Overview of Nitrogen Dioxide (NO2) Air Quality in the United States Updated: June 09, 2022 1. Introduction The overall purpose of this document is to maintain an up-to-date graphical summary of air quality information that supports the review of the National Ambient Air Quality Standards (NAAQS) for nitrogen dioxide (NO2). In previous reviews of the NO2 NAAQS, this type of information has generally been included in atmospheric sections of the Integrated Science Assessment (ISA) and Policy Assessment (PA) for Oxides of Nitrogen. This stand-alone document will either replace or complement the air quality emissions and monitoring data in the atmospheric sections of future NO2 NAAQS documents, and will be updated at regular intervals as new data becomes available. The content of past NAAQS documents' atmospheric sections has included major sections on emissions and concentration trends utilizing maps and data from the EPA's National Emissions Inventory (NEI) and the EPA's Air Quality System (AQS) database. In past NAAQS reviews, this often involved adaptation of figures and tables prepared for other reports, or development of new figures and tables using data analysis and mapping software. Additionally, the release of updated emission inventories and ambient monitoring data may not coincide with the schedule for the development of NAAQS documents. As a result, data access and resources can limit the availability of the most recent information for inclusion in NAAQS documents. This stand-alone document allows the content to be updated as soon as new data becomes available, rather than pulling from whatever is available at the time of publication. It also ensures that the public will have access to a consistent set of maps and figures for each NAAQS pollutant that are updated on a routine basis, rather than separated by several years following the disparate schedules of the various NAAQS reviews for each pollutant. Moreover, a stand-alone document can be expanded to include new air quality analyses as they are completed, rather than following the timeline for the public release of the NAAQS documents. Finally, this document takes advantage of a more flexible digital format for the routinely prepared maps and trends figures, with an end product that more strongly emphasizes visual presentation of data and reduces the amount text, while also creating a more interactive presentation of the information through the use of external links. This document follows an organization similar to the structure of the atmospheric sections of past NO2 NAAQS documents. The subsequent sections are as follows: 2. Atmospheric Chemistry; 3. Sources and Emissions of NOx; 4. Ambient Air Monitoring Requirements and Monitoring Networks; 5. Data Handling Conventions and Computations for Determining Whether the Standards are Met; and 6. NO2 Concentrations Measured at Ambient Air Monitoring Sites Across the U.S. These sections are broad enough in scope to handle changes in what is known about NO2 atmospheric science as it advances but specific enough that NAAQS-relevant information will be able to be quickly retrieved by users of the document. 2. Atmospheric Chemistry Ambient concentrations of NO2 are influenced by both direct NO2 emissions and by emissions of nitric oxide (NO), with the subsequent conversion of NO to NO2 primarily though reaction with ozone (O3). The initial reaction between NO and O3 to form NO2 occurs fairly quickly during the daytime, with reaction times on the order of minutes. However, NO2 can also be photolyzed to reform NO, creating new O3 in the process. A large number of oxidized nitrogen species in the atmosphere are formed from the oxidation of NO and NO2. These include nitrate radicals (NO3), nitrous acid (HONO), nitric acid (HNO3), dinitrogen pentoxide (N2O5), nitryl chloride (CINO2), peroxynitric acid (HNO4), peroxyacetyl nitrate and its homologues (PANs), other organic nitrates, such as alkyl nitrates (including isoprene nitrates), and PNO3. The sum of these reactive oxidation products (collectively referred to as NOz) and NO plus NO2 (collectively referred to as NOx) comprise NOy- Due to the close relationship between NO and NO2, and their ready interconversion, these species are often grouped together and referred to as NOx- The majority of NOx emissions are in the form of NO. For example, 90% or more of tail-pipe NOx emissions are in the form of NO, with only about 2 to 10% emitted as NO2. As noted above, NOx emissions require time and sufficient O3 concentrations for the conversion of NO to NO2. Higher temperatures and concentrations of reactants result in shorter conversion times (e.g., less than one minute under some conditions), while dispersion and depletion of reactants results in longer conversion times. The time required to transport emissions away from a roadway can vary from less than one 1 ------- minute (e.g., under open conditions) to about one hour (e.g., for certain urban street canyons). These factors can affect the locations where the highest NO2 concentrations occur. In particular, while ambient NO2 concentrations are often elevated near important sources of NOx emissions, such as major roadways, the highest measured ambient concentrations in a given urban area may not always occur immediately adjacent to those sources. Ambient NO2 concentrations around stationary sources of NOx emissions are similarly impacted by the availability of O3 and by meteorological conditions, although surface- level NO2 concentrations can be less impacted in cases where stationary source NOx emissions are emitted from locations elevated substantially above ground level. The near-road environment provides a clear example of the interplay between NOx emissions, meteorology, and the atmospheric chemistry that impacts ambient NO2 concentrations. Vehicular emissions tend to peak during the morning and afternoon commutes, while peak O3 concentrations generally occur in the late morning to early evenings. In addition, atmospheric mixing tends to be the strongest during the daytime, rapidly diluting roadway emissions. Given the relative timing of O3 availability and peak atmospheric mixing conditions, the highest near-road NO2 concentrations often occur during the early morning hours (i.e., before atmospheric mixing can rapidly dilute emissions). The conversion of NOx into the species that make up NOz typically takes place on a much longer time scale than do interconversions between NO and NO2. NOx emitted during morning rush hour by vehicles can be converted almost completely into these products by late afternoon during warm, sunny conditions. Oxidized nitrogen compounds are ultimately lost from the atmosphere by wet and dry deposition to the Earth's surface. Soluble species are taken up by aqueous aerosols and cloud droplets and are removed by wet deposition by rainout (i.e., incorporation into cloud droplets that eventually coagulate into falling rain drops). Both soluble and insoluble species are removed by washout (i.e., impaction with falling rain drops, another component of wet deposition), and by dry deposition (i.e., impaction with the surface and gas exchange with plants). NO and NO2 are not very soluble, and therefore wet deposition is not a major removal process for them. However, a major NOx reservoir species, HNO3, is extremely soluble, and its deposition (both wet and dry) represents a major sink for NOy- Sources: Integrated Science Assessment for Oxides of Nitrogen, January 2016 (Chapter 2) Policy Assessment for the Review of the Primary NAAQS for Oxides of Nitrogen, April 2017 (Chapter 2) 3. Sources and Emissions of NOx Anthropogenic sources account for a large majority of NOx emissions in the U.S., with highway vehicles, off-highway vehicles, and stationary fuel combustion identified as the largest contributors (Figure 1). More specifically, highway ve- hicles include all on-road vehicles, including light duty as well as heavy duty vehicles, both gasoline- and diesel-powered. Non-road mobile sources include aircraft, commercial marine vessels, locomotives, and non-road equipment. Fuel com- bustion includes electric generating units (EGUs), which generate electricity from fossil fuels, primarily coal, as well as commercial/institutional, industrial, and residential combustion of biomass, coal, natural gas, oil, and other fuels. Other an- thropogenic NOx sources include agricultural field burning, prescribed fires, and various industrial processes such as cement manufacturing and oil and gas production. Natural sources of NOx include emissions from plants and soil (biogenics) and wildfires. Nationwide estimates indicate a 68% decrease in anthropogenic NOx emissions from 2002 to 2021 (Figure 2) as a result of multiple regulatory programs implemented over the past two decades, including the NOx SIP Call, the Cross-State Air Pollution Rule (CSAPR), and the Tier 3 Light-duty Vehicle Emissions and Fuel Standards. The overall decrease in NOx emissions has been driven primarily by decreases from the three largest emissions sectors. Specifically, compared to the 2002 NEI, estimates for 2021 indicate a 78% reduction in NOx emissions from highway vehicles, a 62% reduction in NOx emissions from non-road mobile sources, and a 69% reduction in NOx emissions from stationary fuel combustion. The National Emissions Inventory (NEI) is a comprehensive and detailed estimate of air emissions of criteria pollutants, precursors to criteria pollutants, and hazardous air pollutants from air emissions sources. The NEI is released every three years based primarily upon data provided by State, Local, and Tribal air agencies for sources in their jurisdictions and supplemented by data developed by the US EPA. The NEI is built using the EPA's Emissions Inventory System (EIS) first to collect the data from State, Local, and Tribal air agencies and then to blend that data with other data sources. Accuracy in an emissions inventory reflects the extent to which the inventory represents the actual emissions that occurred. Anthropogenic emissions of air pollutants result from a variety of sources such as power plants, industrial sources, motor vehicles and agriculture. The emissions from any individual source typically varies in both time and space. For the thousands of sources that make up the NEI, there is uncertainty in one or both of these factors. For some sources, such as power plants, direct emission measurements enable the emission factors derived from them to be more certain than sources without such direct measurements. However, it is not practically possible to directly monitor each of the emission sources individually and, 2 ------- therefore, emission inventories necessarily contain assumptions, interpolation and extrapolation from a limited set of sample data. NOx Emissions (11,786 kTon/year) Stationary Fuel Combustion 22% Industrial Processes 10% Highway Vehicles 30% All Fires 3% Biogenics 12% Other 1% Non-Road Mobile 22% Figure 1. U.S. NOx emissions (tons/year) by sector. Source: 2017 NEI. 24000 CO 0 >1 £ CO E LU X O 20000 - 16000 - 12000 - 8000 - 4000 - 0 - CM O O CM LO O O CM ~ Highway Vehicles ~ Non-Road Mobile ~ Stationary Fuel Combustion Industrial and Other Processes Other Anthropogenic Sources 00 o o CM o cm I o CM o CM o CM o CM Inventory Year Figure 2. U.S. anthropogenic NOx emissions trend, 2002-2021. Source: EPA's Air Pollutant Emissions Trends Data 3 ------- Figure 3 below shows the NOx emissions density in tons/mi2/year for each U.S. county based on the 2017 NEI. The majority of NOx emissions tend to be located near urban areas, which tend to have the most vehicle traffic and industrial sources. However, there are also some counties in rural areas with higher NOx emissions due to the presence of large stationary sources such as EGUs or oil and gas extraction. ~ 0-1.9 (1024) ~ 2-4.9(1264) ~ 5-9.9 (499) ¦ 10-19.9(251) ¦ 20-826(182) Figure 3. U.S. county-level NOx emissions density estimates in tons/year/mi2. Source: 2017 NEI 4. Ambient Air Monitoring Requirements and Monitoring Networks Ambient NO2 concentrations are measured by monitoring networks operated by state, local, and tribal air agencies, which are typically funded in part by the EPA. There were 491 monitoring sites reporting hourly NO2 concentration data to the EPA during the 2019-2021 period. The locations of these monitoring sites are shown in Figure 4. The main network of monitors providing ambient data for use in implementation activities related to the NAAQS is the State and Local Air Monitoring Stations (SLAMS) network, which comprises over 80% of all NO2 monitoring sites. This network relies on a chemiluminescent Federal Reference Method (FRM) and on Federal Equivalent Methods (FEM) that use either chemiluminescence or direct measurement methods of NO2. Data produced by chemiluminescent analyzers include NO, NO2, and NOx measurements, which are all routinely reported by state and local air monitoring agencies. Two important subsets of SLAMS sites separately make up the National Gore (NCore) multi-pollutant monitoring network and the Photochemical Assessment Monitoring Stations (PAMS) network. The NCore network consists of approximately 60 urban monitoring stations and 20 rural monitoring stations, and each state is required to have at least one NCore station. At each NCore site located in a MSA with a population of 1 million or more (based on the most recent census), a PAMS network site is required.1 Monitoring sites in the PAMS network are required to measure NO, NO2, NOy, and other O3 precursors during the months of June, July and August, although some precursor monitoring may be required for longer periods of time. Another important subset of SLAMS sites is the near-road monitoring network, which was required as part of the 2010 NO2 NAAQS review and began operating in 2014. Near-road sites are required in each metropolitan statistical area (MSA) The requirements for PAMS, which were most recently updated in 2015, is fully described in section 5 of Appendix D to 40 C'FR Part 58. 4 ------- with a population of 1,000,000 or greater, and an additional near-road site is required in each MSA with a population of 2,500,000 or greater or with one or more roadway segments that have an average daily traffic volume of 250,000 or more vehicles per day. There were 73 monitors in operation during the 2019-2021 period. Finally, there are also a number of Special Purpose Monitors (SPMs), which are not required but are often operated by air agencies for short periods of time (i.e., less than 3 years) to collect data for human health and welfare studies, as well as other types of monitoring sites, including monitors operated by tribes and industrial sources. The SPMs are typically not used to assess compliance with the NAAQS. • SLAMS (251) • NCORE/PAMS (73) O NEAR ROAD (73) • SPM/OTHER (94) Figure 4: Map of U.S. NO2 monitoring sites reporting data to the EPA during the 2019-2021 period. Source: AQS. The traditional chemiluminescence FRM is subject to potential measurement biases resulting from interference by NOz species. However, within metropolitan areas, where a majority of the NO2 monitoring network is located, NO2 concentrations tend to be most heavily influenced by strong local NOx sources, thus the potential for NOz related measurement bias is relatively small. There have been recent advances in methods that provide measurements of NO2 with less potential for interference. These newer methods include photolytic-chemiluminescent methods that rely on photo dissociation of NO2 using specific wavelengths of light, and direct measurements of NO2, including cavity attenuated phase shift spectrometry and cavity ring-down spectroscopy. It should be noted that the direct NO2 measurement methods do not provide NO or NOx measurements. These newer methods are expected to gradually replace the older FRMs as the older monitors age. 5. Data Handling Conventions and Computations for Determining Whether the Standards are Met To assess whether a monitoring site or geographic area (usually a county or urban area) meets or exceeds a NAAQS, the monitoring data are analyzed consistent with the established regulatory requirements for the handling of monitoring data for the purposes of deriving a design value. A design value summarizes ambient air concentrations for an area in terms of the indicator, averaging time and form for a given standard such that its comparison to the level of the standard indicates whether the area meets or exceeds the standard. There are currently two NO2 NAAQS in effect: the annual NAAQS (established in 5 ------- 1971) and the 1-hour NAAQS (established in 2010). The procedures for calculating design values for both NO2 NAAQS are detailed in Appendix S to 40 GFR Part 50 and are summarized below. Hourly NO2 measurement data collected at an ambient air monitoring site using Federal Reference or Equivalent Methods, meeting all applicable requirements in 40 GFR Part 58, and reported to AQS in parts per billion (ppb) with decimal digits after the first decimal place truncated are used in design value calculations. If multiple monitors collect measurements at the same site, one monitor is designated as the primary monitor. Measurement data collected with the primary monitor are used to calculate the design value, and may be supplemented with data from collocated monitors only if (a) the primary monitor did not collect sufficient data to determine a valid design value, or (b) the primary monitor has been discontinued and replaced by another monitor. The design value for the annual NO2 NAAQS is simply the mean of all hourly concentration values reported for a single year, rounded to the nearest integer in ppb. The annual design value is considered valid when hourly concentrations are reported for at least 75% of the hours in the year, or if the design value is greater than 53 ppb, the level of the NAAQS. For the 1-hour NO2 NAAQS, the maximum hourly concentration is determined for each day (i.e., the "daily maximum value") in a given 3-year period. For each year, the 98th percentile of the daily maximum values is determined, and the design value is the average of the three annual 98th percentile values, rounded to the nearest integer in ppb. The 1-hour NO2 NAAQS are met when the design value is less than or equal to 100 ppb, the level of the NAAQS. In addition, the 1-hour design value must meet data completeness requirements in order to be considered valid. Specifically, a sample day is considered complete when at least 18 hourly measurements are reported. For each calendar quarter (i.e., Jan-Max, Apr-Jun, Jul-Sep, Oct-Dec), the quarter is considered complete if at least 75% of the days in the quarter have complete data. The 1-hour NO2 design value is considered complete when all 12 calendar quarters in the 3-year period have complete data. In addition, there are two data substitution tests specified in Appendix S to 40 GFR Part 50 which may be used to yield a valid design value above or below the NAAQS, respectively, in the event that a site falls short of the minimum data completeness requirement. 6. N02 Concentrations Measured at Ambient Air Monitoring Sites Across the U.S. Table 1 below presents summary statistics based on the two daily NO2 NAAQS metrics, the daily maximum 1-hour (MDA1) metric, and the daily 24-hour average (DA24) metric. These statistics are presented for year-round and each season (winter=Dec/Jan/Feb, spring=Mar/Apr/May, summer=Jun/Jul/Aug, autumn=Sep/Oct/Nov) based on data reported to AQS for 2019-2021. Table 2 presents the same summary statistics for the MDA1 and DA24 metrics for each NOAA Climate Region2. Table 1. National distribution of NO2 concentrations in ppb by season for 2019-2021. Source: AQS. metric season N.sites N.obs mean SD min Pi p5 plO p25 p50 p75 p90 p95 p98 p99 max max.site MDA1 all 491 466,982 16.3 12.3 -5.0 0.5 1.6 2.8 6.2 13.6 24.0 34.0 39.5 45.9 50.3 315.3 201950001 MDA1 winter 485 115,205 19.8 13.0 -5.0 0.7 2.0 3.7 8.8 18.4 29.0 37.5 42.3 48.1 52.8 269.2 220330009 MDA1 spring 480 118,099 15.0 11.8 -5.0 0.4 1.3 2.3 5.4 12.0 22.2 32.4 38.0 43.9 48.0 94.9 340030010 MDA1 summer 481 117,694 12.8 9.9 -3.0 0.6 1.5 2.4 5.1 10.3 18.1 26.7 32.5 38.9 43.1 146.2 720610006 MDA1 autumn 477 115,984 17.7 13.0 -3.4 0.6 1.7 3.0 7.0 15.3 25.8 35.9 42.0 49.1 54.0 315.3 201950001 DA24 all 491 466,982 7.8 7.0 -5.0 0.0 0.6 1.1 2.6 5.7 10.9 17.3 21.9 27.4 31.3 140.7 220330009 DA24 winter 485 115,205 10.2 8.1 -5.0 0.1 0.8 1.6 3.8 8.2 14.5 21.5 26.1 31.7 35.6 140.7 220330009 DA24 spring 480 118,099 6.5 5.9 -5.0 0.0 0.5 1.0 2.2 4.7 9.1 14.6 18.5 23.3 26.3 58.5 080310027 DA24 summer 481 117,694 5.8 5.2 -4.1 0.0 0.6 1.0 2.2 4.4 8.0 12.7 16.1 20.6 24.0 80.6 080677003 DA24 autumn 477 115,984 8.6 7.5 -4.5 0.0 0.7 1.2 3.0 6.6 12.1 18.9 23.5 29.3 33.2 62.0 060374008 N.sites = number of sites; N.obs = number of observations; SD = standard deviation; min = minimum; pi, p5, plO, p25, p50, p90, p95, p98, p99 = 1st, 5th, 10th, 25th, 50th, 90th, 95th, 98th, 99th percentiles; max = maximum; max.site = AQS ID number for the monitoring site corresponding to the observation in the max column, winter = December/January/February; spring = March/April/May; summer = June/July/August; autumn = September/October/November. 2For Table 2, monitoring sites in Alaska were assigned to the Northwest Region, monitoring sites in Hawaii were assigned to the West region, and monitoring sites in Puerto Rico were assigned to the Southeast region. 6 ------- Table 2. National distribution of NO2 concentrations in ppb by climate region for 2019-2021. Source: AQS. metric region N.sites N.obs mean SD mill pi p5 plO p25 p50 p75 p90 p95 p98 p99 max max.site MDA1 MDA1 MDA1 MDA1 MDA1 MDA1 MDA1 MDA1 MDA1 MDA1 DA24 DA24 DA24 DA24 DA24 DA24 DA24 DA24 DA24 DA24 all Central East North Central Northeast Northwest South Southeast Southwest West West North Central all Central East North Central Northeast Northwest South Southeast Southwest West West North Central 491 33 25 71 8 75 51 64 115 49 491 33 25 71 8 75 51 64 115 49 466,982 33,809 19,567 72,228 6,727 74,653 44,463 59,309 113,774 42,452 466,982 33,809 19,567 72,228 6,727 74,653 44,463 59,309 113,774 42,452 16.3 18.9 16.5 18.1 21.2 14.0 15.8 17.3 19.2 5.8 7.8 9.1 8.1 8.8 10.9 6.1 7.4 8.0 9.6 2.1 12.3 11.1 10.8 11.8 10.0 10.8 10.4 13.9 13.2 6.4 7.0 6.1 5.9 6.9 6.0 5.2 5.6 8.2 8.0 2.3 -5.0 -0.6 0.1 -3.0 0.3 -2.6 -1.0 -5.0 -2.0 -1.8 -5.0 -0.8 -0.4 -4.1 0.3 -2.6 -1.6 -5.0 -3.6 -2.2 0.5 1.7 0.9 1.0 3.9 0.7 0.8 0.2 1.0 0.0 0.0 0.8 0.3 0.1 1.7 -0.2 0.4 0.0 1.6 3.6 1.7 2.3 6.1 2.0 2.1 1.3 2.4 0.6 0.6 1.6 0.9 1.0 3.0 0.7 0.9 0.6 0.1 1.0 -0.1 0.1 2.8 5.4 3.0 4.0 8.6 3.1 3.6 2.5 4.0 1.0 1.1 2.4 1.6 1.7 4.0 1.3 1.6 1.0 1.9 0.3 6.2 10.0 7.8 8.3 13.6 5.7 7.4 6.0 8.5 1.7 2.6 4.4 3.6 3.6 6.3 2.4 3.2 2.3 3.7 0.7 13.6 17.5 15.1 16.3 20.5 11.0 14.2 13.2 16.7 3.5 5.7 7.7 6.9 7.2 10.0 4.6 6.0 5.1 7.3 1.4 24.0 26.5 23.7 26.0 27.6 19.8 22.5 26.3 27.9 7.4 10.9 12.4 11.7 12.5 14.5 8.2 10.4 11.0 13.4 2.7 34.0 34.1 31.3 34.4 34.4 29.6 30.1 38.4 38.0 14.0 17.3 17.7 16.2 18.4 18.7 13.1 15.3 20.0 21.0 4.7 39.5 38.6 36.0 39.2 38.4 35.2 34.8 44.0 43.5 19.3 21.9 21.3 19.4 22.2 21.7 16.7 18.5 25.9 25.9 6.4 45.9 44.0 41.5 45.0 43.6 41.1 40.0 50.0 50.2 25.9 27.4 25.1 22.9 26.9 25.3 21.0 22.2 32.3 31.6 9.4 50.3 47.8 45.1 49.0 46.8 45.4 43.3 54.2 55.0 30.5 31.3 27.6 25.3 30.2 28.4 24.0 24.6 36.4 35.5 11.7 315.3 93.1 80.0 110.2 83.3 315.3 171.5 116.0 101.6 87.6 140.7 44.5 48.8 55.8 53.6 140.7 40.7 80.6 62.0 28.3 201950001 290950034 261630098 340030010 530330030 201950001 720250007 490353006 060710027 381010003 220330009 170313103 270530962 340130003 530330030 220330009 517600025 080677003 060374008 460710001 N.sites = number of sites; N.obs = number of observations; SD = standard deviation; min = minimum; pi, p5, plO, p25, p50, p90, p95, p98, p99 = 1st, 5th, 10th, 25th, 50th, 90th, 95th, 98th, 99th percentiles; max = maximum; max.site = AQS ID number for the monitoring site corresponding to the observation in the max column. Central = Illinois, Indiana, Kentucky, Missouri, Ohio, Tennessee, West Virginia; East North Central = Iowa, Minnesota, Michigan, Wisconsin; Northeast = Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont; Northwest = Alaska, Idaho, Oregon, Washington; South = Arkansas, Kansas, Louisiana, Mississippi, Oklahoma, Texas; Southeast = Alabama, Florida, Georgia, North Carolina, South Carolina, Virginia; Southwest = Arizona, Colorado, New Mexico, Utah; West = California, Hawaii, Nevada; West North Central = Montana, Nebraska, North Dakota, South Dakota, Wyoming. 7 ------- Figure 5 below shows a map of the annual NO2 design values at U.S. ambient air monitoring sites based on data from 2021 and Figure 6 shows a map of the 1-hour NO2 design values based on data from the 2019-2021 period. There were no sites with design values exceeding either NAAQS. The maximum annual design value was 30 ppb, while the maximum 1-hour design value was 80 ppb. Both of these maximum design values occurred at near-road sites in the Los Angeles, GA metropolitan area. • 1 - 10 ppb (298 sites) O 11 - 20 ppb (99 sites) O 21 - 30 ppb (8 sites) Figure 5: Annual NO2 design values in ppb based on data from 2021. Source: AQS. 8 ------- °s> o • 3-25 ppb (66 sites) O 26-50 ppb (221 sites) O 51-75 ppb (41 sites) O 76-100 -hour NO2 design values in ppb for the 2019-2021 period. Source: AQS. 9 ------- Figure 7 below shows a map of the site-level trends in the annual NO2 design values at U.S. monitoring sites having valid design values in at least 17 years from 2000 through 2021. Figure 8 shows a map of the site-level trends in the 1-hour NO2 design values at U.S. monitoring sites having valid design values in at least 15 of the 20 3-year periods from 2000 through 2021. The trends were computed using the Thiel-Sen estimator, and tests for significance were computed using the Mann-Kendall test. From these figures it is apparent that NO2 concentrations have been decreasing at nearly all sites in the U.S. Two sites in North Dakota showed an increasing trend in the annual design value (one of these sites also had an increasing trend in the 1-hour design value), which is likely due to an increase in NOx emissions from oil and gas extraction activity in the region. ^ Decreasing > 0.5 ppb/yr (75 sites) o No Significant Trend (7 sites) V Decreasing < 0.5 ppb/yr (132 sites) A Increasing < 0.5 ppb/yr (2 sites) Figure 7: Site-level trends in annual NO2 design values based on data from 2000 through 2021. Source: AQS, trends computed using R statistical software. 10 ------- ^ Decreasing > 1 ppb/yr (94 sites) o No Significant Trend (3 sites) V Decreasing < 1 ppb/yr (45 sites) A Increasing < 1 ppb/yr (1 sites) Figure 8: Site-level trends in 1-hour NO2 design values based on data from 2000 through 2021. Source: AQS, trends computed using R statistical software. 11 ------- Figure 9 below shows the national trends in the annual and 1-hour NO2 design values based on the 216 sites shown in Figure 7 and the 143 sites shown in Figure 8, respectively. The national median of the annual design values has decreased by 57% from about 15.8 ppb in 2000 to about 6.9 ppb in 2021. The national median of the 1-hour design values has decreased by 39% from 61 ppb in 2000 to 37 ppb in 2021. .Q Q. Q. CM O Figure 9: National trends in NO2 design values in ppb, 2000 to 2021. Source: AQS. Additional Resources Nitrogen Dioxide (NO2) Pollution Reviewing National Ambient Air Quality Standards (NAAQS): Scientific and Technical Information Air Emissions Inventories Ambient Monitoring Technology Information Center (AMTIG) Air Quality Design Values National Air Quality: Status and Trends of Key Air Pollutants Air Data: Air Quality Data Collected at Outdoor Monitors Across the U.S. 12 ------- |