A field study using open-path FTIR spectroscopy to measure and map air emissions from volume sources

Open-path Fourier transform infrared (OP-FTIR) spectroscopy is a relatively new measurement technique offering advantages over traditional point samples for monitoring air contaminants and validating dispersion models. In addition, this technology has the potential to produce temporally and spatially resolved chemical concentration maps. In this study, OP-FTIR spectrometer measurements and point samplers were used to evaluate two short-term refined Gaussian dispersion models for predicting the fate of volume source emissions. Additionally, these data were used for a pilot Environmental CAT scanning system using two scanning OP-FTIR spectrometers and eight retroreflectors. An environmental CAT scanning system processes a network of intersecting OP-FTIR spectrometers using a tomographic reconstruction algorithm. When the algorithm is applied to these real-time measurements, two-dimensional chemical concentration maps are created of the area. This technique is noninvasive and is capable of providing real-time spatially and temporally resolved concentration maps of multiple chemicals simultaneously and at low limits of detection (ppb–low ppm). Near real-time chemical concentration maps were generated for the entire field site, and the concentration maps were compared with model maps with respect to plume location and plume shape. We evaluated the Industrial Source Complex–Short Term (Version 3) (ISCST) model and the American Meteorological Society/Environmental Protection Agency Regulatory Model Improvement Committee (AERMOD) model. Sulfur hexafluoride was released at known rates from a simulated volume source and was measured at various points in the study field with the use of a network of Tedlar® bag point samples and OP-FTIR spectrometer measurements. Both ISCST and AERMOD underpredicted SF₆ concentrations when compared to the 5-min averaged OP-FTIR spectrometer measurements (1.3x) and the 1-h integrated Tedlar® bag samples (1.4x). For most time periods, the tomographic concentration maps compared fairly well with the model predictions in terms of plume shape and location. The tomographic maps tracked the impact of changing wind direction better than the model predictions. © 1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3: 69–79, 1999