Sampling Site Selection

Once the initial layout has been developed for an air quality monitoring system, specific sampling sites must be located as close as practical to the ideal locations. The major considerations are the lack of obstruction from local interferences and the adequacy of the site to represent the air mass, accessibility, and security.

Local interferences cause major disruptions to air quality sensor sites. A sampler inlet placed at a sheltered interior corner of a building is not recommended because of poor air motion. Tall buildings or trees immediately adjacent to the sampling site can also invalidate most readings.

The selection of sampling sites in urban areas is complicated by the canyon effect of streets and the high den sity of pollutants, both gaseous and particulate, at street level. In order for the data to represent the air mass sampled, environmental engineers must again review the purpose of the study. If the data are collected to determine areawide pollutant averages, the sampler inlet might best be located in a city park, vacant lot, or other open area. If this location is not possible, the sampler inlet could be at the roof level of a one- or two-story building so that street-level effects are minimized. On the other hand, if the physiological impact of air pollutants is a prime consideration, the samplers should be at or near the breathing level of the people exposed. As a general rule, an elevation of 3 to 6 m above the ground is an optimum elevation.

The sampling site location can be different for the same pollutant depending on the purpose of the sampling. Carbon monoxide sampling is an example. The federally promulgated air quality standards for carbon monoxide include both an 8-hr and a 1-hr concentration limit. Maximum 1-hr concentrations are likely to be found in a high-traffic density, center-city location. People are not ordinarily exposed to these concentrations for 8-hr periods. When sampling for comparison with the 1-hr standard, the environmental engineer should locate the sensor within about 20 ft of a major traffic intersection. When sampling for comparison with the 8-hr standard, the engineer should locate the sensor near a major thoroughfare in either the center-city area or in the suburban area with the sampler less than 50 ft from the intersection. The reason for two different sampling site locations is to be consistent with the physiological effects of carbon monoxide exposure and the living pattern of most of the population. If only one site can be selected, the location described for the 8-hr averaging time is recommended.

When the sampling instruments are located inside a building and an air sample is drawn in from the outside, using a sampling pipe with a small blower is advantageous to bring outside air to the instrument inlets. This technique improves sampler line response time. An air velocity of approximately 700 ft per min in the pipe balances problems of gravitational and inertial deposition of particulate matter when particulates are sampled.

The sampling site should be accessible to operation and maintenance personnel. Since most air pollution monitoring sites are unattended much of the time, sample site security is a real consideration; the risk of vandalism is high in many areas.

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