A novel method to determine the spatial distribution of BTEX using long-path absorption spectroscopy

Accidental releases and fugitive emissions of aromatic hydrocarbons, such as benzene, toluene, ethyl-benzene and xylenes (often summarized as BTEX), from petrochemical facilities have received considerable attention in recent years. The interest in these compounds stems from their well-known detrimental impact on human health, even at fairly low concentrations. Despite the potential impact on the population of adjacent neighborhoods, our ability to monitor these compounds at facility fencelines or on a neighborhood scale has severe limitations. Established monitoring techniques consist of canister or air monitoring station sampling combined with gas-chromatographic analysis. While sampling with multiple canisters can provide a spatial snapshot, these methods typically only allow point measurements and are thus not able to completely describe the spatial distribution of these compounds. In recent years fast mass-spectrometric techniques mounted on mobile platforms have allowed the mapping of neighborhoods in much more detail. This provides unique insights in the dispersion of BTEX and, in combination with inverse dispersion models, have potential to allow source identification. However, long-term around-the-clock measurements with this approach is challenging and expensive. There is thus a need for new methods to provide automatic long-term monitoring of BTEX at the fenceline and on neighborhood-scales to better assess emissions of and exposure to these compounds.

Long-path Differential Optical Absorption Spectroscopy (LP-DOAS) of BTEX offers the unique capability to provide these measurements, but has only been used sporadically in research applications and has not been fully established for this application. Commercial systems to measure BTEX are available, but have proven to be inaccurate and difficult to use, thus hindering their wider adoption. Consequently, we have developed novel LP-DOAS instruments, which overcome many of the challenges faced by older systems. These instruments have been tested extensively along a refinery fenceline to determine their accuracy and long-term behavior. Here we will discuss the novel design features and the performance of these LP-DOAS instruments. We will also present first results from the Fall 2014 Benzene and other Toxics Exposure (BEE‐TEX) Study in Houston, TX, where two LP-DOAS instruments measure BTEX concentrations in a neighborhood adjacent to a refinery. Measurements along multiple, crossed absorption paths, in combination with computer aided tomographic algorithms allow the determination of 2-dimensional BTEX concentration fields. We will show that this novel approach is suitable for long-term monitoring of BTEX exposure on a neighborhood scale.