Gaseous Pollutants - Spectrophotometry

Spectrophotometry is one of the most useful and widely used tools available for quantitative analysis. A spectrophotometer measures the amount of light that a sample absorbs. The instrument operates by passing a beam of light through a sample and measuring the intensity of light reaching a detector. Spectrophotometry relies on colorimetric principles and is commonly used to measure sulfur dioxide (SO2) concentrations. In this method, dyes and chemicals are combined with a solution containing SO2. The color of the solution results in different amounts of light being absorbed. The amount of light absorbed indicates the amount of sulfur dioxide present in the sample.

Figure: Schematic of a UV-VIS spectrophotometer

Ozone can also be analyzed using the monochromatic ultraviolet absorption spectrophotometry principle. As ultraviolet light at 253.7 nm is passed through the optic bench, a fixed quantity of "zero air" and ambient air are drawn into the bench. The intensity of the ultraviolet radiation traversing the optics bench is attenuated by the ozone present in the ambient sample. This attenuated signal is detected and compared with the unattenuated signal from the "zero air" cycle. This difference in intensity is electronically translated into a reading of ozone present in the ambient air.