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The application of low-condensation diesel in cold regions with extremely low ambient temperatures (−14 to −29 °C) has enabled the operation of diesel vehicles. Still, it may contribute to heavy haze pollution in cold regions during winter. Here we examine pollutant emissions from low-condensation diesel in China. We measure the emissions of elemental carbon (EC), organic carbon (OC), and elements, including heavy metals such as arsenic (As). Our results show that low-condensation diesel increased EC and OC emissions by 2.5 and 2.6 times compared to normal diesel fuel, respectively. Indicators of vehicular sources, including EC, As, lead (Pb), cadmium (Cd), chromium (Cr), nickel (Ni), and manganese (Mn), increased by approximately 20.2–162.5% when using low-condensation diesel. Seasonal variation of vehicular source indicators, observed at road site ambient environments revealed the enhancement of PM2.5 pollution by the application of low-condensation diesel in winter. These findings suggest that −35# diesel, a low-cetane index diesel, may enhance air pollution in winter, according to a dynamometer test conducted in laboratory. It raises questions about whether higher emissions are released if −35# diesel is applied to running vehicles in real-world cold ambient environments.

The response to climate change and air pollution control demonstrates strong synergy across scientific mechanisms, targets, strategies, and governance systems. This report, based on a monitoring indicator system for coordinated governance of air pollution and climate change, employs an interdisciplinary approach combining natural and social sciences. It establishes 20 indicators across five key areas: air pollution and climate change, governance systems and practices, structural transformation and technologies, atmospheric components and emission reduction pathways, and health impacts and co-benefits. This report tries to provide actionable insights into the interconnectedness of air pollution and climate governance. It highlights key policy gaps, presents updated indicators, and offers a refined monitoring framework to track progress toward China's dual goals of reducing emissions and improving air quality. Compared to previous editions, this year's report has updated four key indicators: meteorological impacts on air quality, climate change and its effects, governance policies, and low-carbon building energy systems. The aim is to further refine the monitoring framework, track progress, and establish a comprehensive theory for collaborative governance while identifying challenges and proposing solutions for China's pathway to carbon neutrality and clean air. The report comprises six chapters. The executive summary chapter is followed by analyzing air pollution and climate change interactions. Governance systems and practices are discussed in the third chapter, focusing on policy implementation and local experiences. The fourth chapter addresses structural transformations and emission reduction technologies, including energy and industrial shifts, transportation, low-carbon buildings, carbon capture and storage, and power systems. The fifth chapter outlines atmospheric component dynamics and emission pathways, presenting insights into emission drivers and future strategies. The sixth chapter assesses health impacts and the benefits of coordinated actions. Since 2019, China Clean Air Policy Partnership has produced annual reports on China's progress in climate and air pollution governance, receiving positive feedback. In 2023, the report was co-developed with Tsinghua University's Carbon Neutrality Research Institute, involving over 100 experts and multiple academic forums. The collaboration aims to continuously improve the indicator system and establish the report as a key resource supporting China's efforts in pollution reduction, carbon mitigation, greening, and sustainable growth.

Nitrogen oxides (NOx), significant contributors to air pollution and climate change, form aerosols and ozone in the atmosphere. Accurate, timely, and transparent information on NOx emissions is essential for decision-making to mitigate both haze and ozone pollution. However, a comprehensive understanding of the trends and drivers behind anthropogenic NOx emissions from China—the world's largest emitter—has been lacking since 2020 due to delays in emissions reporting. Here we show a consistent decline in China's NOx emissions from 2020 to 2022, despite increased fossil fuel consumption, utilizing satellite observations as constraints for NOx emission estimates through atmospheric inversion. This reduction is corroborated by data from two independent spaceborne instruments: the TROPOspheric Monitoring Instrument (TROPOMI) and the Ozone Monitoring Instrument (OMI). Notably, a reduction in transport emissions, largely due to the COVID-19 lockdowns, slightly decreased China's NOx emissions in 2020. In subsequent years, 2021 and 2022, reductions in NOx emissions were driven by the industry and transport sectors, influenced by stringent air pollution controls. The satellite-based inversion system developed in this study represents a significant advancement in the real-time monitoring of regional air pollution emissions from space.

Wastewater treatment is an important source of greenhouse gases (GHGs). Yet large uncertainties remain in the quantification of GHG emissions from municipal wastewater treatment plants (MWWTPs) in China. A high-resolution and technology-specific emission inventory is still lacking to support mitigation strategies of MWWTPs. Here we develop a plant-level and technology-based MWWTP emission inventory for China covering 8703 plants and 19 treatment technology categories by compiling and harmonizing the most up-to-date facility-level databases. China's methane (CH4) and nitrous oxide (N2O) emissions from MWWTPs in 2020 are estimated to be 150.6 Gg and 22.0 Gg, respectively, with the uncertainty range of −30% to 37% and −30% to 26% at 95% confidence interval. We find an emission inequality across cities, with the richest cities emitting two times more CH4 and N2O per capita from municipal wastewater treatment than the poorest cities. The emitted CH4 and N2O are dominated by Anaerobic/Anoxic/Oxic-, Sequencing Batch Reactor-, Oxidation Ditch-, and Anoxic/Oxic-based MWWTPs of less than 20 years old. Considering the relatively young age structure of China's MWWTPs, the committed emissions highlight the importance of reducing on-site GHG emissions by optimization of operating conditions and innovation management. The emission differences among our estimates, previous studies, and the Intergovernmental Panel on Climate Change guidelines are largely attributed to the uncertainties in emission factors, implying the urgent need for more plant-integrated measurements to improve the accuracy of emission accounting.

Abstract A transition away from coal power always maintains a high level of complexity as there are several overlapping considerations such as technical feasibility, economic costs, and environmental and health impacts. Here, we explore the cost-effectiveness uncertainty brought by policy implementation disturbances of different coal power phaseout and new-built strategies (i.e., the disruption of phaseout priority) in China based on a developed unit-level uncertainty assessment framework. We reveal the opportunity and risk of coal transition decisions by employing preference analysis. We find that, the uncertainty of a policy implementation might lead to potential delays in yielding the initial positive annual net benefits. For example, a delay of six years might occur when implementing the prior phaseout practice. A certain level of risk remains in the implementation of the phaseout policy, as not all strategies can guarantee the cumulative positive net benefits from 2018–2060. Since the unit-level heterogeneities shape diverse orientation of the phaseout, the decision-making preferences would remarkably alter the selection of a coal power transition strategy. More strikingly, the cost-effectiveness uncertainty might lead to missed opportunities in identifying an optimal strategy. Our results highlight the importance of minimizing the policy implementation disturbance, which helps mitigate the risk of negative benefits and strengthen the practicality of phaseout decisions.

Wind energy has become one of the most important measures for China to achieve its carbon neutrality goal. The spatial and temporal evolvement of economic competitiveness for wind energy becomes an important concern in shaping the decarbonization pathway in China. There has been an urgent need in power system planning to model the future dynamics of cost decline and supply potential for wind power in the context of carbon neutrality until 2060. Existing studies often fail to capture the rapid decline in the cost of wind power generation in recent years, and the prediction of wind power cost decline is more conservative than the reality. This study constructs an integrated model to evaluate the cost-competitiveness and grid parity potential of China's onshore wind electricity at fine spatial resolution with updated parameters. Results indicate that the total onshore wind potential amounts to 54.0 PWh. The average levelized cost of wind power is expected to decline from CNY 0.39 kWh−1 in 2020 to CNY 0.30 and CNY 0.21 kWh−1 in 2030 and 2060. 28.3%, 67.6%, and 97.6% of the technical potentials hold power costs lower than coal power in 2020, 2030, and 2060.

China is now confronting the intertwined challenges of air pollution and climate change. Given the high synergies between air pollution abatement and climate change mitigation, the Chinese government is actively promoting synergetic control of these two issues. The Synergetic Roadmap project was launched in 2021 to track and analyze the progress of synergetic control in China by developing and monitoring key indicators. The Synergetic Roadmap 2022 report is the first annual update, featuring 20 indicators across five aspects: synergetic governance system and practices, progress in structural transition, air pollution and associated weather-climate interactions, sources, sinks, and mitigation pathway of atmospheric composition, and health impacts and benefits of coordinated control. Compared to the comprehensive review presented in the 2021 report, the Synergetic Roadmap 2022 report places particular emphasis on progress in 2021 with highlights on actions in key sectors and the relevant milestones. These milestones include the proportion of non-fossil power generation capacity surpassing coal-fired capacity for the first time, a decline in the production of crude steel and cement after years of growth, and the surging penetration of electric vehicles. Additionally, in 2022, China issued the first national policy that synergizes abatements of pollution and carbon emissions, marking a new era for China's pollution-carbon co-control. These changes highlight China's efforts to reshape its energy, economic, and transportation structures to meet the demand for synergetic control and sustainable development. Consequently, the country has witnessed a slowdown in carbon emission growth, improved air quality, and increased health benefits in recent years.

Abstract With the decreasing regional-transported levels, the health risk assessment derived from fine particulate matter (PM2.5) has become insufficient to reflect the contribution of local source heterogeneity to the exposure differences. Here, we combined the both ultra-high-resolution PM2.5 concentration with population distribution to provide the personal daily PM2.5 internal dose considering the indoor/outdoor exposure difference. A 30-m PM2.5 assimilating method was developed fusing multiple auxiliary predictors, achieving higher accuracy (R2 = 0.78–0.82) than the chemical transport model outputs without any post-simulation data-oriented enhancement (R2 = 0.31–0.64). Weekly difference was identified from hourly mobile signaling data in 30-m resolution population distribution. The population-weighted ambient PM2.5 concentrations range among districts but fail to reflect exposure differences. Derived from the indoor/outdoor ratio, the average indoor PM2.5 concentration was 26.5 μg/m3. The internal dose based on the assimilated indoor/outdoor PM2.5 concentration shows high exposure diversity among sub-groups, and the attributed mortality increased by 24.0% than the coarser unassimilated model.

Abstract The global surge in solar photovoltaic (PV) power has featured spatial specialization from manufacturing to installation along its industrial chain. Yet how to improve PV climate benefits are under-investigated. Here we explore the evolution of net greenhouse gas (GHG) mitigation of PV industry from 2009–2060 with a spatialized-dynamic life-cycle-analysis. Results suggest a net GHG mitigation of 1.29 Gt CO2-equivalent from 2009–2019, achieved by 1.97 Gt of mitigation from installation minus 0.68 Gt of emissions from manufacturing. The highest net GHG mitigation among future manufacturing-installation-scenarios to meet 40% global power demand in 2060 is as high as 204.7 Gt from 2020–2060, featuring manufacturing concentrated in Europe and North America and prioritized PV installations in carbon-intensive nations. This represents 97.5 Gt more net mitigation than the worst-case scenario, equivalent to 1.9 times 2020 global GHG emissions. The results call for strategic international coordination of PV industrial chain to increase GHG net mitigation.

Abstract The toughest‐ever clean air actions to date in China have helped to reduce national air pollutant emissions significantly in recent years. However, the heterogeneous mitigation paths and their determinated factors among regions were less concerned. To direct regional mitigation strategies more efficiently, we compiled a time‐series emission inventory of Chinese 30 provinces for SO2, NOx, primary PM2.5 and CO2 at detailed socioeconomic sectors, and then used spatial‐temporal index decomposition analysis to evaluate the driving forces for changes in regional pollutant emissions and emission intensities from 2012 to 2017 and from 2017 to 2020, separately. Our results showed that end‐of‐pipe control measures dominated the decreasing trend of China's pollutant emissions (38%–68% for SO2, NOx, and primary PM2.5), but their relative dominance diminished gradually, and the effects from energy policy emerged, even outweighing the former in southern areas during the second phase. Though driven by different factors, the chemistry, nonmetal, metal, energy and residential sectors dominated regional reduction trend and with varying degrees. Clean air action led to the convergence of regional emission intensity, while optimizing and upgrading industrial structure and improving energy efficiency is still necessary for further reduction in northern and western areas at present.

It is well recognized that carbon dioxide and air pollutants share similar emission sources so that synergetic policies on climate change mitigation and air pollution control can lead to remarkable co-benefits on greenhouse gas reduction, air quality improvement, and improved health. In the context of carbon peak, carbon neutrality, and clean air policies, this perspective tracks and analyzes the process of the synergetic governance of air pollution and climate change in China by developing and monitoring 18 indicators. The 18 indicators cover the following five aspects: air pollution and associated weather-climate conditions, progress in structural transition, sources, inks, and mitigation pathway of atmospheric composition, health impacts and benefits of coordinated control, and synergetic governance system and practices. By tracking the progress in each indicator, this perspective presents the major accomplishment of coordinated control, identifies the emerging challenges toward the synergetic governance, and provides policy recommendations for designing a synergetic roadmap of Carbon Neutrality and Clean Air for China.

Product trade plays an increasing role in relocating production and the associated air pollution impact among sectors and regions. While a comprehensive depiction of atmospheric pollution redistribution through trade chains is missing, which may hinder targeted clean air cooperation among sectors and regions. Here, we combined five state-of-the-art models from physics, economy, and epidemiology to track the anthropogenic fine particle matters (PM2.5) related premature mortality along the supply chains within China in 2017. Our results highlight the key sectors that affect PM2.5-related mortality from both production and consumption perspectives. The consumption-based effects from food, light industry, equipment, construction, and services sectors, caused 2–22 times higher deaths than those from a production perspective and totally contributed 63% of the national total. From a cross-boundary perspective, 25.7% of China's PM2.5-related deaths were caused by interprovincial trade, with the largest transfer occurring from the central and northern regions to well-developed east coast provinces. Capital investment dominated the cross-boundary effect (56% of the total) by involving substantial equipment and construction products, which greatly rely on product exports from regions with specific resources. This supply chain-based analysis provides a comprehensive quantification and may inform more effective joint-control efforts among associated regions and sectors from a health risk perspective.

The Chishui River Valley is a microscale ecology-preservation area with industrial clusters. The significance of evaluating vehicular emissions has been gradually highlighted with the rapid development of the local transportation and tourism sectors. This study provides the first estimates based on both bottom-up and top-down approaches. The annual total emissions of CO, NOx, hydrocarbons (HC), PM10, and PM2.5 in 2019 were 347.8, 189.6, 46.3, 6.9, and 6.3 Mg, respectively. Trucks contributed the most (55%) to the NOx emissions, followed by heavy-duty passenger vehicles (26%). In contrast, light-duty passenger vehicles and motorcycles generated 75% of the HC emissions. The superior accuracy of highly spatial and temporal bottom-up estimates versus top-down estimates is validated by the similar variation trends of hourly emission intensities and enhanced concentrations relative to background observations for both NOx and CO, with Pearson correlation coefficients between the intensities and concentrations ranging from 0.79 to 0.85. Historical HC emissions peaked in 2013, followed by a sharp decline in 2014 and a continuous rise since then, whereas NOx emissions have kept increasing since 2010. These indicated the necessity and urgency of effective vehicular emission mitigations. Based on scenario analysis, traffic restrictions combined with upgrading the emission standards of admitted vehicles will possess huge emission reduction potentials. Future recommendations about establishing a low emission zone in the valley and supporting policies were introduced.

China’s clean air action stimulated a net accumulative reduction of 2.43 Gt CO2 emission from 2013-2020. Phase-out and upgrades of outdated, polluting, and inefficient combustion facilities have promoted the transition of the country’s energy system.

Abstract Globally consistent measurements of airborne metal concentrations in fine particulate matter (PM2.5) are important for understanding potential health impacts, prioritizing air pollution mitigation strategies, and enabling global chemical transport model development. PM2.5 filter samples (N ~ 800 from 19 locations) collected from a globally distributed surface particulate matter sampling network (SPARTAN) between January 2013 and April 2019 were analyzed for particulate mass and trace metals content. Metal concentrations exhibited pronounced spatial variation, primarily driven by anthropogenic activities. PM2.5 levels of lead, arsenic, chromium, and zinc were significantly enriched at some locations by factors of 100–3000 compared to crustal concentrations. Levels of metals in PM2.5 and PM10 exceeded health guidelines at multiple sites. For example, Dhaka and Kanpur sites exceeded the US National Ambient Air 3-month Quality Standard for lead (150 ng m−3). Kanpur, Hanoi, Beijing and Dhaka sites had annual mean arsenic concentrations that approached or exceeded the World Health Organization’s risk level for arsenic (6.6 ng m−3). The high concentrations of several potentially harmful metals in densely populated cites worldwide motivates expanded measurements and analyses.

The COVID-19 pandemic has stopped many human activities, which has had significant impact on emissions of greenhouse gases. Here, the authors present daily estimates of country-level CO2 emissions for different economic sectors and show that there has been a 8.8% decrease in global CO2 emissions in the first half of 2020.

Understanding the compositions and evolution of atmospheric organic vapors is crucial for exploring their impact on air quality. However, the molecular and seasonal characteristics of organic vapors in urban areas, with complex anthropogenic emissions and high variability, remain inadequately understood. In this study, we conducted measurements in urban Beijing during 2021-2022 covering four seasons using a Vocus-PTR, an improved Proton Transfer Reaction-Mass Spectrometry (PTR-MS). During the measurement period, a total of 895 peaks are observed, and 543 of them can be assigned to formulas. The contribution of CxHyOz species is most significant, which compose up to 53.7% of the number and 76.0% of the mass of total organics. With enhanced sensitivity and mass resolution, various sub-ppt level species and organics with multiple oxygens (≥3) were discovered. When counting the species number, 42.2% of the organics measured are at sub-ppt level and 37.8% of the species contain 3-8 oxygens. Organic vapors with multiple oxygens mainly consist of intermediate volatility and semi-volatile compounds, and many of them are found to be the multi-generational oxidation products of various volatile organic precursors. In summer, the fast photooxidation process generates organic vapors with multiple oxygens, and leads to an increase in both their concentration and proportion. While in other seasons, the variations of organic vapors with multiple oxygens are closely correlated with those of organic vapors with 1-2 oxygens, which could be heavily influenced by primary emissions. Organic vapors with low oxygen content (≤ 2 oxygens) are comparable to the results obtained by traditional PTR-MS measurements in both urban Beijing and neighboring regions. [ABSTRACT FROM AUTHOR]

Air pollution can affect people’s emotional status and well-being. Here, the authors simulate fixed-scene images to show that under the atmospheric conditions in Beijing, negative emotions occur when air quality index of PM2.5 increases to approximately 150.

Considering air pollution-induced health risks from a consumption perspective is important. Here the authors evaluated the premature deaths resulting from household consumption across 30 Chinese provinces and find that rural households can cause a similar number of pollution-induced deaths as urban households despite a larger and wealthier urban population, due to the combustion of solid fuel.

To improve air quality, China has been implementing strict clean air policies since 2013. These policies not only substantially improved air quality but may also modify the spatial distribution of air pollution, since urban emission sources were under stricter control and some were moved to rural regions with lower air quality improvement targets and lacking of monitoring. Here, we predicted satellite-based monthly PM2.5 concentrations during 2000–2018 at a 1-km resolution with complete spatial-temporal coverage to analyze changes in the spatial pattern of PM2.5 pollution in China. We found that the PM2.5 concentration in urban regions was higher than that in rural regions of the same city by an average of 3.3 μg/m3 during 2000–2018. This urban-rural disparity in PM2.5 concentration significantly increased from 2.5 μg/m3 in 2000 and peaked in 2007 of 3.8 μg/m3, then it sharply declined by 49% during 2013–2018 with the implementation of clean air policies. This shrinkage in the urban-rural PM2.5 gap was partly due to the 1.3 μg/m3 greater average decrease in the PM2.5 level in the urban region than in the rural region of the same town during 2013–2018 on average. We also observed that cities that started monitoring earlier experienced greater decreases in the urban-rural PM2.5 difference, and regions surrounding monitor showed significantly greater PM2.5 decrease than regions far away from monitor during 2013–2018. Additionally, clean air policies modified the relationship between PM2.5 concentrations and per capita gross domestic product (GDP), leading to a lower PM2.5 level with the same per capita GDP after 2013. Emissions in rural and suburban regions should be considered to further improve air quality in China.

A second wave of coronavirus disease 2019 (COVID-19) infections emerged in Beijing in summer 2020, which provided an opportunity to explore the response of air pollution to reduced human activity. Proton-transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) coupled with positive matrix factorization (PMF) source apportionment were applied to evaluate the pollution pattern and capture the detailed dynamic emission characteristics of volatile organic compounds (VOCs) during the representative period, with the occurrence of O _3 pollution episodes and the Beijing resurgence of COVID-19. The level of anthropogenic VOC was lower than during the same period in previous years due to the pandemic and emission reduction measures. More than two thirds of the days during the observation period were identified as high-O _3 days and VOCs exhibited higher mixing ratios and faster consumption rates in the daytime on high-O _3 days. The identified VOC emission sources and the corresponding contributions during the whole observation period included: vehicle + fuel (12.41 ± 9.43%), industrial process (9.40 ± 8.65%), solvent usage (19.58 ± 13.46%), biogenic (6.03 ± 5.40%), background + long-lived (5.62 ± 11.37%), and two groups of oxygenated VOC (OVOC) factors (primary emission and secondary formation, 26.14 ± 15.20% and 20.84 ± 14.0%, respectively). Refined dynamic source apportionment results show that the ‘stay at home’ tendency led to decreased emission (−34.47 ± 1.90%) and a weakened morning peak of vehicle + fuel during the Beijing resurgence. However, a growing emission of primary OVOCs (+51.10 ± 8.28%) with similar diurnal variation was observed in the new outbreak and afterwards, which might be related to the enhanced usage of products intended to clean and disinfect. The present study illustrated that more stringent VOC reduction measures towards pandemic products should be carried out to achieve the balanced emission abatement of NO _x and VOC when adhering to regular epidemic prevention and control measures.

A Correction to this paper has been published: https://doi.org/10.1038/s41467-020-20254-5

Fugitive particle emission of enclosed storage yards in iron and steel plant is a complicated and multivariable problem, which will have negative impacts on the environment and economy. Researchers have discussed methodologies of emission estimation in open storage yards, but rarely focused on enclosed ones. However, enclosed storage yards are commonly adopted in most industries in China. This paper links onsite observation and computational fluid dynamics (CFD) to estimate the impact of fugitive PM10 emissions from enclosed storage yards on the open air. By collecting and analyzing PM10 samples at three sites inside the yard and one site outside, The result shows that PM10 concentration is in the range of 7.3 ± 1.5~13.4 ± 4.2 mg/m3, which is extremely high in an enclosed storage yard, and significantly influences workers’ health inside and outside atmospheric aerosols. The CFD model simulation is conducted by considering particle deposition, particle emission sources of shovel loader and road dust emission, as well as different wind direction and wind speed. The result shows that PM10 discharge rate from the enclosed area to open-air is significantly influenced by wind velocity and direction, e.g., the result of northwest wind with wind speed in 12.7 m/s is eight times higher than wind speed in 2.5 m/s with the same wind direction, and are 47 and 62 times higher than the east and west wind direction with the same wind speed in 12.7 m/s, respectively. In this case, the PM10 discharge rate is about 131.7 ton/year, which contains about 38~55 ton/year iron-relating particles. This will directly contribute PM10 to open-air and may produce secondary aerosols, due to heterogeneous catalytic reaction. This work identifies the important contribution of fugitive emissions and provides an approach for fugitive emission estimation of industries to the surrounding air. The results provide a reference for material yard zoning and fugitive emission control from minimizing influence from the meteorological condition and reducing source discharge inside.

Revealing the determinants and associated impacts of the transboundary pollution caused by trade is a critical issue when promoting the joint control among regions. This interdisciplinary study utilizes physical, economic and epidemiological methods to explore the anthropogenic PM _2.5 related mortality driven by interprovincial trade within China and its determinants. The results showed that 68% of the mortality flow in China was from the central and north plain area, with 29% occurring within these regions and 39% flowing to other eastern and western provinces. The high death intensity resulting from higher exports of heavily polluted agricultural and heavy industry products dominated the trade surplus of PM _2.5 mortality for the central and northern plains of China; these bring an imbalanced economic return for these regions, with only 43% of the value added generated in interprovincial trade being retained in these regions. Our study provides a more comprehensive picture of how atmospheric pollution deaths were caused by domestic trade within China, which may facilitate the multilateral pollution mitigation actions from an environmentally economic balanced perspective.

It is increasingly recognized that various chemical components of PM _2.5 might have differential toxicities to human health, although such studies are hindered by the sparse or non-existent coverage of ground PM _2.5 speciation monitors. The Multi-angle Imaging SpectroRadiometer (MISR) onboard the Terra satellite has an innovative design to provide information about aerosol shape, size and extinction that are more related to PM _2.5 speciation concentrations. In this study, we developed random forest models that incorporated ground measurements of PM _2.5 species, MISR fractional AODs, simulated PM _2.5 speciation concentrations from a chemical transport model (CTM), land use variables and meteorological fields, to predict ground-level daily PM _2.5 sulfate, nitrate, organic carbon (OC) and elemental carbon (EC) concentrations in California between 2005 and 2014. Our models had out-of-bag R ^2 of 0.72, 0.70, 0.68 and 0.70 for sulfate, nitrate, OC and EC, respectively. We also conducted sensitivity tests to explore the influence of variable selection on model performance. Results show that if there are sufficient ground measurements and predictor data to support the most sophisticated model structure, fractional AODs and total AOD have similar predicting power in estimating PM _2.5 species. Otherwise, models using fractional AODs outperform those with total AOD. PM _2.5 speciation concentrations are more sensitive to land use variables than other supporting data (e.g., CTM simulations and meteorological information).

Daily PM2.5 mass concentrations and chemical compositions together with the aerosol optical properties were measured from 8–28 November 2011 in Beijing. PM2.5 mass concentration varied from 15.6–237.5 μg∙m−3 and showed a mean value of 111.2 ± 73.4 μg∙m−3. Organic matter, NH4NO3 and (NH4)2SO4 were the major constituents of PM2.5, accounting for 39.4%, 15.4%, and 14.9% of the total mass, respectively, while fine soil, chloride salt, and elemental carbon together accounted for 27.7%. Daily scattering and absorption coefficients (σsc and σap) were in the range of 31.1–667 Mm−1 and 8.24–158.0 Mm−1, with mean values of 270 ± 200 Mm−1 and 74.3 ± 43.4 Mm−1. Significant increases in σsc and σap were observed during the pollution accumulation episodes. The revised IMPROVE algorithm was applied to estimate the extinction coefficient (bext). On average, organic matter was the largest contributor, accounting for 44.6% of bext, while (NH4)2SO4, NH4NO3, elemental carbon, and fine soil accounted for 16.3% 18.0%, 18.6%, and 2.34% of bext, respectively. Nevertheless, the contributions of (NH4)2SO4 and NH4NO3 were significantly higher during the heavy pollution periods than those on clean days. Typical pollution episodes were also explored, and it has been characterized that secondary formation of inorganic compounds is more important than carbonaceous pollution for visibility impairment in Beijing.