Reverse Dispersion Modeling (previously termed upwind/downwind monitoring)
An atmospheric dispersion model is applied “in reverse” to calculate the emission rate from an area source that produces the pollutant concentration increment (above background) measured at a downwind monitoring station, taking into account the meteorological conditions observed during the monitoring period. Ideally the meteorological conditions (especially wind speed and direction) are relatively consistent during the measurement period, which generally limits the test period to about an hour. For large area sources with typically non-uniform emissivity, such as a dust storm generated by a dry lake bed, it is important to have an independent measure of the source emissivity, so that the composite emission rate can be spatially partitioned across the area source as an input to the dispersion model.
Plume (flux) profiling
A one-dimensional array of samplers (typically mounted on a single tower) is used to measure time-integrated pollutant concentration and wind speed as a function of height as the basis for calculation of the pollutant flux from a line source (or moving point source). For a fixed “point” or area source, a two-dimensional array (multiple towers) is required. The calculation scheme is based on a mass-balance approach and does not require assumptions about the specifics of the vertical plume profile as long as it can be shown that about 80 percent of the plume mass lies within the sampling array.
A pollutant monitor is mounted on a vehicle that measures lateral profiles or distance profiles of concentration downwind of a source (typically an active roadway) with a steady-state emission rate. In a special application for road dust emissions, the particulate-monitoring vehicle travels over the roadway system with GPS resolution so that emissivity maps of the roadway system can be generated. In most cases, the monitoring vehicle travels at constant speed while gathering data.