Measurement of NOx and NOy with a thermal dissociation cavity ring-down spectrometer (TD-CRDS): Instrument characterisation and first deployment

Abstract. We present a newly constructed, two channel Thermal Dissociation Cavity Ring-Down Spectrometer (TD-CRDS) for the detection of NOx (NO + NO2), NOy (NOx + HNO3 + RO2NO2 + N2O5 etc.), NOz (NOy – NOx) and particulate nitrate (pNit). NOy-containing trace gases are detected as NO2 by CRDS at 405 nm following sampling through inlets at ambient temperature (NOx), or at 850 °C (NOy). In both cases, O3 was added to the air sample directly upstream of the cavities to convert NO (either ambient, or formed in the 850 °C oven) to NO2. An activated carbon denuder was used to remove gas-phase components of NOy when sampling pNit. Detection limits (1 minute averaging) for NOx, NOy and NOz are 98, 51 and 110 pptv, respectively. The total measurement uncertainties (at 50 % RH) in the NOx and NOy channels are 11 % + 10 pptv and 16 % + 14 pptv for NOz, respectively. Thermograms of various NOz species (peroxyacetyl nitrate, isopropyl nitrate, and HNO3) confirm stoichiometric conversion to NO2 (and/or NO) at the oven temperature and rule out significant interferences from NH3 detection or radical recombination reactions under ambient conditions. While fulfilling the requirement of high particle transmission and essentially complete removal of reactive nitrogen under dry conditions, the denuder suffered from NOx breakthrough and memory effects (i.e. release of stored NOy) under humid conditions. NOx measurements obtained from a ship sailing through the Red Sea, Indian Ocean and Arabian Gulf (NOx levels from