One of the main factors affecting visibility in polluted air is light scattering due to fine particles in the air. A nephelometer operates on the basis of this light scattering.
The Nephelometer is used as a standard part of Air quality monitoring networks in many parts of the world, including the Environment Monitoring and Research Centre, India Meteorological Department (IMD). Tesscorn has installed the IMD Multi-wavelength Integrating Nephelometer Network for measurement of aerosol scattering coefficient at twelve locations across the country. The output from the nephelometer is usually expressed as the coefficient of particle light scattering, bsp (in m-1). The results of the nephelometer readings should preferably be reported as either hourly or 24-hour average measurements of bsp and visual range. This monitoring is extremely useful when reporting data to the general public who can relate bad air quality to visible distance more than traditional parts per billion or Mg/m3(pollution).
Visibility, in the most general sense, reduces to understanding the effect that various types of aerosol and lighting conditions have on the appearance of landscape features. To extract quantitative information, the direct measure of a fundamental optical property of the atmosphere is desirable. Therefore, most visibility programs include some measure of either atmospheric extinction or scattering.
The scattering coefficient is a measure of the ability of particles to scatter photons out of a beam of light, while the absorption coefficient is a measure of how many photons are absorbed. Each parameter is expressed as a number proportional to the amount of photons scattered or absorbed per distance. The sum of scattering and absorption is referred to as extinction or attenuation.
A light source-detector configuration can be used to measure just the scattering ability of particles and gases. If the detector is placed parallel to the incident photons, only those photons that are scattered will be detected. This type of instrument is called a nephelometer. If the detector is so aligned as to measure scattering in only one direction it is referred to as a polar nephelometer. On the other hand, if all photons scattered in forward, side, and back directions are allowed to hit the detector, the instrument is referred to as an integrating (summing) nephelometer. The instrument is constructed in such a way as to have the sampling chamber and light source confined to a small volume so that the instrument makes a “point” or localized measurement of scattering.
One of the most basic forms of air pollution – haze – degrades visibility in many Indian cities. Haze is caused when sunlight encounters tiny pollution particles in the air, which reduce the clarity and colour of what we see, especially during humid conditions. Common visibility deterioration is due to high particulate concentration or high humidity. Particles can be of natural or anthropogenic origin, for example, from car emissions, wood burning or smog. Integrating nephelometers are an ideal solution to help us in understanding, identifying and planning ways to control pollution and the imbalance in or degradation of the environment.
Monitoring with a nephelometer is a long-term option for measuring visibility so that sufficient baseline data can be obtained.
The talk, summarized the state-of-the art integrating nephelometer measurements and the greater concern of visibility degradation resulting from emissions of contaminants into the air. These can be particulates or gases, which can undergo relatively complex transport patterns and reactions in the atmosphere. They are becoming increasingly concentrated over urban areas, and have the potential to seriously affect visibility in areas. We hoped you enjoyed our webinar “Nephelometer – Monitoring Visibility” as much as we did! If you weren’t able to make it, catch our replay.