It is easy in most cases to suspect that a region has a significant air quality problem based on human senses. Particulate matter, ozone, volatile organic compounds, and nitrogen and sulfur oxides all impact visibility to some degree and can have a distinct odor as well. Thus, residents might perceive a persistent haze or unusual smell in the region at certain times of the year that leads them to conclude that the air that they breathe is somehow compromised. In addition, certain organic compounds often associated with the formation of ozone can create a burning sensation in the eyes and chest. In a few cases such as Donora, Pennsylvania in the late 1940s and London, England in the early 1950s residents can experience episodes of pollution where hundreds or thousands of deaths and hospitalizations occur resulting in overloaded hospitals and mortuaries along with reduced visibility and breathing difficulty.

However, human senses cannot quantify the levels of air pollution, and air pollution can exist at levels that can impact public health and welfare and yet be barely perceivable by human senses. Conversely, fog episodes can be interpreted by the public as episodes of high air pollution when little air pollution actually exists. Thus, a more rigorous science based program is needed to monitor the air in order:

* To understand the overall air quality in the region

* To determine how air pollution might impact public health and welfare

* To evaluate how the levels of air pollution might be changing

* To provide clues to the sources of the air pollution problem

The many facets of air monitoring are discussed in Chapter 4. The purpose of this section is to introduce the concept in the context of developing an air quality management framework.

Before moving to the modern technology of air pollution monitoring it is worth considering that there are some cases where human senses have been used effectively to gage certain types of air pollution. In the case of odors, there are too many odor-causing pollutants to come up with an acceptable concentration for each possible pollutant. An alternate has been to create a device that dilutes polluted air with clean air. This diluter is designed to be adjustable so that the user can dilute the air to the point that the human nose can no longer detect the odor. The amount of dilution that is required is an indicator of the strength of the odor. A common and effective way to evaluate visibility is to determine the distance that major objects can be discerned. In this case, an observation post is set up so that large buildings and mountains are visible at a distance. The viewer records the most distant object that can be discerned. Knowing the distance to this object allows the viewer to estimate the “visual range” associated with the air quality. Many airports use this approach to estimating visual range.

Air monitoring technology can vary from manual methods that use small glass tubes through which air samples are drawn to sophisticated electronic systems that monitor air quality levels on a second by second basis. While the manual use of glass tubes is relatively inexpensive and can be useful in some cases, the modern electronic systems are the preferred approach to establishing levels of the common air pollutants. There are wide ranges of technologies available to measure the levels of air pollution. The determination of the best choice for monitoring a particular pollutant is not always obvious. However, due to equipment stability, ease of use, and cost considerations certain monitors have become favored for use in monitoring today’s most common air pollutants. Non-dispersive infrared absorption spectrometers are typically used to monitor levels of carbon monoxide and carbon dioxide and on occasion hydrocarbon compounds. Ultraviolet photometric devices are normally preferred for ozone and nitrogen oxide monitoring. Flame ionization detectors are typically the preferred approach to monitor for hydrocarbon compounds; although chromatography can provide better speciation. Modern digital technology has allowed companies and government agencies to assemble collections of monitors that test for all of the common gaseous pollutants on a second by second basis, to record and transmit this information to a central location, and to zero and span the equipment daily with little human intervention.

The monitoring of particulate matter requires more human interaction with the monitoring equipment, but this is changing as well. The traditional way to monitor for particulate matter is to draw a known amount of air through a filter and to weigh the filter to measure the increase in weight due to the particulate matter filtered from the air. The way that the air is delivered to the monitor can impact the size range of particles that are collected. Thus, particulate matter can be broken into different size fractions. Modern equipment can now make second by second particulate measurements using for example the change in crystal vibration rates as particles collect on a vibrating crystal or the attenuation of beta radiation as particles collect on a plate. Finally, light scattering has been used to estimate particulate levels using short-range and long-range lasers. However, the use of light scattering to estimate particulate mass is still in much debate.

In the United States specific reference methods have been established for the monitoring of air pollutants so that there is consistency in the methodologies used by different industrial, government, and research group measurement activities. These reference methods include calibration and maintenance requirements as well as a definition of the equipment to use. The EPA reference methods can be found at http://www.epa.gov/ttn/amtic/criteria.html at the writing of this manual.

Before reference methods and procedures were established in the United States, a large variety of differing equipment was employed and the maintenance and calibration of the equipment was inconsistent. Data collected before 1976 in the United States falls into this category and is suspect because of the poor quality of some of the equipment used along with the poor maintenance and calibration procedures that were used. The establishment of reference air pollution monitoring methods and associated maintenance and calibration requirements are critical to providing reliable air monitoring data.

Locating monitoring stations can become very complex. Air monitors tend to be used to accomplish one of four purposes. First, air monitors are used to look for the highest levels of air pollutants of interest; second, they are used to look for the typical or average levels that the public is exposed to; third, they are used to track the impact of sources of air pollution; and forth, they are used to establish the natural or baseline levels of air pollution in a region. It is not unusual to designate air-monitoring stations as peak, area, trend, baseline, or special purpose stations. Funding for air monitoring is virtually always limited. Thus, a determination must be made for the purpose of each station so that it can be properly located. The USEPA has set standards for the number of monitors that should be set up in an urban area based on its size, population, and air pollution problems. The most recent USEPA air monitoring discussion can be found at http://www.epa.gov/particles/pdfs/naam_strategy_20051222.pdf in a draft document called “National Ambient Air Monitoring Strategy.”