Particle nitrate measurement using a thermal-dissociation, cavity-ringdown-spectrometer with gas-phase denuder

Abstract. Ambient inorganic and organic particulate nitrate has been connected to cardiovascular and respiratory illness and accurate measurement of its concentration is essential for our understanding of its impact on human health and the partitioning of reactive nitrogen between the gas- and particle-phases. We report modifications to an existing Denuded-Thermal-Dissociation-Cavity-Ring-Down Spectrometer system (D-TD-CRDS) system that reliably measured gas-phase NO_X and NOy but suffered from a positive bias in particle nitrate measurement owing to denuder breakthrough and memory effects associated with changes in relative humidity. We describe an air drying system with low particle transmission losses that reduces the relative humidity at the inlet of the denuder to < 5 % so that no measurable denuder breakthrough of NO_Y (even of highly volatile species such as NO) was observed after continuous use over the course of month-long campaigns. The D-TD-CRDS measurement of particulate nitrate has a limit of detection (1 minute) of ~ 0.035 mg m^-3 under laboratory conditions and ~ 0.085 µg m^-3 during field deployment. The associated uncertainty is estimated to be < 15 %. Laboratory experiments in which either inorganic nitrate aerosol (ammonium nitrate) or organic nitrate aerosol (generated from the NO₃-induced oxidation of limonene) were sampled simultaneously from an environmental chamber by the D-TD-CRDS and with an Aerosol Mass Spectrometer (AMS) showed excellent agreement.