Large-scale ozone episodes in Europe: Decreasing sizes in the last decades but diverging changes in the future

Episodes of high near-surface ozone concentrations tend to cover large areas for several days. They are strongly dependent on meteorology, precursor emissions, and the ambient photochemical conditions. This study introduces a new pseudo-Lagrangian algorithm that identifies the spatiotemporal patterns of episodes, allowing for a good characterization of their areal extent and an assessment of their drivers. The algorithm has been used to identify ozone episodes in Europe from April to September over the last twenty years (2003-2022) in the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis as well as in the historical simulation (1950-2014) and four shared socio-economic pathways (SSPs, spanning 2015-2100) of three Earth system models (UKESM1-0-LL, EC-Earth3-AerChem and GFDL-ESM4). While the total number of episodes has increased in recent years, the frequency of large episodes has decreased following European precursor emission reductions. The analysis of the 100 largest episodes shows that they tend to occur in Northern Europe during spring and in the center and south of the continent from June onwards. Most of the top 10 episodes occurred in the first years of the century and were associated with high temperatures, enhanced solar radiation, and anticyclonic conditions. Despite the decrease in large episodes in recent years, there is uncertainty regarding future European episodes. Episodes of reduced size are found for SSPs with weak greenhouse forcing and low precursor emissions, whereas episode sizes increase in scenarios with high methane concentrations and enhanced radiative forcing, even exceeding the maximum historical size. However, the three models project episodes of different sizes for any given scenario, probably associated with their differing warming trends and the varying level of complexity in the implementation of processes. These results point to the need to implement both effective climate and air quality policies to address the ozone air pollution problem in Europe in a warming climate.

Air quality trends and implications pre and post Covid-19 restrictions

Air pollution causes millions of premature deaths every year. Thus, air quality assessment is essential to preserve human health and support authorities to identify proper policies. In this study, concentration levels of 6 air contaminants (benzene, carbon monoxide, nitrogen dioxide, ground level ozone, particulate matters) as monitored in 2019, 2020 and 2021 by 37 stations, located in Campania (Italy) were analysed. Particular attention has been paid to March-April 2020 period to get clues on the possible effects of the lockdown regulations, imposed in Italy from March 9th to May 4th to limit COVID-19 spread, on atmospheric pollution. Air Quality Index (AQI), an algorithm developed by the United States Environmental Protection Agency (US-EPA), allowed us to classify the air quality from moderately unhealthy to good for sensitive groups. The evaluation of air pollution impact on human health by using the AirQ+ software evidenced a significant decrement of adult mortality in 2020 respect to 2019 and 2021. Among the six pollutants considered, PM₁₀ and PM_2.5 resulted the less affected by the lockdown restrictions. Finally, a comparison between NO₂ ground level concentration and the reprocessed Level 2 NO₂ tropospheric column concentration obtained from satellite surveys highlighted as concentration measured at the ground level stations can be strongly influenced by the station position and its surroundings.

Assessing air pollution in European cities to support a citizen centered approach to air quality management

European cities have made significant progress over the last decades towards clean air. Despite this progress, several cities are still facing acute air pollution episodes, with various urban areas frequently exceeding air quality levels allowed by the European legal standards and WHO guidelines. In this paper, six European cities/ regions (Bristol, UK; Amsterdam, NL; Sosnowiec, PL; Ljubljana, SI; Aveiro, PT; Liguria, IT) are studied in terms of air quality, namely particulate matter, nitrogen dioxide and ozone. The concentrations trends from 2008 to 2017 in the different typology of monitoring stations are addressed, together with the knowledge of daily, weekly and seasonal pollution patterns to better understand the city specific profiles and to characterize pollutant dynamics and variations in multiple locations. Additionally, an analysis of the duration and severity of air pollution episodes is also discussed, followed by an analysis of the fulfillment of the legislated limit values. Each of our 6 case study locations face different air pollution problems, but all these case studies have made some progress in reducing ambient concentrations. In Bristol, there have been strong downward trends in many air pollutants, but the levels of NO₂ remain persistently high and of concern. In recent years, decreasing concentration levels point to some success of Amsterdam air quality policies. PM₁₀ exceedances are a seasonal pollution problem in Ljubljana, Sosnowiec and Aveiro region (even if with different levels of severity). While, exceedances of NO₂ and O₃ concentrations are still problematic in Liguria region. The main findings of this paper are particular relevant to define and compare future citizen-led strategies and policy initiatives that may be implemented to improve and fulfill the EU legislation and the WHO guidelines.

Health and economic impacts from PM2.5 pollution transfer attributed to domestic trade in China: a provincial-level analysis

In recent years, severe air pollution has had a serious impact on the health and economy of residents and has attracted great attention. Due to the spatial separation between consumption and production, the transfer of PM_2.5 pollution and its health and economic effects caused by interprovincial trade have not been taken seriously. In this study, economic, atmospheric, and epidemiological models were combined to assess air pollution transfer and its health and economic impacts that are attributed to provincial trade in China. The analyses were performed under the PM_2.5 transfer scenario in which economically developed areas in eastern China transferred many health and economic impacts to inland areas through interprovincial trade in 2012. As a result of interprovincial trade, 1980 (95% CI 0, 4114) extra deaths and 208,000 (95% CI 74.5, 395.6) additional illnesses accrued, but 0.184 (95% CI 0.017, 0.372) billion USD of residents' economic loss was avoided in China. The results illustrate the serious impact of domestic trade on regional health and economics. It is necessary to comprehensively consider supply chains in designing policies to mitigate the negative health and economic impacts of air pollution across China.

Ship emission of nitrous acid (HONO) and its impacts on the marine atmospheric oxidation chemistry

Nitrous acid (HONO) is an important reservoir of the hydroxyl radical (OH) and thus plays a central role in tropospheric chemistry. Exhaust from engines has long been known as a major primary source of HONO, yet most previous studies focused on vehicle emissions on land. In comparison, ship emissions of HONO have been rarely characterized, and their impacts on the tropospheric oxidation chemistry have not been quantified. In this study, we conducted cruise measurements of HONO and related species over the East China Sea. Contrasting air masses from pristine marine background air to highly polluted ship plumes were encountered. The emission ratio of ΔHONO/ΔNO_x (0.51 ± 0.18%) was derived from a large number of fresh ship plumes. Using the in-situ measured emission ratio, a global ship emission inventory of HONO was developed based on the international shipping emissions of NO_x in the Community Emission Data System inventory. The global shipping voyage emits approximately 63.9 ± 22.2 Gg yr^-1 of HONO to the atmosphere. GEOS-Chem modelling with the addition of ship-emitted HONO showed that HONO concentrations could increase up to 40-100% over the navigation areas, leading to about 5-15% increases of primary OH production in the early-morning time. This study elucidates the potentially considerable effects of ship HONO emissions on the marine atmospheric chemistry, and calls for further studies to better characterize the ship emissions of HONO and other reactive species, which should be taken into account by global and regional models.

Satellite isoprene retrievals constrain emissions and atmospheric oxidation

Isoprene is the dominant non-methane organic compound emitted to the atmosphere1-3. It drives ozone and aerosol production, modulates atmospheric oxidation and interacts with the global nitrogen cycle4-8. Isoprene emissions are highly uncertain1,9, as is the nonlinear chemistry coupling isoprene and the hydroxyl radical, OH-its primary sink10-13. Here we present global isoprene measurements taken from space using the Cross-track Infrared Sounder. Together with observations of formaldehyde, an isoprene oxidation product, these measurements provide constraints on isoprene emissions and atmospheric oxidation. We find that the isoprene-formaldehyde relationships measured from space are broadly consistent with the current understanding of isoprene-OH chemistry, with no indication of missing OH recycling at low nitrogen oxide concentrations. We analyse these datasets over four global isoprene hotspots in relation to model predictions, and present a quantification of isoprene emissions based directly on satellite measurements of isoprene itself. A major discrepancy emerges over Amazonia, where current underestimates of natural nitrogen oxide emissions bias modelled OH and hence isoprene. Over southern Africa, we find that a prominent isoprene hotspot is missing from bottom-up predictions. A multi-year analysis sheds light on interannual isoprene variability, and suggests the influence of the El Niño/Southern Oscillation.

Elevated Ozone Concentration Reduces Photosynthetic Carbon Gain but Does Not Alter Leaf Structural Traits, Nutrient Composition or Biomass in Switchgrass

Elevated tropospheric ozone concentration (O₃) increases oxidative stress in vegetation and threatens the stability of crop production. Current O₃ pollution in the United States is estimated to decrease the yields of maize (Zea mays) up to 10%, however, many bioenergy feedstocks including switchgrass (Panicum virgatum) have not been studied for response to O₃ stress. Using Free Air Concentration Enrichment (FACE) technology, we investigated the impacts of elevated O₃ (~100 nmol mol^-1) on leaf photosynthetic traits and capacity, chlorophyll fluorescence, the Ball⁻Woodrow⁻Berry (BWB) relationship, respiration, leaf structure, biomass and nutrient composition of switchgrass. Elevated O₃ concentration reduced net CO₂ assimilation rate (A), stomatal conductance (g_s), and maximum CO₂ saturated photosynthetic capacity (V_max), but did not affect other functional and structural traits in switchgrass or the macro- (except potassium) and micronutrient content of leaves. These results suggest that switchgrass exhibits a greater O₃ tolerance than maize, and provide important fundamental data for evaluating the yield stability of a bioenergy feedstock crop and for exploring O₃ sensitivity among bioenergy feedstocks.

Tropospheric ozone in CCMI models and Gaussian process emulation to understand biases in the SOCOLv3 chemistry-climate model

Previous multi-model intercomparisons have shown that chemistry-climate models exhibit significant biases in tropospheric ozone compared with observations. We investigate annual-mean tropospheric column ozone in 15 models participating in the SPARC/IGAC (Stratosphere-troposphere Processes and their Role in Climate/International Global Atmospheric Chemistry) Chemistry-Climate Model Initiative (CCMI). These models exhibit a positive bias, on average, of up to 40-50% in the Northern Hemisphere compared with observations derived from the Ozone Monitoring Instrument and Microwave Limb Sounder (OMI/MLS), and a negative bias of up to ~30% in the Southern Hemisphere. SOCOLv3.0 (version 3 of the Solar-Climate Ozone Links CCM), which participated in CCMI, simulates global-mean tropospheric ozone columns of 40.2 DU - approximately 33% larger than the CCMI multi-model mean. Here we introduce an updated version of SOCOLv3.0, "SOCOLv3.1", which includes an improved treatment of ozone sink processes, and results in a reduction in the tropospheric column ozone bias of up to 8 DU, mostly due to the inclusion of N₂O₅ hydrolysis on tropospheric aerosols. As a result of these developments, tropospheric column ozone amounts simulated by SOCOLv3.1 are comparable with several other CCMI models. We apply Gaussian process emulation and sensitivity analysis to understand the remaining ozone bias in SOCOLv3.1. This shows that ozone precursors (nitrogen oxides (NO_x), carbon monoxide, methane and other volatile organic compounds) are responsible for more than 90% of the variance in tropospheric ozone. However, it may not be the emissions inventories themselves that result in the bias, but how the emissions are handled in SOCOLv3.1, and we discuss this in the wider context of the other CCMI models. Given that the emissions data set to be used for phase ₆ of the Coupled Model Intercomparison Project includes approximately 20% more NO_x than the data set used for CCMI, further work is urgently needed to address the challenges of simulating sub-grid processes of importance to tropospheric ozone in the current generation of chemistry-climate models.

Evaluating a Space-Based Indicator of Surface Ozone-NO x -VOC Sensitivity Over Midlatitude Source Regions and Application to Decadal Trends

Determining effective strategies for mitigating surface ozone (O₃) pollution requires knowledge of the relative ambient concentrations of its precursors, NO _x , and VOCs. The space-based tropospheric column ratio of formaldehyde to NO₂ (FNR) has been used as an indicator to identify NO _x -limited versus NO _x -saturated O₃ formation regimes. Quantitative use of this indicator ratio is subject to three major uncertainties: (1) the split between NO _x -limited and NO _x -saturated conditions may shift in space and time, (2) the ratio of the vertically integrated column may not represent the near-surface environment, and (3) satellite products contain errors. We use the GEOS-Chem global chemical transport model to evaluate the quantitative utility of FNR observed from the Ozone Monitoring Instrument over three northern midlatitude source regions. We find that FNR in the model surface layer is a robust predictor of the simulated near-surface O₃ production regime. Extending this surface-based predictor to a column-based FNR requires accounting for differences in the HCHO and NO₂ vertical profiles. We compare four combinations of two OMI HCHO and NO₂ retrievals with modeled FNR. The spatial and temporal correlations between the modeled and satellite-derived FNR vary with the choice of NO₂ product, while the mean offset depends on the choice of HCHO product. Space-based FNR indicates that the spring transition to NO _x -limited regimes has shifted at least a month earlier over major cities (e.g., New York, London, and Seoul) between 2005 and 2015. This increase in NO _x sensitivity implies that NO _x emission controls will improve O₃ air quality more now than it would have a decade ago.

Tropospheric methanol observations from space: retrieval evaluation and constraints on the seasonality of biogenic emissions

Methanol retrievals from nadir-viewing space-based sensors offer powerful new information for quantifying methanol emissions on a global scale. Here we apply an ensemble of aircraft observations over North America to evaluate new methanol measurements from the Tropospheric Emission Spectrometer (TES) on the Aura satellite, and combine the TES data with observations from the Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp-A satellite to investigate the seasonality of methanol emissions from northern midlatitude ecosystems. Using the GEOS-Chem chemical transport model as an intercomparison platform, we find that the TES retrieval performs well when the degrees of freedom for signal (DOFS) are above 0.5, in which case the model:TES regressions are generally consistent with the model:aircraft comparisons. Including retrievals with DOFS below 0.5 degrades the comparisons, as these are excessively influenced by the a priori. The comparisons suggest DOFS >0.5 as a minimum threshold for interpreting retrievals of trace gases with a weak tropospheric signal. We analyze one full year of satellite observations and find that GEOS-Chem, driven with MEGANv2.1 biogenic emissions, underestimates observed methanol concentrations throughout the midlatitudes in springtime, with the timing of the seasonal peak in model emissions 1-2 months too late. We attribute this discrepancy to an underestimate of emissions from new leaves in MEGAN, and apply the satellite data to better quantify the seasonal change in methanol emissions for midlatitude ecosystems. The derived parameters (relative emission factors of 11.0, 0.26, 0.12 and 3.0 for new, growing, mature, and old leaves, respectively, plus a leaf area index activity factor of 0.5 for expanding canopies with leaf area index <1.2) provide a more realistic simulation of seasonal methanol concentrations in midlatitudes on the basis of both the IASI and TES measurements.

Tropospheric ozone and plants: absorption, responses, and consequences

Ozone is now considered to be the second most important gaseous pollutant in our environment. The phytotoxic potential of O₃ was first observed on grape foliage by B.L. Richards and coworkers in 1958 (Richards et al. 1958). To date, unsustainable resource utilization has turned this secondary pollutant into a major component of global climate change and a prime threat to agricultural production. The projected levels to which O₃ will increase are critically alarming and have become a major issue of concern for agriculturalists, biologists, environmentalists and others plants are soft targets for O₃. Ozone enters plants through stomata, where it disolves in the apoplastic fluid. O₃ has several potential effects on plants: direct reaction with cell membranes; conversion into ROS and H₂O₂ (which alters cellular function by causing cell death); induction of premature senescence; and induction of and up- or down-regulation of responsive components such as genes , proteins and metabolites. In this review we attempt to present an overview picture of plant O₃ interactions. We summarize the vast number of available reports on plant responses to O₃ at the morphological, physiological, cellular, biochemical levels, and address effects on crop yield, and on genes, proteins and metabolites. it is now clear that the machinery of photosynthesis, thereby decreasing the economic yield of most plants and inducing a common morphological symptom, called the "foliar injury". The "foliar injury" symptoms can be authentically utilized for biomonitoring of O₃ under natural conditions. Elevated O₃ stress has been convincingly demonstrated to trigger an antioxidative defense system in plants. The past several years have seen the development and application of high-throughput omics technologies (transcriptomics, proteomics, and metabolomics) that are capable of identifying and prolifiling the O₃-responsive components in model and nonmodel plants. Such studies have been carried out ans have generated an inventory of O₃-Responsive components--a great resource to the scientific community. Recently, it has been shown that certain organic chemicals ans elevated CO₂ levels are effective in ameliorating O₃-generated stress. Both targeted and highthroughput approaches have advanced our knowledge concerning what O₃-triggerred signaling and metabolic pathways exist in plants. Moreover, recently generated information, and several biomarkers for O₃, may, in the future, be exploited to better screen and develop O₃-tolerant plants.

Background ozone in the southern Europe and Mediterranean area: influence of the transport processes

The troposphere is subject to continuous inputs, production and removal processes of ozone and its precursors from natural processes and human activities acting together within a very complex system. In order to assess the behaviour of background ozone in the Mediterranean area, a description of trends, seasonal and diurnal behaviours of free tropospheric ozone is provided. In the Mediterranean area and southern Europe the background tropospheric ozone concentration appears significantly affected by three main air mass transport processes: (i) transport of polluted air masses on regional and long-range scales, (ii) downward transport of stratospheric air masses, and (iii) transport of mineral dust from the Sahara desert. In this review of the literature of the last two decades, we present an overview of these phenomena, mainly monitored at high baseline mountain stations representative of background atmospheric conditions.

Effects of ozone on inter- and intra-species competition and photosynthesis in mesocosms of Lolium perenne and Trifolium repens

Trifolium repens and Lolium perenne were exposed as both monocultures and two-species mixtures to an episodic rural ozone regime in large, well-watered containers within solardomes for 12 weeks. There were reductions in biomass for T. repens, but not L. perenne, and the proportion of T. repens decreased in ozone-exposed mixtures compared to the control. In addition, leaf biomass of T. repens was maintained at the expense of biomass partitioning to the stolons. The decreased growth corresponded with decreased photosynthetic capacity for T. repens, however, by the end of the exposure there was also decreased photosynthetic capacity of L. perenne, a species previously considered insensitive to ozone. The observed decreases in photosynthetic efficiency and capacity in elevated ozone indicate that the ability of such ubiquitous vegetation to act as a sink for atmospheric carbon may be reduced in future climates.

Interactions of forests with secondary air pollutants: some challenges for future research

The effects of ozone and other photochemical oxidants on individual trees have been studied for several decades, but there has been much less research on the potential effects on entire forest ecosystems. Given that ozone and other oxidants affect the production and subsequent fate of biogenic volatile organic compounds that act as signalling molecules, there is a need for more detailed study of the role of oxidants in modifying trophic interactions in forests. Deposition of fine particulates to forests may act as a source of nutrients, but also changes leaf surface properties, increasing the duration of surface wetness and modifying the habitat for epiphytic organisms, leading to increased risks from pathogens. Even where this pathway contributes a relatively small input of nutrients to forests, the indirect effects on canopy processes and subsequent deposition to the forest floor in throughfall and litter may play a more important role that has yet to be fully investigated.

Surface ozone concentrations and ecosystem health: past trends and a guide to future projections

This paper reviews current understanding of the sources and sinks of ozone in the troposphere, recent studies of long-term trends, and the factors which have to be taken into consideration when constructing and interpreting future models of ozone concentration. The factors controlling surface O(3) concentrations are discussed initially to provide a basis for the ensuing discussion, followed by a summary of the evidence for recent trends in ground-level ozone concentrations, i.e. over the past 3 decades, which have shown a significant increase in the annual average in 'background' air typical of the unpolluted northern hemisphere. Closer to precursor sources, although urban winter concentrations have increased, rural peak spring and summer concentrations during ozone 'episodes' have decreased markedly in response to emissions reductions. In order to determine whether such trends are meaningful, the statistical techniques for determining temporal trends are reviewed. The possible causes of long-term trends in ozone are then discussed, with particular reference to the use of chemistry-transport models to interpret past trends. Such models are also used to make predictions of future trends in surface ozone concentrations, but few are comprehensive in integrating future climate changes with changes in land use and in emissions of ozone precursors. Guidance is given on the likely effects of climate/precursor/chemistry interactions so that model predictions can be judged.