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Assessing the Potential for the Occurrence
of Hourly Maximum 5-Minute Concentrations Greater than or Equal to 0.5 ppm Near SO2 Emission Sources
in the United States

A.S.L. & Associates performed a study that reviewed ambient SO2 data available from public sources in the United States for the purpose of characterizing the measured frequency and magnitude of hourly maximum 5-minute average concentrations greater than or equal to (hereafter referred to as "GE") 0.5 ppm in the vicinity of SO2 sources. The hourly maximum 5-minute SO2 average value is defined as the maximum 5-minute average concentration that occurs over an hour interval.

The objectives of the study were

  • Review the SO2 data associated with the monitors described in the ICF Kaiser Final Report (1996) to the U.S. EPA that quantified hourly maximum 5-minute peaks;


  • Review the data associated with the monitors in the U.S. EPA AIRS database reporting hourly maximum 5-minute SO2 peaks;


  • For monitors reporting 5-minute peaks GE 0.5 ppm based on the first two items, identify the nearest major source(s) of SO2 and the distances to such sources from each monitor;


  • Using the data from the above item, identify which specific categories are most associated with the exceedance of the 5-minute SO2 levels observed; and


  • Describe the potential for high hourly maximum 5-minute SO2 concentrations to occur at a variety of hourly SO2 average concentrations.


The EPA’s AIRS database was queried for hourly maximum 5-minute SO2 values and the ICF Kaiser Final Report was reviewed to quantifying hourly maximum 5-minute peaks. The SO2 emissions inventory listing was downloaded from the EPA Internet AIRS site ( A GIS database was used to identify all SO2 sources in the emissions database that were within a radius of 10 miles from each monitoring site that experienced at least 1 hourly maximum 5-minute value GE 0.5 ppm.

Some of the key findings of the report are


  • No relationship could be found between the hourly maximum 5-minute and hourly maximum SO2 values;


  • It is possible to experience very high hourly maximum 5-minute values (e.g., GE 0.5 or GE .6 ppm) at any time of the day and at very low hourly average SO2 concentrations;


  • Using data collected for the U.S. EPA, hourly average SO2 concentrations as low as 0.04 ppm were associated with hourly maximum 5-minute values of 0.573 ppm;


  • Hourly average SO2 concentrations in the 0.125 ppm range were associated with hourly maximum 5-minute values higher than 0.9 ppm; and


  • In most cases, power plants and smelters appear to be the major SO2 emissions sources responsible for influencing those monitoring sites that experience highly hourly average SO2 concentrations and numerous occurrences of 5-minute values GE 0.5 ppm;

It is important to note that a majority of the sulfur dioxide monitoring sites in the United States experience maximum hourly average concentrations GE 0.04 ppm during any given year. It appears that at this hourly average concentration level, there is an unknown potential for high 5-minute peak sulfur dioxide concentrations (i.e., 0.5 ppm or higher) to occur. The U.S. EPA had previously assumed that there was a relationship between the number of 5-minute peaks and hourly average concentrations. However, the fact that no relationship could be found between the hourly maximum 5-minute and hourly maximum SO2 values indicates that this assumption should be further explored.

For the period 1990-1998, the number of sulfur dioxide monitors reporting 5-minute average concentrations was low. There were only 80 monitoring sites in the AIRS database that reported hourly maximum 5-minute values. In contrast, for the period 1990-1998, there were approximately 1101 individual monitoring sites in the AIRS database that reported SO2 hourly average concentrations.

For more information about the study, please email A.S.L. & Associates. Should you wish additional information about A.S.L. & Associates' ongoing research efforts, please visit our Table of Contents web page.

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