Evaluating PM2.5 at a Construction Site Using Terahertz Radiation

Economic and industrial development has led to increasing problems with particulate pollution in many countries. Particulate matter with the diameter of less than 2.5 μm (PM2.5) is of concern in many cities. In this study, a total of 70 samples of PM2.5 were collected from dusty environments and analyzed using terahertz (THz) radiation. The transmission spectrum of PM2.5 had two distinct absorption bands between 2.5 and 7.5 THz. Their center frequencies were 3.36 and 6.91 THz, respectively. Based on the THz absorbance spectra, the elemental compositions were studied by monitoring PM2.5 masses in conjunction with two-dimensional correlation spectroscopy. Correlations between absorption bands and cross peaks in the synchronous and asynchronous plots indicated that the metallic oxides showed absorption features in the range from 2.5 to 7.5 THz. The vibration modes of anions and cations were located at relatively low and high frequencies, respectively. Statistical methods, including partial least squares and back propagation artificial neural network, were used to quantitatively characterize the PM2.5 content with the input of absorbance over the whole frequency range. This research indicates that THz spectral analysis of PM2.5 is a promising tool for investigating the composition and mass of pollutants.