Aircraft based four-channel thermal dissociation laser induced fluorescence instrument for simultaneous measurements of NO<sub>2</sub>, total peroxy nitrate, total alkyl nitrate, and HNO<sub>3</sub>

Abstract

Abstract. A four-channel thermal dissociation laser induced fluorescence (TD-LIF) instrument has been developed for simultaneous measurements of nitrogen dioxide (NO2), total peroxy nitrate (∑PNs), total alkyl nitrate (∑ANs) and nitric acid (HNO3). NO2 is measured directly by LIF at 532 nm, whereas organic nitrates and nitric acid are thermally dissociated at distinct temperatures in the inlet to form NO2, which is then measured by LIF. The concentrations of each dissociated species are derived by the differences in measured NO2 relative to the reference colder inlet channel. The TD-LIF was adapted to fly on board the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe 146-301 atmospheric research aircraft in summer 2010, and to date has successfully flown in five field campaigns. This paper reports novel improvements in the TD-LIF instrumentations, including (1) the use of a single wavelength laser, which makes the system compact and relatively cheap; (2) the use of a single beam laser that allows easy alignment and optical stability against the vibrational aircraft environment; and (3) the optical assembly of four detection cells that allow simultaneous and fast (time resolution up to 0.1 s) measurements of NO2, ∑PNs, ∑ANs and HNO3. Laboratory-generated mixtures of PNs, ANs and HNO3 in zero air are converted into NO2 and used to fix the dissociation temperatures of each heated inlet to test the selectivity of the instrument and potential interferences due to recombination reactions of the dissociated products. The effectiveness of the TD-LIF was demonstrated during the RONOCO aircraft campaign (summer 2010). A chemiluminescence system that was measuring NO2 and a broadband cavity enhanced absorption spectrometer (BBCEAS) that was measuring one of the PNs (N2O5) were installed on the same aircraft during the campaign. The in-flight intercomparison of the new TD-LIF with the chemiluminescence system for NO2 measurements and the intercomparison between ∑PNs measured by the TD-LIF and N2O5 by the BBCEAS are used to assess the performance of the TD-LIF.

Interferences in photolytic NO<sub>2</sub> measurements: explanation for an apparent missing oxidant?

“…A detailed description of the TD-LIF instrument can be found in Di Carlo et al (2013), with a short description given here. The instrument uses LIF to detect NO 2 concentrations directly (Dari-Salisburgo et al, 2009;Matsumoto and Kajii, 2003;Matsumoto et al, 2001;Thornton et al, 2000) and, coupled with a thermal dissociation inlet system, allows measurement of peroxy nitrates ( PNs), alkyl nitrates ( ANs), and HNO 3 after conversion into NO 2 (Day et al, 2002).…”

“…The TD-LIF is routinely checked for background by an overflow of zero air in the detection cells and is calibrated by standard addition of a known amount of NO 2 from a cylinder (NIST traceable) diluted in zero air -the flow of both zero air and NO 2 being MFC-controlled. The time resolution of the measurements is 10 Hz and the detection limits are 9.8, 18.4, 28.1, and 49.7 pptv (1 s, S/N = 2) for NO 2 , PNs, ANs, and HNO 3 cells respectively (Di Carlo et al, 2013).…”

“…The pump system includes a Roots blower coupled to a rotary vane pump to maintain a flow of 6 L min −1 . The common inlet system is split into four channels: one at ambient temperature to measure NO 2 , and the last three heated at 200, 400, and 550 • C to thermally dissociate PNs, ANs, and HNO 3 respectively into NO 2 (Di Carlo et al, 2013). To minimize quenching due to atmospheric molecules, and therefore increase the sensitivity of the TD-LIF, each cell is kept at low pressure (3-4 Torr).…”

“…Each cell is formed by a cube and two arms where the laser beam passes through the sample air flow in the centre of the cell. Perpendicular to both (laser beam and air flow) there is the detector that is a gated photomultiplier with lens and long-pass filters to optimize the fluorescence detection, minimizing the non-fluorescence light that reaches the detector (Di Carlo et al, 2013;DariSalisburgo et al, 2009). The pump system includes a Roots blower coupled to a rotary vane pump to maintain a flow of 6 L min −1 .…”

Radical chemistry at night: comparisons between observed and modelled HO<sub>x</sub>, NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> during the RONOCO project

“…Supporting measurements made onboard the BAe 146 aircraft during the RONOCO project and used in the model analysis presented here. a The NO 2 measurements used to constrain the model were made by the LIF instrument (Dari-Salisburgo et al, 2009;Di Carlo et al, 2013). ANs -sum of alkyl nitrates; PNs -sum of peroxy nitrates; PAN -peroxy acetyl nitrate; VOCs -volatile organic compounds.…”