March 2013: Criteria Pollutants and National Ambient Air Quality Standards

This is the second article about air quality in general and how it applies to Indiana. I plan to publish an article every two weeks to cover topics related to the air quality field. My first article covered the definition of air pollution and listed different types of air pollutants. This article covers criteria pollutants and national ambient air quality standards.

Shortly after the U.S. Environmental Protection Agency (U.S EPA) was established, they began to set standards for levels of air pollutants that were safe for the public to breathe. In 1971 they established national ambient air quality standards (NAAQS) for the following pollutants: total suspended particulates (dust in the air), sulfur dioxide, nitrogen dioxide, hydrocarbons and carbon monoxide. The Clean Air Act requires that U.S. EPA review these NAAQS every five years and determine whether they need to be modified. In 1978 U.S. EPA added a NAAQS for lead. In 1979 U.S. EPA dropped the hydrocarbon standard and replaced it with an ozone standard. In 1987 U.S. EPA revised the NAAQS for dust to include PM-10 (particles with a diameter less than 10 microns). In 1997 U.S. EPA revised the particulate standard to include particles with a diameter less than 2.5 microns (PM-2.5).

A national ambient air quality standard consists of various parts. First it has a level (for example, 40 micrograms per cubic meter). Next it has an averaging time (for example, 24-hours). Third it has a frequency (for example, second high over a year). Each of these parts of the standard is important in how a standard is applied. Over time the levels, averaging times and frequencies have been revised for the national ambient air quality standards. The following table lists the current NAAQS.

National Ambient Air Quality Standards
Pollutant Averaging Period Level Frequency
PM-10 24-hours 150 ug/m3 4th high
PM-2.5 24-hours 35 ug/m3 98th percentile
Annual 15.0 ug/m3 Annual average
Sulfur dioxide (SO2) 1-hour 75 ppb 99th percentile
Nitrogen dioxide (NO2) 1-hour 100 ppb 98th percentile
Ozone 8-hour 0.075 ppm 4th high
Carbon Monoxide (CO) 1-hour 35 ppm 2nd high
8-hour 9 ppm 2nd high
Lead 3-months 0.15 ug/m3 Highest average

Techie Notes:

  • A ug/m3 is a microgram per cubic meter. A microgram is 1 millionth of a gram. One pound equals 453.6 grams. A cubic meter is a volume approximately 3 feet on each side, approximately the volume of a couch.
  • A ppm is a part per million. This is equivalent to one second in about 12 days.
  • A ppb is a part per billion. This is equivalent to one second in 32 years.
  • U.S. EPA revised the 24-hour standard for PM-2.5 in December of 2012, but this will not be the official standard for approximately two years (December 2014).

Compliance with the NAAQS is determined on a three year average basis for all pollutants except for carbon monoxide. So for example, for PM-10 you would determine the 4th highest daily value for each of three consecutive years and then average them. If the value were less than 150 ug/m3, the monitor would be judged to be meeting the standard. This is done on a monitor by monitor basis. To determine whether a county meets a standard or not, the highest monitored average in that county is the value that is used to judge the county.

The daily standard for PM-2.5 needs a little more explanation. We normally measure PM-2.5 every sixth day. This means that in a year we will have about 60 samples. This means that the 98th percentile is the second highest value during the year.

The hourly sulfur dioxide standard works similarly. For each day you determine the highest hourly value. These 365 values are ranked from highest to lowest. The fifth highest value is approximately the 99th percentile value.

The hourly nitrogen dioxide standard works similarly. For each day you determine the highest hourly value. These 365 values are ranked from highest to lowest. The seventh highest value is approximately the 98th percentile value.

U.S. EPA provides reports that give yearly values that can be compared to the standard (the math is done). However, the reader is left to average values for three consecutive years to determine whether an area meets the standard or not. This is necessary for all pollutants except carbon monoxide. The U.S. EPA provides air quality data and summaries. Data can be downloaded to a spreadsheet so it is possible to merge spreadsheets for different years, sort by monitor and then use the spreadsheet to calculate the three year average values.

A short summary of the Clean Air Act, written in plain English, can be found on U.S. EPA’s website. This provides access to a document called The Plain English Guide to the Clean Air Act.

More information on criteria pollutants and more information on NAAQS can be found on the IDEM Air Quality in Indiana website.

Next article we will discuss ambient air quality monitoring.

Keith Baugues