, relative humidity (RH), aerosol liquid water content (LWC) as well as particle in-situ pH (pH_is), indicating that aqueous-phase oxidation is the major formation pathway of these SOA, and is driven by acid-catalyzed oxidation. Concentrations and relative abundances of secondary species including SNA (SO₄^2-, NO₃^- and NH₄⁺), dicarboxylic acids, and aerosol LWC in PM_2.5 are much higher in the haze periods than in the clean periods, suggesting that aqueous reaction is a vital role in the haze formation. In comparison with those in the clean periods, stable carbon isotopic compositions (δ¹³C) of major dicarboxylic acids and related SOA and the mass ratios of C₂/diacids, C₂/Gly and C₂/mGly are higher in the haze periods, indicating that haze particles were more aged and enriched in secondary species.

The Impact of Water-Soluble Inorganic Ions in Particulate Matter (PM2.5) on Litter Decomposition in Chinese Subtropical Forests

Although numerous studies have demonstrated the toxic effects of fine particulates less than 2.5 µm (PM2.5) on the health of humans, little information is available on the ecotoxicity of PM2.5. Water-soluble inorganic ions (WSII, including Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−) can compose more than 60% of PM2.5. To better understand the possible impacts of WSII-PM2.5 on leaf litter decomposition, we conducted an experiment in which two leaf litters from oak (Quercus variabilis) and pine (Pinus massoniana) dominant forests in subtropical China were incubated in microcosms containing their respective forest soils and treated with WSII-PM2.5. Our results showed that, after six-months of decomposition, the WSII-PM2.5 treatments inhibited leaf litter decomposition rates, carbon and nitrogen loss, microbial biomass, and enzyme activities in the two forests. In addition, higher WSII-PM2.5 concentration led to stronger negative effects. Comparative analysis showed that the negative effects of WSII-PM2.5 on oak forest were greater than on pine forest, relating to the higher susceptibility to changes of soil microenvironment in oak forests. WSII-PM2.5 may influence decomposition through soil acidification and salinization, which could also cause a sub-lethal depression in soil isopod activity. However, in the first month of decomposition, mass loss of the oak and pine leaf litters under the low concentration WSII-PM2.5 were 21.63% and 35.64% higher than that under the control, respectively. This suggests that transitory low concentrations of WSII-PM2.5 have a promoting effect on decomposition. Long-term PM2.5 exposure, therefore, may have profound ecosystem consequences by altering the balance of ecosystem carbon flux, nutrient cycling, and humus formation in the future.

Aerosol optical properties under different pollution levels in the Pearl River Delta (PRD) region of China

To clarify the aerosol hygroscopic growth and optical properties of the Pearl River Delta (PRD) region, integrated observations were conducted in Heshan City of Guangdong Province from October 19 to November 17, 2014. The concentrations and chemical compositions of PM_2.5, aerosol optical properties and meteorological parameters were measured. The mean value of PM_2.5 increased from less than 35 (excellent) to 35-75 μg/m³ (good) and then to greater than 75 μg/m³ (pollution), corresponding to mean PM_2.5 values of 24.9, 51.2, and 93.3 μg/m³, respectively. The aerosol scattering hygroscopic growth factor (f(RH = 80%)) values were 2.0, 2.12, and 2.18 for the excellent, good, and pollution levels, respectively. The atmospheric extinction coefficient (σ_ext) and the absorption coefficient of aerosols (σ_ap) increased, and the single scattering albedo (SSA) decreased from the excellent to the pollution levels. For different air mass sources, under excellent and good levels, the land air mass from northern Heshan had lower f(RH) and σ_sp values. In addition, the mixed aerosol from the sea and coastal cities had lower f(RH) and showed that the local sources of coastal cities have higher scattering characteristics in pollution periods.

Insights into the phenomenon of an explosive growth and sharp decline in haze: A case study in Beijing

A severe haze episode occurred in winter in the North China Plain (NCP), and the phenomenon of an explosive growth and sharp decline in PM_2.5 (particulate matter with an aerodynamic diameter equal to or less than 2.5 μm) concentration was observed. To study the systematic causes for this phenomenon, comprehensive observations were conducted in Beijing from November 26 to December 2, 2015; during this period, meteorological parameters, LIDAR data, and the chemical compositions of aerosols were determined. The haze episode was characterized by rapidly varying PM_2.5 concentration, and the highest PM_2.5 concentration reached 667 μg/m³. During the haze episode, the NCP was dominated by a weak high-pressure system and continuously low PBL (planetary boundary layer) heights, which are unfavorable conditions for the diffusion of pollutants. The large increases in the concentrations of SNA (SO₄^2-, NO₃^- and NH₄⁺) during the haze implied that the formation of SNA was the largest contribution. Water vapor also played a vital role in the formation of haze by promoting the chemical transformation of secondary pollutants, which led to higher PM_2.5 concentrations. The spatial distributions of PM_2.5 in Beijing at different times and the backward trajectories of the air masses also indicated that pollutants from surrounding provinces in particular, contributed to the higher PM_2.5 concentration.

Characteristics of size distributions and sources of water-soluble ions in Lhasa during monsoon and non-monsoon seasons

To understand the physical and chemical characteristics, particle size distribution and sources of size-separated aerosols in Lhasa, which is located on the Tibetan Plateau (TP), six sizes of aerosol samples were collected in Lhasa in 2014. Ca²⁺, NH₄⁺, NO₃^-, SO₄^2- and Cl^- were the dominant ions. The ratio of cation equivalents (CE) to anion equivalents (AE) for each particle size segment indicated that the atmospheric aerosols in Lhasa were alkaline. SO₄^2- and NO₃^- could be neutralized by Ca²⁺, but could not be neutralized by NH₄⁺, according to the [NH₄⁺]/[NO₃^- + SO₄^2-] and [Ca²⁺]/[NO₃^- + SO₄^2-] ratios. Mobile sources were dominant in PM_0.95-1.5, PM_1.5-3 and PM_3-7.2, while stationary sources were dominant in the other three size fractions according to the [NO₃^-]/[SO₄^2-] ratios. The particle size distribution of all water-soluble ions during monsoon and non-monsoon periods was characterized by a bimodal distribution due to the different sources and formation mechanisms, and it was revealed that different ions had different sources in different seasons and different particle size segments by combining particle size distribution with correlation analysis. Source analysis of aerosols in Lhasa was performed using the Principal component analysis (PCA) for the first time, which revealed that combustion sources, motor vehicle exhaust, photochemical reaction sources and various types of dust were the main sources of Lhasa aerosols. Furthermore, Lhasa's air quality was also affected by long-distance transmission, expressed as pollutants from South Asia and West Asia, which were transmitted to Lhasa according to backward trajectory analysis.

Differences of the oxidation process and secondary organic aerosol formation at low and high precursor concentrations

Current atmospheric quality models usually underestimate the level of ambient secondary organic aerosol (SOA), one of the possible reasons is that the precursors at different concentrations may undergo different oxidation processes and further affect SOA formation. Therefore, there is a need to perform more chamber studies to disclose the influence. In this work, SOA formation over a wide range of initial precursor concentrations (tens of ppb to hundreds of ppb levels) was investigated in a 30 m³ indoor smog chamber, and mainly through the analysis of multiple generations of VOCs detected from HR-ToF-PTRMS to expound the difference in the oxidation process between low and high precursor concentrations. Compared to high initial concentrations, gas-phase intermediates formed at low concentrations had a higher intensity by about one order of magnitude, and the low-volatility compounds also had a higher formation potential due to the competition between semi-volatile intermediates and precursors with oxidants. In addition, the formed SOA was more oxidized with higher f₄₄ value (0.14 ± 0.02) and more relevant to real atmosphere than that formed at high concentrations. This work should help to deeply understand SOA formation and improve the performance of air quality models for SOA simulation.

Synergistic effects of sulfur dioxide and polycyclic aromatic hydrocarbons on pulmonary pro-fibrosis via mir-30c-1-3p/ transforming growth factor β type II receptor axis

SO₂ and PAHs are well-known pollutants of coal burning and significant contributors to haze episodes. The purpose of the study is to determine whether the combined effects of SO₂ and BaP are synergetic and to investigate the pro-fibrotic influences and possible mechanism from the aspect of microRNAs. In the present study cellular metabolic activity of BEAS-2B was assessed using MTT probe. C57BL/6 mice were exposed to BaP (40 mg/kg b.w.) for 5 days or SO₂ (7 mg/m³) inhalation for 4 weeks alone or together. Lung tissues were processed for histology to assess pulmonary fibrosis. The protein level of pulmonary pro-fibrotic genes (Col1a1, Col3a1, alpha-SMA, fibronectin) and TGFβR2 were analyzed by Western blot and immunofluorescence in vivo and in vitro. Furthermore, we clarified that the microRNA expression of mir-30c-1-3p by real-time RT-PCR. The luciferase reporter assay was used to determine the binding sites of mir-30c-1-3p in the 3'-UTR of TGFβR2. It was confirmed that SO₂ and BaP acted together to produce synergistic effects in cellular metabolic activity. Coexisting of SO₂ and BaP increased the protein expression of pro-fibrotic genes and TGFβR2 and decreased mir-30c-1-3p in vivo and in vitro. Dual-luciferase reporter gene assays showed that TGFβR2 was a validated target of mir-30c-1-3p. All above results demonstrated that mir-30c-1-3p was involved in the synergistic pro-fibrotic effects of SO₂ and BaP in lung via targeting TGFβR2. This work implies the potential risk of pulmonary fibrosis from the co-existence of SO₂ and PAHs and provides new insights into the molecular markers for relevant diseases.

Impacts of potential HONO sources on the concentrations of oxidants and secondary organic aerosols in the Beijing-Tianjin-Hebei region of China

We first coupled indoor emissions and biomass burning emissions into the WRF-Chem model besides the other four potential HONO sources (i.e., traffic emissions, soil emissions, and heterogeneous reactions on aerosol and ground surfaces). Eight simulations were performed in the Beijing-Tianjin-Hebei region (BTH) of China in August of 2006. The results indicated that traffic emissions and heterogeneous reactions on ground and aerosol surfaces were the key sources of HONO at night, accounting for ~41%, ~27% and ~20% of the nighttime simulated HONO concentrations, respectively. The two heterogeneous reactions were the main contributors during the day, accounting for ~66% (ground surfaces) and ~19% (aerosol surfaces) of the daytime simulated HONO concentrations. The indoor emission source could be the second largest contributor during nighttime and led to a maximum hourly enhancement of 0.59 and 0.76 ppb at the central urban sites of Beijing and Tianjin, respectively. The six potential HONO sources enhanced the monthly meridional-mean concentrations of O₃, OH and HO₂ by 5-44%, 5~>150% and 5~>200%, respectively, leading to an enhancement of 1-3 μg m^-3 in the monthly averaged concentrations of secondary organic aerosol (SOA), and that of 10-35 μg m^-3 in the largest hourly concentrations of SOA within 1000 m above the ground in the BTH. The major precursors of the enhanced SOA were Xylenes, Toluene and BIGALK (lumped alkanes C > 3). The inclusion of the six potential HONO sources in the WRF-Chem model considerably improved the HONO simulations at both urban and suburban sites compared with the corresponding observations. The above results suggested that the six potential HONO sources significantly enhanced the atmospheric oxidation capacity and thus accelerated SOA chemical aging in the BTH of China, leading to large enhancements in the hourly SOA concentrations and aggravating haze events in this region.

Biological and chemical compositions of atmospheric particulate matter during hazardous haze days in Beijing

Particulate matter (PM), a major air pollutant in Beijing in recent years, poses a formidable public health threat. Even through many studies have documented the chemical and biological characteristics of PM, less is known about these characteristics on hazardous haze days (Air Quality Index, AQI 301-500) and the difference with sunny or unhealthy haze day (AQI 151-200) characteristics. Herein, studies were performed during a red alert air pollution event (continuous hazardous haze days) and the first few days following the event (sunny days first and then unhealthy haze days) in Beijing from December 19 to 25, 2016. A laser particle counter and an ANDERSEM-6 sampler were used to study the concentration and size distributions dynamics of the PM and the culturable airborne bacteria and fungi, respectively. PM_2.5 was sampled by a high-volume air sampler and the chemical compositions, bacterial and fungal community structures, and endotoxin levels were analyzed. The results showed that the PM concentrations on the hazardous haze days and unhealthy haze days were 10.7 and 8.0 times higher, respectively, than those on the sunny days. The chemical composition of PM_2.5 was highly correlated with the AQI. The concentration and percentage of water-soluble inorganic ions (WSII), which dominated the PM_2.5 constituents, as well as the levels of endotoxin were higher on hazardous haze days than on unhealthy haze days and sunny days. Interestingly, the abundances of bacteria and fungi demonstrated the following order: unhealthy haze days> sunny days> hazardous haze days. Most culturable bacteria and fungi were distributed in particles with aerodynamic diameters of 2.1-4.7 μm. Redundancy analysis found total organic carbon explained 30.0% and 27.1% of total variations in bacterial composition and fungal composition at the genera level, respectively. Our results facilitate a better understanding of the biological and chemical composition dynamics of PM in Beijing.

Comparison of Measurement-Based Methodologies to Apportion Secondary Organic Carbon (SOC) in PM2.5: A Review of Recent Studies

Secondary organic aerosol (SOA) is known to account for a major fraction of airborne particulate matter, with significant impacts on air quality and climate at the global scale. Despite the substantial amount of research studies achieved during these last decades, the source apportionment of the SOA fraction remains difficult due to the complexity of the physicochemical processes involved. The selection and use of appropriate approaches are a major challenge for the atmospheric science community. Several methodologies are nowadays available to perform quantitative and/or predictive assessments of the SOA amount and composition. This review summarizes the current knowledge on the most commonly used approaches to evaluate secondary organic carbon (SOC) contents: elemental carbon (EC) tracer method, chemical mass balance (CMB), SOA tracer method, radiocarbon (14C) measurement and positive matrix factorization (PMF). The principles, limitations, challenges and good practices of each of these methodologies are discussed in the present article. Based on a comprehensive—although not exhaustive—review of research papers published during the last decade (2006–2016), SOC estimates obtained using these methodologies are also summarized for different regions across the world. Conclusions of some studies which are directly comparing the performances of different methodologies are then specifically discussed. An overall picture of SOC contributions and concentrations obtained worldwide for urban sites under similar conditions (i.e., geographical and seasonal ones) is also proposed here. Finally, further needs to improve SOC apportionment methodologies are also identified and discussed.

Impacts of meteorological conditions on wintertime PM2.5 pollution in Taiyuan, North China

Taiyuan frequently experiences heavy PM_2.5 pollution in winter under unfavorable meteorological conditions. To understand how the meteorological factors influence the pollution in Taiyuan, this study involved a systematic analysis for a continuous period from November 2016 to January 2017, using near-surface meteorological observations, radiosonde soundings, PM_2.5 measurements, and three-dimension numerical simulation, in combination with backward trajectory calculations. The results show that PM_2.5 concentration positively correlates with surface temperature and relative humidity and anti-correlates with near-surface wind speed and boundary layer height (BLH). The low BLH is often associated with a strong thermal inversion layer capping over. In addition to the high local emissions, it is found that under certain synoptic conditions, the southwesterly and southerly winds could bring pollutants from Linfen to Taiyuan, leading to a near-surface PM_2.5 concentration higher than 200 μg m^-3. Another pollution enhancing issue is due to the semi-closed basin of Taiyuan affecting the planetary boundary layer (PBL): the surrounding mountains favor the formation of a cold air pool in the basin, which inhibits vertical exchanges of heat, flux, and momentum between PBL and the free troposphere, resulting in stagnant conditions and poor air quality in Taiyuan. These findings can be utilized to improve the understanding of PM_2.5 pollution in Taiyuan, to enhance the accuracy of forecasting pollution, and to provide scientific support for policy makers to mitigate the pollution.

The indoor-outdoor characteristics of water-soluble ion in PM2.5 in Tianjin wintertime

The indoor and outdoor PM_2.5 mass concentration, water-soluble ion by filter sampler was analyzed on December 3-21, 2015 during wintertime in Tianjin, China. The results indicate that high humidity conditions result in the accumulation of atmospheric pollutants and reduce atmosphere visibility. The I/O ratio for PM_2.5 concentration in dormitory and lab are less than 1 in haze days. Indoor PM_2.5 concentration increases rapidly with outdoor PM_2.5 concentration increasing in haze days. The filtration factors of the dormitory and lab indicate nearly half of the outdoor PM_2.5 enters indoor environment. The human activities in dormitory could cause more the formation of PM_2.5 than those in lab. The concentration of SO₄^2- is the highest ion in water-soluble ion for outdoor PM_2.5. The SO₄^2-, NO₃^-, NH₄⁺, and Cl^- are generated mainly by outdoor sources; however, the Na⁺, Ca²⁺, and Mg²⁺ are generated mainly by indoor sources. The NH₄NO₃, (NH₄)₂SO₄, and NH₄Cl accounts for 20.2~41.8%, 32.0~51.4%, and 6.4~10.6% of the total water-soluble ion in different indoor-outdoor environment. The total secondary aerosols including NH₄NO₃, (NH₄)₂SO₄, and NH₄Cl in PM_2.5 are 28.3, 42.1, 28.2, 31.0, and 33.9% in outdoor environment for haze days, outdoor environment for non-haze days, dormitory for haze days, dormitory for non-haze days, and lab for haze days, respectively.

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 PM_2.5 of 187.51μg/m³, was the most severe. NO₃^- was always higher than SO₄^2-, which indicated that motor vehicles played an important role in the haze, even though the oxidation rate from SO₂ to SO₄^2- was faster than that of NO_X 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.

Seasonal Variation in Water-Soluble Ions in Airborne Particulate Deposition in the Suburban Nanjing Area, Yangtze River Delta, China, During Haze Days and Normal Days

To investigate the seasonal variation and characterization of water-soluble ions (WSIs) present in airborne particle deposition (APD) during Haze Days (visibility ≤7.5 km) and Normal Days (visibility >7.5 km) in suburban Nanjing area, 151 filter samples were collected from 18 May 2013 to 26 May 2014. Ten different WSIs from the samples were determined by Ion Chromatography. The results indicated that secondary WSIs (NH₄⁺, NO₃^-, and SO₄^2-) were the main ions in the WSIs, averaging 17.2, 18.5, and 17.1 μg/m³, respectively, and accounting respectively 20.9, 22.5, and 20.8% of the total WSIs. On Haze Days, the concentration of WSIs increased dramatically in fine size (particle size <2.1 μm), especially for NH₄⁺, NO₃^-, and SO₄^2- (increased by 52.6, 71.3, and 73.1%, respectively), whereas the concentrations of WSIs increased slowly in coarse size (2.1 μm < particle size < 10 μm), in which NH₄⁺, NO₃^-, and SO₄^2- increased by 14.7, 27.2, and 54.5%, respectively. According to the backward trajectories and the principal component analysis analysis, Nanjing APD were mainly derived from the soil dust in northern China (35%) in the spring, from ocean air masses (61 and 55%) in the summer and the autumn, and from local air masses (73%) in the winter. On summer Haze Days, secondary components in PM_2.1 consisted mainly of (NH₄)₂SO₄ and NH₄NO₃, whereas secondary components in PM_2.1-10 consisted mainly of (NH₄)₂SO₄, NH₄Cl, and NH₄NO₃. The increasing concentrations of secondary components increase the light extinction coefficients of aerosol on winter and autumn Haze Days. The concentrations of WSIs in fine size rose sharply on Haze Days, leading the visibility to exponential decline. Differently, the concentrations of WSIs in coarse size were not the main cause in the change of the visibility.

Roles of regional transport and heterogeneous reactions in the PM2.5 increase during winter haze episodes in Beijing

Regional transport and chemical conversions are two major processes that lead to the severe haze pollution in China. Our observations during five haze episodes in Beijing between February 19 and March 12 of 2014 show that the two processes played different roles as PM_2.5 increased from the clean (<75μgm^-3) to the light-medium pollution level (75-150μg m-3) and to levels of heavy (150-250μgm^-3) and severe (>250μgm^-3) pollution. In the initial twelve hours of each episode, the PM_2.5 reached the light-medium level with an increase of approximately 120μgm^-3. At the same time, the particle (~10-700nm) number concentration also showed a distinct increase accompanied by a rapid increase in the mean diameter. A light-medium PM_2.5 occurred in the south areas prior to the haze occurrence in Beijing and the southerly winds were predominant, indicating the rapid increase of PM_2.5 in the initial stage was caused by the regional transport from the south. Subsequently, PM_2.5 elevated to the heavy and severe levels when the wind was weak, relative humidity was high and ozone concentration was low. The increase of PM_2.5 in the elevated stages was characterized by a high percentage (45% for the heavy level and 55% for the severe level) of secondary inorganic components, indicating the substantial contribution of the formation of secondary aerosols. In addition, the increases of the mean diameter (from 108nm to 120nm) and the total volume concentration (by 67%) are regarded as a consequence of heterogeneous reactions on the surfaces of aerosol particles because the particle number concentration remained nearly constant in these two stages. Our results indicate that, during the five winter haze episodes, the regional transport from the south was the major reason for the initial-stage PM_2.5 increase, while heterogeneous reactions dominated the later elevation.

Evolutionary processes and sources of high-nitrate haze episodes over Beijing, Spring

Rare and consecutive high-nitrate haze pollution episodes were observed in Beijing in spring 2012. We present detailed characterization of the sources and evolutionary mechanisms of this haze pollution, and focus on an episode that occurred between 15 and 26 April. Submicron aerosol species were found to be substantially elevated during haze episodes, and nitrates showed the largest increase and occupation (average: 32.2%) in non-refractory submicron particles (NR-PM₁), which did not occur in other seasons as previously reported. The haze episode (HE) was divided into three sub-episodes, HEa, HEb, and HEc. During HEa and HEc, a shallow boundary layer, stagnant meteorological conditions, and high humidity favored the formation of high-nitrate concentrations, which were mainly produced by three different processes - daytime photochemical production, gas-particle partitioning, and nighttime heterogeneous reactions - and the decline in visibility was mainly induced by NR-PM_1. However, unlike HEa and HEc, during HEb, the contribution of high nitrates was partly from the transport of haze from the southeast of Beijing - the transport pathway was observed at ~800-1000m by aerosol Lidar - and the decline in visibility during HEb was primarily caused by PM_2.5. Our results provide useful information for air quality improvement strategies in Beijing during Spring.

Modeling the feedback between aerosol and boundary layer processes: a case study in Beijing, China

Rapid development has led to frequent haze in Beijing. With mountains and sea surrounding Beijing, the pollution is found to be influenced by the mountain-plain breeze and sea-land breeze in complex ways. Meanwhile, the presence of aerosols may affect the surface energy balance and impact these boundary layer (BL) processes. The effects of BL processes on aerosol pollution and the feedback between aerosol and BL processes are not yet clearly understood. Thus, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) is used to investigate the possible effects and feedbacks during a haze episode on 23 September 2011. Influenced by the onshore prevailing wind, sea-breeze, and upslope breeze, about 45% of surface particulate matter (PM)2.5 in Beijing are found to be contributed by its neighbor cities through regional transport. In the afternoon, the development of upslope breeze suppresses the growth of BL in Beijing by imposing a relatively low thermal stable layer above the BL, which exacerbates the pollution. Two kinds of feedback during the daytime are revealed as follows: (1) as the aerosols absorb and scatter the solar radiation, the surface net radiation and sensible heat flux are decreased, while BL temperature is increased, resulting in a more stable and shallower BL, which leads to a higher surface PM2.5 concentration in the morning and (2) in the afternoon, as the presence of aerosols increases the BL temperature over plains, the upslope breeze is weakened, and the boundary layer height (BLH) over Beijing is heightened, resulting in the decrease of the surface PM2.5 concentration there.