Improving sensitivity and reliability of optical PM2.5 measurement

Abstract

Air pollution is a major problem in many industrialized and urbanized areas. Its health hazards are currently the leading environmental cause of death. One of the key components of air pollution is aerosol particulate matter, which is further divided by measurement of diameter into PM10 and PM2.5 subclasses. Smaller PM1 particles are attaining intensive interest now, as their health hazards are being unraveled. However, the need for real-time and local PM2.5 measurement is not yet fulfilled. Further developing the sensitivity and reliability of a real-time semi-low-cost optical PM2.5 sensing technology is of key interest in this thesis, as the technology has room for improvement.

Laser particle counters are common among continuous particulate matter measurement techniques. They are based on measuring the scattering of light by particle. The particulate matter mass concentration in the air is derived from the characteristics of scattering. However, this measurement method is prone to errors, especially in ambient conditions with a lot of variation in temperature, humidity, and in features of particles. As a part of this thesis, there is an empirical study with an aim to improve the sensitivity and reliability of Vaisala Air Quality Transmitter AQT420. The improvements of optical PM2.5 measurement are based on theory, simulation, and prototyping results.

It was discovered, that the sensitivity of a typical laser particle counter is mostly defined by the intensity (W/m^2) of the laser beam. Other key parameters are laser wavelength, polarization, sensitive area of detector and collecting mirror, and electronics amplification capability, when PM1 measurement is performed. The uncertainty of measurement can be roughly divided to aerosol-, and instrument-related. Of these two, aerosol related uncertainty is usually larger, but with a semi-low-cost instrument, like AQT420, instrument-related uncertainty can play a large role.