Characterization of aerosol chemical composition and the reconstruction of light extinction coefficients during winter in Wuhan, China

Changes in wintertime visibility across China over 2013-2019 and the drivers: A comprehensive assessment using machine learning method

Effective emission reduction measures have largely lowered the particulate concentration in China, but low-visibility events still occur frequently, greatly affecting people's daily life, travel, and health. In the context of carbon neutrality strategy and climate change, the mechanisms governing visibility changes may be undergoing a transformation. To address this critical issue, we have undertaken a comprehensive assessment by employing a novel approach that combines site observations, model-derived datasets, and machine learning techniques. Our analysis of the dataset shows varying degrees of improvement in wintertime visibility in regions such as North China, South China, and the Fenwei Plain over 2013-2019, but an unexpected deterioration (approximately 1 km yr-1) in central and southern China (CSC). We further elucidate key roles of PM2.5 reduction in these regions with visibility improvement; whereas the unsatisfactory visibility trend in CSC was caused by combined effect of relative humidity (RH) increase (47 %), aerosol hygroscopicity (κ) enhancement (9 %), and boundary layer (BLH) reduction (8 %), which greatly overwhelms the effect of PM2.5 reduction recently. Moreover, the study reveals a growing influence of RH on the wintertime visibility, reaching 40 % ± 24 % to the total contribution in 2019, while that of PM2.5 declined to 18 % ± 19 % and is expected to further diminish with emission reduction. Note those often-neglected factors-temperature, wind speed, BLH, and κ, account for over 40 % of the total contribution. Though the importance of aerosol hygroscopic growth to visibility was found decreasing recently, it retains non-negligible impacts on driving inter-annual visibility trends. This study yields innovative insights for air pollution control, underscoring the imperative of region-specific strategies to mitigate low-visibility events.

Outdoor Health Risk of Atmospheric Particulate Matter at Night in Xi'an, Northwestern China

The deterioration of air quality via anthropogenic activities during the night period has been deemed a serious concern among the scientific community. Thereby, we explored the outdoor particulate matter (PM) concentration and the contributions from various sources during the day and night in winter and spring 2021 in a megacity, northwestern China. The results revealed that the changes in chemical compositions of PM and sources (motor vehicles, industrial emissions, coal combustion) at night lead to substantial PM toxicity, oxidative potential (OP), and OP/PM per unit mass, indicating high oxidative toxicity and exposure risk at nighttime. Furthermore, higher environmentally persistent free radical (EPFR) concentration and its significant correlation with OP were observed, suggesting that EPFRs cause reactive oxygen species (ROS) formation. Moreover, the noncarcinogenic and carcinogenic risks were systematically explained and spatialized to children and adults, highlighting intensified hotspots to epidemiological researchers. This better understanding of day-night-based PM formation pathways and their hazardous impact will assist to guide measures to diminish the toxicity of PM and reduce the disease led by air pollution.

Enhanced nitrate contribution to light extinction during haze pollution in Chengdu: Insights based on an improved multiple linear regression model

In recent years, the annual mean concentration of PM_2.5 has decreased in Chengdu, China; however, atmospheric visibility has not improved accordingly. Low-visibility events occurred even when the PM_2.5 mass concentrations were below the national ambient air quality secondary standard (daily mean concentration, 75 μg/m³). In this study, the non-linear relationship between PM_2.5 and visibility was analyzed under different NO₃^- mass fractions in PM_2.5 based on 2-year field observation data. The results indicated that NO₃^- formation contributed to particulate pollution events and reduced atmospheric visibility. Multiple linear regression was used to propose a localized reconstruction equation for the light-scattering coefficient. According to the maximum likelihood estimation method and log-transformed residuals, the mass scattering coefficients (MSEs) of organic matter (OM), NH₄NO₃, and (NH₄)₂SO₄ in Chengdu were 7.42, 3.83, and 3.80, respectively. OM and NH₄NO₃ contributed to more than 50% of the light-extinction coefficient (b_ext). NH₄NO₃ was the main pollutant causing the substantial increase in b_ext. Chengdu has a high relative humidity (annual mean 70%), and under such conditions, the contribution of NH₄NO₃ to b_ext was considerably enhanced through hygroscopic growth and heterogeneous reactions. This study estimated the localized MSEs of OM, NH₄NO₃, and (NH₄)₂SO₄ in Chengdu and emphasized that effective control measures to reduce nitrate and its precursors could simultaneously ameliorate air quality and visibility in humid regions with poor atmospheric visibility.

Variations of the urban PM2.5 chemical components and corresponding light extinction for three heating seasons in the Guanzhong Plain, China

In order to investigate the variations of PM_2.5 (particulate matter with an aerodynamic diameter less than 2.5 μm) chemical components responding to the pollution control strategy and their effect on light extinction (b_ext) in the Guanzhong Plain (GZP), the comparisons of urban atmospheric chemical components during the heating seasons were extensively conducted for three years. The average concentration of PM_2.5 decreased significantly from 117.9 ± 57.3 μg m^-3 in the heating season 1 (HS1) to 53.5 ± 31.3 μg m^-3 in the heating season 3 (HS3), which implied that the effective strategies were implemented in recent years. The greatest contribution to PM_2.5 (∼30%) was from Organic matter (OM). The heightened contributions of the secondary inorganic ions (SNA, including NO₃^-, SO₄^2-, and NH₄⁺) to PM_2.5 were observed with the values of 34% (HS1), 41% (HS2), and 42% (HS3), respectively. The increased percentages of NO₃^- contributions indicated that the emission of NOx should be received special attention in the GZP. The comparison of PM_2.5 chemical compositions and implications across major regions of China and the globe were investigated. NH₄NO₃ was the most important contributor to b_ext in three heating seasons. The average b_ext was decreased from 694.3 ± 399.1 Mm^-1 (HS1) to 359.3 ± 202.3 Mm^-1 (HS3). PM_2.5 had a threshold concentration of 75 μg m^-3, 64 μg m^-3, and 57 μg m^-3 corresponding to the visual range (VR) < 10 km in HS1, HS2, and HS3, respectively. The enhanced impacts of the oxidant on PM_2.5 and O₃ were observed based on the long-term variations in PM_2.5 and OX (Oxidant, the sum of O₃ and NO₂ mixing ratios) over the five heating seasons and PM_2.5 and O₃ over six summers from 2016 to 2021. The importance of coordinated control of PM_2.5 and O₃ was also investigated in the GZP.

Significant reduction in atmospheric organic and elemental carbon in PM2.5 in 2+26 cities in northern China

To better understand the change characteristics and reduction in organic carbon (OC) and elemental carbon (EC) in particulate matter (PM) with a diameter of ≤2.5 μm (PM_2.5) driven by the most stringent clean air policies and pandemic-related lockdown measures in China, a comprehensive field campaign was performed to measure the carbonaceous components in PM_2.5 on an hourly basis via harmonized analytical methods in the Beijing-Tianjin-Hebei and its surrounding region (including 2 + 26 cities) from January 1 to December 31, 2020. The results indicated that the annual average concentrations of OC and EC reached as low as 6.6 ± 5.7 and 1.8 ± 1.9 μg/m³, respectively, lower than those obtained in previous studies, which could be attributed to the effectiveness of the Clean Air Action Plan and the impact of the COVID-19-related lockdown measures implemented in China. Marked seasonal and diurnal variations in OC and EC were observed in the 2 + 26 cities. Significant correlations (p < 0.001) between OC and EC were found. The annual average secondary OC levels level ranged from 1.8-5.4 μg/m³, accounting for 37.7-73.0% of the OC concentration in the 2 + 26 cities estimated with the minimum R squared method. Based on Interagency Monitoring of Protected Visual Environments (IMPROVE) algorithms, the light extinction contribution of carbonaceous PM to the total amount reached 21.1% and 26.0% on average, suggesting that carbonaceous PM played a less important role in visibility impairment than did the other chemical components in PM_2.5. This study is expected to provide an important real-time dataset and in-depth analysis of the significant reduction in OC and EC in PM_2.5 driven by both the Clean Air Action Plan and COVID-19-related lockdown policies over the past few years, which could represent an insightful comparative case study for other developing countries/regions facing similar carbonaceous PM pollution.

Network approach reveals the spatiotemporal influence of traffic on air pollution under COVID-19

Air pollution causes widespread environmental and health problems and severely hinders the quality of life of urban residents. Traffic is critical for human life, but its emissions are a major source of pollution, aggravating urban air pollution. However, the complex interaction between traffic emissions and air pollution in cities and regions has not yet been revealed. In particular, the spread of COVID-19 has led various cities and regions to implement different traffic restriction policies according to the local epidemic situation, which provides the possibility to explore the relationship between urban traffic and air pollution. Here, we explore the influence of traffic on air pollution by reconstructing a multi-layer complex network base on the traffic index and air quality index. We uncover that air quality in the Beijing-Tianjin-Hebei (BTH), Chengdu-Chongqing Economic Circle (CCS), and Central China (CC) regions is significantly influenced by the surrounding traffic conditions after the outbreak. Under different stages of the fight against the epidemic, the influence of traffic in some regions on air pollution reaches the maximum in stage 2 (also called Initial Progress in Containing the Virus). For the BTH and CC regions, the impact of traffic on air quality becomes bigger in the first two stages and then decreases, while for CC, a significant impact occurs in phase 3 among the other regions. For other regions in the country, however, the changes are not evident. Our presented network-based framework provides a new perspective in the field of transportation and environment and may be helpful in guiding the government to formulate air pollution mitigation and traffic restriction policies.

Dynamic Metabolic Risk Profiling of World Trade Center Lung Disease: A Longitudinal Cohort Study

Rationale: Metabolic syndrome (MetSyn) increases the risk of World Trade Center (WTC) lung injury (LI). However, the temporal relationship of MetSyn, exposure intensity, and lung dysfunction is not well understood. Objective: To model the association of longitudinal MetSyn characteristics with WTC lung disease to define modifiable risk. Methods: Firefighters, for whom consent was obtained (N = 5,738), were active duty on September 11, 2001 (9/11). WTC-LI (n = 1,475; FEV₁% predicted <lower limit of normal [LLN]) and non-WTC-LI (n = 4,263; FEV₁% predicted ⩾LLN at all exams) was the primary outcome, and FVC% predicted <LLN and FEV₁/FVC <0.70 were secondary outcomes. We assessed 1) the effect of concurrent MetSyn on longitudinal lung function by linear mixed models, 2) the temporal effect of MetSyn and exposure by Weibull proportional hazards, 3) the effects of MetSyn's rate of change by two-stage models, and 4) the nonlinear joint effect of longitudinal MetSyn components by a partially linear single-index model (PLSI). Measurements and Main Results: WTC-LI cases were more often ever-smokers, arrived in the morning (9/11), and had MetSyn. Body mass index ⩾30 kg/m² and high-density lipoprotein <40 mg/dl were most contributory to concurrent loss of FEV₁% predicted and FVC% predicted while conserving FEV₁/FVC. Body mass index ⩾30 kg/m² and dyslipidemia significantly predicted WTC-LI, FVC% predicted <LLN in a Weibull proportional hazards model. Dynamic risk assessment of WTC-LI on the basis of MetSyn and exposure showed how reduction of MetSyn factors further reduces WTC-LI likelihood in susceptible populations. PLSI demonstrates that MetSyn has a nonlinear relationship with WTC lung disease, and increases in cumulative MetSyn risk factors exponentially increase WTC-LI risk. An interactive metabolic-risk modeling application was developed to simplify PLSI interpretation. Conclusions: MetSyn and WTC exposure contribute to the development of lung disease. Dynamic risk assessment may be used to encourage treatment of MetSyn in susceptible populations. Future studies will focus on dietary intervention as a disease modifier.

Atmospheric ozonolysis of crotonaldehyde in the absence and presence of hydroxylated silica oligomer cluster adsorption

As one of the main components of combustion of tobacco products occurs (CARB), crotonaldehyde has an acute toxicity and widely exists in the atmosphere, which is harmful to human health. The removal efficiency of VOCs by ozonation can reach 80-90%. Based on the theory of quantum chemistry, the degradation mechanism, kinetics and toxicity of crotonaldehyde by ozonation in gas phase and heterogeneous phase were studied. Ozone was added to the olefins unsaturated double bond to form a five-membered ring primary ozonide, which was further fractured due to its unstable structure to form a Criegee intermediate and an aldehyde compound. The reaction rate constant of crotonaldehyde with ozone was 1.24 × 10^-17 cm³ molecule^-1 s^-1 at 298 K and 1 atm, which was an order of magnitude higher than the experimental value. From toxicity assessment, it was found that the ozonation of crotonaldehyde is beneficial to the removal of toxicity. Mineral dust aerosol exists in the atmosphere in large quantities, and SiO₂ is the most abundant component. VOCs are transformed into particle state on their surface through homogeneous nucleation and heterogeneous nucleation. Referring to the crystal structure of SiO₂, five hydroxylated silica oligomer cluster structures were simulated and the adsorption configurations of crotonaldehyde on their surface were simulated. The adsorption of crotonaldehyde on the surface of the clusters was achieved by forming hydrogen bonds and had good adsorption effects. The adsorption of hydroxylated silica oligomer clusters didn't change the ozonation mechanism of crotonldehyde, but had a certain effect on the reaction rate.

Wintertime chemical characteristics of aerosol and their role in light extinction during clear and polluted days in rural Indo Gangetic plain

This paper reports the chemical and light extinction characteristics of fine aerosol (PM_2.5) during the winter period (2017-18) at Lumbini, Nepal, a rural site on the Indo Gangetic Plains. A modified IMPROVE algorithm was employed to reconstruct light extinction by chemical constituents of aerosol. The fine aerosol levels impacted visibility adversely during daytime, but during nighttime visibility was controlled by fog droplets rather than by aerosols. The PM_2.5 chemical constituents showed varying characteristics during clear and polluted days. The average NO₃^-/SO₄^2- concentration ratio was 0.57 during clear and 1.36 and polluted days, signifying a change in secondary inorganics and formation processes mainly due to decreasing photochemical production and due to increased partitioning of nitrate particles at a lower temperature. The increased secondary organics contribution and the higher OM/OC ratio (2.2) during polluted days showed the vital role of aqueous processing and biomass burning emissions in determining the concentration of organics. Total light extinction was 2.3 times higher on polluted days compared to clear days, while the PM_2.5 mass concentration was 1.5 times higher. This variation in mass and extinction order signifies that various chemical components in fine particles have a more considerable impact on light extinction. On clear days we found that carbonaceous particles (OM and EC) made a major contribution to light extinction. In contrast, the extinction contribution by secondary inorganic (especially NH₄NO₃) increased significantly during polluted days, with hygroscopic growth and enhanced scattering efficiency at higher RH conditions playing a major role. The comparison between clear and polluted days altogether suggests that regulating the nitrate sources can help significantly in improving the visibility levels and restrict fog haze development during wintertime in rural IGP.

Light absorption enhancement of particulate matters and their source apportionment over the Asian continental outflow site and South Yellow Sea

Light absorption enhancement of black carbon due to the aerosol mixing states is an important parameterization for climate modeling, while emission source contributions to the enhancement factor are unclear. An intensive campaign was conducted simultaneously at a China coastal site (Qingdao city) and maritime sites (South Yellow Sea, SYS) in August and Nov to Dec 2018. The absorption enhancement (E_MAC) of the black carbon was calculated using a two-step solvent dissolution protocol and found 1.96 ± 0.68, 1.64 ± 0.38, and 2.40 ± 0.76 for Qingdao summer (QS), Qingdao autumn (QA), and SYS, respectively. Positive matrix factorization (PMF) model identified six sources of PM_2.5 and E_MAC, which were secondary aerosol (with contribution 27.9% and 29.2%), coal combustion (24.9% and 20.2%), industrial emissions (15.2% and 25.4%), sea salt (6.9% and 9.6%), vehicle emissions (12.1% and 10.9%), and soil dust (13.0% and 4.7%), respectively. These sources increased the absorption of black carbon by a factor of 1.25 ± 0.11 (secondary aerosol), 1.21 ± 0.20 (industrial emissions), 1.17 ± 0.08 (coal combustion), 1.09 ± 0.07 (vehicle emissions), 1.08 ± 0.17 (sea salt), and 1.04 ± 0.10 (soil dust). Based on the correlation between PM and E_MAC source contributions, we estimated that secondary aerosols, industrial emissions, and coal combustion contributed to 74.8% of absorption enhancement at a regional scale in China. The source apportionment for E_MAC offers a new diagnosis for each source regarding aerosol forcing simulation which inputs from the individual emission sector.

Impact of city lockdown on the air quality of COVID-19-hit of Wuhan city

A series of strict lockdown measures were implemented in the areas of China worst affected by coronavirus disease 19, including Wuhan, to prevent the disease spreading. The lockdown had a substantial environmental impact, because traffic pollution and industrial emissions are important factors affecting air quality and public health in the region. After the lockdown, the average monthly air quality index (AQI) in Wuhan was 59.7, which is 33.9% lower than that before the lockdown (January 23, 2020) and 47.5% lower than that during the corresponding period (113.6) from 2015 to 2019. Compared with the conditions before the lockdown, fine particulate matter (PM_2.5) decreased by 36.9% and remained the main pollutant. Nitrogen dioxide (NO₂) showed the largest decrease of approximately 53.3%, and ozone (O₃) increased by 116.6%. The proportions of fixed-source emissions and transported external-source emissions in this area increased. After the lockdown, O₃ pollution was highly negatively correlated with the NO₂ concentration, and the radiation increase caused by the PM_2.5 reduction was not the main reason for the increase in O₃. This indicates that the generation of secondary pollutants is influenced by multiple factors and is not only governed by emission reduction.