Quantification of Non-refractory Aerosol Nitrate in Ambient Air by Thermal Dissociation Cavity Ring-Down Spectroscopy

A thermal dissociation cavity ring-down spectrometer (TD-CRDS) for real-time quantification of non-refractory aerosol nitrate in ambient air is described. The instrument uses four parallel detection channels and heated quartz inlets to convert particulate organic nitrate (pON) (at 350 °C) and ammonium nitrate (NH₄NO₃) aerosol (at 540 °C) to nitrogen dioxide (NO₂), whose mixing ratio is monitored via its absorption at 405 nm. Concentrations of aerosol nitrate are determined by difference relative to a parallel TD-CRDS channel in which aerosol are removed by in-line filtering. The method was validated by sampling gas streams containing laboratory-generated NH₄NO₃ aerosol in parallel to a scanning mobility particle sizer (SMPS). Scatter plots of TD-CRDS and SMPS data correlated (r² > 0.9) with slopes near unity, confirming quantitative TD-CRDS response to NH4NO3 aerosol. In contrast, no response was observed when sampling (NH₄)₂SO₄ aerosol. Instrument limits of detection (LOD; 2σ, 10 s) were 120 parts per trillion by volume (10^-12, pptv) for NO₂ and 148 pptv for ammonium nitrate. Partial and unsustained conversion of refractory sodium nitrate (NaNO3) was observed at the inlet temperature used for complete dissociation of HNO₃ and NH₄NO₃, suggesting that this channel may not constitute a robust measurement of total odd nitrogen (NO_y) in environments where NaNO₃ particles may be present (e.g., the polluted marine boundary layer). A potential application of TD-CRDS is the calibration of particle counters, for which convenient methods are not currently available. Sample ambient air measurements of pON and total aerosol nitrate in Calgary are presented.