Photochemical ozone, the primary constituent of photochemical oxidants, is a key air pollutant that affects human health and ecosystems. It is also recognized as a short-lived climate pollutant (SLCP). Primarily produced through photochemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) at the urban and regional scales, photochemical ozone remains a serious environmental issue not only in Japan but also globally.

　In Japan, various emission reduction measures targeting NOx and VOCs have been implemented, leading to mitigation of ozone pollution. Attainment rates of the environmental quality standard (EQS) for photochemical oxidants, however, remain extremely low and pollutant concentrations continue to stay high.

　Against this background, discussions on revising the EQS for photochemical oxidants are currently underway. East Asia has been identified as a global hotspot, exhibiting the fastest increase in tropospheric ozone since the 1990s, as reported by the Intergovernmental Panel on Climate Change (IPCC) in its Sixth Assessment Report (AR6). These findings indicate that unless background ozone levels are reduced at the East-Asia scale, it will be difficult to achieve sufficient reductions in the concentration of photochemical oxidants in Japan. Furthermore, as climate change progresses, understanding the interactions between tropospheric ozone and climate change, and addressing both issues simultaneously, has become a critical global challenge.

　This special issue focuses on the current state of photochemical ozone at both the global and regional scales, summarizing its impacts and discussing potential mitigation strategies. The articles included in this issue address a wide range of topics related to photochemical ozone, including present ozone concentration levels, effects on human health and vegetation, and approaches toward mitigation. We are deeply grateful to the authors for contributing their valuable scientific insights and technical expertise toward improving ozone pollution. We sincerely hope that the knowledge presented herein will contribute to ongoing and future efforts to mitigate photochemical ozone at both the global and regional scales.

Guest Editors

Toshimasa OHARA

Asia Center for Air Pollution Research, Japan Environmental Sanitation Center

Hiroshi NITTA

Association of International Research Initiative for Environmental Studies

Mitsuko AONO

Mukogawa Women’s University

　Tropospheric ozone levels are changing dynamically over the entire globe, including the Asia-Pacific region, due to changes in precursor emissions and environmental conditions. These changes have implications for climate and air quality. These global and regional-scale variabilities determine background ozone levels in Japan. Local ozone buildup on top of the background levels may result in worrisome ozone levels. This study summarizes the latest understanding of the global and regional ozone abundance and trends, emphasizing chemical conditions. It uses the latest IPCC assessment report (AR6) and publications from the Tropospheric Ozone Assessment Report Phases I and II (TOAR and TOAR-II). The overall positive global tropospheric ozone burden trend from the pre-industrial era, which has contributed to global warming alongside CO₂ and methane, has recently been found to have positive and negative regional trends, particularly for the surface levels over the last decade. Positive trends in the tropospheric ozone burden have been evident in Asia, but a slowdown has been suggested in China since around 2019. This aligns with the most recent leveling-off trend in the number of high ozone events (e.g., 90 ppb) observed in western Japan during spring, when transboundary air pollution from the continent has been a concern. Photochemical smog events with warning levels of ozone (120 ppb) in urban areas of Japan in the summertime arise with clean marine air masses south of Japan containing very low ozone levels (10–20 ppb) as its background, for which predictive capability has been limited. Recently, iodine chemistry was identified as a potential cause of such low background ozone levels. Closing knowledge gaps in other areas is also needed to improve our understanding of ozone formation processes. This includes the potential role of the heterogeneous loss of HO₂ radicals on aerosol surfaces in affecting ozone production efficiency. Recent progress with satellite observations and reanalysis has also been significant. Further studies on ozone processes could help improve our understanding of its current status and the potential implications of its reduction for climate and human health.

　Surface ozone is a major air pollutant that affects human health and ecosystems. It has become a significant atmospheric environmental issue not only in Japan, but also in East Asia and globally. According to an analysis of continuous air quality monitoring data in Japan, surface ozone concentrations have shown a long-term decreasing trend due to reductions in emissions of ozone precursors. The current concentration levels, however, greatly exceed the new National Ambient Air Quality Standard (NAAQS) for photochemical oxidants that has been proposed by the Ministry of the Environment, Japan. The annual 99th percentile of daily maximum 8-hour averages at five remote sites around Japan ranged from 59.9 ppb to 89.5 ppb over the most recent three years (FY2020–FY2022). At two sites, the levels exceeded the proposed short-term NAAQS of 70 ppb, while the remaining three sites all exceeded or nearly reached 60 ppb. Notably, even the site with the lowest concentration substantially exceeded the World Health Organization’s recommended short-term air quality guideline level of 50 ppb. Because of the high background ozone levels, domestic measures alone would be insufficient to achieve the new NAAQS for photochemical oxidants in Japan. We thus explore the importance of reducing background ozone levels at the East-Asia scale through international cooperation, and emphasize the value of co-benefit approaches that simultaneously address decarbonization and surface ozone mitigation. Finally, based on our analytical results, we have presented policy recommendations for reducing surface ozone in Japan.

　Effectiveness of mitigation measures implemented in Japan on ambient ozone concentrations were evaluated by performing long-term air quality simulations for the years 2000 to 2020. The long-term emission inventories fed into the long-term air quality simulations indicated that precursor emissions had been effectively reduced by mitigation measures implemented in Japan. The long-term air quality simulations showed good performance in reproducing long-term trends in annual mean and annual top 10-day mean values of daily maximum 8-hour (MDA8) ozone concentrations. Simulations for multiple cases suggested that mitigation measures implemented in Japan were effective at suppressing annual high ozone concentrations, whereas annual mean ozone concentrations were increased as a result of suppressed titration of ozone due to reduced NO_x emissions. The evidence obtained through these long-term air quality simulations is valuable when considering effective strategies to suppress ambient ozone concentrations further in Japan.

　Much scientific knowledge has accumulated on the health effects of ozone (O₃), the main component of photochemical oxidants produced by photochemical reactions. These findings are mainly based on controlled human exposure studies and epidemiological studies. This paper presents an overview of this scientific evidence and discusses the O₃ concentration levels at which health effects are observed. In Japan, the Ministry of the Environment has been reevaluating Japan’s Ambient Air Quality Standards for photochemical oxidants, and has recently established an 8-hour environmental target value for O₃ as a short-term exposure standard, as well as an annual average environmental target value for O₃ as a long-term exposure standard.

　Ozone (O₃) is the main component of photochemical oxidants, and is a gaseous air pollutant that is harmful to plants. In Japan, tropospheric O₃ concentrations have been relatively high in recent years, and are likely to have negative impacts on plants. In this review, I will examine 1) the effects of O₃ on dry matter growth of Japanese tree species, 2) the effects of O₃ on the physiological functions, such as photosynthesis, of Japanese tree species, 3) O₃ risk assessments for Japanese forest trees, and 4) recent studies on the evaluation of canopy O₃ uptake in mature Japanese trees, based on the experimental studies and field surveys conducted in Japan over the past nearly 40 years.

　Ground-level ozone (O₃), a major photochemical oxidant, is found at elevated levels in many regions of the world, including Japan. Ozone is believed to adversely affect the growth and yield of various plants worldwide. This review highlights the diverse effects of ozone on visible foliar injury, leaf physiology, growth and plant yield, with a focus on agricultural and horticultural crops in Japan and an evaluation of the risks from ozone. Several studies have reported adverse effects of ozone on crops such as japonica rice, soybeans, potatoes, cucumbers, Japanese mustard spinach and radishes in Japan. According to these studies, the extent of effects caused by ozone varies not only among different crop species but also among their varieties. The level of visible foliar damage to crops caused by ozone, however, does not necessarily align with the extent of growth reduction or yield loss. Ozone risk assessment reports for Japanese crops are limited and only a few studies have assessed rice. Some reports suggest that japonica rice yields in urban areas of Japan may have decreased by up to 10% owing to current ozone levels. These findings provide valuable insights for future crop- and environment-specific studies on ozone risk assessment and agricultural planning.

　Ozone (O₃) is a main component of photochemical oxidants. In Japan, O₃ at current levels has negative impacts on Japanese agricultural crops and trees. It is well known that environmental factors such as air temperature alter the negative impacts of O₃. Since the Industrial Revolution, long-term shifts in temperature and weather patterns, known as climate change, have occurred. In the future, because climate change and an increase in nitrogen (N) deposition are projected, the negative impact of O₃ will be altered by these environmental changes. This review summarizes experimental studies conducted in Japan on the combined effects of O₃ and elevated air temperature, soil water stress, elevated CO₂ and soil N supply on plants. It was found that the negative impacts of O₃ are often exacerbated by elevated air temperatures, whereas these effects are mitigated or partially offset by soil water stress and elevated CO₂. Soil N supply reduces and increases the O₃ sensitivity of Japanese larch and Japanese beech, respectively. Although the degree of modification differs among plant species, growing conditions and the extent of changes in these factors, it is obvious that the degree of negative O₃ effects on plants is modified by environmental changes. Because the modification of negative O₃ effects can be caused by altered stomatal O₃ uptake and/or O₃ detoxification capacity in the leaves, O₃ risk assessments for plants under changing environments should be based on effective O₃ flux, which can be evaluated by both stomatal O₃ uptake and O₃ detoxification capacity in the leaves.

　This study investigated the climate impacts of regional reductions in anthropogenic emissions of NOx, CO and VOCs using a chemistry–climate model. Sensitivity simulations were conducted to evaluate changes in surface and tropospheric ozone concentrations and associated degrees of radiative forcing, with a focus on key source regions including Asia, North America and Europe, as well as emissions from aircraft (for nitrogen oxides, NOx) and biomass burning (for carbon monoxide, CO). The results showed that NOx emission reductions in Asia lead to the largest regional decrease in surface ozone (~15 ppbv), while reductions in CO and VOCs yield broader but smaller-scale decreases due to their longer atmospheric lifetimes. However, although reducing NOx decreases tropospheric ozone and provides an associated negative radiative forcing (cooling), they simultaneously cause significant warming via aerosol–radiation interactions and increased methane concentrations driven by reduced hydroxyl radicals (OH). As a result, the net radiative forcing from NOx reduction is positive (warming), particularly in Asia. In contrast, CO and VOC reductions consistently produce net cooling effects through decreased ozone and methane levels, with CO reduction in Asia and from biomass burning offering particularly strong mitigation benefits. These findings highlight the complex trade-offs involved in precursor emission control and emphasize the importance of prioritizing CO and VOC reduction for effective climate mitigation.

　Progress toward a decarbonized society aimed at mitigating climate change will have a significant impact on air pollutant emissions and is therefore a crucial factor to take into account when considering the future of ozone (O₃) concentrations in Japan. To assess the impact of changes in air pollutant emissions resulting from decarbonization in Japan toward the 2050 Carbon Neutrality (2050CN) target and those in other Asian countries as well as the potential impact of climate change up to 2050 on future surface ozone (O₃) concentrations in Japan, we developed future emission and climate change scenarios and performed regional-scale air quality model simulations. Four future emission scenarios were developed, including one that achieves the 2050CN target and others that do not: two baseline scenarios (FIX and BAU) and two decarbonization scenarios (20D and 15D), where the 15D scenario corresponds to achieving 2050CN target in Japan. In the decarbonization scenarios O₃ precursor emissions such as NOx and NMVOCs both decrease in Japan, China, and South Korea. Air quality model projections of surface O₃ concentrations in Japan varied significantly depending on which future emission scenario was used. In the baseline scenarios (FIX, BAU), surface O₃ concentrations in 2050 would increase slightly compared to the present, while in the decarbonization scenarios (20D, 15D), they would decrease significantly nationwide in Japan, with a maximum decrease of 20 ppb compared to the present. These results clearly demonstrate the synergistic effects of decarbonization and air pollution mitigation in Japan.