Abstract. 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+2N2O5 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 ∘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, defined as the
2σ 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.