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Ozone holds a certain fascination in atmospheric science. It is ubiquitous in the atmosphere, central to tropospheric oxidation chemistry, yet harmful to human and ecosystem health as well as being an important greenhouse gas. It is not emitted into the atmosphere but is a byproduct of the very oxidation chemistry it largely initiates. Much effort is focused on the reduction of surface levels of ozone owing to its health and vegetation impacts, but recent efforts to achieve reductions in exposure at a country scale have proved difficult to achieve owing to increases in background ozone at the zonal hemispheric scale. There is also a growing realisation that the role of ozone as a short-lived climate pollutant could be important in integrated air quality climate change mitigation. This review examines current understanding of the processes regulating tropospheric ozone at global to local scales from both measurements and models. It takes the view that knowledge across the scales is important for dealing with air quality and climate change in a synergistic manner. The review shows that there remain a number of clear challenges for ozone such as explaining surface trends, incorporating new chemical understanding, ozone–climate coupling, and a better assessment of impacts. There is a clear and present need to treat ozone across the range of scales, a transboundary issue, but with an emphasis on the hemispheric scales. New observational opportunities are offered both by satellites and small sensors that bridge the scales.

Dry deposition to the Earth's surface is an important process from both an atmospheric and biospheric perspective. Dry deposition controls the atmospheric abundance of many compounds as well as their input to vegetative surfaces, thus linking the atmosphere and biosphere. In many atmospheric and Earth system models it is represented using \"resistance in series\" schemes developed in the 1980s. These methods have remained relatively unchanged since their development and do not take into account more recent understanding of the underlying processes that have been gained through field and laboratory based studies. In this study we compare dry deposition of ozone across 15 models which contributed to the TF HTAP model intercomparison to identify where differences occur. We compare modelled dry deposition of ozone to measurements made at a variety of locations in Europe and North America, noting differences of up to a factor of two but no clear systematic bias over the sites examined. We identify a number of measures that are needed to provide a more critical evaluation of dry deposition fluxes and advance model development.

Surface ozone measurements have been made for the first time at Nainital (29.37°N, 79.45°E, 1958 m amsl), a high‐altitude site in the central Himalayas, between October 2006 and December 2008. Diurnal variations in ozone do not show the daytime photochemical build‐up typical of urban or rural sites. The seasonal variation shows a distinct ozone maximum in late spring (May; 67.2 ± 14.2 ppbv) with values sometimes exceeding 100 ppbv and a minimum in the summer/monsoon season (August; 24.9 ± 8.4 ppbv). Springtime ozone values in the central Himalayas are significantly higher than those at another high‐altitude site (Mt. Abu) in the western part of India. Seasonal variations in ozone and the processes responsible for the springtime peak are studied using meteorological parameters, insolation, spatial and temporal classifications of air mass trajectories, fire counts, and simulations with a chemical transport model. Net ozone production over the Northern Indian Subcontinent in regionally polluted air masses is estimated to be 3.2 ppbv/day in spring but no clear build‐up is seen at other times of year. Annual average ozone values in regionally polluted air masses (47.1 ± 16.7 ppbv) and on high insolation days (46.8 ± 17.3 ppbv) are similar. Background ozone levels are estimated to be 30–35 ppbv. Regional pollution is shown to have maximum contribution (16.5 ppbv) to ozone levels during May–June and is about 7 ppbv on an annual basis, while the contribution of long‐range transport is greatest during January–March (8–11 ppbv). The modeled stratospheric ozone contribution is 2–16 ppbv. Both the trajectory analysis and the model suggest that the stratospheric contribution is 4–6 ppbv greater than the contribution from regional pollution. Differences in the seasonal variation of ozone over high‐altitude sites in the central Himalayas (Nainital) and western India (Mt. Abu) suggest diverse regional emission sources in India and highlight the large spatial and temporal variability in ozone over the Indian region.

Lightning is an important natural source of nitrogen oxide especially in the middle and upper troposphere. Hence, it is essential to represent lightning in chemistry transport and coupled chemistry–climate models. Using ERA-Interim meteorological reanalysis data we compare the lightning flash density distributions produced using several existing lightning parametrisations, as well as a new parametrisation developed on the basis of upward cloud ice flux at 440 hPa. The use of ice flux forms a link to the non-inductive charging mechanism of thunderstorms. Spatial and temporal distributions of lightning flash density are compared to tropical and subtropical observations for 2007–2011 from the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite. The well-used lightning flash parametrisation based on cloud-top height has large biases but the derived annual total flash density has a better spatial correlation with the LIS observations than other existing parametrisations. A comparison of flash density simulated by the different schemes shows that the cloud-top height parametrisation has many more instances of moderate flash densities and fewer low and high extremes compared to the other parametrisations. Other studies in the literature have shown that this feature of the cloud-top height parametrisation is in contrast to lightning observations over certain regions. Our new ice flux parametrisation shows a clear improvement over all the existing parametrisations with lower root mean square errors (RMSEs) and better spatial correlations with the observations for distributions of annual total, and seasonal and interannual variations. The greatest improvement with the new parametrisation is a more realistic representation of the zonal distribution with a better balance between tropical and subtropical lightning flash estimates. The new parametrisation is appropriate for testing in chemistry transport and chemistry–climate models that use a lightning parametrisation.

Stratospheric ozone and associated climate impacts in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) simulations are evaluated in the recent past (1980–2000), and examined in the long-term (1850–2100) using the Representative Concentration Pathways (RCPs) low- and high-emission scenarios (RCP2.6 and RCP8.5, respectively) for the period 2000–2100. ACCMIP multi-model mean total column ozone (TCO) trends compare favourably, within uncertainty estimates, against observations. Particularly good agreement is seen in the Antarctic austral spring (−11.9 % dec−1 compared to observed  ∼  −13.9 ± 10.4 % dec−1), although larger deviations are found in the Arctic's boreal spring (−2.1 % dec−1 compared to observed  ∼  −5.3 ± 3.3 % dec−1). The simulated ozone hole has cooled the lower stratosphere during austral spring in the last few decades (−2.2 K dec−1). This cooling results in Southern Hemisphere summertime tropospheric circulation changes captured by an increase in the Southern Annular Mode (SAM) index (1.3 hPa dec−1). In the future, the interplay between the ozone hole recovery and greenhouse gases (GHGs) concentrations may result in the SAM index returning to pre-ozone hole levels or even with a more positive phase from around the second half of the century (−0.4 and 0.3 hPa dec−1 for the RCP2.6 and RCP8.5, respectively). By 2100, stratospheric ozone sensitivity to GHG concentrations is greatest in the Arctic and Northern Hemisphere midlatitudes (37.7 and 16.1 DU difference between the RCP2.6 and RCP8.5, respectively), and smallest over the tropics and Antarctica continent (2.5 and 8.1 DU respectively). Future TCO changes in the tropics are mainly determined by the upper stratospheric ozone sensitivity to GHG concentrations, due to a large compensation between tropospheric and lower stratospheric column ozone changes in the two RCP scenarios. These results demonstrate how changes in stratospheric ozone are tightly linked to climate and show the benefit of including the processes interactively in climate models.

A lightning parametrisation based on upward cloud ice flux is implemented in a chemistry–climate model (CCM) for the first time. The UK Chemistry and Aerosols model is used to study the impact of these lightning nitric oxide (NO) emissions on ozone. Comparisons are then made between the new ice flux parametrisation and the commonly used, cloud-top height parametrisation. The ice flux approach improves the simulation of lightning and the temporal correlations with ozone sonde measurements in the middle and upper troposphere. Peak values of ozone in these regions are attributed to high lightning NO emissions. The ice flux approach reduces the overestimation of tropical lightning apparent in this CCM when using the cloud-top approach. This results in less NO emission in the tropical upper troposphere and more in the extratropics when using the ice flux scheme. In the tropical upper troposphere the reduction in ozone concentration is around 5–10 %. Surprisingly, there is only a small reduction in tropospheric ozone burden when using the ice flux approach. The greatest absolute change in ozone burden is found in the lower stratosphere, suggesting that much of the ozone produced in the upper troposphere is transported to higher altitudes. Major differences in the frequency distribution of flash rates for the two approaches are found. The cloud-top height scheme has lower maximum flash rates and more mid-range flash rates than the ice flux scheme. The initial Ox (odd oxygen species) production associated with the frequency distribution of continental lightning is analysed to show that higher flash rates are less efficient at producing Ox; low flash rates initially produce around 10 times more Ox per flash than high-end flash rates. We find that the newly implemented lightning scheme performs favourably compared to the cloud-top scheme with respect to simulation of lightning and tropospheric ozone. This alternative lightning scheme shows spatial and temporal differences in ozone chemistry which may have implications for comparison between models and observations, as well as for simulation of future changes in tropospheric ozone.

Many plant species used for biofuel emit more isoprene—an ozone precursor—than the traditional crops they are replacing. A modelling study now indicates the potential for significant human mortality and crop losses due to changes in ground-level ozone concentrations that could arise from large-scale biofuel cultivation in Europe. These findings suggest that biofuel policies could have adverse consequences that should be evaluated alongside carbon-budgeting considerations before large-scale policies are implemented. Ground-level ozone is a priority air pollutant, causing ∼ 22,000 excess deaths per year in Europe 1 , significant reductions in crop yields 2 and loss of biodiversity 3 . It is produced in the troposphere through photochemical reactions involving oxides of nitrogen (NO x ) and volatile organic compounds (VOCs). The biosphere is the main source of VOCs, with an estimated 1,150 TgC yr −1 (∼ 90% of total VOC emissions) released from vegetation globally 4 . Isoprene (2-methyl-1,3-butadiene) is the most significant biogenic VOC in terms of mass (around 500 TgC yr −1 ) and chemical reactivity 4 and plays an important role in the mediation of ground-level ozone concentrations 5 . Concerns about climate change and energy security are driving an aggressive expansion of bioenergy crop production and many of these plant species emit more isoprene than the traditional crops they are replacing. Here we quantify the increases in isoprene emission rates caused by cultivation of 72 Mha of biofuel crops in Europe. We then estimate the resultant changes in ground-level ozone concentrations and the impacts on human mortality and crop yields that these could cause. Our study highlights the need to consider more than simple carbon budgets when considering the cultivation of biofuel feedstock crops for greenhouse-gas mitigation.

This study describes a simple parameterization to estimate regionally averaged changes in surface ozone due to past or future changes in anthropogenic precursor emissions based on results from 14 global chemistry transport models. The method successfully reproduces the results of full simulations with these models. For a given emission scenario it provides the ensemble mean surface ozone change, a regional source attribution for each change, and an estimate of the associated uncertainty as represented by the variation between models. Using the Representative Concentration Pathway (RCP) emission scenarios as an example, we show how regional surface ozone is likely to respond to emission changes by 2050 and how changes in precursor emissions and atmospheric methane contribute to this. Surface ozone changes are substantially smaller than expected with the SRES A1B, A2 and B2 scenarios, with annual global mean reductions of as much as 2 ppb by 2050 vs. increases of 4–6 ppb under SRES, and this reflects the assumptions of more stringent precursor emission controls under the RCP scenarios. We find an average difference of around 5 ppb between the outlying RCP 2.6 and RCP 8.5 scenarios, about 75% of which can be attributed to differences in methane abundance. The study reveals the increasing importance of limiting atmospheric methane growth as emissions of other precursors are controlled, but highlights differences in modelled ozone responses to methane changes of as much as a factor of two, indicating that this remains a major uncertainty in current models.

Large-scale production of feedstock crops for biofuels will lead to land use changes. We quantify the effects of realistic land use change scenarios for biofuel feedstock production on isoprene emissions and hence atmospheric composition and chemistry using the HadGEM2 model. Two feedstocks are considered: oil palm for biodiesel in the tropics and short rotation coppice (SRC) in the mid-latitudes. In total, 69 Mha of oil palm and 9 Mha of SRC are planted, each sufficient to replace just over 1% of projected global fossil fuel demand in 2020. Both planting scenarios result in increases in total global annual isoprene emissions of about 1%. In each case, changes in surface concentrations of ozone and biogenic secondary organic aerosol (bSOA) are substantial at the regional scale, with implications for air quality standards. However, the changes in tropospheric burden of ozone and the OH radical, and hence effects on global climate, are negligible. Over SE Asia, one region of oil palm planting, increases in annual mean surface ozone and bSOA concentrations reach over 3 ppbv (+11%) and 0.4 μg m−3 (+10%) respectively for parts of Borneo, with monthly mean increases of up to 6.5 ppbv (+25%) and 0.5 μg m−3 (+12%). Under the SRC scenario, Europe experiences monthly mean changes of over 0.6 ppbv (+1%) and 0.1 μg m−3 (+5%) in June and July, with peak increases of over 2 ppbv (+3%) and 0.5 μg m−3 (+8 %). That appreciable regional atmospheric impacts result from low level planting scenarios demonstrates the need to include changes in emissions of reactive trace gases such as isoprene in life cycle assessments performed on potential biofuel feedstocks.

Japan lies downwind of the Asian continent and for much of the year air quality is directly influenced by emissions of ozone precursors over these heavily-populated and rapidly-industrializing regions. This study examines the extent to which oxidant transport from regional and distant anthropogenic sources influences air quality in Japan in springtime, when these contributions are largest. We find that European and North American contributions to surface ozone over Japan in spring are persistent, averaging 3.5±1.1 ppb and 2.8±0.5 ppb respectively, and are greatest in cold continental outflow conditions following the passage of cold fronts. Contributions from China are larger, 4.0±2.8 ppb, and more variable, as expected for a closer source region, and are generally highest near cold fronts preceding the influence of more distant sources. The stratosphere provides a varying but ever-present background of ozone of about 11.2±2.5 ppb during spring. Local sources over Japan and Korea have a relatively small impact on mean ozone, 2.4±7.6 ppb, but this masks a strong diurnal signal, and local sources clearly dominate during episodes of high daytime ozone. By examining the meteorological mechanisms that favour transport from different source regions, we demonstrate that while maximum foreign influence generally does not occur at the same time as the greatest buildup of oxidants from local sources, it retains a significant influence under these conditions. It is thus clear that while meteorological boundaries provide some protection from foreign influence during oxidant outbreaks in Tokyo, these distant sources still make a substantial contribution to exceedance of the Japanese ozone air quality standard in springtime.