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 (NHNO) aerosol (at 540 °C) to nitrogen dioxide (NO), 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 is removed by in-line filtering. The method was validated by sampling gas streams containing laboratory-generated NHNO aerosol in parallel to a scanning mobility particle sizer (SMPS). Scatter plots of TD-CRDS and SMPS data correlated ( > 0.9) with slopes near unity, confirming quantitative TD-CRDS response to NHNO aerosol. In contrast, no response was observed when sampling (NH)SO aerosol. Instrument limits of detection (LOD; 2σ, 10 s) were 120 parts per trillion by volume (10, pptv) for NO and 148 pptv for ammonium nitrate. Partial and unsustained conversion of refractory sodium nitrate (NaNO) was observed at the inlet temperature used for complete dissociation of HNO and NHNO, suggesting that this channel may not constitute a robust measurement of total odd nitrogen (NO) in environments where NaNO particles may be present (e.g., the polluted marine boundary layer). A potential application of the 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.