Air Pollutants and Monitoring - Criteria Pollutants
Sulfur Dioxide (SO2) Current and Historical Monitoring
DHEC monitors and tracks sulfur dioxide levels in South Carolina's air. Find out how monitoring is performed and why we do it.
DHEC has developed an ambient (outdoor) air quality monitoring network for sulfur dioxide and other pollutants to:
- measure maximum expected concentrations
- measure concentrations in areas where people live, work and play,
- provide background information in rural areas,
- help determine the impacts of sulfur dioxide emissions from specific sources, and
- monitor interstate and intrastate transport of pollutants.
In 2011, there were 7 SO2 monitors strategically located in 5 counties throughout the state. DHEC chose the location for each monitor in accordance with EPA monitor-siting requirements found in the Code of Federal Regulations Title 40, Part 58, Appendix E.
How SO2 monitoring is performed
Monitors are operated in several areas of the state to provide representative measurements of sulfur dioxide concentrations. Data is available for areas in cities and near sources where there is a higher potential for population exposure and health impacts. Monitoring is also done in more rural areas to provide background concentration data to support evaluation of ecological effects.
All of the Monitoring Network's sulfur dioxide monitors measure ambient concentrations 24 hours a day. Air is continuously drawn into the instrument where it is exposed to ultraviolet light that excites the sulfur dioxide molecules. The sulfur dioxide molecules fluoresce (glow) at a unique wavelength that is detected and measured by the instrument. The measurements are averaged over 5 minutes (to detect short concentration peaks) and one hour periods and stored in a computer at the site. The central data polling system collects the data from the monitoring computers each hour for review and reporting.
Minimum Monitoring Requirements
EPA determines the minimum number of sulfur dioxide monitors that can adequately represent an area based on the Metropolitan Statistical Area's (MSA) most recently measured sulfur dioxide emissions and its population. The MSA may include heavily populated counties and the adjacent counties that are closely related economically.
The minimum number of required monitors for each MSA is based on an EPA index called the "Population Weighted Emissions Index", or PWEI. To determine the PWEI for an area, the population of the MSA is multiplied by the total amount of sulfur dioxide emission for the MSA. This product is then divided by 1,000,000 to make the number easier to read. EPA requires MSAs with PWEI scores between 5,000 and 100,000 to have one monitor. MSAs with PWEI scores between 100,000 and 1,000,000 are required to have two sulfur dioxide monitors.
The image below shows the number of sulfur dioxide monitoring sites required in each of the MSAs in our state as of January 1, 2011.
Historical Sulfur Dioxide Monitoring Data and Trends
The monitoring network has recorded a modest decline in sulfur dioxide concentrations in South Carolina over the past ten years. A significant portion of this improvement occurred between 2005 and 2008, as a result of better industrial controls.
Improvements in air quality have lead to reduced sulfur dioxide concentrations. On average, statewide maximum daily sulfur dioxide concentrations have decreased approximately 14% from 2001 to 2010, due in large part to the cooperative efforts of state agencies, local governments, and industry to control emissions.
Sulfur Dioxide Design Values
Every year, a design value is calculated for each monitoring site and compared to the National Ambient Air Quality Standard for the air pollutants it monitors. The design value for sulfur dioxide is the average of the 99th percentile (approximately 4th highest) daily maximum 1-hour sulfur dioxide concentrations for each year over a three year period. For example, a monitoring site's design value for 2009 is the average of the annual 99th percentile daily maximum 1-hour sulfur dioxide concentrations from 2007, 2008, and 2009. See the annual 99th percentile sulfur dioxide concentrations and calculated design values for each monitoring site since 2000 below.
Statewide Sulfur Dioxide Design Value Trends (2001 - 2010)
In the chart below, the blue band shows the range and trend of average South Carolina sulfur dioxide design values over a ten-year period. All sulfur dioxide monitoring data between 2001 and 2010 are included in the chart. Eighty percent of the monitoring sites have design values within the blue band. The middle white line is the state's average sulfur dioxide design values by year.
For more information please contact the Bureau of Air Quality at (803) 898-4123 or by email.