A two-channel, Thermal Dissociation Cavity-Ringdown Spectrometer for the detection of ambient NO<sub>2</sub>, RO<sub>2</sub>NO<sub>2</sub> and RONO<sub>2</sub>

Abstract

Abstract. We describe a Thermal Dissociation Cavity-Ring-Down Spectrometer (TD-CRDS) for measurement of ambient NO2, total peroxy nitrates (ΣPNs) and total alkyl nitrates (ΣANs). The spectrometer has two separate cavities operating at ~ 405.2 and 408.5 nm, one cavity (reference) samples NO2 continuously from an inlet at ambient temperature, the other samples sequentially from an inlet at 473 K in which PNs are converted to NO2 or from an inlet at 723 K in which both PNs and ANs are converted to NO2, difference signals being used to derive mixing ratios of ΣPNs and ΣANs. We describe an extensive set of laboratory experiments and numerical simulations to characterise the fate of organic radicals in the hot inlets and cavity and derive correction factors to account for the bias resulting from interaction of peroxy radicals with ambient NO and NO2. Finally, we present the first measurements and comparison with other instruments during a field campaign, outline the limitations of the present instrument and provide an outlook for future improvements.

Day and night-time formation of organic nitrates at a forested mountain site in south-west Germany

“…The average correction factor for the ANs was 1.2, with maximum values of 2. The uncertainty associated with the correction procedure is estimated as ∼ 30 % (Thieser et al, 2016).…”

“…The instruments deployed during both field campaigns have been described in Schuster et al (2009) and Thieser et al (2016) for PARADE and in Sobanski et al (2016a) for NOTOMO. During the PARADE campaign, all instruments used collocated inlets of perfluoralkoxy (PFA) piping.…”

“…During PARADE, a two-cavity TD-CRDS instrument was deployed (Thieser et al, 2016). The instrument was located in a container and sampled air from 5 m long PFA tube (1/2 in outer diameter) acting as a bypass flow through which ambient air was drawn at ∼ 40 SLM (standard litres per minute).…”

“…Sampling via the 450 • C inlet results in detection of NO 2 + PNs + ANs. Mixing ratios of PNs and ANs were obtained by the difference in NO 2 measured in the two cavities and by applying corrections to account for conversion (to NO 2 ) of ClNO 2 (measured by chemical ionization mass spectrometry; Phillips et al, 2012) and N 2 O 5 (TD-CRDS, Sobanski et al, 2016b) in the hot inlets and also for biases related to reactions involving peroxy radicals, NO, NO 2 , and O 3 , as outlined in detail in Thieser et al (2016). The average correction factor for the ANs was 1.2, with maximum values of 2.…”

Estimating N<sub>2</sub>O<sub>5</sub> uptake coefficients using ambient measurements of NO<sub>3</sub>, N<sub>2</sub>O<sub>5</sub>, ClNO<sub>2</sub> and particle-phase nitrate

“…The concentration of the standard was determined using thermal dissociation cavity ring-down absorption spectroscopy (TD-CRDS) using the instrument described by Thieser et al (2016). A zero measurement, using a bypass with 25 cm of metal wool heated to 473 K, was made once an hour and the accuracy of the ClNO 2 measurement is 25 % (Phillips et al, 2012).…”