Measurement of Airborne Particulate Matter (PM10 and PM2.5) Using a Near Forward Light Scattering Nephalometer

摘要

The measurement of airborne particulate matter (PM10 and PM2. 5) in the ambient atmosphere presents the air quality specialist with a wide choice of monitoring techniques. Each has a very different principle of operation, and whilst it is not within the remit of this discussion to recommend one over another, it is perhaps more beneficial to consider their relative merits. The use of a nephalometer is discussed here, with particular reference to instrument response and data repeatability. The principle of light scatter for the measurement of airborne particulate is well documented and has formed the basis of many commercial instruments over the last two decades. According to Mie theory of light scatter, particles passing through a light beam scatter a proportion of this light in all directions, its intensity proportional to the nature and concentration of the particles. Narrow angle forward scatter nephalometers tend to be less sensitive to refractive index than large angle measuring instruments and substantially reduce the response derived from varying material composition of particles. By measuring only this intensity, the response can be detected and related to mass concentration. Generally, as the angle increases, the backscatter component becomes weaker and the forward scattering increases in intensity. In addition, the detector response for a given dust type and composition is linear up to about 200 mgm ~(-3). One of the main advantages of using nephalometers over mass measurement systems is that a near instantaneous response is achieved over the entire range of mass concentration.