Particle nitrate measurement using a thermal-dissociation, cavity-ringdown-spectrometer with gas-phase denuder (D-TD-CRDS)

Abstract

Ambient inorganic and organic particulate nitrate has been connected to cardiovascular and respiratory illness. Accurate measurement of nitrate in both gas- and aerosol-phases is essential for understanding its partitioning and thus its impact on human health. We report modifications to an existing Denuded-Thermal-Dissociation-Cavity-Ring-Down Spectrometer (D-TD-CRDS) system that reliably measured gas-phase NOX (NO and NO2) and NOY (NOX plus other reactive nitrogen species, see below) 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 NOY (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 min) of ∼ 0.035 µg 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.