Opacity (optics) - the degree to which light is blocked. (Wiki)
Of pollutants for which there are no proven continuous mass measurement technologies, particulate matter rates as most harmful. The first opacity standards were promulgated a millennium ago in London. Testers would determine whether a smoke plume was black, some gradation of gray, or white. In the 20th century, this visual test was automated using opacity monitors. The opacity monitor also is used to calculate the weight of emissions. A sample is extracted from the stack and weighed. The results are compared to the opacity at the time and an emission rate is extrapolated.
One problem is to adapt current monitors designed to measure just the fine fraction. Another problem is to define and measure condensable particulates. Whereas regulations have only addressed solid particles, the present definition includes droplets. The condensable 2.5 fraction is often larger than the solid particle fraction.The introduction of condensable or liquid droplets creates many complications. Many stacks discharge gases at (250 to 350˚). Vapors then form droplets some distance from the stack. It is necessary to cool the air to ambient temperatures before measuring the droplets. But there is another problem: some of the non-condensable gases such as NOX and SO2 move downwind and then react with ammonia, calcium or other airborne particles to form sulfates and nitrates. It is necessary to distinguish between primary and secondary particulates. Primary particulates include solids, liquids or potential liquids at ambient temperatures. Secondary particulates include gases that react with other elements to form particulates downwind.Despite the many hurdles, ongoing efforts to solve the problem of measuring particulates will likely lead to major advances. Some day soon, we should finally be able to abandon the smoke testing method; after a thousand years, it’s about time.
By Robert McIlvane, Pollution Engineering, January 2007
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