Measurement of NO<sub><i>x</i></sub> and NO<sub><i>y</i></sub> with a thermal dissociation cavity ring-down spectrometer (TD-CRDS): instrument characterisation and first deployment

Abstract

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.

Comment on acp-2021-42

“…frequent occurrence of NOx plumes resulting from emissions of nearby ships. The NOx mixing ratios measured by TD-CRDS and CLD were in good agreement (see Friedrich et al (2020)) and the bias of the TD-CRDS to higher values reflects the exclusion of data below the detection limit. A histogram of the NOx measurements made by the CLD is displayed in Fig.…”

Reactive nitrogen around the Arabian Peninsula and in the Mediterranean Sea during the 2017 AQABA ship campaign

“…The TD-CRDS instrument, its operating principles and laboratory characterisation, and a validation of the NOx measurements versus an independent CLD instrument have recently been presented (Friedrich et al, 2020). The TD-CRDS (located in an airconditioned research container on the front deck of the vessel) has two separate cavities operating at a wavelength of 405 nm and at sub-ambient pressure (720 to 770 hPa) to prevent condensation of water in inlet lines under humid conditions.…”

“…One of the heated inlets was equipped with an activated carbon denuder in order to remove gas-phase NOy species from the sample stream and thus, in principal, was used to measure only particulate nitrate. The oven connected to the inlet featuring the denuder malfunctioned very early in the campaign so that TD-CRDS measurements of particulate nitrate are not available (a short time frame is discussed in Friedrich et al (2020)). In the Red Sea and Arabian Gulf, the oven heating the inlet of the total-NOy channel was also switched off occasionally during the hottest hours of the day, to prevent damage to the oven electronics.…”

“…NOx measurements and thus contain a contribution from particulate nitrate. Friedrich et al (2020) have shown that this instrument measures ammonium nitrate quantitatively, but detects only a fraction (≈ 25 %) of sodium nitrate (NaNO3) of 200-300 nm diameter as NOx. The inefficient detection of some non-refractory nitrate species (e.g.…”

“…HNO3 was not measured directly but calculated from HNO3 = NOy -(NOx + ON + pNit + ClNO2 + HONO), where pNit refers to sub-micron particulate nitrate as measured by the HR-ToF-AMS. Detection of coarse mode pNit by the TD-CRDS (see Friedrich et al (2020)) would lead to an overestimation of HNO3. However, given that the thermal dissociation to NO2 of NaNO3 particles with 300 nm diameter is inefficient (~ 20 %) with this instrument, a significant bias by coarse mode nitrate (e.g.…”