[ Review Article ]

Asian Journal of Atmospheric Environment - Vol. 16, No. 3, pp.1-11

ISSN: 1976-6912 (Print) 2287-1160 (Online)

Print publication date 30 Sep 2022

Received 10 Mar 2022 Revised 07 Jun 2022 Accepted 01 Aug 2022

DOI: https://doi.org/10.5572/ajae.2022.021

Current Status of Ozone Control Measures in the United States and Europe and Implications for Japan

Hideki Hasunuma¹⁾^{, 2)}^{, *}

; Liliana Martinez Rivera¹⁾^{, 3)} ; Hirosato Kobayashi¹⁾ ; Kenji Aizu¹⁾ ; Kazunori Oshima¹⁾ ; Jun Shibutani¹⁾ ; Yasuyuki Itano⁴⁾ ; Satoru Chatani⁵⁾ ; Shuichi Hasegawa⁶⁾ ; Makiko Yamagami⁷⁾ ; Junya Hoshi⁸⁾

1)Center for Environmental Information Science, Tokyo, Japan
2)Department of Public Health, Hyogo Medical University, Nishinomiya, Hyogo, Japan
3)Graduate School of Public Policy and Social Governance, Hosei University, Tokyo, Japan
4)Osaka City Institute of Public Health and Environmental Sciences, Osaka, Japan
5)National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan
6)Center for Environmental Science in Saitama, Kazo, Saitama, Japan
7)Nagoya City Institute for Environmental Sciences, Nagoya, Aichi, Japan
8)Tokyo Metropolitan Research Institute for Environmental Protection, Tokyo, Japan

Correspondence to: ^* Tel: +81-798-45-6566 E-mail: hi-hasunuma@hyo-med.ac.jp

Copyright © 2022 by Asian Association for Atmospheric Environment
This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In Japan, the achievement rate of environmental standards for air pollution has been improving in recent years, but for photochemical oxidants, including ozone, improvement are required. In this study, we investigated trends in ozone countermeasures in the United States and Europe from the State Implementation Plan (SIP) and National Air Pollution Control Program (NAPCP) and examined whether there are any points recommended in Japan. The United States and Europe have different policies on environmental standards for air pollution and ozone control. In the United States, states that do not meet the environmental standards for ozone concentration are required to develop an SIP to attain the standards. There was an urgent need to find a cost-effective approach to addressing the ozone problem, and forest control measures were one of the possibilities. In Europe, the European Union has set “target values” for ozone and NAPCP does not mention forest control measures or ozone sensitivity regimes. The main focus in Europe is on the win-win solution of reducing emissions of air pollutants at the same time as greenhouse gases. Japan should consider a framework including setting feasible step-by-step goals to attain the desired standards. As ozone is greatly affected by advection, wide-regional measures against ozone precursors and prediction of the future precursor reduction and concentration are required. The preparation of an emissions inventory and estimated reduction amount is prerequisites as basic data for simulation. To achieve this, cooperation between national, local and private research institutes is crucial. Since the international community has agreed to prioritize greenhouse gas countermeasures, Japan could consider a win-win solution for both ozone and greenhouse gas reduction.

Keywords:

Ozone, Photochemical oxidants, Policy, Control measure, State implementation plan, National air pollution control program, Countermeasure strategy

1. INTRODUCTION

In Japan, the achievement rate of environmental standards for air pollutants such as nitrogen dioxide (NO₂) and fine particulate matter (PM_2.5) has been improving in recent years. However, progress in achieving environmental standards for photochemical oxidants remains slow. In our previous research, we found that implementing measures against photochemical oxidants was a problem for local governments (CEIS, 2016). Although some reports focus on overseas trends of PM_2.5 countermeasures (Wakamatsu, 2011), no studies systematically report trends in photochemical oxidant countermeasures.

The purpose of this study is to understand the current situation of photochemical oxidant countermeasures in the United States (US) and Europe and to examine whether there are any points recommended in Japan. In the US and Europe, the environmental standard is set by the ozone concentration, whereas in Japan, it is set by the photochemical oxidant concentration. Most photochemical oxidants are ozone. Due to the differences among the US, Europe and Japan, the terms ozone and photochemical oxidant are used interchangeably in this study.

2. METHODS

The trends in ozone countermeasures in the US and Europe were investigated using State Implementation Plan (SIP) sources in 50 states and National Air Pollution Control Program (NAPCP) sources in 28 European Union (EU) countries, respectively. To examine the background and processes to develop the SIPs and NAPCPs, we selected several regions and conducted interviews with the regulatory bodies of the administrative agencies cooperating with our study.

2. 1 Ozone Control Measures in the United States

In the US, the Clean Air Act requires the United States Environmental Protection Agency (US EPA) to set National Ambient Air Quality Standards (NAAQS) for pollutants common in outdoor air and considered harmful to public health and the environment. The Clean Air Act requires states to develop SIPs: both a general plan to attain and maintain the NAAQS in all areas of the country and a specific plan to attain the NAAQS for areas designated nonattainment. In this study, we obtained SIPs for states in areas where environmental standards were not met in each base year of the ozone standard (1997, 2008, and 2015). This was done by accessing the official website of the environmental department of each state and searching the web page related to atmospheric environment protection. Each SIP showed the current state of air pollutant concentration, source analysis, countermeasure plans for achievement, and future forecasts. In addition, we examined the trend of ozone countermeasures in the US by investigating the SIP of each state over time. California and Washington DC were selected as the target states because they have warm climates similar to Japan’s and their SIPs showed a reduction due to their countermeasures. The field interview surveys were conducted on January 13, 2020, January 15, 2020, and January 17, 2020, with the cooperation from California, US EPA, and Washington DC, respectively.

2. 2 Ozone Control Measures in the European Union

In the EU, ambient air quality standards are set out in the Ambient Air Quality Directives, to protect human health and the environment. The directives require member states to assess air quality in all their territories and adopt and implement plans to improve air quality where standards are not met and to maintain air quality where it is good. National emission reduction commitments were established in the National Emission Ceilings (NEC) Directive, which required member states to develop NAPCPs to comply with these commitments (EEA, 2019b).

In this study, we examined the laws and regulations from the whole of Europe using information released by EU government agencies and examined the NAPCP of each member state. Germany, which has abundant ozone measures, was selected as the target country. The interview survey was conducted on October 13, 2021, and November 8, 2021, in cooperation with Germany and the European Commission.

3. RESULTS

3. 1 United States

3. 1. 1 Framework of Ozone Control Measures

In the US, the Clean Air Act requires states with nonattainment areas to adopt additional regulatory programs designed to achieve and maintain the NAAQS standards. If a state fails to meet its obligations, penalties are applied until the standard is attained. Four elements shown in Table 1 are important in a SIP attainment demonstration. Air quality monitoring is required to determine whether areas meet the NAAQS. The Act require nonattainment areas to submit emissions inventory for the baseline year and every three-year period thereafter until the attainment date. Air Quality Modeling and attainment demonstration by control measures is also required (CARB, 2018, 2017; US EPA, 2018).

Table 1.

Four elements in a SIP attainment demonstration.

3. 1. 2 Ozone Standards and Achievement Status

The US statute established two types of national air quality standards: primary and secondary. Primary standards are designed to protect public health, while secondary standards are designed to protect the public welfare from adverse effects on soils, water, crops, vegetation, anthropogenic materials, animals, wildlife, weather, visibility, and climate. The Clean Air Act requires the US EPA to review the ozone standards every five years, to determine whether they should be revised in light of the latest science. The ozone National Ambient Air Quality Standards were set at 0.08, 0.075, and 0.070 ppm (averaged over 8 h) in 1997, 2008, and 2015, respectively. This standard is met at an air quality monitor when the 3-year average of the annual fourth-highest daily maximum 8-h average ozone concentration is less than or equal to the current standard (US EPA, 2015).

The number of states with 8-h ozone nonattainment areas has increased over time: with 14 states in 1997, 17 in 2008, and 23 in 2015. The 8-h ozone nonattainment areas occurred on the east and west coasts.

3. 1. 3 Sources, Causes, Status of Efforts, and Countermeasures in California

Fifty years ago, California had a history of serious air pollution. In order to overcome this, it became one of the most advanced states in relation to pollution control by strengthening measures against road transport sources. The causes of air pollution are thought to be transport, forest fires, weather, global warming, geographic factors (sea breezes and mountains), and cross-border pollution from Mexico. The California Air Resources Board and local governments are responsible for SIP development in California.

As control strategy from the above four elements, various stakeholders such as the public, private companies, and scientists, were involved in developing measures from the perspectives of economic and technological feasibility, and enforceability. These measures combined regulations, financial incentives, and voluntary actions. For instance, the regulation requires older vehicles to have exhaust filters installed. The Carl Moyer Program as incentive program provides monetary grants to those who clean up their heavy-duty engines beyond the level required by law through retrofitting, re-powering, or replacing their engines with newer and cleaner models. Other examples include vessels voluntarily slowing down when approaching a port to reduce emissions, and carpooling to reduce traffic congestion. In California, incentive programs involving the public are regarded as crucial, and in recent years, the budget for incentive programs has significantly increased (CARB, 2018, 2017).

The Zero Emission Vehicle (ZEV) regulation is a measure that has received particular attention in California. This regulation is designed to achieve the state’s long-term emission reduction goals by requiring auto manufacturers to sell specific numbers of the cleanest cars available. These vehicle technologies include full battery-electric vehicles, hydrogen fuel cells, and plug-in hybrid electric vehicles. The ZEV regulation requires auto manufacturers in California to produce a certain percentage of ZEVs and plug-in hybrids each year, ranging from 4.5% in 2018 to 22% by 2025.

3. 1. 4 Sources, Causes, Status of Efforts, and Countermeasures in Washington DC

The SIP for the 8-h ozone standard for the Washington nonattainment area was developed by the Metropolitan Washington Air Quality Committee in cooperation with Maryland, Virginia, and the District of Columbia (MWCOG, 2017, 2007). They have taken the stance that the ozone problem is regional, given that smog travels across state lines. Ozone countermeasures cannot, therefore, be implemented by just one state. As a result local governments in the Northeast and Mid-Atlantic regions formed a multi-state organization called the Ozone Transport Commission (OTC). The OTC brings states together to coordinate reductions in air pollution that benefit the entire region. They provide air pollution assessments, technical support, and a forum through which states can collaborate to harmonize their pollution reduction strategies. The OTC members include Connecticut, Delaware, the District of Columbia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and Virginia. The atmospheric environment model is implemented by OTC and supported by the US EPA. The OTC has three main roles. First, it estimates the ozone concentration using an estimation model because each state does not have sufficient resources (e.g., such as staff) to do this. Second, it evaluates the cost of the countermeasures. Third, representatives of corporate organizations participate in the SIP plan while discussing and communicating how stakeholders (for example, companies) can support on the state’s efforts. An extensive list of potential control measures is analyzed and evaluated against the following criteria: a) economic feasibility proposed as a cost of $3,500-$5,000 per ton or less of nitrogen oxides (NOx) or non-methane volatile organic compounds (NMVOC), b) emissions from the source under control exceed a threshold, proposed as 0.1 tons per day, and c) enforceability and technical feasibility. As a particularly noteworthy measure, starting in the summer of 2021, power plants in 12 states will be required to cut smog-forming emissions of NOx contributing to unhealthy air quality in communities downwind by installing, improving or upgrading pollution controls (US EPA, 2021).

3. 2 European Union

3. 2. 1 Framework of Ozone Control Measures

The clean air legislation framework in the EU is based on three main pillars: a) the Ambient Air Quality Directive (2008/50/EC), b) the National Emissions Ceilings Directive (2016/2284/EU), and c) source-specific legislation on emission standards such as the Industrial Emissions Directive, Medium Combustion Plants Directive, energy efficiency, vehicle emission standards (Euro) and fuel quality standards. The Ambient Air Quality Directive sets out ambient air quality standards and requires member states to assess air quality and implement plans to improve or maintain it. The NEC Directive requires member states to develop NAPCP to comply with their emission reduction commitments for five main air pollutants (NOx, NMVOC, PM_2.5, sulfur dioxide (SO₂), and ammonia (NH₃)) and was enforced on December 31, 2016. The reduction commitments were designed to reduce the number of premature deaths caused by exposure to air pollution by half by 2030 (compared to 2005). The source-specific legislation on emission standards is set out in EU legislation and targets industrial emissions, emissions from power plants, vehicles, non-road mobile machinery and transport fuels, as well as the energy performance of products. Member states work on domestic measures based on their respective directives (EEA, 2019b).

3. 2. 2 Ozone Standards and Achievement Status

Air quality standards in the EU were set for the protection of health and the protection of vegetation. The EU has set a target value for ozone to protect human health: the maximum daily 8-h mean may not exceed 120 micrograms per cubic meter (μg/m³) on more than 25 days per calendar year averaged over three years. The 93.2 percentile of the ozone maximum daily 8-h mean, representing the 26th highest value in a complete series of 365 days, is related to the ozone target value. Out of 28 member states, 17 had monitoring stations above the ozone target value in 2017. One of the regions where the highest ozone concentrations were measured is the Mediterranean areas (EEA, 2019a). Trend analysis for the period 2009-2018 showed no significant increasing or decreasing trend for the 93.2 percentile of the maximum daily 8-h mean ozone concentration in 95% of the measuring stations in the EU. Emissions of NOx, a precursor of ozone, in the EU region were reduced by about 20% (from 9,515 to 7,532 Gg) from 2010 to 2017, and non-methane volatile organic compound (NMVOC) emissions were also reduced by approximately 13% (from 7,993 to 6,964 Gg). However, this is inconsistent with the annual trend of ozone concentration (EEA, 2020).

3. 2. 3 Sources, Causes, Status of Efforts, and Countermeasures in Germany

In Germany, reduction targets by 2030 compared to the base year 2005 were set at 65% and 28% for the ozone precursors NOx and NMVOC, respectively. The focus was on air pollution by nitrogen oxides because NO₂ levels were still high in many urban areas.

Table 2.

Comparison of ozone measures among the United States, Europe and Japan.

Between 2005 and 2016, NOx and NMVOC emissions were reduced by more than 20%, showing that the measures were effective. The measures taken by the NAPCP in Germany against mobile sources of pollution were the strongest for both NOx and NMVOC. These measures include tightening emission limits, consistent vehicle fleet renewal, and the introduction of environmental zones in many German cities. However, the average number of days per year when the daily maximum 8-h ozone value exceeds 120 μg/m³ has hardly changed since 2005, even if annual variations due to weather are considered (BMU, 2019).

The NEC Directive stipulates that member states shall provide a “with measures (WM)” scenario and a “with additional measures (WAM)” scenario for specific pollutants. The NAPCP in Germany provided ozone concentration forecasts for 2030 for both the WM and WAM scenarios. The WM scenario is based on measures relating to climate protection that were adopted by July 31, 2016, and measures relating to air pollution control that were adopted by September 1, 2017. These measures comprise: a) lignite power plants on standby mode, b) carbon trading, c) a market incentive program for renewable energy in the construction sector, d) energy-efficient construction and renovation, e) energy-saving regulations, and f) energy consultancy for medium-sized companies. The WAM scenario comprises the following options for action: a) climate protection measures, b) phasing out of power generation using hard coal and lignite, and c) air pollutant emission regulations for medium-sized industrial facilities (Medium Combustion Plant Directive).

4. DISCUSSION

The purpose of this study is to understand the current situation of ozone countermeasures used in the US and Europe and to examine whether there are any points recommended in Japan. The original research plan was to obtain information contributing to regional measures, but surveys in the US and Europe showed that ozone is a secondary pollutant that is very widespread. Due to the large impact of advection, measures at the state level in the US and at the national level in Europe were mainstream, and measures at the small regional or city level were very limited. That is, wide-regional measures are being taken for ozone measures. The points recommended for Japan were examined by comparing with the US and Europe from the perspectives of the atmospheric environment policy framework, air quality standards, emissions inventory, countermeasure strategy, evaluation methods for countermeasure effects, and ozone sensitivity regime.

4. 1 Atmospheric Environment Policy Framework

In this research, we consider the introduction of the atmospheric environment policy frameworks of the US and Europe into Japan.

Introducing an European framework based on the NEC Directive would be acceptable because it evaluates and visualizes the reduction of air pollution emissions. In that case, the scientific validity of the emission reduction target amount and emissions inventory, calculation technology, and quality control is considered to be issues for Japan.

Introducing the US air pollution policy SIP framework (that is, each state/local government requires measures to be implemented until the environmental standards are achieved) in Japan is more challenging. The reason is that ozone concentration would not be reduced even if one local government took measures, and there is no prospect of achieving environmental standards at this time. As in the case of Washington DC, the work with the surrounding municipalities was thought to be one of the solution. Considering that local governments will implement measures, achievable targets could be set in stages, and reduction targets could be assigned to each local government, thereby reducing emissions nationwide.

4. 2 Air Quality Standards

In the US, the Clean Air Act requires that scientific findings, such as the health effects of air pollutants, be reviewed every five years and that changes in air quality standards should be considered. In the EU, PM_2.5, PM₁₀, NO₂, SO₂, and carbon monoxide (CO) have a “limit value” in the air quality standards for protecting human health. However, the environmental standard for ozone concentration is a “target value”. The World Health Organization updated its air quality guidelines on September 22, 2021, to offer global guidance on thresholds and limits for key air pollutants, including ozone, that pose health risks. The recommended level was set at a 100 μg/m³ (≈ 0.05 ppm) average daily maximum 8-h mean ozone concentration. Further, the level for the peak season was set at 60 μg/m³ (≈ 0.03 ppm).

In Japan, the environmental standard for photochemical oxidants was established in 1973, and the hourly value was set at 0.06 ppm or less. Since then, the environmental quality standards for photochemical oxidants have not been revised. The achievement rates of the environmental standard for photochemical oxidant concentration in Japan (Fiscal Year 2020) were 0.2% for ambient air pollution monitoring stations and 0% for roadside monitoring stations. Therefore, there is no prospect of achieving the environmental standard at this time. Thus, it is advisable to consider a framework that includes setting feasible step-by-step goals to achieve the environmental standards. In addition, adding standards for vegetation protection, as applied in the US and the EU, should also be considered because the increase in ozone concentration due to global warming will affect vegetation.

4. 3 Air Pollution Emissions Inventory

The states in the US and EU member countries have prepared air pollution emissions inventories, and the US EPA and the EU are providing support for inventory preparation. The EU aims to halve the number of early deaths due to particulate matter and ozone by 2030 (compared to 2005). The upper emissions limit is set for each country and the reduction target is set by the model based on the emissions inventory.

In Japan, NMVOC reduction targets were set in the 2000s. The revised Air Pollution Control Act of 2006 stated that by the end of 2010, NMVOC emissions from fixed sources should be reduced by approximately 30% of the 2000 level in Japan as a whole, using measures that combined legal regulations and voluntary actions. Ministry of the Environment has developed and maintains the specific inventory for NMVOC emissions to check progresses of the actions. According to this emission inventory, emissions during this period were reduced by approximately 44%, achieving the target. On the other hand, the percentage of measuring stations that did not exceed the photochemical oxidant warning level remained unchanged at 50-60% in the 10 years following 2000. However, it has been shown that the law was prematurely enforced when there was a heavy financial burden on the industry, such as the increase in energy consumption due to the introduction and operation of NMVOC processing equipment (Hashimori, 2014; Endo, 2013; Yui, 2011). Therefore, developing a reliable emission inventory and estimation model has been a challenge. Since 2013, the development of emission inventories for secondary air pollutants has been promoted. The target substances are PM_2.5 (as primary particles), suspended particulate matter (SPM), sulfur oxides (SOx), CO, NOx, NMVOC, and NH₃. It is updated regularly and is beginning to be used for research on photochemical oxidants. Tokyo Metropolitan Government has also developed its own inventory, conducted simulations, and is studying what the local government can do.

Even if the reduction target is not indicated, it is important to estimate the increase or decrease of ozone precursors and the ozone concentration in the future with the policy of maintaining the status quo. This exercise could forecast the time needed to achieve the environmental standard and to consider additional measures. Inoue et al. (2019) pointed out regional countermeasures should be considered rather than the existing uniform nationwide measures to mitigate ozone pollution more efficiently in Japan. So, if local governments, such as Tokyo Metropolitan Government, take the lead in predicting ozone concentration and developing control measures, preparing an emissions inventory and reduction amount will be prerequisites as basic data for simulation. As in the case of the OTC in Washington DC, it is difficult to develop by prefecture in terms of cost and technology, so cooperation between national, local and private research institutes is important, such as utilizing the emission inventory maintained by the national government.

4. 4 Countermeasure Strategy

In the US, ozone countermeasures are not taken by just one state/district but by larger regions. In collaboration with other states, Washington DC, Maryland, and Virginia formed a multi-state organization called the Ozone Transport Commission (OTC) to act against the arrival of ozone precursors due to cross-border pollution. Research on forest control measures has been conducted in California (Taha et al., 2016; Simpson and Mcpherson, 2011) and Texas (Kroeger et al., 2014), showing that environmental standards could not be achieved without measures against BVOC derived from nature. Kroeger et al. (2014) stated that there was an urgent need to find a cost-effective approach to addressing the ozone problem, and forest control measures were one of the possibilities. In the EU, the focus is on the win-win solution, which reduces emissions of air pollutants at the same time as greenhouse gases, owing to the consistency of air conservation measures and energy policies. Specifically, the win-win solution with greenhouse gas countermeasures includes phasing out coal in power generation, increasing the share of renewable energy power generation, and popularizing electric vehicles. The achievability of the emission reduction commitment is estimated in the WM and WAM scenarios, and the win-win solution with greenhouse gas countermeasures is expected to achieve the target value.

As it is the consensus of the international community to promote greenhouse gas countermeasures as a priority issue, public support for reducing air pollutants at the same time as greenhouse gases will be obtained in Japan as well. Therefore, the first step for Japan’s future ozone countermeasures is to maintain the status quo, estimate what percentage of air pollutants will be reduced or increased, and the future ozone concentration. This could be considered the Japanese version of the WM scenario. The second step is to estimate the ozone concentration if additional measures centered on the win-win solution for greenhouse gas reduction, which is a climate change measure, are promoted (Japanese version of the WAM scenario).

4. 5 Evaluation Methods for Countermeasure Effects

An analysis of the 93.2 percentile annual change in maximum daily 8-h mean ozone concentration (2009-2018) in the EU showed no significant increase or decrease in 95% of the stations. The NOx emissions were reduced by approximately 20% from 2010 to 2017, and NMVOC emissions were reduced by approximately 13%, but this was inconsistent with the trend of annual changes in ozone concentration (EEA, 2020). This is because reductions in NOx and NMVOC were offset by rising temperatures due to climate change, increased ozone production in East Asia, and increased global methane emissions (BMNT, 2019). Recent research has also shown that advection due to long-distance movement and vertical diffusion in the troposphere, and the effects of global warming, are significant (Pay et al., 2019; Sicard et al., 2016; Barros et al., 2015; Colette et al., 2015). Reports from the EU and Germany state the difficulty of detecting the effect of countermeasures on a frequency exceeding the ozone standard value even if the precursor was reduced (EEA, 2020; BMU, 2019).

In Japan, the Ministry of the Environment has proposed a new indicator: the three-year moving average of the annual 99th percentile of the daily 8-h maximum for photochemical oxidants for adequately assessing the effectiveness of Ox-related environmental improvements. Fukunaga et al. (2021) explain that because this indicator is often strongly influenced by transboundary pollutants due to meteorological conditions, another new indicator independent of meteorological conditions has been proposed to assess photochemical oxidant formation in local areas. Since ozone is greatly affected by advection, it is necessary to maintain and share inventories in East Asian countries and evaluate and promote research on indicators that can predict the effects of countermeasures.

4. 6 Ozone Sensitivity Regime

The NAPCP of EU member states did not mention the ozone sensitivity regime as a matter to be considered for ozone concentration measures. This may be because a reduction of both NOx and NMVOC is necessary and because NO₂ levels have a set limit while ozone concentration is the target value. However, Barros et al. (2015) pointed out that the NEC Directive should impose emissions reductions with respect to the NOx/NMVOC ratio, instead of considering it as a mere guideline value, because of its non-linear interactions with the ozone chemical balance. In the US, the ozone sensitivity regime was considered in SIP by the Metropolitan Washington Air Quality Committee. When considering the ozone sensitivity regime, it is conceivable to control the emissions reduction balance between NOx and NMVOC. The methodology might be established by simulating the emissions reduction amount and estimated concentrations, using the Japanese version of the WM and WAM scenarios for the area.

Japan’s NO₂ environmental standard achievement rate (Fiscal Year 2019) is as high as 100% for ambient air pollution monitoring stations and for roadside monitoring stations, and the ozone sensitivity regime in urban areas is mainly NMVOC rate-determined (Sakamoto et al., 2019; Sadanaga et al., 2012; Kannari and Ohara, 2010). Therefore, ozone countermeasures focus on reducing NMVOC emissions (rather than NOx). However, when the WHO global air quality guidelines updated in September 2021 (annual average value of 10 μg/m³ (≈ 0.005 ppm)) (WHO, 2021) was applied for NO₂, only 29% of ambient air pollution monitoring stations and 2% of roadside monitoring stations nationwide achieved it according to monitoring data (MOE, Japan, 2021). Hence, it is considered necessary to promote further emission reduction for both NOx and NMVOCs in the future.

5. CONCLUSION

This paper summarizes the current situation of ozone countermeasures in the US and Europe. It is clear that the US and Europe differ in their approach to the legal system and environmental standards for air pollution control and that the framework and policies for ozone control vary. In the US, states that do not meet the environmental standards for ozone concentration are obliged to develop an SIP to attain the standards. There was an urgent need to find a cost-effective approach to the ozone problem, and forest control measures were one of the possibilities. In Europe, the EU has set “target values” for ozone and NAPCP does not mention forest control measures or ozone sensitivity regimes. The main focus is on the win-win solution, which reduces emissions of air pollutants at the same time as greenhouse gases because of the consistency of air conservation measures and energy policies. In Japan, the environmental standard for photochemical oxidants was established in 1973. The achievement rates (Fiscal Year 2020) were 0.2% for ambient air pollution monitoring stations, and there is currently no prospect of achieving the environmental standard. Therefore, it is advisable to consider a framework that includes setting feasible step-by-step goals that lead to the achievement of environmental standards. As ozone is greatly affected by advection and wide-regional measures are being taken in the US and Europe, it is necessary for Japan to take wide-regional measures against precursors and predict the future precursor reduction and concentration. The preparation of emission inventory and reduction amount will be prerequisites as basic data for simulation. In terms of cost and technology, cooperation between national, local and private research institutes is important. There is an international consensus to promote greenhouse gas countermeasures as a priority issue. Thus, it is appropriate to consider a win-win solution with greenhouse gas reduction (Japanese version of the WM/WAM scenario).

Acknowledgments

The authors thank Dr. Sunil Kumar from the Metropolitan Washington Council of Governments (MWCG), Dr. Marcel Langner from the German Environment Agency, Ms. Viviane Andre and Mr. Thomas Henrichs from the Directorate-General for the Environment, European Commission, the California Air Resources Board (CARB), and US EPA North Carolina for their cooperation with the surveys undertaken in this study. They also thank the external review committee: Shinji Wakamatsu (chair; Institute of Integrated Atmospheric Environment, Japan), Yasuhiro Daisyo (Waseda University), Toshimasa Ohara (Center for Environmental Science in Saitama, Japan), and Tokuhisa Yoshida (Waseda University, Japan).

FUNDING

This study was funded by the Environmental Restoration and Conservation Agency (ERCA), Japan. The findings and conclusions in this article are the sole responsibility of the authors and do not represent the official views of the above agency.

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Air quality monitoring	Determine if areas meet federal air quality standards. Help determine the cause of the problem. Demonstrate that air pollution controls are working.
Emissions inventory	Estimate the current and future emissions to predict from which sources reduction can be expected in future.
Air quality modeling	Calculate the emission reductions needed to meet acceptable air pollution levels.
Control strategy	Develop a strategy in collaboration with industry and the public. It must consider economic and technological feasibility and be enforceable. Emission reductions can come from control regulations, financial incentives, and voluntary actions.

United States			European Union	Japan
Framework	The Clean Air Act requires states with nonattainment areas to adopt additional regulatory programs designed to achieve and maintain attainment of the NAAQS		Three main pillars: 1) the Ambient Air Quality Directive, 2) the National Emissions Ceilings Directive, 3) source-specific legislation on emission standards	The Basic Environment Law stipulates that the government must strive to ensure environmental standards.
Air quality standards	National Ambient Air Quality Standards (NAAQS)		Ambient Air Quality Directive	Article 16 of the Basic Environment Law
	Primary/Secondary 8-h 0.070 ppm		Target value: the maximum daily 8-h mean 120 μg/m³ (≈ 0.06 ppm）	Hourly value: 0.06 ppm
	Standard is met at an air quality monitor when the 3-year average of the annual fourth-highest daily maximum 8-h average ozone concentration		On more than 25 days per calendar year averaged over 3 years (93.2 percentile).	-
Achievement of air quality standards	The number of states that contain 8-h ozone nonattainment areas (2015 Standard) was 23 (as of April 30, 2019）		17 out of 28 member states had monitoring stations above the ozone target value in 2017.	Achievement rate was 0.2% for ambient monitoring stations (Fiscal Year 2020)
Countermeasure plan	State Implementation Plan (SIP) by State		National Air Pollution Control Program (NAPCP) by member country	Ordinances, etc.
State/country subject to detailed investigation	California	Washington, D.C.	Germany
Emissions	Transport, forest fires, weather, global warming, geographic factors (sea breezes and mountains), cross-border pollution from Mexico	Cross-border pollution (NOx emitted from coal combustion from power plants), transport	Emissions of NOx and NMVOC account for 15.8% and 15.3% of the EU as a whole, both of which are the largest emitters. 40% of NOx is from transport. 56% of NMVOC emissions are from industrial processes.
Effort status	California had a history of serious air pollution 50 years ago. In order to overcome this, it became one of the most advanced states, strengthening measures against road transport sources.	Citizens are highly conscious of environmental protection, and are actively engaged in voluntary actions. In addition to national measures, they are implementing unique measures in a well-balanced manner.	Measures against transport sources are the largest for both NOx and NMVOC. The vehicle was renewed due to tightening of emission limits, consistent fleet renewal, and the environment zones.
Cross-border pollution	California holds work group meeting with Mexico, due to cross-border pollution from Mexico.	Ozone countermeasures cannot be taken by just one state, so local governments in the Northeast and Mid-Atlantic regions formed a multi-state organization called the Ozone Transport Commission (OTC).	Since the ozone background concentration in the westernmost part of Europe is increasing, it is pointed out that advection (long-distance transport and vertical diffusion) has a large effect. Reducing NOx and NMVOC will be offset by rising temperatures due to climate change, increased ozone production in East Asia, and increased global methane emissions.