Development of and Improved Detector for use with a Gas Chromatograph to Measure NOx and PAN in the Atmosphere

Although nitrogen dioxide (NO2) is a priority pollutant, the Federal Reference Method is based on the chemiluminescent measurement of nitric oxide (NO) with the assumption that NO2 is the difference between nitrogen oxides (NOx) reduced to NO with a converter and the NO originally in the sampled air. These converters also are effective in reducing a wide variety of nitrogenous species found in polluted atmospheres such as HNO3, HONO, peroxyacetyl nitrate (PAN), and even NH3 at low efficiency. The presence of these nitrogenous compounds results in a positive bias when measuring NO2. Indeed, well-aged photochemical smog contains very little NO2 when the analyzer is measuring tens of parts per billion (ppb). While this bias was well understood when the reference method was chosen, the availability and reliability of relatively low-maintenance and low-cost chemiluminescent analyzers was the determining factor in the choice. While wet chemical methods were available, they were complex, unreliable, and required highly trained personnel. The development of luminal-based chemiluminescent direct detection of NO2 was an improvement, but these analyzers were subject to interferences such as ozone and other oxidized nitrogenous species. Coupling these luminal detectors with gas chromatography resulted in specific detection of NO2 (and other nitrogenous species), but the complexity and reliability of these instruments precluded wide-spread adoption. A new detector has been developed specifically for the low-flow rates of capillary gas chromatography and while it has many advantages, it still requires regular replacement of reagents. The objective of this proposed research is to develop a luminal-based chemiluminescent gas chromatographic detector that is a sealed system. The amount of luminal in a liter of reagent is enough for tens of thousands of parts per million (ppm)-hours of detection in a gas chromatograph. The primary problem is maintaining a consistent wetted surface for contact with the effluent of the chromatographic column. In the past, this was done by continuously pumping fresh luminal reagent to the top of the wick and allowing the excess reagent to flow out by gravity with the effluent carrier gas. An alternative method is proposed to maintain a wetted surface and eliminate the liquid pump. The development of a no-maintenance, easily constructed, reliable detector for NO2 would greatly facilitate the development of the chromatographic approach as a more accurate commercial analyzer to measure NO2 in the atmosphere. As an added benefit, it could simultaneously measure PAN in the atmosphere using a method that requires neither a radioactive source nor purified PAN for calibration. Finally, the general approach may be adaptable to other types of wet chemical methods to reduce the maintenance and improve the reliability.