Overcoming humidity dependence of the chromium trioxide converter used in luminol-based nitric oxide detection

The surface-atmosphere exchange of gaseous NOx (NO+NO2) is commonly measured by chamber methods that employ a portable luminol-based NO2 detector. Detection of NO depends on its prior oxidation to NO2 by CrO3 with efficiency that depends on sample relative humidity. We found that inadequate drying of wet chamber air samples, and consequent changes in NO oxidation efficiency, often contributes to observed apparent shifts in the response of commercially available detectors in the field. Our instrument’s performance was improved when the water vapor gradient across its membrane dryer was enhanced by replacing the restrictor on the air inlet of the dryer’s shell with a desiccant column, and then increasing ambient air flow through the shell to approximately equal the sample air flow rate. We also demonstrated that pumice is the preferred CrO3 support material for luminol-based detection of NO in chamber air samples; advantages include low cost, superior physical properties, and the simplicity of fabricating a CrO3/pumice converter by the user.