Laboratory Testing of a Continuous Emissions Monitor for Trace Level Sulfur Dioxide 2016-01-0986

The measurement of SO₂ levels in vehicle exhaust can provide important information in understanding the relative contribution of sulfur and sulfate from fuel vs. oil source to PM. For this study, a differential optical absorption spectrometer (DOAS) that can measure SO₂ down to 20 ppbV in real-time was built and evaluated. The DOAS consisted of an extractive sampling train, a cylindrical sampling cell with a single-path design to minimize cell volume, a spectrometer, and a deuterium lamp light source with a UVC range of ∼200-230 nanometer (nm). Laboratory tests showed detection limits were approximately in the range of 12 to 15 ppbV and showed good linearity over SO₂ concentration ranges of 20 to 953 ppbV. Interference tests showed some interference by NO and by NH₃, at levels of 300 ppmV and 16.6 ppmV, respectively. The software was modified to a allow for measurements in a narrower spectral range from 206 to 214 nm, when the DOAS interference SO₂ response was less than 1.5 ppbV with 300 ppmV of NO injected with, and was slight less than 8 ppbV with 16.6 ppmV of NH₃ injected. The DOAS was evaluated using exhaust from a vehicle. The results showed no appreciable interferences on the SO₂ measurement for tests on a diesel truck, with very low NH₃ emissions, demonstrating the ability of the instrument to measure at 20 ppbV levels without interferences in a low NH₃ environment. The gasoline vehicles, on the other hand, had higher levels of NH₃, and showed a significant interference, especially for NH₃ values exceeding 11 ppmV. For vehicles with higher levels of NH₃, an NH₃ scrubber such as permeation dryer would likely be needed to scrub NH₃ levels to <10 ppmV, as well as some additional updates to the software that have recently been implemented.