Monitoring Air Quality and Communicating Air-Quality Information to the Public 

The Environmental Protection Agency sets National Ambient Air Quality Standards (NAAQS).  Air-quality data (which include concentrations of nitrogen oxides, ozone, etc.) are collected at monitoring stations, and reported to the EPA.  When levels of air pollution exceed legal limits, the state must develop a plan to reduce emissions.  St. Louis, Missouri is slightly out of compliance on ozone levels, but has attained NAAQS for the other major outdoor pollutants.  (Hey!  I thought ozone was a good thing!  Click here for a discussion of when and where ozone is beneficial.)  St. Louis is typical of many urban areas; ozone concentrations above the levels set by the EPA are the most prevalent air-quality problem in the U.S.  

    You may have noticed that the local news in the summer includes an "air-quality forecast."  (See Figure 2 for an example from the St. Louis Post-Dispatch and click here for air-quality forecasts of many U.S. cities, compiled by the EPA.)  The air-quality rating is essentially a predictor of how much smog (and, hence ozone) could be formed that day.  (Both concentrations of pollutants and weather patterns are used in air-quality forecasting.)  Especially on poor air-quality days, people are encouraged to carpool, use public transportation or postpone non-essential car trips.  >We are discouraged from refueling cars and using lighter fluid on outdoor grills.  Besides publicizing the times when citizens should make an extra effort not to contribute to air pollution, the air-quality forecasts also serve as a warning for people whose health is especially sensitive to poor air quality.  On "bad air" days, some people need to avoid outdoor activities or use medications.  

Figure 2 This January 23, 2001 Forecast from the St. Louis Post-Dispatch gives readers a prediction of air quality for the day. Reprinted with permission of the St. Louis Post-Dispatch, copyright 2001.

    The limits set by the EPA for the components of air pollution differ.  For example, the EPA has set the limit for ozone at 0.12 parts per million (ppm), measured as the average concentration over a one-hour period.  This means that in one gram of air, the amount of ozone should not exceed 1.2 x 10^-7 grams, or 2.5 x 10^-9 moles, on average.  The density of air varies with temperature and humidity, but is on the order of 0.001 g/mL.  This means that one gram of air will occupy a volume of about 1 L.  Therefore, the maximum concentration of ozone is about 3 x 10^-9 moles/L.  From your experience with solutions in lab, you will recognize this as a very small concentration.  Why is the acceptable limit for ozone so low?