Ambient Concentrations of Nitrogen Dioxide

Nitrogen dioxide (NO₂) is one of many highly reactive gases collectively referred to as “nitrogen oxides” (NO_x), all of which contain nitrogen and oxygen in varying amounts. Direct air emissions of NO_x generally comprise a mix of nitric oxide (NO) and NO₂, with a ratio in favor of NO. Specific emissions sources of NO_x that can affect local air quality include on-road vehicles, airports, railyards, shipping ports, home wood burning, intense industrial and chemical processes, activities for oil and gas development, and wildfires (see Nitrogen Oxides Emissions indicator). Once emitted to the air, NO reacts rapidly in the presence of ozone to form NO₂. In fact, in U.S. urban locations, most measured airborne NO₂ comes from the reaction of these two precursors, rather than from direct NO₂ emissions (U.S. EPA, 2008b).

Scientific evidence links short-term NO₂ exposures, ranging from 30 minutes to 24 hours, with an array of adverse respiratory effects including increased asthma symptoms, more difficulty controlling asthma, and an increase in respiratory illnesses and symptoms. Studies also show a connection between short-term NO₂ exposure and increased visits to emergency departments and hospital admissions for respiratory illnesses, particularly in at-risk populations and lifestages including children, the elderly, and persons with pre-existing respiratory disease (e.g., asthmatics). More limited evidence is available regarding long-term NO₂ exposure and respiratory effects (e.g., development of asthma) (U.S. EPA, 2008b).

Atmospheric transformation of NO_x and other pollutants leads to the formation of nitrogen-bearing particles that can eventually deposit to the surface, causing acidification, nitrogen enrichment, and other ecological effects. Acidification from the deposition resulting from current emission levels causes a cascade of effects that harm susceptible aquatic and terrestrial ecosystems, including slower growth and injury to forests and localized extinction of fishes and other aquatic species (see Acid Deposition indicator). Further, deposition of reactive nitrogen particles along with other non-atmospheric sources (e.g., fertilizers and wastewater) causes a suite of ecological changes within sensitive ecosystems. Examples include increased primary productivity in most nitrogen-limited ecosystems, biodiversity losses, changes in carbon cycling, and eutrophication and harmful algal blooms in freshwater, estuarine, and ocean ecosystems (U.S. EPA, 2008a).

This indicator presents ambient NO₂ concentrations in parts per billion (ppb) from 1980 to 2013 using two averaging times: 1-hour averages to be consistent with the short-term primary National Ambient Air Quality Standard (NAAQS) and annual averages to present trends consistent with the long-term NO₂ NAAQS. Trend data are based on measurements from the State and Local Air Monitoring Stations network and from other special purpose monitors. The number and spatial coverage of monitoring sites vary: between 1980 and 2013, 69 monitoring sites in 51 counties nationwide have sufficient data to characterize annual average trends; and between 1980 and 2013, 29 monitoring sites in 24 counties nationwide have sufficient data to characterize 1-hour trends. Trends are displayed for the entire nation and for each EPA Region with monitoring sites with sufficient long-term data.

Trends in 1-hour NO₂ concentrations are presented for the 98^th percentile of 1-hour daily maximum concentrations. All exhibits in this indicator present the NO₂ NAAQS as a point of reference. The fact that the national or regional concentrations fall below the standards does not mean that all monitoring sites nationally or in the EPA Region also are below the standards. The indicator displays trends in the number of trend sites nationwide at which NO₂ concentrations exceeded the level of the standards, but these statistics are not displayed for the EPA Regions.

The national annual average NO₂ concentration in 2013 was 58 percent lower than that recorded in 1980 (Exhibit 1). Also shown on this graph are the 90^th and 10^th percentiles of NO₂ concentrations based on the distribution of annual statistics at the monitoring sites. This provides additional graphical representation of the distribution of measured concentrations across the monitoring sites for a given year. Thus, for each year, the graphic displays the concentration range where 80 percent of measured values occurred. The highest annual average NO₂ concentrations are typically found in urban areas. In addition, of the 69 sites used to determine the trend for annual average concentrations (out of 357 total monitoring sites that were operating in 2013), the number reporting NO₂ concentrations above the level of the NO₂ standard declined from six sites in 1981 to zero sites since 1992 (Exhibit 2). Annual average NO₂ levels in all EPA Regions with trend sites have steadily decreased since 1980, with percent reductions over this time ranging from 37 percent in Region 8 to 62 percent in Region 1 (Exhibit 3). The decrease in NO₂ concentrations in this indicator is consistent with the decreasing NO_x emissions observed between 1990 and 2011 (the Nitrogen Oxides Emissions indicator).

The 98^th percentile of 1-hour NO₂ concentrations also exhibited a downward trend. From 1980 to 2013, the concentrations decreased by 60 percent across the 29 sites with sufficient data (Exhibit 4). Among these sites, the number reporting concentrations above the level of the 2010 1-hour NAAQS decreased from 12 sites in 1982 to zero sites since 2004 (Exhibit 5).

• Because ambient monitoring for NO₂ occurs almost exclusively in high-traffic urban areas, the average concentrations presented in this indicator likely may not reflect NO₂ concentrations in rural areas. Also, in rural areas, air mass aging could foster greater relative levels of peroxyacetyl nitrate (PAN) and nitric acid which can cause a positive interference in NO₂ measurements.
• The measurement of NO₂ is based on the conversion of NO₂ to NO and the subsequent detection of NO using the chemiluminescence technique. Because there are other nitrogen-containing compounds, such as PAN and nitric acid that can be converted to NO, the chemiluminescence technique may overestimate NO₂ concentrations due to these interferences. Measurement devices with ultraviolet photolytic converters are less prone to interferences than devices with heated surfaces (or catalysts) upstream of the chemiluminescence detector, but are not in widespread use.
• Because of the relatively small number of trend sites in some EPA Regions, the regional trends are subject to greater uncertainty than the national trends. Some EPA Regions with low average concentrations may include areas with high local concentrations, and vice versa. In addition, the 69 trend sites with sufficient data for annual trends are not dispersed uniformly across all states in the EPA Regions. The 69 sites are located in 19 states. In the remaining 31 states, there currently are insufficient long-term NO₂ data from monitoring sites to include in this indicator.
• To ensure that long-term trends are based on a consistent set of monitoring sites, selection criteria were applied to identify the subset of NO₂ monitoring sites with sufficient data to assess trends since 1980. Monitoring sites without sufficient data are not included in the trend analysis. Some excluded monitoring sites reported NO₂ concentrations above the level of the NO₂ standard over the time frame covered by this indicator. In 2013, however, no monitoring sites in the U.S. measured annual average NO₂ concentrations above the level of the NAAQS.

Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, based on NO₂ ambient air monitoring data in EPA’s Air Quality System (U.S. EPA, 2014a) (https://www.epa.gov/aqs). National and regional trends in this indicator are based on the subset of NO₂ monitoring stations that have sufficient data to assess trends since 1980.