EPA's Report on the Environment: External Review Draft
Ambient Concentrations of Sulfur Dioxide
Note to reviewers of this draft revised ROE: This is a new indicator that reflects data through 2011. EPA anticipates updating this indicator in 2014.
Sulfur dioxide (SO2) is one of many sulfur oxide gases that form during the combustion of fuels containing sulfur, primarily coal and oil. The largest anthropogenic source of SO2 emissions in the U.S. is fossil fuel combustion at electric utilities and other industrial facilities. SO2 is also emitted from certain manufacturing processes and mobile sources, including locomotives, large ships, and construction equipment (see SO2 Emissions indicator). The highest concentrations of SO2 are typically recorded in the vicinity of large emissions sources.
Short-term exposure to airborne SO2 has been associated with various adverse health effects (U.S. EPA, 1994; ATSDR, 1998). Multiple human clinical studies, epidemiological studies, and toxicological studies support a causal relationship between short-term exposure to airborne SO2 and respiratory morbidity. The observed health effects have included respiratory symptoms, airway inflammation, and increased emergency department visits and hospitalizations for all respiratory causes. These studies further suggest that asthmatics, children, older adults, and people who spend a lot of time outdoors at increased exertion levels are potentially susceptible or vulnerable to these health effects (U.S. EPA, 2008a). In addition, SO2 reacts with other air pollutants to form sulfate particles, which are constituents of fine particulate matter (PM2.5). Inhalation exposure to PM2.5 has been associated with various cardiovascular and respiratory health effects (see PM Concentrations indicator).
Airborne SO2 also causes or contributes to numerous environmental impacts. For instance, airborne SO2 along with airborne nitrogen oxides contributes to acidic deposition, and this deposition can harm susceptible aquatic and terrestrial ecosystems, including injury to forests and localized extinction of fishes and other aquatic species (see Acid Deposition indicator). In some watersheds, sulfate deposition increases mercury methylation rates, which leads to formation of methylmercury—the chemical form of mercury that accumulates in the aquatic food chain (EPA, 2008b). In addition, SO2 contributes to the formation of fine airborne particles that can impair visibility—an issue of particular concern in National Parks and Wilderness Areas (see Regional Haze indicator).
This indicator presents ambient SO2 concentrations in parts per billion (ppb) from 1978 to 2011 using two averaging times: 1-hour averaging times to be consistent with the current primary National Ambient Air Quality Standard (NAAQS) and annual averaging times to present trends in long-term exposure levels. 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 depend on the time horizon for the trends: for 1980 through 2011, 107 monitoring sites in 90 counties nationwide have sufficient data to characterize annual average trends; and for 1998 through 2011, 246 monitoring sites in 173 counties nationwide have sufficient data to characterize 1-hour trends. Trends are displayed for the entire nation and for each EPA Region. Refer to the technical documentation for the selection criteria that were applied to identify the sites with sufficient data to characterize air quality trends.
Trends in 1-hour SO2 concentrations are presented for the annual 99th percentile 1-hour daily maximum, averaged over 3 consecutive years. This averaging time and statistic is consistent with the primary NAAQS, which was derived to protect public health, including the health of sensitive populations such as asthmatics, children, and the elderly. All exhibits in this indicator present the SO2 NAAQS as a point of reference. The exhibits showing trends in 1-hour concentrations display the current NAAQS, and the exhibits based on annual average concentrations show the former 1971 NAAQS, which was revoked in 2010. The fact that the national or regional concentrations fall below the standards does not mean that all monitoring sites nationally or in any EPA Region also are below the standards. The indicator displays trends in the number of trend sites nationwide at which SO2 concentrations exceeded the level of the standards, but these statistics are not displayed for each EPA Region.
What the Data Show
Annual average SO2 concentrations at 107 monitoring sites decreased by 83 percent between 1980 and 2011, and the 2011 levels are the lowest over the 32-year period of record (Exhibit 1). Annual average SO2 levels have also steadily decreased in every EPA Region, with the greatest reductions (88 percent) observed in Region 2 and Region 8 (Exhibit 2). This downward trend in annual SO2 concentrations parallels the downward trend observed in SO2 emissions from 1990 to 2008, which has been attributed largely to decreased emissions from electric utilities (the SO2 Emissions indicator). Decreased emissions from mobile sources due to use of low-sulfur fuels has also contributed to the ambient concentration trend.
The annual 99th percentile of daily maximum 1-hour SO2 concentrations, averaged over 3 consecutive years, also exhibited a downward trend. These data were displayed for two different time horizons in order to account for as many monitoring sites as possible, because more sites meet the site selection criteria when considering more recent time, shorter frames:
- For 1978 to 2011, the 3-year average of the 99th percentile of daily maximum 1-hour SO2 concentrations decreased by 74 percent across the 30 sites with sufficient data (Exhibit 3). Among these sites, the number reporting concentrations above the level of the 1-hour NAAQS decreased by 85 percent (Exhibit 4).
- For 1998 to 2011, the 3-year average of the 99th percentile of daily maximum 1-hour SO2 concentrations decreased by 49 percent across the 246 sites with sufficient data (Exhibit 5). Among these sites, the number reporting concentrations above the level of the 1-hour NAAQS decreased by 63 percent (Exhibit 6). Consistent with the nationwide trend, the 99th percentile of daily maximum 1-hour SO2 concentrations averaged over 3 consecutive years also steadily decreased between 1998 and 2011 in the EPA Regions, with the greatest percent reduction observed in Region 2 (Exhibit 7).
Also shown in Exhibit 1, 3, and 5 are the 90th and 10th percentiles based on the annual measurements at the monitoring sites. This provides additional graphical representation of the variability of measured concentrations across the monitoring sites for a given year. The shaded areas in these exhibits display the concentration range where 80 percent of measured values occurred for that year.
- Because most SO2 monitoring sites are in urban areas, the trends might not accurately reflect conditions outside the immediate urban monitoring areas.
- 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 trend sites in this indicator are not dispersed uniformly across all states in the EPA Regions. The 246 sites for the 1998-2011 trends are located in 37 states. In the remaining 13 states, there currently are insufficient long-term data from the existing 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 SO2 monitoring sites with sufficient data to assess trends since 1978. Monitoring sites without sufficient data are not included in the trend analysis. Some excluded monitoring sites reported SO2 concentrations above the level of the NAAQS over the time frame covered by this indicator. In 2011, 33 sites in the U.S. measured SO2 concentrations above the level of the 1-hour NAAQS.
- Because of the relatively small number of trend sites for the long-term period 1980-2011, the national trends in Exhibits 1-4 may not necessarily be representative of the entire U.S.
Summary data in this indicator were provided by EPA’s Office of Air Quality Planning and Standards, based on SO2 ambient air monitoring data in EPA’s Air Quality System (U.S. EPA, 2012) (http://www.epa.gov/ttn/airs/airsaqs/). National and regional trends in this indicator are based on the subset of SO2 monitoring stations that have sufficient data to assess trends.
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