Measurements of Volatile Organic Carbons Using Open Path Fourier Transform Infrared Spectrometry

초록

Recently, there have been growing concerns on non-criteria air pollutants for their role in affecting human health as well as in determining oxidant concentrations in urban areas. Because the conventional monitoring methods using gas chromatography was often unable to satisfy temporal and spatial resolutions required to evaluate the air quality. The open path Fourier Transform Infrared Spectrometry (hereafter denoted as open path FTIR) may be remedy these resolution related problems by enabling on-line measurements and by providing path averaged concentration. However, the open path Fourier Transform suffers low detection limits due to inhibition of various chemical species including water vapors. The open path FTIR commonly employs a classical least square regression method as multivariate calibration routine. However, the classical least square regression often results in physical not-feasible concentrations, when several chemical species are overlapped into the same band, which arises quite often for the analysis of volatile organic carbons. In the present study, the problems associated with overlapping bands are first examined for alkane species first in urban conditions. Then, a new method was introduced to limit all the analyzed chemical species concentrations within the physically possible range. This new method applies constraints to the classical least square regression to remove all the abnormal concentrations such as negative concentrations and too-large-concentrations. Furthermore, the proposed constraint classical least square regression method incorporates various information including typical ratios among alkane species and concentration ranges of alkane species. The proposed constraint classical least square regression can be easily extended to incorporate chemical equilibrium among reactive chemical species, such as NO, NO2 and O3. This constraint classical least square regression was applied field measurements carried out on the part locat