HONO Measurement by Catalytic Conversion to NO on Nafion Surfaces

A selective catalytic converter has been developed to quantify nitrous acid (HONO), a photochemical precursor to NO and OH radicals that drives the formation of ozone and other pollutants in the troposphere. The converter is made from a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nafion) that was found to convert HONO to NO with unity yield under specific conditions. When coupled to a commercially available NO_x (=NO + NO₂) chemiluminescence (CL) analyzer, the system measures HONO with a limit of detection as low as 64 parts-per-trillion (ppt) (1 min average) in addition to NO_x. The converter is selective for HONO when tested against other common gas-phase reactive nitrogen species, although loss of O₃ on Nafion is a potential interference. The sensitivity and selectivity of this method allow for accurate measurement of atmospherically relevant concentrations of HONO. This was demonstrated by good agreement between HONO measurements made with the Nafion-CL method and those made with chemical ionization mass spectrometry in a simulation chamber and in indoor air. The observed reactivity of HONO on Nafion also has significant implications for the accuracy of CL NO_x analyzers that use Nafion to remove water from sampling lines.