Accurately quantifying carbon dynamics in peatlands is critical to assess their role in regulating global climate. Within hotspots of peatland degradation, such as in Europe and South-east Asia, skilful assessment of the spatial and temporal impacts of climate change and different land management options is required to meet emissions reductions targets and improve regional management planning.To address this challenge, a random forest-based metamodel was evaluated to assess its utility in simulating various greenhouse gas (CO2) emission components, including Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), and Ecosystem Respiration (ER) across two Scottish peatlands. The metamodel mimicked the complex Wetland-DNDC model at a higher level of abstraction with increased efficiency and lower computational time.While Wetland-DNDC also simulates NEE, GPP and ER, it typically involves a considerable number of parameters related to soil properties, climate data, vegetation characteristics, biogeochemical processes, hydrology, nutrient cycling, and microbial activity. Many of these parameters (more than 100) are challenging to measure in the field, and literature values are often adopted, which may not necessarily reflect local site conditions. In essence, this multidimensional parameter space introduces high uncertainties in modelling carbon fluxes.In contrast, random forest-based metamodel preserved the key relationships between NEE and input variables (air and soil temperature, water table, precipitation, vegetation, and soil properties) as described in the Wetland-DNDC model with lower parameter requirements (less than 20) and increased accuracy. Similar unique relationships were established for GPP and ER. The random forest-based metamodel represented the Wetland-DNDC model within the spectrum of input values and parameters across which it was simulated.The simulation was conducted in two locations across Scotland with contrasting contemporary carbon dynamics: a near natural blanket bog in Cross Lochs, Forsinard, currently functioning as a resilient net carbon dioxide sink (UK-CLS; Lat. = 58.37, Long. = -3.96; altitude = 207 m) and an eroding oceanic blanket bog located in the Cairngorms, currently net emitting carbon dioxide (UK-BAM; Lat. = 56.92, Long. = -3.15, altitude = 642 m). The simulation was validated against eddy covariance flux measurements under varying climate conditions.In contrast to Wetland-DNDC (R2 = 0.43), the metamodel provided a much-improved fit to the 1:1 line for NEE (R2 = 0.83). Model accuracy was slightly lower for the former (RMSE = 0.72) compared to its metamodel version (RMSE = 0.699). Similar trends were observed for GPP and ER simulations. At a monthly resolution, Wetland-DNDC-derived NEE, GPP, and ER consistently deviated by more than 20% from the eddy covariance-derived estimates, whereas its metamodel version showed deviations of less than 10%. Currently, work is in progress to incorporate management and drought simulation within a metamodel framework, as well as to upscale carbon fluxes from tower to landscape resolution.The simulation of carbon fluxes using the metamodel-based approach holds the promise of enhancing emission reporting to Tier 3 standards and offers a hopeful avenue for modelling carbon dynamics in peatlands.

The measurement method of NO2 with continuous analysers is specified for EU Ambient Air Quality Directive compliance reporting, which provides a consistent methodology and concurrent NO measurements (85/203/EEC-NO2). While the established method of measurement of NO2, following conversion of NO2 to NO using a molybdenum-conversion process, has known interference uncertainties (due to conversion of other oxidised nitrogen (NOy) chemicals, the consistency and traceability of compliance measurement is important. This study compared three continuous NO2 analyser instruments: a Thermo-NOx molybdenum convertor chemiluminescence analyser (Model 42C, ThermoFisher Scientific Inc., MA, USA), a photolytic chemiluminescence analyser (T200UP, Teledyne Technologies Inc., San Diego, USA) and a Cavity Attenuated Phase Shift (CAPS) analyser (T500U, Teledyne Technologies Inc., CA, USA). The instruments were run for over a year at the Auchencorth Moss long-term peatland monitoring site (Southeast Scotland) which is a low NOx atmosphere away from sources. NOy and NHx chemicals were also measured concurrently. This study concludes that there is a strong artefact in molybdenum catalyst chemiluminescent instruments as a result of unselective catalysis of airborne NOy compounds that causes an overestimate of NO2 measured in the atmosphere. The observed artefact in concentration measurements is likely to be observed at the entire UK scale as almost the entirety of the rural air network relies on molybdenum catalyst instruments. We therefore recommend that molybdenum catalyst instruments should be phased out and replaced in air quality monitoring networks with molecule specific (spectroscopy) instrumentation (equivalent in cost, such as those described in this study) that do not suffer from the same measurement artefacts.

A primary sink of air pollutants and their precursors is dry deposition. Dry deposition estimates differ across chemical transport models, yet an understanding of the model spread is incomplete. Here, we introduce Activity 2 of the Air Quality Model Evaluation International Initiative Phase 4 (AQMEII4). We examine 18 dry deposition schemes from regional and global chemical transport models as well as standalone models used for impact assessments or process understanding. We configure the schemes as single-point models at eight Northern Hemisphere locations with observed ozone fluxes. Single-point models are driven by a common set of site-specific meteorological and environmental conditions. Five of eight sites have at least 3 years and up to 12 years of ozone fluxes. The interquartile range across models in multiyear mean ozone deposition velocities ranges from a factor of 1.2 to 1.9 annually across sites and tends to be highest during winter compared with summer. No model is within 50 % of observed multiyear averages across all sites and seasons, but some models perform well for some sites and seasons. For the first time, we demonstrate how contributions from depositional pathways vary across models. Models can disagree with respect to relative contributions from the pathways, even when they predict similar deposition velocities, or agree with respect to the relative contributions but predict different deposition velocities. Both stomatal and nonstomatal uptake contribute to the large model spread across sites. Our findings are the beginning of results from AQMEII4 Activity 2, which brings scientists who model air quality and dry deposition together with scientists who measure ozone fluxes to evaluate and improve dry deposition schemes in the chemical transport models used for research, planning, and regulatory purposes.

<p>Peatlands occupy 12% of the UK territory and can store large amounts of carbon (C). However, drainage, peat extraction, and other management activities have turned these ecosystems into greenhouse gas (GHG) emitters. Currently, peatlands account for ~ 4% of the UK’s total annual GHG emissions. Eddy covariance is considered the best method to measure landscape scale GHG exchange (CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O), between the Earth’s surface and the atmosphere. Recently many flux towers have been installed on UK peatlands under different land-use and in different condition, with some undergoing restoration. In total there are currently 30 operating, with 9 in Scotland (SCO2FLUX managed by The James Hutton Institute, JHI) and 21 across England, Wales and Northern Ireland (managed by UKCEH), including the Auchencorth Moss ICOS site. As part of the projects, NERC-MOTHERSHIP and SRC-CENTREPEAT, these peatland sites are being harmonised into a network. The data is being analysed using standard protocols in order to generate a powerful dataset to examine the exchange of CO<sub>2</sub> and CH<sub>4 </sub>over UK peatlands. Some of the topics being investigated are: the spatial and temporal variability of emissions for all peatland classifications; the main drivers and controlling mechanisms of GHG exchange, such as the effect of water table depth on gas exchange and restoration impacts (e.g. raising water levels in agricultural peatlands); the value and effectiveness of restoration techniques (e.g. the timeline of recovery in the transition from forest to bog); improving the modelling of peatlands in JULES and other land-surface models; ground-proofing data for Earth observation techniques; assessing the contribution of peatlands to achieving net zero; examining the impact of wildfire on restoration from forest to bog.</p> <p>An overview of the network of sites and some highlights of the analysis to date will be presented.</p> <p> </p>

The net impact of greenhouse gas emissions from degraded peatland environments on national Inventories and subsequent mitigation of such emissions has only been seriously considered within the last decade. Data on greenhouse gas emissions from special cases of peatland degradation, such as eroding peatlands, are particularly scarce. Here, we report the first eddy covariance-based monitoring of carbon dioxide (CO 2 ) emissions from an eroding Atlantic blanket bog. The CO 2 budget across the period July 2018–November 2019 was 147 (± 9) g C m −2 . For an annual budget that contained proportionally more of the extreme 2018 drought and heat wave, cumulative CO 2 emissions were nearly double (191 g C m −2 ) of that of an annual period without drought (106 g C m −2 ), suggesting that direct CO 2 emissions from eroded peatlands are at risk of increasing with projected changes in temperatures and precipitation due to global climate change. The results of this study are consistent with chamber-based and modelling studies that suggest degraded blanket bogs to be a net source of CO 2 to the atmosphere, and provide baseline data against which to assess future peatland restoration efforts in this region.

In May 2019, a major wildfire event affected >60 km2 within the 4000 km2 Flow Country in Northern Scotland, UK, a flagship blanket bog peatland that is being considered for UNESCO World Heritage Status. While the fire itself created significant damage, it also led to an extraordinary and unique opportunity to compare burned and unburned landscape scale greenhouse gas flux and surface energy dynamics using sites that, crucially, have otherwise identical biophysical characteristics (slope, aspect, peat depth) and land management histories. Since September 2019, carbon dioxide and methane flux data have been collected alongside other micrometeorological variables. Due to the COVID-19 lockdown in the UK, the team had severe difficulties in maintaining the equipment and hence, only partial and preliminary data will be reported here to showcase the findings from this project to date. The data obtained so far suggest a post-fire reduction in net CO2 emissions for a period of one year since the beginning of our monitoring campaign.

Global peatlands store more carbon than is naturally present in the atmosphere1,2. However, many peatlands are under pressure from drainage-based agriculture, plantation development and fire, with the equivalent of around 3 per cent of all anthropogenic greenhouse gases emitted from drained peatland3-5. Efforts to curb such emissions are intensifying through the conservation of undrained peatlands and re-wetting of drained systems6. Here we report eddy covariance data for carbon dioxide from 16 locations and static chamber measurements for methane from 41 locations in the UK and Ireland. We combine these with published data from sites across all major peatland biomes. We find that the mean annual effective water table depth (WTDe; that is, the average depth of the aerated peat layer) overrides all other ecosystem- and management-related controls on greenhouse gas fluxes. We estimate that every 10 centimetres of reduction in WTDe could reduce the net warming impact of CO2 and CH4 emissions (100-year global warming potentials) by the equivalent of at least 3 tonnes of CO2 per hectare per year, until WTDe is less than 30 centimetres. Raising water levels further would continue to have a net cooling effect until WTDe is within 10 centimetres of the surface. Our results suggest that greenhouse gas emissions from peatlands drained for agriculture could be greatly reduced without necessarily halting their productive use. Halving WTDe in all drained agricultural peatlands, for example, could reduce emissions by the equivalent of over 1 per cent of global anthropogenic emissions.

You have accessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Fowler David, Brimblecombe Peter, Burrows John, Heal Mathew R., Grennfelt Peringe, Stevenson David S., Jowett Alan, Nemitz Eiko, Coyle Mhairi, Liu Xuejun, Chang Yunhua, Fuller Gary W., Sutton Mark A., Klimont Zbigniew, Unsworth Mike H. and Vieno Massimo 2021Correction to ‘A chronology of global air quality’Phil. Trans. R. Soc. A.3792021011320210113http://doi.org/10.1098/rsta.2021.0113SectionYou have accessCorrectionCorrection to ‘A chronology of global air quality’ David Fowler David Fowler Google Scholar Find this author on PubMed Search for more papers by this author , Peter Brimblecombe Peter Brimblecombe Google Scholar Find this author on PubMed Search for more papers by this author , John Burrows John Burrows Google Scholar Find this author on PubMed Search for more papers by this author , Mathew R. Heal Mathew R. Heal Google Scholar Find this author on PubMed Search for more papers by this author , Peringe Grennfelt Peringe Grennfelt Google Scholar Find this author on PubMed Search for more papers by this author , David S. Stevenson David S. Stevenson Google Scholar Find this author on PubMed Search for more papers by this author , Alan Jowett Alan Jowett Google Scholar Find this author on PubMed Search for more papers by this author , Eiko Nemitz Eiko Nemitz Google Scholar Find this author on PubMed Search for more papers by this author , Mhairi Coyle Mhairi Coyle Google Scholar Find this author on PubMed Search for more papers by this author , Xuejun Liu Xuejun Liu Google Scholar Find this author on PubMed Search for more papers by this author , Yunhua Chang Yunhua Chang Google Scholar Find this author on PubMed Search for more papers by this author , Gary W. Fuller Gary W. Fuller Google Scholar Find this author on PubMed Search for more papers by this author , Mark A. Sutton Mark A. Sutton Google Scholar Find this author on PubMed Search for more papers by this author , Zbigniew Klimont Zbigniew Klimont Google Scholar Find this author on PubMed Search for more papers by this author , Mike H. Unsworth Mike H. Unsworth Google Scholar Find this author on PubMed Search for more papers by this author and Massimo Vieno Massimo Vieno Google Scholar Find this author on PubMed Search for more papers by this author David Fowler David Fowler Google Scholar Find this author on PubMed Search for more papers by this author , Peter Brimblecombe Peter Brimblecombe Google Scholar Find this author on PubMed Search for more papers by this author , John Burrows John Burrows Google Scholar Find this author on PubMed Search for more papers by this author , Mathew R. Heal Mathew R. Heal Google Scholar Find this author on PubMed Search for more papers by this author , Peringe Grennfelt Peringe Grennfelt Google Scholar Find this author on PubMed Search for more papers by this author , David S. Stevenson David S. Stevenson Google Scholar Find this author on PubMed Search for more papers by this author , Alan Jowett Alan Jowett Google Scholar Find this author on PubMed Search for more papers by this author , Eiko Nemitz Eiko Nemitz Google Scholar Find this author on PubMed Search for more papers by this author , Mhairi Coyle Mhairi Coyle Google Scholar Find this author on PubMed Search for more papers by this author , Xuejun Liu Xuejun Liu Google Scholar Find this author on PubMed Search for more papers by this author , Yunhua Chang Yunhua Chang Google Scholar Find this author on PubMed Search for more papers by this author , Gary W. Fuller Gary W. Fuller Google Scholar Find this author on PubMed Search for more papers by this author , Mark A. Sutton Mark A. Sutton Google Scholar Find this author on PubMed Search for more papers by this author , Zbigniew Klimont Zbigniew Klimont Google Scholar Find this author on PubMed Search for more papers by this author , Mike H. Unsworth Mike H. Unsworth Google Scholar Find this author on PubMed Search for more papers by this author and Massimo Vieno Massimo Vieno Google Scholar Find this author on PubMed Search for more papers by this author Published:24 May 2021https://doi.org/10.1098/rsta.2021.0113This article corrects the followingReview ArticleA chronology of global air qualityhttps://doi.org/10.1098/rsta.2019.0314 David Fowler, Peter Brimblecombe, John Burrows, Mathew R. Heal, Peringe Grennfelt, David S. Stevenson, Alan Jowett, Eiko Nemitz, Mhairi Coyle, Xuejun Liu, Yunhua Chang, Gary W. Fuller, Mark A. Sutton, Zbigniew Klimont, Mike H. Unsworth and Massimo Vieno volume 378issue 2183Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences28 September 2020Phil. Trans. R. Soc. A378, 20190314 (Published online 28 September 2020). (doi:10.1098/rsta.2019.0314)In the original version of this article, there was a typographical error in the author list. This has now been corrected. Previous Article VIEW FULL TEXT DOWNLOAD PDF FiguresRelatedReferencesDetailsRelated articlesA chronology of global air quality28 September 2020Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences This Issue12 July 2021Volume 379Issue 2201Theme issue ‘Topics in mathematical design of complex materials’ compiled and edited by Xian Chen, Miha Ravnik, Valeriy Slastikov and Arghir Zarnescu Article InformationDOI:https://doi.org/10.1098/rsta.2021.0113PubMed:34024135Published by:Royal SocietyPrint ISSN:1364-503XOnline ISSN:1471-2962History: Published online24/05/2021Published in print12/07/2021 License:© 2021 The Author(s)Published by the Royal Society. All rights reserved. Citations and impact Subjectsatmospheric chemistrybiogeochemistryenvironmental chemistry

Peatlands North of 45˚ represent one of the largest terrestrial carbon (C) stores. They play an important role in the global C-cycle, and their ability to sequester carbon is controlled by multiple, often competing, factors including precipitation, temperature and phenology. Land-atmosphere exchange of carbon dioxide (CO2) is dynamic, and exhibits marked seasonal and inter-annual variations which can effect the overall carbon sink strength in both the short- and long-term.Due to increased incidences of climate anomalies in recent years, long-term datasets are essential to disambiguate natural variability in Net Ecosystem Exchange (NEE) from shorter-term fluctuations. This is particularly important at high latitudes (>45˚N) where the majority of global peatlands are found. With increasing pressure from stressors such as climate and land-use change, it has been predicted that with a ca. 3oC global temperature rise by 2100, UK peatlands could become a net source of C.NEE of CO2 has been measured using the eddy-covariance (EC) method at Auchencorth Moss (55°47’32 N, 3°14’35 W, 267 m a.s.l.), a temperate, lowland, ombrotrophic peatland in central Scotland, continuously since 2002. Alongside EC data, we present a range of meteorological parameters measured at site including soil temperature, total solar and photosynthetically active radiation (PAR), rainfall, and, since April 2007, half-hourly water table depth readings. The length of record and range of measurements make this dataset an important resource as one of the longest term records of CO2 fluxes from a temperate peatland.Although seasonal cycles of gross primary productivity (GPP) were highly variable between years, the site was a consistent CO2 sink for the period 2002-2012. However, net annual losses of CO2 have been recorded on several occasions since 2013. Whilst NEE tends to be positively correlated with the length of growing season, anomalies in winter weather also explain some of the variability in CO2 sink strength the following summer.Additionally, water table depth (WTD) plays a crucial role, affecting both GPP and ecosystem respiration (Reco). Relatively dry summers in recent years have contributed to shifting the balance between Reco and GPP: prolonged periods of low WTD were typically accompanied by an increase in Reco, and a decrease in GPP, hence weakening the overall CO2 sink strength. Extreme events such as drought periods and cold winter temperatures can have significant and complex effects on NEE, particularly when such meteorological anomalies co-occur. For example, a positive annual NEE occurred in 2003 when Europe experienced heatwave and summer drought. More recently, an unusually long spell of snow lasting until the end of March delayed the onset of the 2018 growing season by up to 1.5 months compared to previous years. This was followed by a prolonged dry spell in summer 2018, which weakened GPP, increased Reco and led to a net annual loss of 47.4 ton CO2-C km-2. It is clear that the role of Northern peatlands within the carbon cycle is being modified, driven by changes in climate at both local and global scales.

Greenhouse gas emissions from damaged peatlands in the UK contribute around 5% to the annual national UK emissions. This has prompted a large national effort to restore these ecosystems as part of the package of action that aims to deliver net zero by 2050 in the UK and 2045 in Scotland. Eroded peatlands cover an estimated 275kha in Scotland, yet continuous monitoring data on the carbon losses from such sites are very sparse, in part due to the challenge in instrumenting such remote and complex terrain with eddy covariance equipment. We present a full, pre-restoration, 18-month data series of carbon dioxide and energy budget from a typical Scottish eroded peatland and show initial data that suggests sensitivity of the sign of the net annual CO2 budget to interannual climate variability.

Abstract. Production of hydroxyl (OH) radicals is frequently dominated by the photolysis of tropospheric ozone (O3). However, photolysis of nocturnal radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO2), also produces radicals (OH and Cl atoms) that contribute to the oxidising capacity of the local atmosphere, and initiate many radical-chain reactions that lead to the formation of harmful secondary pollutants. Photolysis of nitric acid (HNO3) is also a minor radical production mechanism. In this paper, locally representative photolysis rate constants (j-values) for these molecules are shown to be critical for quantifying and understanding the rate of radical production in a local atmosphere. The first long-term 4-π filter radiometer dataset in the UK (21 November 2018–20 November 2019) available for direct atmospheric model validation is reported. Measurements were made at Auchencorth Moss, a Scottish rural background site, and j(NO2) is used to generate a measurement-driven adjustment factor (MDAF) for calculated j-values that accounts for local changes in meteorological variables without significantly increasing computational cost. Modelled clear-sky j-values and actinic flux for Auchencorth Moss were generated using the Tropospheric Ultraviolet and Visible radiation model (TUV; v.5.3.1). Applying the MDAF metric resulted in the calculated photolytic production rate of OH radicals, from all sources considered, being ~40 % lower over the year. Photolysis of HONO resulted in an increased rate of OH production compared to that from O3 in low-light conditions, such as sunrise and sunset (Solar Zenith Angle > 80°). Hydroxyl radical production from HONO photolysis exceeded that from O3 consistently throughout the day during the winter and autumn (by a factor of 5 and 2.1, respectively). Radical production rates from HONO and ClNO2 reached maximum values during the early morning hours of summer (06:00–09:00 UTC), with OH produced at a rate of 1.06 × 106 OH radicals cm−3 s−1, and Cl radicals at 3.20 × 104 Cl radicals cm−3 s−1, with the MDAF metric applied. This first application of the MDAF j-values demonstrates an efficient measurement and computational approach to improve modelling of the local atmospheric photochemistry that drives NO2, O3 and PM pollution levels. The incorporation of local radiation measurements in measurement networks, and the consequent greater spatial resolution of locally-relevant photolysis coefficients in model photolysis parameterisations, will improve the accuracy of assessment of air pollution and policy-intervention impacts.

Tropospheric ozone (O-3) is probably the air pollutant most damaging to vegetation. Understanding how plants respond to O(3)pollution under different climate conditions is of central importance for predicting the interactions between climate change, ozone impact and vegetation. This work analyses the effect of O(3)fluxes on net ecosystem productivity (NEP), measured directly at the ecosystem level with the eddy covariance (EC) technique. The relationship was explored with artificial neural networks (ANNs), which were used to model NEP using environmental and phenological variables as inputs in addition to stomatal O(3)uptake in Spring and Summer, when O(3)pollution is expected to be highest. A sensitivity analysis allowed us to isolate the effect of O-3, visualize the shape of the O-3-NEP functional relationship and explore how climatic variables affect NEP response to O-3. This approach has been applied to eleven ecosystems covering a range of climatic areas. The analysis highlighted that O(3)effects over NEP are highly non-linear and site-specific. A significant but small NEP reduction was found during Spring in a Scottish shrubland (-0.67%), in two Italian forests (up to -1.37%) and during Summer in a Californian orange orchard (-1.25%). Although the overall seasonal effect of O(3)on NEP was not found to be negative for the other sites, with episodic O(3)detrimental effect still identified. These episodes were correlated with meteorological variables showing that O(3)damage depends on weather conditions. By identifying O(3)damage under field conditions and the environmental factors influencing to that damage, this work provides an insight into O(3)pollution, climate and weather conditions.

Dry deposition of ozone is an important sink of ozone in near-surface air. When dry deposition occurs through plant stomata, ozone can injure the plant, altering water and carbon cycling and reducing crop yields. Quantifying both stomatal and nonstomatal uptake accurately is relevant for understanding ozone's impact on human health as an air pollutant and on climate as a potent short-lived greenhouse gas and primary control on the removal of several reactive greenhouse gases and air pollutants. Robust ozone dry deposition estimates require knowledge of the relative importance of individual deposition pathways, but spatiotemporal variability in nonstomatal deposition is poorly understood. Here we integrate understanding of ozone deposition processes by synthesizing research from fields such as atmospheric chemistry, ecology, and meteorology. We critically review methods for measurements and modeling, highlighting the empiricism that underpins modeling and thus the interpretation of observations. Our unprecedented synthesis of knowledge on deposition pathways, particularly soil and leaf cuticles, reveals process understanding not yet included in widely used models. If coordinated with short-term field intensives, laboratory studies, and mechanistic modeling, measurements from a few long-term sites would bridge the molecular to ecosystem scales necessary to establish the relative importance of individual deposition pathways and the extent to which they vary in space and time. Our recommended approaches seek to close knowledge gaps that currently limit quantifying the impact of ozone dry deposition on air quality, ecosystems, and climate.

Air pollution has been recognized as a threat to human health since the time of Hippocrates, ca 400 BC. Successive written accounts of air pollution occur in different countries through the following two millennia until measurements, from the eighteenth century onwards, show the growing scale of poor air quality in urban centres and close to industry, and the chemical characteristics of the gases and particulate matter. The industrial revolution accelerated both the magnitude of emissions of the primary pollutants and the geographical spread of contributing countries as highly polluted cities became the defining issue, culminating with the great smog of London in 1952. Europe and North America dominated emissions and suffered the majority of adverse effects until the latter decades of the twentieth century, by which time the transboundary issues of acid rain, forest decline and ground-level ozone became the main environmental and political air quality issues. As controls on emissions of sulfur and nitrogen oxides (SO2 and NOx) began to take effect in Europe and North America, emissions in East and South Asia grew strongly and dominated global emissions by the early years of the twenty-first century. The effects of air quality on human health had also returned to the top of the priorities by 2000 as new epidemiological evidence emerged. By this time, extensive networks of surface measurements and satellite remote sensing provided global measurements of both primary and secondary pollutants. Global emissions of SO2 and NOx peaked, respectively, in ca 1990 and 2018 and have since declined to 2020 as a result of widespread emission controls. By contrast, with a lack of actions to abate ammonia, global emissions have continued to grow. This article is part of a discussion meeting issue 'Air quality, past present and future'.

In this study, we used eight sites from across Europe to investigate the implications of a future climate (2 °C warmer and 20% drier) and a changing ozone profile (increased background concentrations and reduced peaks) on stomatal ozone fluxes of three widely occurring plant species. A changing ozone profile with small increases in background ozone concentrations over the course of a growing season could have significant impacts on the annual accumulated stomatal ozone uptake, even if peak concentrations of ozone are reduced. Predicted increases in stomatal ozone uptake showed a strong relationship with latitude and were larger at sites from northern and mid-Europe than those from southern Europe. At the sites from central and northern regions of Europe, including the UK and Sweden, climatic conditions were highly conducive to stomatal ozone uptake by vegetation during the summer months and therefore an increase in daily mean ozone concentration of 3–16% during this time of year (from increased background concentrations, reduced peaks) would have a large impact on stomatal ozone uptake. In contrast, during spring and autumn, the climatic conditions can limit ozone uptake for many species. Although small increases in ozone concentration during these seasons could cause a modest increase in ozone uptake, for those species that are active at low temperatures, a 2 °C increase in temperature would increase stomatal ozone uptake even in the absence of further increases in ozone concentration. Predicted changes in climate could alter ozone uptake even with no change in ozone profile. For some southern regions of Europe, where temperatures are close to or above optimum for stomatal opening, an increase in temperature of 2 °C could limit stomatal ozone uptake by enhancing stomatal closure during the summer months, whereas during the spring, when many plants are actively growing, a small increase in temperature would increase stomatal ozone uptake.