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There has been a continuously growing trend in international commercial air traffic, with the exception of COVID-19 crises; however, after the recovery, the trend is expected to even sharpen. The consequences of released emissions and by-products in the environment range from human health hazards, low air quality and global warming. This study is aimed to investigate the role of aviation emissions in global warming. For this purpose, data on different variables including global air traffic and growth rate, air traffic in different continents, total global CO 2 emissions of different airlines, direct and indirect emissions, air traffic in various UK airports and fuel-efficient aircraft was collected from various sources like EU member states, Statista, Eurostat, IATA, CAA and EUROCONTROL. The results indicated that in 2019, commercial airlines carried over 4.5 × 10 9 passengers on scheduled flights. However, due to the COVID-19 pandemic in 2020, the global number of passengers was reduced to 1.8 × 10 9 , representing around a 60% reduction in air traffic. Germany was the largest contributor to greenhouse gas (GHG) from the EU, releasing 927 kt of emissions in 3 years. In the UK, Heathrow airport had the highest number of passengers in 2019 with over 80 million, and the study of monthly aircraft movement revealed that Heathrow Airport also had the highest number of EU and International flights, while Edinburgh had the domestic flights in 2018. These research findings could be beneficial for airlines, policymakers and governments targeting the reduction of aircraft emissions. Graphical abstract

Purpose of Review Fine particulate matter (PM 2.5 ) and ground-level ozone (O 3 ) pose a significant risk to human health. The World Health Organization (WHO) has recently revised healthy thresholds for both pollutants. The formation and evolution of PM 2.5 and O 3 are however governed by complex physical and multiphase chemical processes, and therefore, it is extremely challenging to mitigate both pollutants simultaneously. Here, we review mechanisms and discuss the science-informed pathways for effective and simultaneous mitigation of PM 2.5 and O 3 . Recent Findings Global warming has led to a general increase in biogenic emissions, which can enhance the formation of O 3 and secondary organic aerosols. Reductions in anthropogenic emissions during the COVID-19 lockdown reduced PM 2.5 ; however, O 3 was enhanced in several polluted regions. This was attributed to more intense sunlight due to low aerosol loading and non-linear response of O 3 to NO x . Such contrasting physical and chemical interactions hinder the formulation of a clear roadmap for clean air over such regions. Summary Atmospheric chemistry including the role of biogenic emissions, aerosol-radiation interactions, boundary layer, and regional-scale transport are the key aspects that need to be carefully considered in the formulation of mitigation pathways. Therefore, a thorough understanding of the chemical effects of the emission reductions, changes in photolytic rates and boundary layer due to perturbation of solar radiation, and the effect of meteorological/seasonal changes are needed on a regional basis. Statistical emulators and machine learning approaches can aid the cumbersome process of multi-sector multi-species source attribution.

Tropospheric ozone is a key air pollutant and greenhouse gas. Its fate strongly depends on meteorological conditions and therefore subject to climate change influences. Such dependences through biogenic, chemical, and dynamic processes on different spatiotemporal scales have been unraveled from observations and modeling studies. In this process-oriented review, we summarize three dominant pathways of meteorological and climatic impacts on tropospheric ozone and present their recent progress. The three pathways are influences through changes in the natural precursor emissions, the kinetics and partitioning of chemistry and deposition, and the transport of ozone and its precursors. Tropospheric ozone levels have shown significant global or regional responses to meteorological/climatic changes (e.g., changes in the Brewer-Dobson Circulation, the Hadley Circulation, and El Niño–Southern Oscillation) and can be explained through the conjunction of these pathways. Most recent model projections predict that future climate will increase surface ozone in polluted regions and decrease ozone at a global scale due to stronger ozone chemical loss. However, uncertainties in climate-ozone responses and limitations in model capability still challenge the magnitude and even the sign of such projections. We highlight the rising importance of future increase of stratosphere-troposphere exchange in modulating tropospheric ozone that may largely compensate the predicted chemical loss of tropospheric ozone burden. We also highlight that uncertainties in isoprene chemistry, biogenic emissions in changing CO 2 levels and vegetation, and interactions between ozone and vegetation may largely affect the surface ozone response to climate change. Future research and model improvements are required to fill these gaps.

Purpose of Review The purpose of this review is to summarize the current understandings of atmospheric VOC characteristics in China and put forward the methodological drawbacks of the VOC measurement that need to be resolved and the research gaps that need to be filled. Recent Findings Whereas in recent investigations in the North China Plain (NCP) a reduction (20–66%) in total VOC concentration is noticed compared with the ones published before 2015, an increase (13–127%) is observed for the Yangtze River Delta (YRD) region. Aromatics and oxygenated VOCs are frequently appearing as the most abundant VOC group in recent investigations. Industry-related VOC sources are more dominant in the YRD regions while vehicle-related sources are more influential in the NCP, Central China, and Pearl River Delta regions. Benzene, 1,3,5-trimethylbenzene, ethylbenzene, naphthalene, dichloromethane, 1,2-dichloroethane, 1,2-dichloropropane, chloroform, carbon tetrachloride, and 1,2-dibromoethane pose carcinogenic risk to exposed population in China and the most risk-prone areas are affected by the petrochemical industry, biomass burning, waste management, and vehicle emissions. Formaldehyde and toluene have relatively high concentrations among the different indoor VOCs observed and their concentrations noticed to be exceeded the national air quality standard. Summary More investigations have to be performed on rarely studied health risk assessment of VOCs and characterization of indoor VOCs. BVOC studies are rarely conducted in China, which has to be performed on common plant species, different forest, and agricultural crops. VOC characterization in forest fire smokes and more process-specific emission characteristics in common industries need to be conducted.

Purpose of Review During the past decades, the number and size of megacities have been growing dramatically in China. Most of Chinese megacities are suffering from heavy PM 2.5 pollution. In the pollution formation, the planetary boundary layer (PBL) plays an important role. This review is aimed at presenting the current state of understanding of the PBL-PM 2.5 interaction in megacities, as well as to identify the main gaps in current knowledge and further research needs. Recent Findings The PBL is critical to the formation of urban PM 2.5 pollution at multiple temporal scales, ranging from diurnal change to seasonal variation. For the essential PBL structure/process in pollution, the coastal megacities have different concerns from the mountainous or land-locked megacities. In the coastal cities, the recirculation induced by sea-land breeze can accumulate pollutants, whereas in the valley/basin, the blocking effects of terrains can lead to stagnant conditions and thermal inversion. Within a megacity, although the urbanization-induced land use change can cause thermodynamic perturbations and facilitate the development of PBL, the increases in emissions outweigh this impact, resulting in a net increase of aerosol concentration. Moreover, the aerosol radiative effects can modify the PBL by heating the upper layers and reducing the surface heat flux, suppressing the PBL and exacerbating the pollution. Summary This review presented the PBL-PM 2.5 interaction in 13 Chinese megacities with various geographic conditions and elucidated the critical influencing processes. To further understand the complicated interactions, long-term observations of meteorology and aerosol properties with multi-layers in the PBL need to be implemented.

Purpose of Review The direct radiative effect of brown carbon (BrC) absorption predicated by climate-modeling studies is highly uncertain, with values ranging between +0.03 W/m 2 and + 0.57 W/m 2 . This review strives to identify sources of this uncertainty stemming from challenges in translating measurements into model inputs and to draw lessons from recent advances that lead to improved BrC representation in models. Recent Findings Previously thought to absorb only short-visible and UV light, BrC was recently shown to comprise components that are strongly absorptive in the mid- and long-visible wavelengths, with light-absorption efficiencies approaching that of black carbon. The classic picture of biomass and biofuel combustion being the major sources of atmospheric BrC still holds, with recent measurements indicating a strong correlation between BrC optical properties and combustion conditions. Other combustion sources of BrC, currently not accounted for in models, include low-efficiency coal combustion and ship engines utilizing heavy fuel oil. Gas-phase, aqueous, and particle-phase reactions in the atmosphere produce secondary BrC and bleach/darken the primary BrC. Climate-modeling studies revealed that predicted BrC radiative effects are sensitive to the assumed optical properties and atmospheric aging mechanisms. Summary BrC can be grouped into four optical classes, each separated by an order of magnitude in mid-visible light absorption. The classes are approximately mapped to BrC sources, with secondary BrC being the least absorbing and BrC from high-temperature combustion the most absorbing. There is evidence that each class exhibits characteristic physicochemical properties (molecular size, volatility, and solubility), which can be leveraged to design measurements that quantify distributions of BrC across classes as well as rates of photobleaching/darkening for each class. Utilizing this framework to develop BrC parameterizations promises to enhance its representation in climate models.

Air pollution is one of major environmental issues in the twenty-first century due to global industrialization and urbanization. Its mitigation necessitates accurate air quality forecasts. However, current state-of-the-art air quality forecasts are limited from highly uncertain chemistry-transport models (CTMs), shallow statistical methods, and heterogeneous and incomplete observing networks. Recently, deep learning has emerged as a general-purpose technology to extract complex knowledge using massive amount of data and very large networks of neurons and thus has the potential to break the limits of air quality forecasts. Here, we provide a brief review of recent attempts on using deep learning techniques in air quality forecasts. We first introduce architectures of deep networks (e.g., convolutional neural networks, recurrent neural networks, long short-term memory neural networks, and spatiotemporal deep network) and their relevance to explore the nonlinear spatiotemporal features across multiple scales of air pollution. We then examine the potential of deep learning techniques for air quality forecasts in diverse aspects, namely, data gap filling, prediction algorithms, improvements of CTMs, estimations with satellite data, and source estimations for atmospheric dispersion forecasts. Finally, we point out some prospects and challenges for future attempts on improving air quality forecasts using deep learning techniques.

Purpose of Review Climate warming may bear a penalty on future ozone air quality, even in the absence of changes in anthropogenic activities. This penalty has important implications for policy-making, but its quantification involves complex meteorological, chemical, and biological processes and feedbacks that are not well understood. We examined how climate-sensitive processes may affect surface ozone, identified key knowledge gaps uncovered by recent studies, and summarized latest assessments of the climate change penalty on ozone air quality. Recent Findings Recent analyses have challenged earlier paradigms on how climate change may affect surface ozone. The widely accepted associations of high ozone events with stagnation and heat waves require re-examination. Emission responses of natural precursors to climate warming may be significantly modulated by CO 2 levels and ecosystem feedbacks, such that the direction of emission changes cannot be robustly determined at this time. Climate variability may drive fluctuations in surface ozone, which has implications for near-term air quality management. Recent studies have generally projected a climate change penalty on ozone air quality, although the magnitudes are smaller than those projected by earlier studies. Summary This review examined the latest understanding on the climate change penalty to surface ozone. Critical uncertainties are associated with the meteorological, chemical, and biological processes linking climate warming and ozone, and many of the known feedbacks are not yet included in models. Further research is needed to examine those processes in order to better quantify the climate change penalty on surface ozone to inform policy-making.

Background Air pollution is a global problem with PM 2.5 being one of the major pollutants causing many diseases. The concentrations of PM 2.5 are found to exceed the World Health Organization (WHO) standards especially in lower middle-income countries (LMICs) that house around 40% of the global population. Materials and Methods Studies conducted globally in the past 5 years (2015–2019) on health effects of PM2.5 were systematically reviewed to understand the current research gaps. For this systematic search, Web of Science and PubMed were used to obtain 247 articles. Results Systematic review of these studies revealed that PM 2.5 and other air pollutants have been found to be associated with increased mortality and morbidity due to respiratory, cardiovascular, cerebrovascular disorders and diabetes. However, most of the total studies (~ 69%) were carried out in the high-income countries (HICs) despite the fact that PM 2.5 concentrations are higher in the LMICs (annual mean exposure (2011–2017) of 48.42 µg/m 3 ) and lower in the HICs (annual mean exposure (2011–2017) of 20.02 µg/m 3 ). Therefore, the exposure response functions for mortality estimates associated with PM 2.5 and developed using the exposure data from the HICs will not have predictive value in the LMICs. Furthermore, very few studies relate chemical components and source apportionment of PM 2.5 to the associated toxicity. Conclusions More studies on morbidity and mortality associated with PM2.5 and its components are needed in LMICs for better estimation of the overall risks.

Purpose of Review The air quality management within urban street canyons can be improved by enhancing ventilation for dispersion of pollutants. The purpose of this review is to summarize effects of various impact factors on airflow and pollutant dispersion in urban street canyons. The relative intensity of different influence factors is reviewed, which should provide a useful comprehensive guide for modelling of these effects for urban developments. Recent Findings All reviewed numerical simulations, wind tunnel and outdoor scaled model experiments show that the various building heights and incoming airflow conditions could produce a clear influence on airflow and pollutant dispersion in urban street canyon. Outdoor scaled experiments have provided complex turbulent data and illustrated the complexity of airflow within urban street canyons, which would require comprehensive simulations to investigate the microclimate within these urban street canyons. Summary Impacts of thermal and/or wall heating conditions have been fully studied, while the impact of inflow variation, building height difference, model scale and the coupling effect of different factors are current hot topics for research. Building height difference and time-varying inflow conditions are factors of most significant influence, while tree planting, vehicle-induced turbulence, thermal and/or wall heat conditions have a relatively weak influence.