Nitrogen Dioxide and Ozone Pollution in the Chicago Metropolitan Area

Ground-level ozone is a harmful air pollutant associated with several health issues. Ozone concentrations have exceeded the National Ambient Air Quality Standards (NAAQS) in the Chicago metropolitan area on hot summer days for many years because of nitrogen oxide and volatile organic compound emissions. Annual fourth highest 8-hour ozone concentrations have been between 0.070 and 0.084 ppm at several monitoring sites in Cook county, during the 2016-2018 time period. The continuous measurement of nitrogen dioxide (NO 2 ) and ozone (O 3 ) was conducted in several communities in Chicago in 2017. The air pollution impacts the health of all who live in the area. The data were used to analyze correlations between the O 3 distribution and its association with ambient concentrations of NO 2 from transportation emissions. Higher concentrations in NO 2 and O 3 occurred in succession in the daytime. The diurnal variation of O 3 concentration was analyzed. The daily cycle of NO 2 concentration reaches a maximum in the late morning and has smaller nighttime concentrations. The daily cycle of ozone concentration reaches the maximum in the afternoon and also becomes smaller for nighttime concentrations. In addition, relationships were found between O 3 and NO 2 . Monthly variations of ozone and NO 2 are presented. Some options to reduce ozone pollution are presented.


Introduction
Ozone (O 3 ) is a key secondary air pollutant associated with a number of health issues including asthma and premature death [1] [2] [3]. Because of its effects on people and the environment, ground-level ozone is a harmful air pollutant and the main ingredient in smog [2]. According to the American Lung Association, more than 40% of Americans currently live in areas with unhealthy levels of ozone pollution [3]. The importance of climate change for O 3 air pollution has been highlighted by past studies estimating health impacts of projected O 3 increases over the 21st century [1] [4]. However, the problem of ozone pollution related to NO 2 emissions continues to be a serious issue. Jing et al. pointed out that despite strenuous efforts to reduce the emissions of ozone precursors such as nitrogen oxides (NO x ), concentrations of ground-level ozone (O 3 ) still often exceed the National Ambient Air Quality Standard in U.S. cities in summertime, including Chicago [4]. Pollution of ozone is caused by the photochemical reaction of precursor volatile organic compounds (VOCs) and nitrogen oxides (NO x ), with a major source of nitrogen dioxide (NO 2 ) being traffic emissions. In many areas, nitrogen oxides in air have been a pollution problem for more than 20 years [3] [4] [5]. There are three nitrogen oxides-nitrous oxide (N 2 O), nitric oxide (NO), and nitrogen dioxide-that are found in air as gas [6]. NO 2 is the most important of these air pollutants because of concentration and health impacts [6]. The chemical reactions related to ozone formation are presented in Discussion.
To determine the optimal emission control strategy, understanding the complexity of the diurnal ozone process and its relationship to precursors is critical [7]. In recent years, the governments of the world have been more concerned about ozone pollution and its harm to human health. They have tried to make policies and management measures to reduce ozone pollution. We need to continue to advance public policies and cooperative efforts to have better control of ozone pollution. Exposure of vegetation to tropospheric ground level ozone causes an alteration in the allocation of resources, which can affect all aspects of plant growth. Urban heat island phenomenon and the related problems of ozone in Chicago's metropolitan area are also major problems. The thermal processes that are active in a heat island start early morning with the sunrise, continue throughout the day, and often last through the night [8]. Temperature is important because it affects ozone formation and human health.
A county is defined as out of attainment with the National Ambient Air Quality Standard (NAAQS) for ozone if the 3-year average of the annual 4th highest maximum 8-hour average exceeds 70 ppb [9]. Since the Industrial Revolution, Chicago has struggled with air pollution like many other major U.S. cities. Although emissions of ozone-precursors and summertime ozone levels have decreased significantly since the late 1970's, ozone in Chicago has never complied fully with standards set by the EPA to protect public health [3] [10]. Transportation has a negative impact on health because of its negative impact on air quality [11]. The main objective of this work is to analyze the issues of ozone air pollution caused by NO 2 emissions from transportation in Chicago. Emissions of volatile organic compounds (VOCs) contribute to ozone formation also [4] [6].

Methods, Study Region & Data Scope
The study regions include two communities in Chicago, which are Little Village (LV) and South Loop (SL). All collected data were provided by Shared Air/Shared Action (SASA) project [13]. The data may be found at http://fingolfin.kdd.cs.ksu.edu:8080/airquality/download/. SASA is Community Empowerment through Low-Cost Air Pollution Monitoring, an EPA Air Pollution Monitoring for Communities Grant [14]. Low-cost monitors "Aeroqual" for the SASA project (SASA_AQ) were utilized to monitor NO 2 and O 3 concentration in the communities, and many regulatory monitors from EPA also were used to monitor the neighborhood of the communities [13] [14] [22].
The locations of the EPA nitrogen dioxide and ozone monitoring sites in Thus, the transportation activities will be influenced by the students' traffic behavior and school hours. There will be more regular variations of NO 2 concentrations due to emissions from transportation. Additionally, there are also some shops. They also can bring much traffic to the area. Moreover, the monitoring sensors are close to the Perspectives Charter schools and other schools, so the data collected on NO 2 concentrations has time variations, as they are influenced by school hours. SASA project involved discontinuous 390 hours to monitor NO 2 concentration of South Loop by sensor "SASA_AQ1" from 9/13/2017 13:00 to 10/2/2017 17:00. Regulatory monitor close to South Loop is site "17310063", which collect discontinuous 460 hours of NO 2 concentration from 9/13/2017 13:00 to 10/2/2017 17:00 [14].
All of the communities involved in the project are impacted by diesel exhaust. Air quality is impacted by stationary point sources such as coal fired power plants, industrial smoke stacks and mobile sources such as vehicles [15]. Little Village is a Latino neighborhood, which is heavily impacted by industrial activity. South Loop, the non-environmental justice community, is a gentrified neighborhood bordering transportation areas. The area has heavy construction activities and considerable vehicle emissions during rush hours [14]. The monitors involved are as close to the main roads as possible but far away from the heavy industries for focusing the discussion on the influence of transportation emission of NO x . 1-hr average NO 2 concentration in NAAQS (0.1 ppm), and also lower than the regulatory level of annual average NO 2 concentration in NAAQS (0.053 ppm).
The data of NO 2 contains results for 509 hours, and the data of O 3 contains results from 576 hours. Although there are some differences in some days, the time is enough to obtain reasonable estimates of the values for this period of time. Medians are very close to means in Table 2, but they are slightly larger.
In Table 3, the regulatory monitors by EPA have more hours of data than the SASA sensors. The data from sites "17310063" and "17314002" have more than 1500 hourly observations. In a purely statistical sense, estimates of averages and medians are more precise. The mean values of EPA monitors in Table 3 are smaller than the mean values of SASA sensors for NO 2 which is to be expected because the SASA data are from closer to the road.
For example, the mean and maximum of the site "17310063" in Table 3 are lower than the data from the SASA monitor in SL as shown in Table 3 collected in the same period. Similarly, the mean and maximum of The EPA monitor "17314002" are lower than the values from the SASA monitor in LV as shown in Table 3. The nitrogen oxide emissions from vehicles are the cause of the larger NO 2 concentrations close to streets and roads [4] [6] [13] [15] [22]. Figure 1 shows how the concentrations varied by day at the LV community and at nearby regulatory site. The figure shows the 24-hr average values for the sensor SASA_AQ1_LV, and the 24-hr average value at the regulatory monitor in the area. Figure 2 shows how the concentrations varied by day at the SL community, and at nearby regulatory sites. The figure shows the 24-hr average values for the sensor SASA_AQ1_SL, and the 24-hr average value at the regulatory monitor in the area. Figure 1 and Figure 2 show that the values from the SASA data are higher than those from EPA data for daily average NO 2 concentration.
Overall, the data of SASA sensors and EPA monitors are very close over the same periods for NO 2 , though some data shows the SASA collecting values are larger. The monitors by the SASA project are closer to main roads/streets. Erickson et al. state that there will be variations in air quality that depend on how close vehicle emissions are to the instruments that are making air quality measurements [15]. The data values for NO 2 are larger when they are taken near the road because of the emission from transportation, and this may explain why some of the SASA values are larger. SASA sensors could be used widely because they have economical and portable advantages.   Ozone concentrations from SASA sensors were similar to those at nearby regulatory monitors shown in Table 4, but they still have some differences. The EPA monitors "17314201" and "17314002" were used to collect data of O 3 concentration from nearby fixed site of Little Village. The site "17314201" is at 750 Dundee Rd, in Northbrook, approximately 20 miles north of the Little Village Environmental Justice organization [16]. Compared with data from Table 2, the mean, median, and maximum of EPA monitor "17314201" are larger than values from the SASA sensor in Little Village in same period. Besides, the minimum of the EPA monitor is lower than the minimum of the SASA sensor, and the standard deviation of the EPA monitor is larger than that of the SASA sensor.

Discussion of Ozone Formation
Photochemical smog is a mixture of ozone, nitrogen oxides, and volatile organic compounds (VOCs) that react with sunlight to form mist [17]. Nitrogen dioxide (NO 2 ) can be broken down by sunlight to form nitric oxide (NO) and an oxygen radical (O) in Equation (2). There are no serious anthropogenic ozone emissions. Oxygen radicals can then react with atmospheric oxygen (O 2 ) to form ozone (O 3 ) in Equation (3). Ozone is consumed by nitric oxide to produce nitrogen dioxide and oxygen in Equation (4). The main source of hydrocarbons is the VOCs. Similarly, oxygenated organic and inorganic compounds (RO x ) react with nitric oxide to produce more nitrogen oxides in Equation (6). Ozone is normally consumed by nitric oxide, as in Equation (4). In daytime, the sufficient sunlight makes more O as shown in Equation (2). O reacts with O 2 to produce O 3 shown in Equation (3), so the increasing O 3 might happen in the late morning and afternoon. In Equation (4), NO will be depleted and more NO 2 is produced. Because the mixing ratio of NO is much lower than O 3 , Equation (4) does not deplete ozone during the day in background tropospheric air. However, in urban air, Equation (4) can deplete local ozone at night because NO is produced at night and NO mixing ratios at night may exceed those of O 3 (g) [19]. At the same time, if VOCs are present, nitric oxide and nitrogen dioxide can be consumed as in Equations (5) and (6), allowing the buildup of ground level ozone in Equations (3) O 3 also can be removed by producing more OH in Equation (7) & (8). Therefore, NO 2 and O 3 concentrations will decrease in daytime due to Equation (7)-(9).

( )
More O 3 is formed by Equations (1)-(3) by more NO 2 in daytime. In nighttime, a lot of ozone production shuts down because of lacking O and light.
At the same time, NO 2 could react with ozone in Equation (9) to consume ozone and reduce ozone concentration.

Diurnal Variation of NO2 and O3 Concentration
The daily cycle of NO 2 concentration reaches the maximum in the late morning and has smaller nighttime concentrations. The daily cycle of ozone concentration reaches the maximum in the afternoon and also becomes smaller for nighttime concentrations. The observed average diurnal variation of NO 2 and O 3 concentration during the period are shown in Figure 4 and Figure 5. The ozone concentration slowly increases after the beginning of sun shining in a day, getting to the higher concentration during the daytime, after which it starts to decrease slowly until the next morning. The NO 2 concentration also becomes higher during the daytime than during the nighttime, but it usually increases more rapidly than the ozone concentration after the early morning and reaches its peak earlier.    In Figure 4, there is the highest NO 2 concentration around 10:00, but the highest O 3 value is at 15:00. The ozone peak is about 5 hours after the NO 2 peak.
Although the formation of ozone is deeply influenced by NO 2 , the tendency of change between NO 2 and O 3 is not the same. An obvious time lag exists. The period for high values is different because the reactions (shown in Equations (2)  In Figure 5, the maximum ozone concentration appears at about 15:00. The diurnal cycle of NO 2 is shaped like multiple waves. The peaks exist in the late morning, at noon and in the afternoon.
The SL community has similar tendency of the 24-hr variation of NO 2 and O 3 concentration to LV. The obvious time lag also exists in Figure 5. The area of high NO 2 concentration is during 10:00 to 16:00, but the area of high O 3 concentration is during 14:00 to 18:00. The NO 2 decrease correlates with an increase in O 3 from 10:00 to 14:00; it indicates that there is an inverse relationship between NO 2 and O 3 at some times of the day.     Figure 6 shows the period from 6:00 to 10:00. Figure 7 shows the period from 10:00 to 15:00. They individually present the variation in daytime O 3 concentration as a function of NO 2 . According to Figure 4, O 3 concentration increases with rapid NO 2 increasing from 6:00 to 10:00. NO 2 concentration decreases with O 3 increasing from 10:00 to 15:00. We fitted the data to linear functions. In Figure 6 the  Figure 6, the relationship of O 3 with NO 2 was statistically significant (p value < 0.0001, which was <0.05) from 6:00 to 10:00 in LV, indicating that O 3 was sensitive to NO 2 .     [20]. Marr and Harley suggest there is a decrease in absorption of sunlight due to lower fine particle concentration during weekends, resulting in enhanced ozone formation [21]. The NO 2 concentration varies with traffic intensity. Erickson et al. also stated that since traffic density varies with location and time of day, there will be variations in air quality [15]. Figure 9 shows the NO 2 concentrations increase starting at 6 am. The maximum is at 10 am. At night, the concentrations are lower because most of people don't go outside [22]. In Figure 10, on weekends, the concentrations at night are also low but they don't have a consistent moment of peak value. This indicates that people's behavior differs from during the workdays. People like to go outside at any time instead of focusing on hours associated with work. The workdays and weekends differ in terms of the highest value of the 1-hr average NO 2 concentrations. On workdays, there are obvious peaks at 10:00. However, on some Saturdays and Sundays, there may be two peaks within a day. There is a possible reason for these two peaks: Dining time produces more traffic [22]. More vehicle emissions on the road cause higher NO 2 concentration at more than one time during the day. For example, there is one peak at 10:00 while the other peak is at 15:00 for NO 2 values in Figure 10. This is likely caused by traffic flow. Less traffic flow creates fewer NO 2 emissions; thus, there is a period of lower concentration between the peaks [22].

Weekday/Weekends Comparision
Also, the ozone concentration curve for weekends in Figure 10 shows more peaks than the ozone concentration curve for workdays in Figure 9. At the same time, the NO 2 concentration curve for weekends in Figure 10 also has more peaks than the NO 2 concentration for workdays in Figure 9. The results of comparison indicate ozone concentration on weekends is due to the influence of NO 2 .

Monthly Variation
Higher concentrations of nitrogen dioxide were measured at all measuring points in the urban setting during the winter than during the summer since the winter weather conditions reduce the self-cleaning capacity of the air in the city and increase the use of energy for heating; fuel consumption by vehicles is also higher. Nitrogen dioxide concentrations greatly exceed the measured value of annual average, during the winter, due to emissions from heating buildings.
In Figure 11, According to the results of monthly variation, ozone concentrations have inverse relationship with NO 2 concentration for monthly change. When the NO 2 concentration is higher in winter, the ozone concentration is lower.

Improving Air Quality
There is a need to improve air quality in the Chicago metropolitan area because of the significant health benefits associated with better air [3] [4] [6] [16] [23]- [28]. Table 5 shows that there are several counties that are not meeting the NAAQS for ozone [3]. Cook county has a number of sites where ozone concentrations often exceed regulatory values because of large emissions of NO x and VOCs; see Table 6 and Table 7. Table 8 and Table 9 provide information on sources of VOCs that contribute to ozone formation.     The ozone pollution problems can be addressed by reducing nitrogen oxide emissions and releases of VOCs [4] [6]. One action to take is to reduce combustion emissions in the Chicago metropolitan area. There is already interest and action to replace diesel powered buses with electric buses. The Chicago Transit Authority has already reported that operating costs and maintenance costs are less for their electric buses [27]. When health and quality of life are considered, electric buses are the best choice when it is time to purchase a new bus. The plan is to have all electric buses by 2040 [28].
The transition to electric power for school buses has started, and many school districts have demonstrated interest in participating in the transition to electric school buses. Since school buses are needed in mornings and afternoons, there is time for charging during the day and at night, and battery capital costs are less than for transit buses because of the shorter range that is needed.
The electrification of transportation reduces both NO x and VOCs [23]. The transition to electric passenger cars has made great progress [6]. Electric vehicles are being used for taxis in many locations [6]. Since EVs are less expensive to operate and maintain, there is significant interest in using EVs in metropolitan areas for a number of applications including delivery vehicles, police cars, and service vehicles [6].
In China, there has been a major transition to electric bicycles for transportation to work, shopping, and other destinations [6]. There are often significant benefits in communities where improvements are made for biking and walking.
Since wind and solar energy are now the low cost options to add electric generating capacity, the portion of electricity being generated without combustion emissions continues to increase. Electric vehicle charging can take place whenever the vehicle is parked, and there are benefits in doing it when the grid has power that needs to be delivered.
One of the sources of VOC emissions is from consumer products. These emissions can be addressed through public education on how to reduce them. There are benefits in helping the public understand ozone pollution, the process of ozone formation, and what can be done to reduce emissions.
There are many beneficial policy choices that will help improve air quality. Some examples include incentives to purchase and use electric vehicles, a tax on carbon emissions, and a permit requirement to operate an internal combustion vehicle in the congested part of the city.

Conclusion
This article analyzes the concentrations of NO 2 and O 3 collected in Chicago in 2017. The daily cycle of NO 2 concentration reaches the maximum in the late morning and has smaller nighttime concentrations, and the daily cycle of ozone concentration reaches the maximum in the afternoon and also becomes smaller for nighttime concentrations. After the sun rises, the ozone concentration rises slowly, and reaches a peak in the afternoon, and then drops until the next morning. Ozone formation involves photochemical reactions that are impacted by solar radiation and temperature. The diurnal ozone change is influenced by NO 2 , and the NO 2 concentration is influenced by transportation emissions. The NO 2 is from transportation emissions and other emissions. Both NO 2 and VOCs impact the daily cycle of ozone levels. The weekend effects of NO 2 concentration on ozone concentration are different because traffic patterns are different. Ozone concentration has an inverse relationship with NO 2 concentration for monthly change. Reducing greenhouse gas emissions and improving air quality are significant challenges [23]. The regional background O 3 concentration in Chicago is of concern because ozone air quality does not meet environmental protection standards. Ozone pollution can be reduced by transitioning to electric buses, taxis, and other changes that reduce emissions from combustion processes.