Aircraft based four-channel thermal dissociation laser induced fluorescence instrument for simultaneous measurements of NO 2, total peroxy nitrate, total alkyl nitrate, and HNO 3
A four-channel thermal dissociation laser induced fluorescence (TD-LIF) instrument has been developed for simultaneous measurements of nitrogen dioxide (NO 2), total peroxy nitrate (∑PNs), total alkyl nitrate (∑ANs) and nitric acid (HNO 3). NO 2 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 NO 2, which is then measured by LIF. The concentrations of each dissociated species are derived by the differences in measured NO 2 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 NO 2, ∑PNs, ∑ANs and HNO 3. Laboratory-generated mixtures of PNs, ANs and HNO 3 in zero air are converted into NO 2 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 NO 2 and a broadband cavity enhanced absorption spectrometer (BBCEAS) that was measuring one of the PNs (N 2O 5) were installed on the same aircraft during the campaign. The in-flight intercomparison of the new TD-LIF with the chemiluminescence system for NO 2 measurements and the intercomparison between ∑PNs measured by the TD-LIF and N 2O 5 by the BBCEAS are used to assess the performance of the TD-LIF.