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The paper summarizes many of the scientific achievements of Professor Sergej S. Zilitinkevich (1936–2021). It first focuses on his basic and applied atmospheric boundary layer research contributions. It then reviews their applications within research and operational numerical weather prediction and air quality modeling, showing their contribution to solving modeling problems related to extremely-stable and -unstable boundary layers.

Turbulence is the key process transporting material and energy in the atmosphere. Furthermore, turbulence causes concentration fluctuations, influencing different atmospheric processes such as deposition, chemical reactions, formation of low-volatile vapours, formation of new aerosol particles and their growth in the atmosphere, and the effect of aerosol particles on boundary-layer meteorology. In order to analyse the connections, interactions and feedbacks relating those different processes require a deep understanding of atmospheric turbulence mechanisms, atmospheric chemistry and aerosol dynamics. All these processes will further influence air pollution and climate. The better we understand these processes and their interactions and associated feedback, the more effectively we can mitigate air pollution as well as mitigate climate forcers and adapt to climate change. We present several aspects on the importance of turbulence including how turbulence is crucial for atmospheric phenomena and feedbacks in different environments. Furthermore, we discuss how boundary-layer dynamics links to aerosols and air pollution. Here, we present also a roadmap from deep understanding to practical solutions.

Air pollution is estimated to contribute to approximately 7 million premature deaths, of which around 4.5 million deaths are linked to ambient (outdoor) air pollution. The deaths attributed to air pollution rank the highest in the Asian region, and thus, the implementation of the stricter World Health Organization (WHO) Global Air Quality Guidelines (AQGs) released on 22 September 2021 will generate the greatest health benefits in the Asian region. Here we present some key messages and recommendations at national, regional, and global levels to promote the strategies for implementation of the ambitious WHO 2021 AQGs in the Asian region.

The Enviro-HIRLAM (Environment - HIgh Resolution Limited Area Model) is seamless/ online integrated numerical weather prediction and atmospheric chemical transport modelling system. It is capable of simultaneous simulation of meteorology – atmospheric composition and downscaling/nesting for regional–subregional–urban scales. The research and development are focused on: multi-scale modelling up to fine resolution; improving parameterizations describing urban processes, boundary/surface layer structures; implementation of emissions, aerosol/chemistry mechanisms, aerosol feedback and interactions. The Enviro-components includes: gas-phase chemistry; aerosol microphysics and deposition processes; urban sublayer physics parameterisations; direct/indirect/combined aerosol feedbacks due to radiation; locally mass-conserving semi-Lagrangian numerical advection scheme; natural and anthropogenic emission inventories. The model has modules for pre-processing of the ECMWF’s initial/ boundary conditions for meteorology-chemistry-aerosols, observations for data assimilation, and selected emission inventories. The model has been developed through HPC projects such as Enviro-HIRLAM at CSC and Enviro-PEEX(Plus) at ECMWF, as well as other research projects.The research and development of Enviro-HILRAM and its application will be demonstrated on examples, where this model is used as a research tool for studies in domain of the Pan-Eurasian Experiment (PEEX; https://www.atm.helsinki.fi/peex) programme. The examples include: Integrated modelling for assessment of potential pollution regional atmospheric transport as result of accidental wildfires; Integrated modelling and analysis of influence of land cover changes on regional weather conditions/ patterns; High-resolution integrated urban environmental modelling with integration of the urban large-eddy simulation (PALM model) and meteorological simulations into a seamless modelling chain; Effects of spring air pollution and weather on Covid-19 infection/situation in Finland; Meteorology integration between seamless and trajectory (FLEXPART model) models; and others. The Enviro-HIRLAM model generated output provides valuable input (3D meteorology and atmospheric composition) to assessment studies, and it as can be integrated into GIS environment for further risk/ vulnerability/ consequences/ etc. estimation, and other studies.

While overall global warming with the causes and global processes connected to well-mixed CO2, and its impacts on global to continental scales are well understood with a high level of confidence, there are knowledge gaps concerning the impact of many other non-CO2radiative forcers leading to low confidence in the conclusions. This relates mainly to specific anthropogenic and natural precursor emissions of short-lived GHGs and aerosols and their precursors. These gaps and uncertainties also exist in their subsequent effects on atmospheric chemistry and climate, through direct emissions dependent on changes in e.g., agriculture production and technologies based on scenarios for future development as well as feedback of global warming on emissions, e.g., permafrost thaw. In addition to the atmospheric radiative forcing (gaseous or aerosols), albedo changes connected to land use and land cover can play a role, depending on the adaptation or mitigation measures included in different scenarios. The main goal of the EC Horizon Europe project FOCI (from the call HORIZON-CL5-2021-D1-01-0 Improved understanding of greenhouse gas fluxes and radiative forcers, including carbon dioxide removal technologies), is to assess the impact of key radiative forcers, where and how they arise, the processes of their impact on the climate system, to find and test an efficient implementation of these processes into global Earth System Models and into Regional Climate Models, eventually coupled with CTMs, and finally to use the tools developed to investigate mitigation and/or adaptation policies incorporated in selected scenarios of future development targeted at Europe and other regions of the world. We will develop new regionally tuned scenarios based on improved emissions to assess the effects of non-CO2 forcers. Mutual interactions of the results and climate services producers and other end-users will provide feedback for the specific scenarios preparation and potential application to support the decision-making, including climate policy.

An improved understanding of the land - atmosphere - ocean feedbacks and interactions is needed for the future strategies for sustainable development of the Arctic region. An understading of the Arctic climate system and the implementation of the related processes in climate models are required to provide advice for policy actions. In this presenation, we highlight key areas for Arctic research from the atmospheric, oceanic, cryospheric, and social perspectives, and summarize recent developments in a holistic understanding of the Arctic climate system carried out in in a frame of AASCO Arctic Science Collaboration project in 2020-2022. We also provide an outlook of the links between research and its societal impacts.

The global methane pledge paves a fresh, critical way toward carbon neutrality. However, it remains largely invisible and highly controversial due to the fact that planet-scale and plant-level methane retrievals have rarely been coordinated. This has never been more essential within the narrow window to reach the Paris target. Here we present a two-tiered spaceborne architecture to address this issue. Using this framework, we focused on the United States, China, the Middle East, and North Africa, and simultaneously uncovered methane-abundant regions and plumes. These include new super-emitters, potential leakages, and unprecedented multiple plumes in a single source. More importantly, this framework is shown to challenge official emission reports that possibly mislead estimates from global, regional, and site scales, particularly by missing super-emitters. Our results show that, in principle, the above framework can be extended to be multi-tiered by adding upcoming stereoscopic measurements and suitable artificial intelligence, and thus it is sufficiently versatile for immediate and future monitoring of the global methane pledge.

El fuego en la vegetación, incluida la aplicación del fuego en el uso de la tierra, en el cambio de uso de la tierra y los incendios forestales, afectan el funcionamiento del sistema terrestre e imponen amenazas importantes para la salud y la seguridad públicas. Este documento presenta el concepto de un sistema de alerta y evaluación de la contaminación causada por el humo del fuego en la vegetación (VFSP-WAS, por su sigla en inglés). Se presentan el fundamento científico del sistema y directrices para abordar los problemas del fuego en la vegetación y la contaminación por humo, indicándose los principales desafíos de la investigación. El artículo propone el establecimiento de centros regionales VFSP-WAS y describe ejemplos potenciales de este concepto VFSP-WAS de dos regiones (Sudeste de Asia y América del Norte) en donde los centros VFSP-WAS se asocian con Centros Regionales de Monitoreo de Fuego/Manejo de Fuego. O fogo na vegetação, incluindo a aplicação do fogo no uso da terra e na mudança de uso da terra, assim como os incêndios florestais, afetam o funcionamento do sistema terrestre e impõem ameaças significativas à saúde e segurança públicas. Este documento apresenta o conceito de um Sistema de Avaliação e Alerta de Poluição causada por Fumaça decorrente do Fogo na Vegetação (VFSP-WAS, na sigla em inglês). Apresenta-se o fundamento científico do sistema e diretrizes para abordar as questões de fogo na vegetação e poluição por fumaça, indicando-se os principais desafios para a pesquisa. O artigo propõe o estabelecimento de centros regionais VFSP-WAS e mostra exemplos potenciais do conceito VFSP-WAS em duas regiões (sudeste da Ásia e América do Norte), onde centros regionais VFSP-WAS trabalham em parceria com Centros Regionais de Monitoramento de Fogo/Manejo de fogo. Vegetation fires – including the application of fire in land use, land-use change and uncontrolled wildfire – affect the functioning of the Earth System and impose significant threats to public health and security. This paper presents the concept of a Vegetation Fire and Smoke Pollution Warning Advisory and Assessment System (VFSP-WAS). It describes the scientific rationale for the system and provides guidance for addressing the issues of vegetation fire and smoke pollution, including key research challenges. The paper proposes the establishment of VFSP-WAS regional centers and describes potential examples of this VFSP-WAS concept from two regions in (Southeast Asia and North America) where regional centers will partner with Regional Fire Monitoring/Fire Management Resource Centers.

The problem of air pollution in Almaty city in stagnant weather conditions is considered. Such conditions are so frequent, because the city is located at the foothills of the mountain range of Ile Alatau. The WRF-Chem complex model is used for the analysis and allows a high-detail simulation both of the atmospheric circulation and the transport of various pollutants, taking into account their chemical transformations. The results of modeling meteorological processes are compared with observational data. The main attention is paid to the calculation of the sulfur dioxide plume emitted by the pipes of the coal-fired CHP-2. The simulations showed that regional latitudinal wind, whose direction regularly changes to the opposite one, dominates in the northern part of the city atmosphere. During such periods and under unstable atmosphere stratification conditions, the plume dissipates and pollutants penetrate into the surface layer. In some areas, the concentration of SO2 significantly exceeds sanitary standards.

Short summary We summarize results during the last 5 years in the northern Eurasian region, especially from Russia, and introduce recent observations of the air quality in the urban environments in China. Although the scientific knowledge in these regions has increased, there are still gaps in our understanding of large-scale climate–Earth surface interactions and feedbacks. This arises from limitations in research infrastructures and integrative data analyses, hindering a comprehensive system analysis. The Pan-Eurasian Experiment (PEEX) Science Plan, released in 2015, addressed a need for a holistic system understanding and outlined the most urgent research needs for the rapidly changing Arctic-boreal region. Air quality in China, together with the long-range transport of atmospheric pollutants, was also indicated as one of the most crucial topics of the research agenda. These two geographical regions, the northern Eurasian Arctic-boreal region and China, especially the megacities in China, were identified as a “PEEX region”. It is also important to recognize that the PEEX geographical region is an area where science-based policy actions would have significant impacts on the global climate. This paper summarizes results obtained during the last 5 years in the northern Eurasian region, together with recent observations of the air quality in the urban environments in China, in the context of the PEEX programme. The main regions of interest are the Russian Arctic, northern Eurasian boreal forests (Siberia) and peatlands, and the megacities in China. We frame our analysis against research themes introduced in the PEEX Science Plan in 2015. We summarize recent progress towards an enhanced holistic understanding of the land–atmosphere–ocean systems feedbacks. We conclude that although the scientific knowledge in these regions has increased, the new results are in many cases insufficient, and there are still gaps in our understanding of large-scale climate–Earth surface interactions and feedbacks. This arises from limitations in research infrastructures, especially the lack of coordinated, continuous and comprehensive in situ observations of the study region as well as integrative data analyses, hindering a comprehensive system analysis. The fast-changing environment and ecosystem changes driven by climate change, socio-economic activities like the China Silk Road Initiative, and the global trends like urbanization further complicate such analyses. We recognize new topics with an increasing importance in the near future, especially “the enhancing biological sequestration capacity of greenhouse gases into forests and soils to mitigate climate change” and the “socio-economic development to tackle air quality issues”.

The Enviro-HIRLAM (Environment - HIgh Resolution Limited Area Model) is seamless/ online integrated numerical weather prediction and atmospheric chemical transport modelling system capable to simulate simultaneously meteorology – atmospheric composition on regional to subregional – urban scales.The main areas of the model research and development include: downscaling/ nesting for high resolutions; improved resolving boundary and surface layers structures; urbanization and sub-layer processes; improvement of advection schemes; integration of natural and anthropogenic emission inventories; implementation of gas-phase chemistry mechanisms, aerosol dynamics and microphysics, aerosol feedback and interactions mechanisms.The Enviro-components includes: gas-phase chemistry; aerosol microphysics with nucleation, coagulation, condensation of sulfate, mineral dust, sea-salt, black and organic carbon together with aerosols’ dry and wet deposition, sedimentation processes; parameterisations of urban sublayer with modifications of the interaction soil–biosphere–atmosphere scheme; sulfur cycle mechanism with dimethyl sulfide, sulfur dioxide and sulfate; radiation scheme improved to account explicitly for aerosol radiation interactions for aerosol subtypes; aerosol activation implemented in condensation-convection scheme with nucleation dependent on aerosol properties and ice-phase processes; locally mass-conserving semi-Lagrangian numerical advection scheme; natural and anthropogenic emission inventories.The Enviro-HIRLAM utilises extraction and pre-processing of initial/ boundary meteorology-chemistry-aerosol conditions and observations for data assimilation (from ECMWF’s ERA-5 & CAMS), pre-processing of selected emission inventories for anthropogenic and natural emissions. The latest version has been run on CRAY-XC30/40 and Atos BullSequana HPCs machines, and it has been developed through the research and HPC projects such as Enviro-HIRLAM at CSC and Enviro-PEEX & Enviro-PEEX(Plus) at ECMWF, as well as other research projects.The research, development and science education of the modelling system and its applications will be demonstrated on examples, where the Enviro-HIRLAM is used as a research tool for studies in domain of the Pan-Eurasian Experiment (PEEX; https://www.atm.helsinki.fi/peex) programme. Examples of such include: aspects of regional-subregional-urban downscaling with focus on metropolitan areas of St.Petersburg and Moscow; influence of dust transport from artificial tailing dumps and Cu-Ni smelters of the Kola Peninsula on pollution of environment and health of population; aerosol feedbacks and interactions at regional scale in the Arctic-boreal domain; evaluation of atmosphere-land-sea surfaces interactions, and in particular, heat-moisture exchange/ regime between these surfaces and for better understanding and forecasting of local meteorology in the Arctic; analysis of urban meteorology and atmospheric pollution with integrated approach to high-resolution numerical modelling; and others. The modelling output provides meteorology-chemistry related input to assessment studies for population and environment as well as can be integrated into GIS environment for further risk/vulnerability/consequences/etc. estimation, and other studies.The science education component for the model is also realised though short-term visits of young researchers, organization and carrying out research training weeks. The latest face-to-face trainings took place in Apr and Jun 2019 (Helsinki and Tyumen), and online in Nov-Dec 2021 (https://megapolis2021.ru).

Last year the scientific community lost a great scientist; leader in environmental turbulence and planetary boundary layer research; recipient of the 2019 International Meteorological Organization (IMO) Prize and many other science awards; leader of numerous international research projects; outstanding mentor, and dear friend, Professor Sergej Zilitinkevich.Among his numerous outstanding scientific achievements in the boundary layer theory, several theoretical results broadly used in the numerical weather prediction, climate, and air pollution modelling communities, in particular, should be mentioned:The Zilitinkevich formula for the depth of stably stratified PBLs is often called that depth scale by Sergej’s name, which indicates that his result is a truly classical one. The Zilitinkevich correction to the rate equation for the depth of a convectively mixed layer, and the resistance and the heat and mass transfer laws for geophysical turbulent flows are also widely known and used. The Zilitinkevich scale - a length scale of a rotational stratification turbulent mixing in stably stratified PBLs. Conceptual models of new types of atmospheric PBLs, i.e.,: conventionally neutral PBLs settled on the background of the strongly stable stratification typical of the free atmosphere are several times thinner than truly neutral PBLs settled in neutral stratification; and long-lived stable PBLs typical in winter time at high latitudes and affected by the stably stratified free atmosphere. Discovered and described by Zilitinkevich: the “weak turbulence regime,” typical of the free atmosphere, which determines the turbulent transport of energy and momentum and the diffusion of passive scalars. Non-local turbulent transport for BLM and the pollution dispersion aspects of the coherent structure of convective flows. These results have paved the way towards improved theories and parametrizations of boundary layers in many NWP, climate, and ACT models worldwide.Over the last few decades, Sergej Zilitinkevich was deeply concerned with general questions of the physical nature of geophysical (and astrophysical) turbulence. The classical view, pioneered by Kolmogorov, assumes a cascade process from large eddies towards small eddies and eventually to heat. This “chaos out of order” paradigm put forward for shear-generated non-stratified turbulence is shifted towards an “order out of chaos” paradigm more appropriate for real-world turbulence complicated by body forces, where small-scale motions can organize themselves and give rise to quasi-organized coherent structures at larger scales. Sergej made a remarkable contribution to this paradigm shift. He passionately addressed several fundamental issues, such as the: origin and transport properties of coherent motions, effect of buoyancy on turbulent transport, and maintenance of turbulence at strongly stable stratification. This promising and long-awaited scientific revolution in this area of research will allow for a better understanding of the nature of global pollution and climate change.In this presentation we analyze the scientific legacy of Sergej Zilitinkevich for further developments in boundary layer research and modelling.

A deep understanding of the land - atmosphere - ocean feedbacks and interactions is required to make impact on the sustainable and just development of the Arctic region. The science based knowledge of the Arctic environments would lead to improved mitigation and adaptation plans, sustainable services for the Arctic communities and stakeholders and to well targeted policy actions. At the same time with the science approach we need a process of making policies acceptable and normative to the people living in the Artic. AASCO – “Arena gap analysis of the existing Arctic science co-operations” highlights key areas for the Arctic interactions – feedbacks research from the atmospheric, oceanic, cryospheric and social perspectives, and summarizes the potential improvements stemming from the holistic understanding of the Arctic climate system. Furthermore, AASCO aims to provide an outlook and benefits of the bridges between other international approaches like Pan-Eurasian Experiment (PEEX) Program, University of Arctic network (U-Arctic), The Global Atmosphere Watch (GAW) Programme of WMO, Sustainable Arctic Observation Network (SAON) e.g it’s strategy process called “ROADS” and the Digital Belt and Road (DBAR) program and, in general, of bridges between research and society impact.

This chapter describes a new World Meteorological Organisation (WMO) approach to the provision of science-based services to assist the planning of safe, healthy, resilient and climate-friendly cities. The approach is outlined in full in the WMO Guidance on Integrated Urban Hydrometeorological, Climate and Environmental Services. Volume I, Concepts and Methodology publication. WMO Guidance for Urban Integrated Hydrometeorological, Climate and Environmental Services. Volume I: Concept and Methodology. WMO-No: 1234, https://library.wmo.int/doc_num.php?explnum_id=9903, 2019a, which has been approved for promotion and communication to WMO Members. Highlighted here are the underlying concepts and methods that underpin the services to help manage cities on a day-to-day basis, responding to short-term hazards but also to prepare for climate change are described. Heterogeneous dense observation networks, high-resolution forecasts, multi-hazard early warning systems, long-term climate prediction and service applications all working in combination are at the heart of such services, the implementation of which will support mitigation and adaptation strategies for achieving thriving climate resilient cities and promotion of the United Nations Sustainable Development Goals. To meet the needs of urban areas, the concept of Integrated Urban Hydrometeorological, Climate and Environmental Services (IUS) is embedded in a multidisciplinary approach that identifies the requisite partnerships, including research agencies, city governments, international organizations, and private sector stakeholders, to establish and sustain urban services. As this is an emerging and diverse service, the requirements are broad with considerable research, development, capacity building and cross-service integrations yet to be done. In addition to providing an overview of the concept, we set out the challenges and recommended path forward to develop and implement IUS.

Dust particles from high latitudes have a potentially large local, regional, and global significance to climate and the environment as short-lived climate forcers, air pollutants, and nutrient sources. Identifying the locations of local dust sources and their emission, transport, and deposition processes is important for understanding the multiple impacts of high-latitude dust (HLD) on the Earth's systems. Here, we identify, describe, and quantify the source intensity (SI) values, which show the potential of soil surfaces for dust emission scaled to values 0 to 1 concerning globally best productive sources, using the Global Sand and Dust Storms Source Base Map (G-SDS-SBM). This includes 64 HLD sources in our collection for the northern (Alaska, Canada, Denmark, Greenland, Iceland, Svalbard, Sweden, and Russia) and southern (Antarctica and Patagonia) high latitudes. Activity from most of these HLD sources shows seasonal character. It is estimated that high-latitude land areas with higher (SI ≥0.5), very high (SI ≥0.7), and the highest potential (SI ≥0.9) for dust emission cover >1 670 000 km2, >560 000 km2, and >240 000 km2, respectively. In the Arctic HLD region (≥60∘ N), land area with SI ≥0.5 is 5.5 % (1 035 059 km2), area with SI ≥0.7 is 2.3 % (440 804 km2), and area with SI ≥0.9 is 1.1 % (208 701 km2). Minimum SI values in the northern HLD region are about 3 orders of magnitude smaller, indicating that the dust sources of this region greatly depend on weather conditions. Our spatial dust source distribution analysis modeling results showed evidence supporting a northern HLD belt, defined as the area north of 50∘ N, with a “transitional HLD-source area” extending at latitudes 50–58∘ N in Eurasia and 50–55∘ N in Canada and a “cold HLD-source area” including areas north of 60∘ N in Eurasia and north of 58∘ N in Canada, with currently “no dust source” area between the HLD and low-latitude dust (LLD) dust belt, except for British Columbia. Using the global atmospheric transport model SILAM, we estimated that 1.0 % of the global dust emission originated from the high-latitude regions. About 57 % of the dust deposition in snow- and ice-covered Arctic regions was from HLD sources. In the southern HLD region, soil surface conditions are favorable for dust emission during the whole year. Climate change can cause a decrease in the duration of snow cover, retreat of glaciers, and an increase in drought, heatwave intensity, and frequency, leading to the increasing frequency of topsoil conditions favorable for dust emission, which increases the probability of dust storms. Our study provides a step forward to improve the representation of HLD in models and to monitor, quantify, and assess the environmental and climate significance of HLD.

Dust particles emitted from high latitudes (≥ 50° N and ≥ 40° S, including Arctic as a subregion ≥ 60° N), have a potentially large local, regional, and global significance to climate and environment as short-lived climate forcers, air pollutants and nutrient sources. To understand the multiple impacts of the High Latitude Dust (HLD) on the Earth systems, it is foremost to identify the geographic locations and characteristics of local dust sources. Here, we identify, describe, and quantify the Source Intensity (SI) values using the Global Sand and Dust Storms Source Base Map (G-SDS-SBM), for sixty-four HLD sources included in our collection in the Northern (Alaska, Canada, Denmark, Greenland, Iceland, Svalbard, Sweden, and Russia) and Southern (Antarctica and Patagonia) high latitudes. Activity from most of these HLD dust sources show seasonal character. The environmental and climatic effects of dust on clouds and climatic feedbacks, atmospheric chemistry, marine environment, and cryosphere-atmosphere feedbacks at high latitudes are discussed, and regional-scale modelling of dust atmospheric transport from potential Arctic dust sources is demonstrated. It is estimated that high latitude land area with higher (SI ≥ 0.5), very high (SI ≥ 0.7) and the highest potential (SI ≥ 0.9) for dust emission cover >1 670 000 km2, >560 000 km2, and >240 000 km2, respectively. In the Arctic HLD region, land area with SI ≥ 0.5 is 5.5 % (1 035 059 km2), area with SI ≥ 0.7 is 2.3 % (440 804 km2), and with SI ≥ 0.9 it is 1.1 % (208 701 km2). Minimum SI values in the north HLD region are about three orders of magnitude smaller, indicating that the dust sources of this region are highly dependable on weather conditions. In the south HLD region, soil surface conditions are favourable for dust emission during the whole year. Climate change can cause decrease of snow cover duration, retrieval of glaciers, permafrost thaw, and increase of drought and heat waves intensity and frequency, which all lead to the increasing frequency of topsoil conditions favourable for dust emission and thereby increasing probability for dust storms. Our study provides a step forward to improve the representation of HLD in models and to monitor, quantify and assess the environmental and climate significance of HLD in the future.

This review provides a community’s perspective on air quality research focussing mainly on developments over the past decade. The article provides perspectives on current and future challenges as well as research needs for selected key topics. While this paper is not an exhaustive review of all research areas in the field of air quality, we have selected key topics that we feel are important from air quality research and policy perspectives. After providing a short historical overview, this review focuses on improvements in characterising sources and emissions of air pollution, new air quality observations and instrumentation, advances in air quality prediction and forecasting, understanding interactions of air quality with meteorology and climate, exposure and health assessment, and air quality management and policy. In conducting the review, specific objectives were (i) to address current developments that push the boundaries of air quality research forward, (ii) to highlight the emerging prominent gaps of knowledge in air quality research and (iii) and to make recommendations to guide the direction for future research within the wider community. This review also identifies areas of particular importance for air quality policy. The original concept of this review was borne at the International Conference on Air Quality 2020 (held online due to the COVID 19 restrictions during 18–26 May 2020), but the article incorporates a wider landscape of research literature within the field of air quality science. On air pollution emissions the review highlights, in particular, the need to reduce uncertainties in emissions from diffuse sources, particulate matter chemical components, shipping emissions and the importance of considering both indoor and outdoor sources. There is a growing need to have integrated air pollution and related observations from both ground based and remote sensing instruments, including especially those on satellites. The research should also capitalize on the growing area of lower cost sensors, while ensuring a quality of the measurements which are regulated by guidelines. Connecting various physical scales in air quality modelling is still a continual issue, with cities being affected by air pollution gradients at local scales and by long range transport. At the same time, one should allow for the impacts from climate change on a longer timescale. Earth system modelling offers considerable potential by providing a consistent framework for treating scales and processes, especially where there are significant feedbacks, such as those related to aerosols, chemistry and meteorology. Assessment of exposure to air pollution should consider both the impacts of indoor and outdoor emissions, as well as apply more sophisticated, dynamic modelling approaches. With particulate matter being one of the most important pollutants for health, research is indicating the urgent need to understand, in particular, the role of particle number and chemical components in terms of health impact, which in turn requires improved emission inventories and models for predicting high resolution distributions of these metrics over cities. The review also examines, how air pollution management needs to adapt to the above-mentioned new challenges and briefly considers the implications from the COVID-19 pandemic for air quality. Finally, we provide recommendations for air quality research and support for policy.

In this study, the downscaling modeling chain for prediction of weather and atmospheric composition is described and evaluated against observations. The chain consists of interfacing models for forecasting at different spatiotemporal scales that run in a semi-operational mode. The forecasts were performed for European (EU) regional and Danish (DK) subregional-urban scales by the offline coupled numerical weather prediction HIRLAM and atmospheric chemical transport CAMx models, and for Copenhagen city-street scale by the online coupled computational fluid dynamics M2UE model. The results showed elevated NOx and lowered O3 concentrations over major urban, industrial, and transport land and water routes in both the EU and DK domain forecasts. The O3 diurnal cycle predictions in both these domains were equally good, although O3 values were closer to observations for Denmark. At the same time, the DK forecast of NOx and NO2 levels was more biased (with a better prediction score of the diurnal cycle) than the EU forecast, indicating a necessity to adjust emission rates. Further downscaling to the street level (Copenhagen) indicated that the NOx pollution was 2-fold higher on weekends and more than 5 times higher during the working day with high pollution episodes. Despite high uncertainty in road traffic emissions, the street-scale model effectively captured the NOx and NO2 diurnal cycles and the onset of elevated pollution episodes. The demonstrated downscaling system could be used in future online integrated meteorology and air quality research and operational forecasting, as well as for impact assessments on environment, population, and decision making for emergency preparedness and safety measures planning.

Today, every second person lives in a city, and urbanization is continuously increasing. For 2050, it is to be expected that 2 out of 3 people will live in a city and thus the vast majority of the world's population will be affected not only by global climate change but also by locally induced climatic changes. The canopy layer urban heat island (CL-UHI) is one of the most well-known meteorological characteristics of urban areas found in cities small and large around the world. Its characteristics differ between cities, across a city and with time. The climate change induced warming cities experience is additionally impacted by the CL-UHI.Despite the city-scale importance of CL-UHI, the WMO has not had any specific guidance on this. In response to the request of the 18th World Meteorological Congress (Resolutions 32 and 61) experts from WMO GAW (Global Atmosphere Watch) Urban Research Meteorology and Environment (GURME) initiated in 2020 preparation of a guidance on measuring, modelling and monitoring the CL-UHI. The guidance is a community-based development with 30 contributors providing expertise in all different aspects of CL-UHI. This includes a clear definition of what a CL-UHI is and clarifications of what it is not, how it develops (e.g. meteorological and morphological influences), methods to assess CL-UHI intensities (measurements, modelling approaches) as well as when its assessment (applications) is needed and how it can be reduced (or when it is beneficial).The presentation will specifically focus on the key questions addressed in the guidance: what a CL-UHI is and what it is not, where CL-UHI values are relevant for and the many challenges that exist in simulating the CL-UHI with different models.

The Pan-Eurasian Experiment (PEEX) Science Plan, released in 2015, addressed a need for a holistic system understanding and outlined the most urgent research needs for sustainable development in the Artic-boreal region. Air quality in China and long-range transport of the atmospheric pollutants was also indicated as one of the most crucial topics of the research agenda. This paper summarizes results obtained during the last five years in the Northern Eurasian region. It also introduces recent observations on the air quality in the urban environments in China. The main regions of interest are the Russian Arctic, Northern Eurasian boreal forests (Siberia) and peatlands and on the mega cities in China. We frame our analysis against research themes introduced in 2015. We summarize recent progress in the understanding of the land – atmosphere – ocean systems feedbacks. Although the scientific knowledge in these regions has increased, there are still gaps in our understanding of large-scale climate-Earth surface interactions and feedbacks. This arises from limitations in research infrastructures and integrative data analyses, hindering a comprehensive system analysis. The fast-changing environment and ecosystem changes driven by climate change, socio-economic activities like the China Silk Road Initiative, and the global trends like urbanization further complicate such analyses. We recognize new topics with an increasing importance in the near future, such as enhancing biological sequestration capacity of greenhouse gases into forests and soils to mitigate the climate change and the socio-economic development to tackle air quality issues.

The WMO World Weather Research Programme (WWRP) “promotes international and interdisciplinary research for more accurate and reliable forecasts from minutes to seasons, expanding the frontiers of weather science to enhance society’s resilience to high-impact weather and the value of weather information for users. In the 2016-2023 WWRP implementation plan, activities focus on 4 challenges: High-Impact Weather, Water, Urbanization, Evolving technologies. Furthermore, the WMO Global Atmosphere Watch Urban Research Meteorology and Environment (GURME) focus on the development of models and associated research activities to enhance the capabilities in providing urban-environmental forecasting and air quality services, illustrating the linkages between meteorology and air quality (https://public.wmo.int/en/programmes).This talk presents an international Research Demonstration Project (RDP), that will focus on international research on scientific urban issues addressed by both WWRP and GURME. The strategic objective of this RDP is to focus on the Olympic Games of Paris in 2024 in order to advance research on the theme of the “future Meteorological Forecasting systems at 100m (or finer) resolution for urban areas”. Such systems would prefigure the numerical weather prediction at the horizon 2030. The focus will be on themes related to extreme weather events in summer which both are influenced by and impacts urbanization: thunderstorms and strong Urban Heat Islands, and their consequences.There are 5 scientific questions that will be addressed during this Paris RDP:Nowcasting & Numerical Weather Prediction in cities at order 100m resolution High resolution thunderstorm nowcasting (probabilistic and deterministic) in the urban environment, Urban heat islands, cool areas and air quality Nowcasting and forecast in coastal cities (for the Marseilles site) How to improve and better use observational networks in urban areas, including (big) non-conventional data Conception and Communication of tailored weather, climate, environmental information at infra-urban resolution. Several High-Impact weather case studies were selected. Storm cases (starting with one the 10th July 2017) will allow to evaluate the role of the urban area on their enhancement. Extreme Heat wave aggravated by a strong Urban Heat Island are also studied (July 2019). Open urban data describing the agglomerations at very high resolution are provided. New innovative methods to produce maps of urban form characteristics (e.g. from street images) and meteorological data (from personal meteorological stations) will be explored.This talk will describe these scientific questions, as well as the common methodology approach that is being discussed within the partners. A focus will be the international experimental campaign that will take place in 2022 over the Paris agglomeration, with an Intensive Observation Period in the summer 2022. Interactions between urban surface and the atmospheric boundary layer, the interactions between air quality and aerosols between city and biogenic plumes, and the local effect of urban trees on micro-climate and chemistry are some of the axes of the campaign. It will provide additional meteorological and air quality observations, to both help to improve the nowcasting and NWP systems at urban scale, and aim to define the required additional instrumentation that should be deployed during the Olympics games themselves.

Vegetation fires – including the application of fire in land use, land-use change and uncontrolled wildfire – affect the functioning of the Earth System and impose significant threats to public health and security. This paper presents the concept of a Vegetation Fire and Smoke Pollution Warning Advisory and Assessment System (VFSP-WAS*). It describes the scientific rationale for the system and provides guidance for addressing the issues of vegetation fire and smoke pollution, including key research challenges. The paper proposes the establishment of VFSP-WAS regional centers and describes Potential examples of this VFSP-WAS concept are described from two regions in (South-East Asia and North America) where regional centers will partner with Regional Fire Monitoring / Fire Management Resource Centers.*) https://community.wmo.int/activity-areas/gaw/science/modelling-applications/vfsp-was