ausblenden:
Schlagwörter:
-
Zusammenfassung:
We present a newly constructed, two-channel ther-
mal dissociation cavity ring-down spectrometer (TD-CRDS)
for the measurement of NOx (NO + NO2), NOy (NOx
+
HNO3
+RO2NO2
+2N2O5 etc.), NOz (NOy
−NOx) and par-
ticulate nitrate (pNit). NOy-containing trace gases are de-
tected 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 di-
rectly upstream of the cavities to convert NO (either am-
bient 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. Ther-
mograms 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 break-
through and memory effects (i.e. release of stored NOy) un-
der humid conditions, which may potentially bias measure-
ments 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 detec-
tor 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 at-
tributed to the impact of local emissions by road traffic.