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

We present a newly constructed, two-channel thermal dissociation cavity ring-down spectrometer (TD-CRDS) for the measurement of NOx (NO + NO2), NOy (NOx + HNO3 + RO2NO2 + 2N(2)O(5) etc.), NOz (NOy-NOx) and particulate nitrate (pNit). NOy-containing trace gases are detected as NO2 by the CRDS at 405 nm following sampling through inlets at ambient temperature (NOx) or at 850 degrees C (NOy). In both cases, O-3 was added to the air sample directly upstream of the cavities to convert NO (either ambient or formed in the 850 degrees C oven) to NO2. An activated carbon denuder was used to remove gas-phase components of NOy when sampling pNit. Detection limits, defined as the 2 sigma precision for 1 min averaging, are 40 pptv for both NOx and NOy. The total measurement uncertainties (at 50% relative humidity, RH) in the NOx and NOy channels are 11%+10 pptv and 16%+14 pptv for NOz respectively. Thermograms of various trace gases of the NOz family confirm stoichiometric conversion to NO2 (and/or NO) at the oven temperature and rule out significant interferences from NH3 detection (< 2 %) or radical recombination reactions under ambient conditions. While fulfilling the requirement of high particle transmission (> 80% between 30 and 400 nm) and essentially complete removal of reactive nitrogen under dry conditions (> 99 %), the denuder suffered from NOx breakthrough and memory effects (i.e. release of stored NOy) under humid conditions, which may potentially bias measurements of particle nitrate. Summertime NOx measurements obtained from a ship sailing through the Red Sea, Indian Ocean and Arabian Gulf (NOx levels from < 20 pptv to 25 ppbv) were in excellent agreement with those taken by a chemiluminescence detector of NO and NO2. A data set obtained locally under vastly different conditions (urban location in winter) revealed large diel variations in the NOz to NOy ratio which could be attributed to the impact of local emissions by road traffic.