Indirect methods are best suited for the measurement of emission rates from large, heterogeneous sources. The en vironmental engineer measures the emission flux indirectly by collecting ambient concentrations upwind and downwind of the emission source.
The main disadvantage of indirect methods is that the analytical results rely on meteorological conditions, which can invalidate the data or preclude the collection altogether. In addition to weather patterns, buildings and hills also influence dispersion characteristics and limit sampling. The sensitivity of the analytical method is also critical since the ambient concentration of the emission can be low.
The downwind measurement determines the average concentration of contaminants in the plume. The upwind measurement monitors background readings. The volume of air passing over the monitors in a time period is recorded, and a computer model calculates an emission rate from the concentration data. The average emission rate is equal to the difference in mass measurements (downwind minus upward concentration) divided by the transit time across the source. Environmental engineers have tried various approaches for making these estimates using conventional ambient air monitoring methods (Schmidt et al. 1990).
In some cases, gaseous tracers are released at the emission source. These tracers mimic the dispersion of the emissions. When a tracer is released at a known rate, the environmental engineer can determine the emission rates of compounds by comparing the measured pollutant concentration to that of the tracer at the same location. Sections 5.12 and 5.13 describe other techniques such as continuous emission monitoring and remote sensing.
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