Environmental engineers use an emission-flux chamber to make direct measurements of concentrations (see Figure 5.9.6). The atmospheric emissions in an area enter the chamber where they are mixed with clean dry air or nitrogen that is fed in at a fixed rate. The analyzer measures the pollutant concentration. The emission flux is the exit gas concentration multiplied by the flow rate divided by the surface area covered by the chamber.
When the area is several acres in size, measurements must be taken at various points to develop an overall emission rate. The number of measurements needed depends on the precision required and the size of the source.
Flux chambers are best suited to measure small to medium size areas (as large as a few acres) in which the pollutant concentration is fairly homogeneous. Because the flux chamber is isolated, measurements are independent from environmental influences such as wind; therefore, the measurement data are independent of the meteorological conditions at the site and are comparable from day to day and site to site.
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