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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 (SO 2 and NO x ) 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 SO 2 and NO x 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’.

The eddy covariance (EC) method is a standard micrometeorological technique for monitoring the exchange rate of the main greenhouse gases across the interface between the atmosphere and ecosystems. One of the first EC data processing steps is the temporal alignment of the raw, high frequency measurements collected by the sonic anemometer and gas analyser. While different methods have been proposed and are currently applied, the application of the EC method to trace gases measurements highlighted the difficulty of a correct time lag detection when the fluxes are small in magnitude. Failure to correctly synchronise the time series entails a systematic error on covariance estimates and can introduce large uncertainties and biases in the calculated fluxes. This work aims at overcoming these issues by introducing a new time lag detection procedure based on the assessment of the cross-correlation function (CCF) between variables subject to (i) a pre-whitening based on autoregressive filters and (ii) a resampling technique based on block-bootstrapping. Combining pre-whitening and block-bootstrapping facilitates the assessment of the CCF, enhancing the accuracy of time lag detection between variables with correlation of low order of magnitude (i.e. lower than -1) and allowing for a proper estimate of the associated uncertainty. We expect the proposed procedure to significantly improve the temporal alignment of the EC time-series measured by two physically separate sensors, and to be particularly beneficial in centralised data processing pipelines of research infrastructures (e.g. the Integrated Carbon Observation System, ICOS-RI) where the use of robust and fully data-driven methods, like the one we propose, constitutes an essential prerequisite.

This paper reports the fluxes and mixing ratios of biogenically emitted volatile organic compounds (BVOCs) 4 m above a mixed oak and hornbeam forest in northern Italy. Fluxes of methanol, acetaldehyde, isoprene, methyl vinyl ketone + methacrolein, methyl ethyl ketone and monoterpenes were obtained using both a proton-transfer-reaction mass spectrometer (PTR-MS) and a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) together with the methods of virtual disjunct eddy covariance (using PTR-MS) and eddy covariance (using PTR-ToF-MS). Isoprene was the dominant emitted compound with a mean daytime flux of 1.9 mg m−2 h−1. Mixing ratios, recorded 4 m above the canopy, were dominated by methanol with a mean value of 6.2 ppbv over the 28-day measurement period. Comparison of isoprene fluxes calculated using the PTR-MS and PTR-ToF-MS showed very good agreement while comparison of the monoterpene fluxes suggested a slight over estimation of the flux by the PTR-MS. A basal isoprene emission rate for the forest of 1.7 mg m−2 h−1 was calculated using the Model of Emissions of Gases and Aerosols from Nature (MEGAN) isoprene emission algorithms (Guenther et al., 2006). A detailed tree-species distribution map for the site enabled the leaf-level emission of isoprene and monoterpenes recorded using gas-chromatography mass spectrometry (GC–MS) to be scaled up to produce a bottom-up canopy-scale flux. This was compared with the top-down canopy-scale flux obtained by measurements. For monoterpenes, the two estimates were closely correlated and this correlation improved when the plant-species composition in the individual flux footprint was taken into account. However, the bottom-up approach significantly underestimated the isoprene flux, compared with the top-down measurements, suggesting that the leaf-level measurements were not representative of actual emission rates.

The Russian invasion of Ukraine has disrupted crop exports and global food security, overshadowing critical nutrient asymmetry and the associated environmental risks. Here we demonstrate that following nutrient shortages after independence in 1991, fertilizer use increased over 2000-2021, but has decreased sharply following the invasion in early 2022. Input-output balances of nitrogen (N), phosphorus (P) and potassium (K) for staple crops (wheat, maize and sunflower) highlight soil P and K mining since 1991, increasing N surpluses during 2000-2021 and large NPK deficits since the war began in 2022. Based on analysis of five scenarios for 2030, we show how an Integrated Nutrient Management Plan for Ukraine combining manure recycling, precision fertilization and legume expansion is urgently needed, and would maintain crop productivity, significantly reduce nutrient surpluses and improve nutrient use efficiencies up to 80–89%, substantially curtailing environmental pollution and soil degradation. Fertilization use in Ukraine declined sharply following the Ukraine war in early 2022, associated with severe national nutrient deficits and underscoring an urgent need for an integrated nutrient management plan to improve agricultural sustainability, as shown through scenario analysis

Open biomass burning has major impacts on the Earth system, including on air quality via the emission of primary fine particulate matter (PM 2.5 ). Its effect on secondary inorganic PM 2.5 formation is comparatively little investigated. Simulations with the EMEP MSC-W WRF atmospheric chemistry transport model reveal that global biomass burning emissions lead to elevated annual mean ammonium nitrate (NH 4 NO 3 ) concentrations in densely populated regions where biomass burning mostly does not occur. These regions include eastern USA, northwestern Europe, the Indo-Gangetic Plain and eastern China, where NH 4 NO 3 conditional on biomass burning emissions constitutes between 29% and 51% of the annual mean PM 2.5 conditional on biomass burning emissions. Biomass burning emissions of CO, NO x (NO and NO 2 ) and volatile organic compounds perturb the HO x (OH and HO 2 ) cycle globally, such that there is increased oxidation of anthropogenic NO x to HNO 3 . This results in additional contributions to local-scale secondary NH 4 NO 3 in areas with high emissions of anthropogenic NO x and NH 3 . These teleconnections increase, by up to a factor of two, the contribution of biomass burning emissions to long-term PM 2.5 concentrations, which measurements alone cannot identify as an impact of biomass burning activity. This may become relatively more important as anthropogenic sources of PM 2.5 are reduced and as the wildfire component of biomass burning increases under climate change.

Isoprene is emitted from numerous plant species in response to light and temperature and parameterisations of these relationships, based on observations from a few vascular plant species, have been shown to be broadly applicable to many different vegetation types. Here, we investigate their performance when applied to an ecosystem dominated by bryophytes. Over a six-week period, emissions of isoprene were measured above a Scottish peat bog. The light response derived on the basis of both canopy-scale flux and whole-plant enclosure measurements, deviated from the classical response, showing no sign of saturation within the observed range. We attribute this response to the canopy architecture of moss hummocks, which may attenuate light differently compared to a grass canopy. Both existing big-leaf and canopy-level emission algorithms, developed for vascular plants but commonly used for moorland vegetation, failed to replicate the observed fluxes, overestimating at low light intensities (<1000 μmol m−2 s−1 photosynthetically active radiation) and underestimating during daytime clear sky conditions. The light response was optimised for bryophyte-dominated ecosystems using measured fluxes and incorporated into the EMEP4UK chemical transport model and applied exclusively to moorland. The revised parameterisation resulted in a small reduction in the average annual isoprene emissions in the northern latitudes (5%), but peak isoprene emissions and concentrations increased by up to a factor of two. Yet, no significant change in average or maximum surface ozone concentrations was observed, reflecting that the northern latitudes are in a chemical regime that is strongly NOx limited, in part due to the spatial segregation with the urban sources of NO x . We conclude that, the anticipated increase in isoprene emissions from the northern latitudes in response to climate change is unlikely to contribute towards ozone-related air quality issues, as long as NO x pollution does not increase. However, the non-saturating light response may be equally applicable to non-vascular plants elsewhere, including in the tropics.

Volcanic emissions, specifically from Iceland, pose a pan-European risk and are on the UK National Risk Register due to potential impacts on aviation, public health, agriculture, the environment and the economy, from both effusive and explosive activity. During the 2014–2015 fissure eruption at Holuhraun in Iceland, the UK atmosphere was significantly perturbed. This study focuses one major incursion in September 2014, affecting the surface concentrations of both aerosols and gases across the UK, with sites in Scotland experiencing the highest sulfur dioxide (SO2) concentrations. The perturbation event observed was confirmed to originate from the fissure eruption using satellite data from GOME2B and the chemical transport model, EMEP4UK, which was used to establish the spatial distribution of the plume over the UK during the event of interest. At the two UK European Monitoring and Evaluation Program (EMEP) supersite observatories (Auchencorth Moss, SE Scotland, and Harwell, SE England) significant alterations in sulfate (SO42−) content of PM10 and PM2.5 during this event, concurrently with evidence of an increase in ultrafine aerosol most likely due to nucleation and growth of aerosol within the plume, were observed. At Auchencorth Moss, higher hydrochloric acid (HCl) concentrations during the September event (max  =  1.21 µg m−3, cf. annual average 0.12 µg m−3 in 2013), were assessed to be due to acid displacement of chloride (Cl−) from sea salt (NaCl) to form HCl gas rather than due to primary emissions of HCl from Holuhraun. The gas and aerosol partitioning at Auchencorth Moss of inorganic species by thermodynamic modelling confirmed the observed partitioning of HCl. Using the data from the chemical thermodynamic model, ISORROPIA-II, there is evidence that the background aerosol, which is typically basic at this site, became acidic with an estimated pH of 3.8 during the peak of the event.Volcano plume episodes were periodically observed by the majority of the UK air quality monitoring networks during the first 4 months of the eruption (August–December 2014), at both hourly and monthly resolution. In the low-resolution networks, which provide monthly SO2 averages, concentrations were found to be significantly elevated at remote “clean” sites in NE Scotland and SW England, with record-high SO2 concentrations for some sites in September 2014. For sites which are regularly influenced by anthropogenic emissions, taking into account the underlying trends, the eruption led to statistically unremarkable SO2 concentrations (return probabilities  > 0.1, ∼ 10 months). However, for a few sites, SO2 concentrations were clearly much higher than has been previously observed (return probability < 0.005,  > 3000 months). The Holuhraun Icelandic eruption has resulted in a unique study providing direct evidence of atmospheric chemistry perturbation of both gases and aerosols in the UK background atmosphere. The measurements can be used to both challenge and verify existing atmospheric chemistry of volcano plumes, especially those originating from effusive eruptions, which have been underexplored due to limited observations available in the literature. If all European data sets were collated this would allow improved model verification and risk assessments for future volcanic eruptions of this type.

Tropospheric ozone (O3) is probably the air pollutant most damaging to vegetation. Understanding how plants respond to O3 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 O3 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 O3 uptake in Spring and Summer, when O3 pollution is expected to be highest. A sensitivity analysis allowed us to isolate the effect of O3, visualize the shape of the O3-NEP functional relationship and explore how climatic variables affect NEP response to O3. This approach has been applied to eleven ecosystems covering a range of climatic areas. The analysis highlighted that O3 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 O3 on NEP was not found to be negative for the other sites, with episodic O3 detrimental effect still identified. These episodes were correlated with meteorological variables showing that O3 damage depends on weather conditions. By identifying O3 damage under field conditions and the environmental factors influencing to that damage, this work provides an insight into O3 pollution, climate and weather conditions.

Open biomass burning impacts air quality through direct emissions of fine particulate matter (PM.sub.2.5) and its role in secondary PM.sub.2.5 formation. Here the interest is in the long distance and cumulative influences of biomass burning on annual mean concentrations of PM.sub.2.5 in a country far removed from major biomass burning regions: the UK. A novel, globally nested setup of the EMEP4UK atmospheric chemistry transport model is used to isolate contributions to UK PM.sub.2.5 from global biomass burning activity. Long-range influences are found to be considerable, with 0.99 µg m.sup.-3 of UK-averaged PM.sub.2.5 in 2019 being conditional on biomass burning emissions. Of this, 97 % and 73 % are associated with biomass burning outside the UK and outside the model's European domain, respectively - notably from Russia, Asia and boreal North America - which highlights the importance of boundary conditions on regional modelling setups. The simulations suggest some influences of biomass burning have lags of several weeks. The long-range component is enhanced by the role of biomass burning in secondary aerosol formation (58 % of PM.sub.2.5 conditional on biomass burning), of which 55 % is organic; the inorganic component (mainly ammonium nitrate) derives from increased oxidation of local emissions, which may be mitigated through local emissions reductions. The PM.sub.2.5 conditional on biomass burning is highly policy relevant for the UK, constituting (for 2019) 20 % of the current WHO target and 10 % of the contribution from all sources. This relative contribution is likely to increase as anthropogenic PM.sub.2.5 declines and as climate change increases northern-hemispheric extratropical biomass burning.

Open biomass burning impacts air quality through direct emissions of fine particulate matter (PM2.5) and its role in secondary PM2.5 formation. Here the interest is in the long distance and cumulative influences of biomass burning on annual mean concentrations of PM2.5 in a country far removed from major biomass burning regions: the UK. A novel, globally nested setup of the EMEP4UK atmospheric chemistry transport model is used to isolate contributions to UK PM2.5 from global biomass burning activity. Long-range influences are found to be considerable, with 0.99 µg m−3 of UK-averaged PM2.5 in 2019 being conditional on biomass burning emissions. Of this, 97 % and 73 % are associated with biomass burning outside the UK and outside the model's European domain, respectively – notably from Russia, Asia and boreal North America – which highlights the importance of boundary conditions on regional modelling setups. The simulations suggest some influences of biomass burning have lags of several weeks. The long-range component is enhanced by the role of biomass burning in secondary aerosol formation (58 % of PM2.5 conditional on biomass burning), of which 55 % is organic; the inorganic component (mainly ammonium nitrate) derives from increased oxidation of local emissions, which may be mitigated through local emissions reductions. The PM2.5 conditional on biomass burning is highly policy relevant for the UK, constituting (for 2019) 20 % of the current WHO target and 10 % of the contribution from all sources. This relative contribution is likely to increase as anthropogenic PM2.5 declines and as climate change increases northern-hemispheric extratropical biomass burning.