In support of the first Tropospheric Ozone Assessment Report (TOAR) a relational database of global surface ozone observations has been developed and populated with hourly measurement data and enhanced metadata. A comprehensive suite of ozone data products including standard statistics, health and vegetation impact metrics, and trend information, are made available through a common data portal and a web interface. These data form the basis of the TOAR analyses focusing on human health, vegetation, and climate relevant ozone issues, which are part of this special feature. Cooperation among many data centers and individual researchers worldwide made it possible to build the world’s largest collection of in-situ hourly surface ozone data covering the period from 1970 to 2015. By combining the data from almost 10,000 measurement sites around the world with global metadata information, new analyses of surface ozone have become possible, such as the first globally consistent characterisations of measurement sites as either urban or rural/remote. Exploitation of these global metadata allows for new insights into the global distribution, and seasonal and long-term changes of tropospheric ozone and they enable TOAR to perform the first, globally consistent analysis of present-day ozone concentrations and recent ozone changes with relevance to health, agriculture, and climate. Considerable effort was made to harmonize and synthesize data formats and metadata information from various networks and individual data submissions. Extensive quality control was applied to identify questionable and erroneous data, including changes in apparent instrument offsets or calibrations. Such data were excluded from TOAR data products. Limitations of a posteriori data quality assurance are discussed. As a result of the work presented here, global coverage of surface ozone data for scientific analysis has been significantly extended. Yet, large gaps remain in the surface observation network both in terms of regions without monitoring, and in terms of regions that have monitoring programs but no public access to the data archive. Therefore future improvements to the database will require not only improved data harmonization, but also expanded data sharing and increased monitoring in data-sparse regions.

Beijing has experienced a rapid urbanization in the last few decades and has been suffering from serious air pollution during recent years. The Weather Research and Forecasting‐Chem model is used to quantify the effects of urbanization on regional climate and air quality and those of urban heat island (UHI) mitigation strategy on urban air quality in Beijing, with a special focus on the impacts under different weather conditions (heat waves in summer and polluted days in winter). The modification of rural land use into urban impervious surface significantly increases 2‐m temperature (T₂) and planetary boundary layer height but decreases 2‐m relative humidity (RH₂) and 10‐m wind speed (WS₁₀) in urban Beijing, which further leads to the increases in surface‐layer O₃ concentrations of 9.5 ppbv in summer and 1.8 ppbv in winter and the decreases in PM_2.5 concentrations of 16.6 μg m⁻³ in summer and 26.2 μg m⁻³ in winter. Compared with normal days (clean days), the UHI intensity is enhanced by 11.1% during heat waves in summer (by 16.7% during polluted days in winter). Although increasing urban albedo is an effective mitigation strategy to decrease UHI intensity, it worsens the urban air quality. When the urban albedo increases from 0.2 to 0.85, the daily average PM_2.5 concentrations are increased by 10.2 (6.1) μg m⁻³ in summer (in winter), and the daily maximum O₃ concentrations are increased by 12.8 ppbv under heat waves in summer.

Volatile organic compounds (VOCs), ozone (O₃), nitrogen oxides (NOx), carbon monoxide (CO), meteorological parameters, and total non-methane hydrocarbons (NMHC) were analyzed from simultaneous measurements at the MSU-IAP (Moscow State University—Institute of Atmospheric Physics) observational site in Moscow from 2011–2013. Seasonal and diurnal variability of the compounds was studied. The highest O₃ concentration in Moscow was observed in the summer daytime periods in anticyclonic meteorological conditions under poor ventilation of the atmospheric boundary layer and high temperatures (up to 105 ppbv or 210 μg/m³). In contrast, NOx, CO, and benzene decreased from 8 a.m. to 5–6 p.m. local time (LT). The high positive correlation of daytime O₃ with secondary VOCs affirmed an important role of photochemical O₃ production in Moscow during the summers of 2011–2013. The summertime average concentrations of the biogenic VOCs isoprene and monoterpenes were observed to be 0.73 ppbv and 0.53 ppbv, respectively. The principal source of anthropogenic VOCs in Moscow was established to be local vehicle emissions. Yet, only about 5% of the observed isoprene was safely attributed to anthropogenic sources, suggesting significant contribution of biogenic sources into the total levels of ozone precursors. The non-linear O₃–NOx dependence shows a decrease in ground-level O₃ with an increase in NOx during the summers of 2011–2013, which is typical for the VOC-sensitive photochemical regime of O₃ formation. Nevertheless, during the elevated ozone episodes in July 2011, the photochemical regime of ozone production was either transitional or NOx-sensitive. Contribution of various anthropogenic and biogenic VOCs into the measured ozone values was evaluated. The ozone-forming potential (OFP) of total VOCs was 31–67 μg/m³ on average and exceeded 100 μg/m³ in the top 10% of high ozone events, reaching 136 μg/m³. Acetaldehyde, 1.3-butadiene, and isoprene have the highest ozone production potential in Moscow compared to that of other measured VOCs.

The northern Eurasian regions and Arctic Ocean will very likely undergo substantial changes during the next decades. The Arctic–boreal natural environments play a crucial role in the global climate via albedo change, carbon sources and sinks as well as atmospheric aerosol production from biogenic volatile organic compounds. Furthermore, it is expected that global trade activities, demographic movement, and use of natural resources will be increasing in the Arctic regions. There is a need for a novel research approach, which not only identifies and tackles the relevant multi-disciplinary research questions, but also is able to make a holistic system analysis of the expected feedbacks. In this paper, we introduce the research agenda of the Pan-Eurasian Experiment (PEEX), a multi-scale, multi-disciplinary and international program started in 2012 (https://www.atm.helsinki.fi/peex/). PEEX sets a research approach by which large-scale research topics are investigated from a system perspective and which aims to fill the key gaps in our understanding of the feedbacks and interactions between the land–atmosphere–aquatic–society continuum in the northern Eurasian region. We introduce here the state of the art for the key topics in the PEEX research agenda and present the future prospects of the research, which we see relevant in this context.

Volatile organic compounds (VOCs) were measured by proton transfer reaction mass spectrometry (PTR-MS) on a mobile laboratory in a transcontinental TROICA-12 (21 July–4 August 2008) campaign along the Trans-Siberian Railway from Moscow to Vladivostok. Surface concentrations of benzene (C₆H₆) and toluene (C₇H₈) along with non-methane hydrocarbons (NMHCs), CO, O₃, SO₂, NO, NO₂ and meteorology are analyzed in this study to identify the main sources of benzene and toluene along the Trans-Siberian Railway. The most measurements in the TROICA-12 campaign were conducted under low-wind/stagnant conditions in moderately ( ∼ 78 % of measurements) to weakly polluted ( ∼ 20 % of measurements) air directly affected by regional anthropogenic sources adjacent to the railway. Only 2 % of measurements were identified as characteristic of highly polluted urban atmosphere. Maximum values of benzene and toluene during the campaign reached 36.5 and 45.6 ppb, respectively, which is significantly less than their short-term exposure limits (94 and 159 ppb for benzene and toluene, respectively). About 90 % of benzene and 65 % of toluene content is attributed to motor vehicle transport and 10 and 20 %, respectively, provided by the other local- and regional-scale sources. The highest average concentrations of benzene and toluene are measured in the industrial regions of the European Russia (up to 0.3 and 0.4 ppb for benzene and toluene, respectively) and south Siberia (up to 0.2 and 0.4 ppb for benzene and toluene, respectively). Total contribution of benzene and toluene to photochemical ozone production along the Trans-Siberian Railway is about 16 % compared to the most abundant organic VOC – isoprene. This contribution, however, is found to be substantially higher (up to 60–70 %) in urbanized areas along the railway, suggesting an important role of anthropogenic pollutant sources in regional ozone photochemistry and air quality.

In Russian Arctic seas, observations of surface methane concentrations (CH₄), ozone, nitrogen and carbon oxides, as well as the content of \({{\delta }^{{13}}}{{{\text{C}}}_{{{\text{C}}{{{\text{H}}}_{4}}}}}\) isotope and black carbon (BC, soot aerosol), were carried out from onboard of the R/V Academician Mstislav Keldysh. The areas of local methane releasing from bottom sediments were investigated. It was shown that the studied methane releasing on the Arctic shelf are of a local nature and, on the whole, insignificantly affect the composition of the atmosphere in the region. The average concentration of methane in the surface air in the Arctic seas is mainly determined by large-scale processes of air mass transfer. An analysis of the distribution of black carbon along the route of the vessel was carried out. It was found that the excess in the concentration of black carbon over the background values is observed occasionally during advection of air masses from the mainland and from the areas of associated gases burning and forest fires. The effect of emissions from a ship’s chimney on the data obtained was analyzed.