Developing a Framework for Effective Air Quality Management

3.4 Understanding Air Quality Levels in a Region

3.4.2 Location Considerations Carbon Monoxide (CO)

The chief source of CO is normally automobiles. Thus, the first CO monitor is typically located at the point where humans live or work and where there is high traffic volume all combined with high buildings that act as inhibitors to air movement resulting in increased CO concentrations. There are exceptions to this situation, however. In communities where wood burning is an important source of winter heating, the main source of CO can be fireplaces or wood stoves. In this case, the location of the initial monitor would likely be where the air drains from the residential areas at night. Ozone (O3)

Ozone is normally not directly emitted but forms in the atmosphere due to atmospheric chemistry. Peak ozone levels tend to occur downwind of the sources of ozone precursors (VOC and NO). Thus, the location selected for the initial ozone monitor is typically 8-15 kilometers downwind from the main sources of VOC and NO for the region. It must also be considered that NO2 actually temporarily suppresses ozone formation so that ozone monitors located in high NO2 concentrations will measure reduced levels. High volume roadways, especially those carrying many trucks, can have elevated NO2 levels and thus unrepresentative ozone levels. Particulate Matter (PM)

PM can be directly emitted (primary PM) or can form in the atmosphere due to atmospheric chemistry (secondary PM). Thus, it is not easy to make a general rule for finding the peak location of PM in order to monitor. Directly emitted PM can come from coal fired power plants and steel plants for example. In this case, depending upon the height of the stacks associated with the sources, the PM monitor should be located downwind from the identified sources of primary PM. On the other hand, in most urban areas, the most problematic PM is secondary PM. Secondary PM is PM forms from nitrogen oxides, sulfur oxides, and organic matter in the atmosphere. Similar to ozone, this PM will occur several kilometers downwind of the main sources of nitrogen oxides, sulfur oxides, and organic compounds. Also, a source of ammonia such as an animal feedlot or other large collection of decaying matter can rapidly interact with nitrogen and sulfur oxides in the atmosphere to form particulate matter. For example, the worst PM levels in Los Angeles is downwind of Los Angeles and immediately downwind of a large collection of dairies where many cows are confined into a small area. Nitrogen Oxides (NOx) and Sulfur Oxides (SOx)

Except in rare cases such as near copper ore processing, large coal fired power plants or the like, sulfur oxide and nitrogen oxide levels today rarely result in direct violations of air quality standards. Thus, most monitoring for these pollutants is to support modeling studies and to aid in improved understanding of the atmospheric chemistry that is occurring in the region of interest related to ozone and PM formation. Thus, these monitors tend to be located in places to provide estimates of average levels of the pollutant. Toxic Air Pollutants

Most toxic monitoring is carried out in the form of campaigns rather than the use of fixed monitoring sites. In some case, portable trailers are outfitted to monitor for various toxics and these trailers are moved from location to location to look for peak exposures of toxic pollutants. Climate Change Pollutants

Only a few worldwide stations are set up to monitor climate change pollutants. These pollutants are very persistent in the atmosphere and thus do not require local monitoring. The global monitoring is typically carried out at remote sites in order to estimate average levels over the whole atmosphere. Site Selection and Sampling Network Design

Once the monitoring needs are determined for a region, then monitoring sites must be selected that fit the monitoring needs. The number of air monitors to be located in a region depends upon the size and population of the region as well as the available funding. The air quality agency in Los Angeles operates about 34 complete monitoring stations distributed fairly evenly over the 12,000 square kilometer metropolitan area. Santiago, Chile operates about 8 monitoring stations distributed over a 1400 square kilometer area, and Mexico City, Mexico operates about 36 monitoring stations distributed over a 7,800 square kilometer area. Each area also operates additional manual stations as well.

Finally, for air quality management purposes, it is valuable to analyze the monitoring data for air quality trends. This requires that monitors be located in a place for many years. Thus, great care should be taken in determining the location of at least key stations so that they can be kept at the same location for a long period of time.

It is important to note that a monitoring network made up of a few well-maintained stations that are situated in meaningful locations over a long period of time is more valuable than the use of many stations that are poorly maintained or moved often. Quality Assurance

It has been noted earlier in these discussions, but the need for quality assurance programs in all aspects of air quality management is paramount. This is especially true for the case of air monitoring. Almost all air quality monitoring relies on the used of highly sensitive electronic equipment, that can be easily impacted by dirt, vibration, moisture, and the like. Thus, the equipments performance must be checked regularly. This is normally done with the use of calibration gases to check the instruments zero and span. If the instrument cannot be brought into compliance with zero and span requirements then intervention is normally called for to insure proper operation of the equipment.

Air monitors can produce readings as often as every second. While data is normally reported as minute or hourly averages, the amount of data from a large network can quickly become overwhelming. Thus, quality assurance procedures are also required for data handling and storage to minimize the possibility of getting erroneous data into the air monitoring database.