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 (NH 4 NO 3 ) aerosol (at 540 °C) to nitrogen dioxide (NO 2 ), 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 NH 4 NO 3 aerosol in parallel to a scanning mobility particle sizer (SMPS). Scatter plots of TD-CRDS and SMPS data correlated ( r 2 > 0.9) with slopes near unity, confirming quantitative TD-CRDS response to NH 4 NO 3 aerosol. In contrast, no response was observed when sampling (NH 4 ) 2 SO 4 aerosol. Instrument limits of detection (LOD; 2σ, 10 s) were 120 parts per trillion by volume (10 -12 , pptv) for NO 2 and 148 pptv for ammonium nitrate. Partial and unsustained conversion of refractory sodium nitrate (NaNO 3 ) was observed at the inlet temperature used for complete dissociation of HNO 3 and NH 4 NO 3 , suggesting that this channel may not constitute a robust measurement of total odd nitrogen (NO y ) in environments where NaNO 3 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.