Haze Pollution Episodes Research Articles

Direct observation of the complex S(IV) equilibria at the liquid-vapor interface

The multi-phase oxidation of S(IV) plays a crucial role in the atmosphere, leading to the formation of haze and severe pollution episodes. We here contribute to its understanding on a molecular level by reporting experimentally determined pKa values of the various S(IV) tautomers and reaction barriers for SO2 formation pathways. Complementary state-of-the-art molecular-dynamics simulations reveal a depletion of bisulfite at low pH at the liquid-vapor interface, resulting in a different tautomer ratio at the interface compared to the bulk. On a molecular-scale level, we explain this with the formation of a stable contact ion pair between sulfonate and hydronium ions, and with the higher energetic barrier for the dehydration of sulfonic acid at the liquid-vapor interface. Our findings highlight the contrasting physicochemical behavior of interfacial versus bulk environments, where the pH dependence of the tautomer ratio reported here has a significant impact on both SO2 uptake kinetics and reactions involving NOx and H2O2 at aqueous aerosol interfaces.

Reveal persistent haze pollution episodes in hefei: a perspective from ground-based and satellite observation

Reveal persistent haze pollution episodes in hefei: a perspective from ground-based and satellite observation

Influence of aerosol–meteorology interactions on visibility during a wintertime heavily polluted episode in Central‐East, China

AbstractAtmospheric visibility profoundly impacts daily life, and accurate prediction is crucial, particularly in conditions of low visibility characterized by high aerosol loading and humidity. This study employed the WRF‐Chem model to simulate a severe wintertime haze pollution episode that transpired from January 17 to 19, 2010, in Central‐East China (112–122° E, 34–42° N). The results reveal that excluding aerosol–meteorology interactions led to underestimated PM2.5 concentrations and relative humidity in comparison with ground‐based measurement data, accompanied by a significant overestimation of visibility. Aerosols can engage with meteorological elements, particularly humidity, resulting in positive feedback. Upon considering these feedback interactions, the simulation results showed an increase of 5.17% and 1.99% in PM2.5 concentration and relative humidity, respectively, compared with the original simulation. This adjustment narrowed the bias between simulated and measured data. The overestimation of simulated visibility was reduced by 16% and 25% for the entire study period and the severe haze pollution period, respectively. These findings underscore the vital role of incorporating aerosol–meteorology interactions in visibility simulations using the WRF‐Chem model. Notably, the inclusion of aerosol–meteorological feedback significantly enhances the accuracy of visibility predictions, particularly during heavily polluted periods.

Evolution stage identification of haze pollution episodes in beijing using constrained dynamic time warping and multiway principal component analysis

Evolution stage identification of haze pollution episodes in beijing using constrained dynamic time warping and multiway principal component analysis

Comparative Analysis of Secondary Organic Aerosol Formation during PM2.5 Pollution and Complex Pollution of PM2.5 and O3 in Chengdu, China

Nowadays, many cities in China are suffering from both fine particulate matter (PM2.5, particulate matter with an aerodynamic diameter smaller than 2.5 µm) and ozone (O3) pollution. Secondary organic aerosol (SOA) is an important component of PM2.5 and is closely related to the oxidation processes. To investigate the characteristics and formation pathways of SOA during different types of haze pollution episodes, carbonaceous components of PM2.5 and volatile organic compounds (VOCs) were monitored continuously in Chengdu in April 2019, when Chengdu experienced not only PM2.5 pollution (SPP) but also a complex pollution of PM2.5 and O3 (CoP). In the CoP episode, the concentrations of SOA increased by 51.2% as compared to SPP, and the SOA concentrations were positively correlated with PM2.5 mass concentrations. These suggest that SOA drove the increase in PM2.5 levels during the haze event to some extent. The preliminary VOC source analysis based on the feature ratio showed that vehicle emission and fuel volatilization sources were the main sources of VOCs at this urban site. In addition, coal emissions and biomass burning were also important contributors. High-carbon alkanes and aromatic hydrocarbons significantly contributed to the SOA formation. These results provide a preliminary understanding of SOA formation during different types of pollution episodes in Chengdu, which can help us to further understand air pollution in this typical region.

Atmospheric sulfate formation in the Seoul Metropolitan Area during spring/summer: Effect of trace metal ions

Despite the effort to control SO2 emission, sulfate is still one of the major inorganic components of PM2.5 in urban area. Moreover, there is still a lack of understanding of the sulfate formation mechanism via SO2 oxidation under various ambient conditions. In this study, we focus on sulfate formation during a haze pollution episode in the spring/summertime of 2016 in Seoul Metropolitan Area (SMA). During the pollution episode, PM2.5 mass concentration exceeded over 60 μg m−3, and sulfate accounted for about 25% of the total PM2.5 mass concentration. A sharp increase of sulfur oxidation ratio (SOR) values along with aerosol liquid water content (AWC) under humid conditions could be ascribed to an apparent contribution of aqueous-phase oxidation of SO2 of sulfate formation during the pollution period. Comparisons of SOR values with four representative oxidants for the aqueous-phase oxidation (i.e., NO2, H2O2, O3, and TMIs) indicated that TMIs concentration, especially for Mn (II), showed the best positive correlation. Furthermore, for calculating the sulfate production rate, the contribution of TMIs concentration was found to be dominant within the pH range in SMA (2.1–3.0), which was determined by the chemical composition and derived AWC. These results imply that not only the SO2 emission but also other chemical components (e.g., TMI and nitrate) would play a critical combined role in sulfate formation under urban haze condition.

Haze Air Pollution Health Impacts of Breath-Borne VOCs

Here, we investigated the use of breath-borne volatile organic compounds (VOCs) for rapid monitoring of air pollution health effects on humans. Forty-seven healthy college students were recruited, and their exhaled breath samples (n = 235) were collected and analyzed for VOCs before, on, and after two separate haze pollution episodes using gas chromatography-ion mobility spectrometry (GC-IMS). Using a paired t-test and machine learning model (Gradient Boosting Machine, GBM), six exhaled VOC species including propanol and isoprene were revealed to differ significantly among pre-, on-, and post-exposure in both haze episodes, while none was found between clean control days. The GBM model was shown capable of differentiating between pre- and on-exposure to haze pollution with a precision of 90-100% for both haze episodes. However, poor performance was detected for the same model between two different clean days. In addition to gender and particular haze occurrence influences, correlation analysis revealed that NH4+, NO3-, acetic acid, mesylate, CO, NO2, PM2.5, and O3 played important roles in the changes in breath-borne VOC fingerprints following haze air pollution exposure. This work has demonstrated direct evidence of human health impacts of haze pollution while identifying potential breath-borne VOC biomarkers such as propanol and isoprene for haze air pollution exposure.

Formation and evolution of secondary particulate matter during heavy haze pollution episodes in winter in a severe cold climate region of Northeast China.

The formation and evolution of sulfate (SO42-) and nitrate (NO3-) secondary contaminants under different stages of pollution episodes and different meteorological and emission conditions were compared, based on the simultaneous observation of fine particulate matter (PM2.5) and its chemical components in four heavy haze pollution episodes at 14 sampling sites in a severe cold climate region of Northeast China in winter from 2017 to 2019. The results yielded two main findings. (1) Nitrate formation during the day was mainly due to the combination of high emissions and high relative humidity (RH, 50-90%), high temperature (T, 0 to 5°C), high atmospheric oxidizability (ozone (O3) and nitrous acid (HONO) concentrations), and high ammonia (NH3) concentrations. Nitrate was formed by a gas-phase homogeneous reaction of the hydroxyl radical (OH·) with nitrogen dioxide (NO2), sulfur dioxide (SO2), and ammonia (NH3). (2) The main differences in SO42- formation between Northeast China and other regions were that the gas-phase oxidation process played an important role. This was mainly a result of the promotion of the gas-phase oxidation of SO42- due to the high oxidizing ability and the suppression of the aqueous reaction due to the low Ts in winter and low-sulfur coal emissions. Sulfate formation mostly occurred through an aqueous phase reaction in winter, but the highest yield and the fastest production capacity were produced by the gas-phase reaction.

High atmospheric oxidation capacity drives wintertime nitrate pollution in the eastern Yangtze River Delta of China

Abstract. Nitrate aerosol plays an increasingly important role in wintertime haze pollution in China. Despite intensive research on wintertime nitrate chemistry in recent years, quantitative constraints on the formation mechanisms of nitrate aerosol in the Yangtze River Delta (YRD), one of the most developed and densely populated regions in eastern China, remain inadequate. In this study, we identify the major nitrate formation pathways and their key controlling factors during the winter haze pollution period in the eastern YRD using 2-year (2018–2019) field observations and detailed observation-constrained model simulations. We find that the high atmospheric oxidation capacity, coupled with high aerosol liquid water content (ALWC), made both the heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) and the gas-phase OH oxidation of nitrogen dioxide (NO2) important pathways for wintertime nitrate formation in this region, with contribution percentages of 69 % and 29 % in urban areas and 63 % and 35 % in suburban areas during the haze pollution episodes, respectively. We further find that the gas-to-particle partitioning of nitric acid (HNO3) was very efficient so that the rate-determining step in the overall formation process of nitrate aerosol was the oxidation of NOx to HNO3 through both heterogeneous and gas-phase processes. The atmospheric oxidation capacity (i.e., the availability of O3 and OH radicals) was the key factor controlling the production rate of HNO3 from both processes. During the COVID-19 lockdown (January–February 2020), the enhanced atmospheric oxidation capacity greatly promoted the oxidation of NOx to nitrate and hence weakened the response of nitrate aerosol to the emission reductions in urban areas. Our study sheds light on the detailed formation mechanisms of wintertime nitrate aerosol in the eastern YRD and highlights the demand for the synergetic regulation of atmospheric oxidation capacity and NOx emissions to mitigate wintertime nitrate and haze pollution in eastern China.

Aloft Transport of Haze Aerosols to Xuzhou, Eastern China: Optical Properties, Sources, Type, and Components

Rapid industrialization and urbanization have caused frequent haze pollution episodes during winter in eastern China. Considering that the vertical profile of the aerosol properties changes significantly with altitude, investigating aerosol aloft information via satellite remote sensing is essential for studying regional transport, climate radiative effects, and air quality. Through a synergic approach between lidar, the AErosol RObotic NETwork sunphotometer observations, and WRF-Chem simulations, several transboundary aloft transport events of haze aerosols to Xuzhou, eastern China, are investigated in terms of source, type, and composition and the impact on optical properties. Upper-air aerosol layers are short-lived tiny particles that increase the total aerosol optical depth (AOD). The aloft aerosols not only play a critical role during the haze event, enhancing the scattering of aerosol particles significantly but also cause a rise in the AOD and the Ångström exponent (AE), which increases the proportion of fine particles, exacerbating the pollution level near the surface. Based on the model simulation results, our study highlights that the transported aloft aerosols lead to the rapid formation of secondary inorganic substances, such as secondary sulfates, nitrates, and ammonium salts, which strongly contribute to haze event formation. Moreover, the results provide evidence that the haze frequency events associated with polluted dust outbreaks were higher for 2014–2015 winter. A closer analysis shows that the advected dust layers over Xuzhou originated from Inner Mongolia and the Xinjiang Uygur Autonomous Region. The study of the occurrence frequency, height, thickness, and optical properties of aloft anthropogenic haze in China will further deepen our understanding and provide a strong basis to assess aerosol impact on transport and the Earth–atmosphere radiative balance.

Intensified wintertime secondary inorganic aerosol formation during heavy haze pollution episodes (HPEs) in Beijing, China

Air pollution in China is complex, and the formation mechanism of chemical components in particulate matter is still unclear. This study selected three consecutive heavy haze pollution episodes (HPEs) during winter in Beijing for continuous field observation, including an episode with heavy air pollution under red alert. Clean days during the observation period were selected for comparison. The HPE characteristics of Beijing in winter were: under the influence of adverse meteorological conditions such as high relative humidity, temperature inversion and low wind speed; and strengthening of secondary transformation reactions, which further intensified the accumulation of secondary aerosols and other pollutants, promoting the explosive growth of PM2.5. PM2.5/CO values, as indicators of the contribution of secondary transformation in PM2.5, were approximately 2 times higher in the HPEs than the average PM2.5/CO during the clean period. The secondary inorganic aerosols (sulfate nitrate and ammonium salt) were significantly enhanced during the HPEs, and the conversion coefficients were remarkably improved. In addition, it is interesting to observe that the production of sulfate tended to exceed that of nitrate in the late stage of all three HPEs. The existence of aqueous phase reactions led to the explosive growth sulfur oxidation ratio (SOR) and rapid generation of sulfate under high relative humidity (RH>70%).

Evolution Stage Identification of Haze Pollution Episodes Over the North China Plain Using Constrained Dynamic Time Warping and Multiway Principal Component Analysis

Evolution Stage Identification of Haze Pollution Episodes Over the North China Plain Using Constrained Dynamic Time Warping and Multiway Principal Component Analysis

Vertical profile of aerosol number size distribution during a haze pollution episode in Hefei, China

The vertical distribution of aerosols has important implications on haze formation as development, which is manifested to some extent by the planetary boundary layer (PBL)–aerosol interactions. Information on the number concentration and size of particles is essential to understand these processes, but studies on vertical profiles of particle number-size distribution are limited. Herein, an unmanned aerial vehicle (UAV) equipped with a custom-built optical particle counter (0.4–10 μm) was used to investigate the vertical profiles of particle number-size distribution in Hefei (China) during January 20–30, 2021. Combining ground-based scanning mobility particle sizer and meteorological data, the pollution accumulation and diffusion mechanisms were analyzed in depth. Results showed that as the pollution episode developed, the vertical distribution of the particle number concentration changed from a flat profile to a sharp vertical gradient. Under polluted conditions, a three-layer structure was clearly evident: uniform distribution in a mixed layer near the ground, a sharply reduced transition layer, and a low number concentration layer in the free atmosphere. Analysis revealed that fundamental to this conversion is that aerosols are highly affected by the PBL dynamics. Concurrent on-UAV and ground-based observations revealed that the ratio of particle numbers in the accumulation mode to that in the Aitken mode was 0.92 ± 0.05 in polluted days, which was almost three times that of clean days. This difference in the ratio of large to small particles suggests that hygroscopic growth of aerosol particles under high humidity conditions played an important role in haze development. Moreover, the sharp vertical gradient of the particle number concentration in the transition layer was identified as an important parameter for characterizing PBL height. The findings in this study highlight the importance of PBL dynamics on the under-studied vertical profiles of particle number-size distribution, especially during heavy pollution episodes.

Spatial and temporal distribution characteristics of urban air quality index during haze pollution episodes in China

Spatial and temporal distribution characteristics of urban air quality index during haze pollution episodes in China

Study of Persistent Haze Pollution in Winter over Jinan (China) Based on Ground-Based and Satellite Observations

A comprehensive study of the formation process of haze events in the Jinan area of China during winter is conducted based on the ground-based and satellite observation data from 1 December 2020 to 12 January 2021. According to variation of pollutant concentrations, two typical types of haze pollution are found in the Jinan area. During the type 1 haze pollution, the PM2.5 concentrations are greater than 75 μgm−3 and less than 115 μgm−3 with a short duration. The haze is mainly caused by local pollutant emissions and the accumulation of pollutants transported from areas around Jinan. By contrast, type 2 haze pollution episodes have a long duration and peak PM2.5 concentrations between 150 μgm−3 and 250 μgm−3, which is considered heavy pollution. Type 2 haze pollution is mainly caused by a mixture of long-range transported dust with locally emitted pollutants. Moreover, the unfavorable meteorological factors such as stable inversion layer, continuous cold high-pressure system, high relative humidity, and low wind speed play an important role in the formation of both types of haze pollution. In addition, there are significant photochemical processes during the haze pollutions. According to satellite data, the AOD in Jinan and surrounding areas is maintained at a high-level during haze pollution. It indicates that the local pollution is often accompanied by regional pollution during haze pollution events. This study reveals the formation process of haze pollution and promotes the study of regional climate change, which can provide guidance to the government in the prevention and control of haze pollution in East China.

Estimating aerosol optical extinction across eastern China in winter during 2014–2019 using the random forest approach

Air quality has been continuously improved in recent years across eastern China, yet severe haze pollution episodes still frequently occur in winter. Optical extinction of aerosols can provide a proxy for assessing haze pollution levels, especially for submicron particles (i.e., PM1, aerodynamic diameter less than 1.0 μm) because they are close to the wavelength of visible solar radiation and are very efficient in reducing atmospheric visibility. In past decades, many studies had indicated that optical extinction showed an opposite trend to fine aerosol particles (i.e., PM2.5, aerodynamic diameter less than 2.5 μm). However, little work has examined the changing trend of optical extinction since the implementation of the toughest-ever clean air policy in 2013. Has the trend of optical extinction been consistent with PM2.5 in recent years? Here a random forest model was developed to predict optical extinction across eastern China in winter for the period of 2014–2019. The model captured the observed spatiotemporal variations of optical extinction (cross-validation R2 = 0.72 and RMSE = 0.12), taking advantage of the extensive network datasets available including air quality data, meteorological data, land cover data, and other predictors. The wintertime optical extinction was predicted to be the highest in North China, suggesting that the region still suffers from the adverse impacts of haze pollution. From 2014 to 2018, we estimated that optical extinction exhibited a shrinking trend across eastern China, at a decline rate of 3.6% each year, and the spatial coverage affected by haze pollution was also cut down. Moreover, grid-based statistics indicated that the decreasing trends of optical extinction were slower than those of PM2.5 mass concentrations, which could attribute to the enhancement of the ratio of the secondary aerosols to PM2.5 mass concentrations in recent years. This study provides the latest investigation into the trend of aerosol optical properties and highlights the importance of reducing secondary aerosol formation to control haze pollution across eastern China.

Evolution and Potential Source Apportionment of Atmospheric Pollutants of Two Heavy Haze Episodes During the COVID-19 Lockdown in Beijing, China

To control the spread of the 2019 novel coronavirus(COVID-19), China imposed rigorous restrictions, which resulted in great reductions in pollutant emissions. However, two heavy haze pollution episodes still occurred in Beijing. In this study, we use the air pollutants, aerosol number concentration, and meteorological elements data in Beijing, combined with the HYSPLIT model, to calculate the potential source contribution factor(PSCF) and concentration weight trajectory(CWT), and analyze the characteristics of evolution and potential source apportionment of atmospheric pollutants during the two episodes. The COVID-19 lockdown restrictions had great impacts on the diurnal variations of PM2.5 and black carbon(BC), while small impacts on the diurnal variations of CO, NO2, SO2, and O3. The primary pollutant was PM2.5 during the two haze pollution episodes, and the haze1 episode was mainly local pollution, while haze 2 was mainly local and external transportation pollution. The spectrum of aerosol number concentration was unimodal under different processes, with the peak located at 0.3 μm. During the haze episodes, the number concentration in the size range of 0.2-0.5 μm increased 3.3-13.6 times that of the clean days. The mass concentration contributions of BCliquid to BC in different processes were 64.8%-85.1%. This mass concentration of BCliquid ranked in the order of haze 1(5.04 μg·m-3) >haze 1(3.20 μg·m-3) >clean day(before COVID-19) (2.31 μg·m-3) >clean day(COVID-19) (0.76 μg·m-3). The characteristics of PSCF and CWT distribution of PM2.5 and BC were different in different processes. The PSCF high value areas of PM2.5 on the clean day(before COVID-19) and the clean day(COVID-19) were mainly distributed in the southwest and western of Beijing, and the weight concentration exceeded 30 μg·m-3. The PSCF high value areas of PM2.5 during haze 1 and haze 2 were mainly distributed in Beijing and its surrounding areas and southwestern, when the weight concentration exceeded 90 μg·m-3. The PSCF high value areas of BC were mainly distributed in Beijing and its surrounding areas on clean days(before COVID-19), clean days(COVID-19) and haze 1, with weighted concentrations exceeding 2.4, 0.9 and, 4.5 μg·m-3, respectively. The PSCF high value areas of BC on haze 2 was distributed in the southwest of Beijing, and the weight concentration exceeded 5 μg·m-3.

Current challenges of improving visibility due to increasing nitrate fraction in PM2.5 during the haze days in Beijing, China

The annual mean PM2.5 mass concentration has decreased because of the stringent emission controls implemented in Beijing, China in recent years, whereas the nitrate NO3– mass fraction in PM2.5 increases gradually. Low-visibility events occur frequently even though PM2.5 pollution has been mitigated significantly, with the daily mean PM2.5 mass concentration mostly less than 75 μg/m3. In this study, the non-linear relationship was analyzed between atmospheric visibility and PM2.5 based on chemical composition from a two-year field observation. Our results showed that NO3– became the main constituent of PM2.5, especially during the haze pollution episodes. A localized parameterization scheme was proposed between the atmospheric extinction coefficient (σext) and major chemical constituents of PM2.5 by multiple linear regression (MLR). The contribution of NO3– to σext increased with increasing air pollution, and NO3– became the most important contributor for PM2.5 above 75 μg/m3. The visibility decreased with increasing NO3– mass fraction for the same PM2.5 mass concentration when PM2.5 was above 20 μg/m3. The hygroscopicity of PM2.5 increased with increasing mass fraction of hygroscopic NO3–. These results stressed the importance of reducing particulate NO3– and its precursors (for instance, NH3) through effective emission control measures as well as the tightening of PM2.5 standards to further improve air quality and visibility in Beijing.

Ozone pollution in the North China Plain spreading into the late-winter haze season

Surface ozone is a severe air pollution problem in the North China Plain, which is home to 300 million people. Ozone concentrations are highest in summer, driven by fast photochemical production of hydrogen oxide radicals (HOx) that can overcome the radical titration caused by high emissions of nitrogen oxides (NOx) from fuel combustion. Ozone has been very low during winter haze (particulate) pollution episodes. However, the abrupt decrease of NOx emissions following the COVID-19 lockdown in January 2020 reveals a switch to fast ozone production during winter haze episodes with maximum daily 8-h average (MDA8) ozone concentrations of 60 to 70 parts per billion. We reproduce this switch with the GEOS-Chem model, where the fast production of ozone is driven by HOx radicals from photolysis of formaldehyde, overcoming radical titration from the decreased NOx emissions. Formaldehyde is produced by oxidation of reactive volatile organic compounds (VOCs), which have very high emissions in the North China Plain. This remarkable switch to an ozone-producing regime in January-February following the lockdown illustrates a more general tendency from 2013 to 2019 of increasing winter-spring ozone in the North China Plain and increasing association of high ozone with winter haze events, as pollution control efforts have targeted NOx emissions (30% decrease) while VOC emissions have remained constant. Decreasing VOC emissions would avoid further spreading of severe ozone pollution events into the winter-spring season.

Aqueous production of secondary organic aerosol from fossil-fuel emissions in winter Beijing haze

Secondary organic aerosol (SOA) produced by atmospheric oxidation of primary emitted precursors is a major contributor to fine particulate matter (PM2.5) air pollution worldwide. Observations during winter haze pollution episodes in urban China show that most of this SOA originates from fossil-fuel combustion but the chemical mechanisms involved are unclear. Here we report field observations in a Beijing winter haze event that reveal fast aqueous-phase conversion of fossil-fuel primary organic aerosol (POA) to SOA at high relative humidity. Analyses of aerosol mass spectra and elemental ratios indicate that ring-breaking oxidation of POA aromatic species, leading to functionalization as carbonyls and carboxylic acids, may serve as the dominant mechanism for this SOA formation. A POA origin for SOA could explain why SOA has been decreasing over the 2013-2018 period in response to POA emission controls even as emissions of volatile organic compounds (VOCs) have remained flat.