Nitrogen oxides and ozone in urban air: A review of 50 plus years of progress

Nitrogen oxides and ozone impact air quality in many parts of the United States, Europe, China, and many other countries. The greatest air quality challenge in Los Angeles, some other areas of California, and some parts of China is to reduce ozone levels to meet regulations. Background ozone is a major factor which makes it more difficult to reduce urban concentrations in Los Angeles and some other locations. Air pollution from China affects the background ozone entering California. More than 50 years of history are reviewed with an emphasis on reducing concentrations of nitrogen oxides and ozone. During this time period, there has been significant progress in reducing levels of these pollutants in urban air in the United States and Europe; however, ozone concentrations in China have increased since 2013. Cost and benefit analysis has shown that benefits associated with the Clean Air Act of 1970 have greatly exceeded costs to improve air quality in the U.S. over the past 50 years. Further actions to consider to improve air quality include reducing combustion of coal and petroleum products and transitioning to renewable energy. Public education is recommended to inform citizens that the benefits of reducing emissions far exceed costs.

Photochemical oxidation of VOCs produces ozone and other compounds present in smog when the sun shines on warm days. Ozone is a strong oxidant that is responsible for more than 400,000 premature deaths and more than 8 million emergency room visits each year globally. 14,17 Ozone affects respiratory health and has effects on cardiovascular health as well. 10 Many of the deaths are due to respiratory mortality for those with asthma and chronic obstructive pulmonary disease (COPD). Ozone impacts vegetation and reduces the productivity of ecosystems. 38 One of the reasons for this review is to identify cost-effective ways to reduce ozone pollution which is one of the greatest global challenges in 2020. 1,3,8,14,24,25,38,52 The Clean Air Act of 1970 is now 50 years old and great progress has been made in addressing air quality issues in the United States. However, ozone pollution is still above regulatory standards in Los Angeles and many other locations in the United States, in China, and in many other locations. 3,7,8,14,38,52 This review covers over 50 years of progress to improve air quality.
In 1991, the National Research Council published a National Ambient Air Quality Standards (NAAQS) comprehensive report on ozone because of the failure to meet the NAAQS for ozone in 60 locations in the United States in 1987. 46 The report identifies ozone pollution as a "pervasive and stubborn environmental problem". 46 The study was requested by the U.S. Congress. The report recommends efforts to find better regulations and verification methods to reduce emissions of VOCs and additional research. 46 The Clean Air Act Amendments of 1990 established new regulations to address ozone attainment; however progress to reduce ozone concentrations has been poor and measured values of ozone continue to exceed the NAAQS in many locations. 3,7,14 Significant efforts have been made to reduce emissions of methane and other VOCs; however, emissions continue to be greater than reported values. 26 There are ways to reduce ozone concentrations, and this review will include information on positive actions that can be implemented.
Those who live in urban communities should understand that there are cost effective ways to improve air quality.
Since 1970, air pollution has become more important in industry.
Many chemical engineers have contributed to the progress that has been made toward better air quality. Professionals in industry have been very involved in developing new products that have resulted in significant improvements in air quality. 1,16,22,24,46 2 | REGULATORY STANDARDS AND HEALTH GUIDELINES In the United States, the U.S. Environmental Protection Agency (EPA) has set the following values for NO 2 . The average value for 1 hr is 100 ppb where this is the 98th percentile of 1 hr daily maximum concentrations averaged over 3 years. The annual mean value should be less than or equal to 53 ppb. 15 For ozone, the EPA regulatory value is 0.070 ppm = 70 ppb where this value is the annual fourth highest daily maximum 8-hr concentration averaged over 3 years (4MDA8). 15 The average values are recorded for each 8 hr period of the day, and the highest of these is identified and recorded for each day of the year. The fourth highest of these ozone values is recorded for each year. The average over three consecutive years should be less than 70 ppb.
In Europe, for NO 2 , the 1 hr limit value is 200 μg/m 3 which is equal to 106 ppb at 25 C and 1 atm pressure, and is not to be exceeded more than 18 hr/year. The annual average value is 40 μg/ m 3 or 21.3 ppb. For ozone, the maximum daily 8 hr mean value is 120 μg/m 3 , which is equal to 60 ppb, and is not to be exceeded on more than 25 days/year, averaged over 3 years. 10 In China, the regulatory standards for NO 2 in micrograms per cubic meter are 200 for 1 hr and 40 for the annual average value. For ozone, the 1 hr standard is 200 and the 8 hr average value is 160 μg/m 3 . 45 The World Health Organization (WHO) has air quality guidelines, which are 200 μg/m 3 for NO 2 for 1 hr, which is equivalent to 106 ppb. For NO 2 the annual average value is 40 μg/m 3 or 21.3 ppb.
The WHO value for ozone is 100 μg/m 3 or 50 ppb. 10 There are many countries with other air quality standards 38 (see Tables 1 and 2). In Table 1, the annual average value of the nitrogen dioxide standard is largest for the United States. China has the largest regulatory values for ozone (see Table 2).
In the United States and globally, there are many locations where ozone standards and guidelines are exceeded. In the United States, 41.9% of the people live in areas with unhealthy levels of ozone pollution where the regulatory standards are exceeded at least once annually. 7 In these communities, there are frequently more than 3 days each year where the maximum 8 hr average ozone concentration for the day exceeds 70 ppb. In recent years, approximately 90% of the time that ambient air quality standards are exceeded in the United States, it is due to ozone. 7,11 Figure 1 provides information on the air quality index for ozone. 7,62 This guide is used in the United States to provide guidance on ambient air quality of ozone.
In Europe, 14 member states reported exceeding the regulatory value for ozone more than 25 times per year. 10 In Europe, there are more than 50,000 deaths per year from ozone pollution, with Italy, Germany, France and Spain being between 5,000 and 10,000 per year. 18 In 2010 the annual health costs of air pollution in Europe were estimated to be $1.575 trillion. 27 The annual limit value for NO 2 is widely exceeded in Europe, 10,16 and progress in reducing ozone pollution has been poor in Europe and in many other locations in the world. 7,8,16,17 In many parts of the world, the most frequently exceeded air quality guidelines and regulations are those for ozone. 7,8,11,14  locations in 20 countries for the short term study. 55 The American Lung Association has a good summary of health studies related to ozone. 7 The U.S. EPA study to determine the ozone regulatory standard includes an analysis of the results of many studies on the health impacts of ozone. 11 There has been a global effort to assess ozone concentrations and develop a comprehensive database with data from 9,600 sites. 32 There are several different metrics that have been used to describe ozone values in air in different parts of the world. 33 Results for ozone at many different locations in the world have been presented with their impacts on health included. 3 The high concentrations of nitrogen oxides and ozone occur during the day time. With morning traffic, nitrogen oxide emissions cause ambient concentrations to increase to values that are much larger than late night values. The radiant energy from the sun and the nitrogen oxides cause ozone formation in polluted air where VOCs are present. In the daily cycle nitrogen dioxide concentration often has its peak value before noon while ozone concentration exhibits its peak in the afternoon. 22,28,29 There may be a second nitrogen dioxide peak associated with early evening traffic when many people are going home from work and other activities. 29 Figure 2 shows hourly average values for nitrogen dioxide and ozone for Little Village in Chicago. 22 The nitrogen dioxide is directly associated with vehicle emissions and the peak concentration occurs early in the day. The peak in ozone concentration occurs later in the day when the temperature is higher. 22

| FIFTY YEARS OF HISTORY
The Before 1970, ozone concentrations were above 600 ppb and smog was very visible when the weather was right for ozone and smog for- In the last 75 years, there has been great progress in understanding air quality and the chemistry of nitrogen oxides and ozone. 11,13,38 The formation of ozone in air has a well-developed chemistry which is described in many sources; see Ref. 38 and the references therein.
Ozone formation depends on solar radiation, nitrogen oxides, and VOCs; temperature is important because reaction rates are faster at higher temperatures.
One of the major reasons that ozone values do not meet NAAQS in Los Angeles and some other parts of southern California is because of background ozone values. 25,34,38 The prevailing winds in southern California are from the west, and the ozone concentrations in the air are often above 30 ppb because of natural processes and international anthropogenic air pollution. Ozone is formed when there is sunshine, nitrogen oxides, and VOCs. Thus, ozone, nitrogen oxides, and VOCs from other countries and from the emissions of ships have the potential to increase the amount of ozone in the air that enters California after passing over the Pacific ocean. 38,47,48 The concentrations of ozone in the air that enters California are greater at higher elevations. 25 The background concentrations of ozone have been increasing in many locations because of emissions of ozone precursors. 38 The precursors and ozone may travel together from where the precursors are generated to locations where the ozone contributes to the background ozone. 38 Sometimes the tropospheric ozone concentration increases because of stratospheric ozone that comes down and gets mixed into the air near ground level. 25,34 Another source of ozone is that produced because of wildfires. 34 Nitrogen oxides and VOCs from wildfires contribute to ozone formation. Exceptional events such as wildfires may cause communities to experience ozone pollution that impacts health and regulatory compliance.
As of 2019, ozone concentrations still exceed the NAAQS and air pollution is a continuing concern in Los Angeles and other parts of California. 7 Nitrogen dioxide concentrations have been reduced such that they are in compliance with the NAAQS to a much greater extent than ozone. 24 Given the size, population, and vehicle miles traveled, the quality of the air in the Los Angeles metropolitan area is much better than many other large cities with similar populations. 24,57 The quality of life has been improved greatly with significant benefits to health. Ozone pollution is a major issue in large cities in Europe, India, Japan, China, and South Korea. 3 In 2015, about 89% of deaths because of ambient air pollution were in countries with middle level incomes and lower. 35 In the United States, more than one third of the people live in counties where the NAAQS for ozone have been exceeded in recent years. 7 Some major U.S. cities where ozone is a health problem are listed in Table 3 Ozone scores are shown in Table 3  The results in Table 3 show that 12 of the ozone scores increased with time. One concern is that the increased temperatures associated with climate change are making it harder for communities to meet NAAQS for ozone.
In Europe, there has been progress in reducing nitrogen dioxide concentrations in the last 10 years; however, about 12% of reporting sites measured values above the annual standard. 10   In Israel, nitrogen oxides in air have been a pollution problem for more than 20 years. 43,44 Tel Aviv, the largest metropolitan area often has concentrations that exceed target values. 43,44 Emissions from buses are one of the significant sources. Figure 3 shows the percentage of days that the ozone concentration is above 70 ppb for each city. 57

| AIR QUALITY IN CHINA
Air quality is a major environmental concern in China with the first air The increase in temperatures because of climate change is expected to make ozone pollution even worse. 14 In 2008, when Beijing hosted the Olympic Games, significant efforts were made to reduce air pollution by reducing the volume of traffic and reducing industrial operations. As a result, ozone values in August 2008 were about 15 ppb lower at Miyun, which is downwind from Beijing. 22,50 Health benefits associated with the better air quality were reported. 51 This historical result shows that air quality can be improved by reducing emissions.

| IMPACT OF COVID-19 ON AIR QUALITY
Nitrogen dioxide concentrations in ambient air increase with emissions from motor vehicles. A daily cycle is shown in Figure 2. In 2020 there was a significant decrease in travel because of the COVID-19 pandemic. 59,60 In California, nitrogen dioxide concentrations decreased by 34% during the period from March 20 to April 9 compared with 2017-2019 average. 59 Ozone concentrations were 5% lower; these are average values for California. 59 In Wuhan, China, average nitrogen dioxide values decreased by 57% because of the COVID-19 lockdown compared with the average values for the same period in 2017-2019. 60 However, average ozone concentrations increased by 36% because of lower NO concentrations. 60 This is because NO reacts with ozone to remove it from air. 22,61 In the European cities of Nice, Turin, and Valencia, average nitro- however, average ozone concentrations increased by 2.4-27%. 60

| IMPROVING AIR QUALITY
There are many places in the world where there is a great need to improve air quality. In the most impacted countries, about 25% of deaths are due to pollution. 35 One of the most significant challenges is to reduce ozone concentrations. This is an issue in most metropolitan areas of 10 million people and larger where there is significant sunshine. 57 Since ozone is formed when nitrogen oxides and VOCs are present, actions to reduce concentrations of these compounds can be beneficial. Table 4 provides a list of actions to take to reduce ozone concentrations in ambient air. While ozone concentrations have local health impacts, some actions to reduce ozone concentrations will be beneficial globally as well as in the country where action is taken.
Nitrogen oxides are formed during burning of fuels, so transitioning away from combustion processes will reduce nitrogen oxides in air. There are many sources of VOCs, including industrial emissions, solvents, cleaning supplies, alcoholic beverages, and emissions from baking bread.
Because vehicle emissions are important sources of nitrogen oxides and VOCs, actions to reduce these emissions are very important. 24 This can be done through emission control technologies that reduce the amount of nitrogen oxides in diesel exhaust. 22,39,40 There has been significant progress with three technologies: exhaust gas recirculation (EGR), lean NO x trap (LNT), and selective catalytic reduction (SCR) using ammonia. 22,39,40 These technologies have been implemented in Europe and there has been progress in reducing NO x emissions. 22,39,40 The electrification of public transportation with electric trains and buses that reduce emissions compared with diesel buses is receiving attention because when health and climate change costs are included, electric buses are very cost effective. 1,53 In China, there is a major effort to replace diesel buses with electric buses because of the economic benefits. 53 In 2018, more than 90% of the electric buses were in China, but there is progress in adding electric buses in cities in the United States and Europe also. 1 Electric bikes are an inexpensive way to reduce emissions; they have been used in China successfully. 1 In an effort to reduce nitrogen oxide concentrations, there is a transition to electric buses and electric taxicabs that is in progress in London. 1 All air pollution sources should be considered in efforts to reduce ozone concentrations. 24 This should include efforts to reduce methane emissions associated with oil and gas operations 26,38 and sources of VOCs. VOCs in ambient air may need to be regulated more comprehensively in order to meet the standards for ozone. Regulations that reduce methane emissions are beneficial for reducing ozone concentrations in two ways. Methane is a precursor for ozone formation, and methane is a greenhouse gas; increases in temperature because of climate change contribute to greater amounts of ozone formation. 61 One way to reduce methane emissions is to establish a social value of methane tax to encourage reductions in methane emissions.
The social value of a tax on methane emissions that includes impacts on ozone concentrations and climate change has been estimated to be more than $700 per ton of methane. 63 China is moving forward with new regulations designed to reduce VOC emissions. 20,49 Recent research shows that ozone generation in Nanjing can be reduced by controlling VOC emissions. 49 A study of sources of VOC emissions shows that many of the emissions are associated with transportation. 49  Public education and citizen participation are important in efforts to increase community will to improve air quality. 35 The regulations on emissions need to be for the entire geographical area of the air shed, and they need to be enforced effectively on a continuing basis with updates to address emissions that should be added. 24 There are many actions that can be taken that will be beneficial to both air quality and reducing greenhouse gas emissions. 1