Measuring chemical emissions using open-path Fourier transform infrared (OP-FTIR) spectroscopy and computer-assisted tomography

This paper reports on a field study that was part of a large-scale, multi-s easonal research study with the North Carolina Department of Environment, H ealth, and Natural Resources, to measure nitrogen emissions from an intensi ve swine confinement facility. The study measured emission rates using trac er gases and a horizontal network of open-path Fourier transform infrared ( FTIR) optical rays placed less than a meter above the surface of an approxi mately 6 acre intensive swine waste lagoon in Eastern North Carolina. This network of rays simultaneously monitored the ammonia and the tracer gases e very 2 min. The open-path measurements were combined with the mathematical mapping techniques of computer-assisted tomography (CAT) to create two-dime nsional concentration maps of the gases for the entire lagoon surface. For this study, a ratioing technique was applied to the tomographic concentrati on maps to estimate the nitrogen emission rates (from ammonia) using known tracer emission rates. The average concentrations of ammonia measured in Au gust, November, and May were 0.81, 0.25, and 0.74 ppm, respectively. In gen eral, ammonia concentrations were lowest at the center of the lagoon and co uld vary across a lagoon from 2 to 4 times depending upon the time of the d ay and the meteorological conditions. Emission rates were only calculated f or November and May, up until midnight. In November 1997, the average flux was 1910 mug Nm(-2)-min(-1)(range 542-4695 mug Nm(-2)-min(-1)) and in May t he average flux was 4775 mug Nm(-2)-min(-1)(range 2572-8499 mug Nm(-2)-min( -1)). This study was important because it not only provided nitrogen emissi on rate measurements using a new technology which can measure concentration s over large areas in real time, it was the first large-scale outdoor field study using this application. (C) 2001 Elsevier Science Ltd. All rights re served.