High-accuracy and high-sensitivity optical tracing of dinitrogen pentoxide (N2O5) involved in a nocturnal tropospheric chemical reaction process in smog chamber using quantum cascade laser

Dinitrogen pentoxide (N2O5) is an important reactive intermediate in the atmospheric chemistry of nitrogen oxides and nitrate aerosol, especially during night-time. It is also an important reservoir of NO3 radical that can react with various volatile organic compounds (VOCs) including alkenes and dimethyl sulphide (DMS). The heterogeneous removal of N2O5 through the reactions of NO3 (and N2O5) with aerosol particles may lead to a global O3 reduction, thus directly and indirectly impacting on climate. Despite its importance and several decades of research, there are still many open questions about the role of N2O5 in tropospheric chemistry due to the lack of suitable measurement methods for precise quantification of N2O5 concentration.

Tunable laser absorption spectroscopy in the infrared involving fundamental ro-vibrational molecular transitions could provide a useful tool for high-sensitivity and high temporal resolution measurements of N2O5. However, the N2O5 absorption in the infrared exhibits a broad band absorption feature (over ~40 cm1) and the usually used distributed feedback lasers with a tuning range of ~5 cm1 were unable to scan a whole N2O5 absorption feature for accurate quantification. Widely-tunable external-cavity quantum cascade laser (EC-QCL) is mostly suitable for such application. However, the prominent drawback of the commercially available broadband EC-QCL is the unavoidable etalon fringes resulting from its external cavity design structure. These etalon fringes limit the ultimate detection sensitivity and measurement accuracy.

Fig. 1. left: Schematic of the EC-QCL-MPC apparatus used for quantitative measurements of N2O5 in an ASC. A He–Ne laser (λ =632.8 nm) overlapped with the mid-IR EC-QCL beam by means of a dichroic beamsplitter DB (ISP Optics, model BSP-DI-25-3) was used for optical alignment. L: focusing lens; M: reflective mirror; W: CaF2 window; LED-IBBCEAS: Light emitted diode based incoherent broadband absorption spectrometer for NO2 and NO3 measurement; right: Picture of the ASC involved in the present work.

We report recently on the development of an EC-QCL based spectroscopic instrument for N2O5 measurement by direct long path absorption spectroscopy near 8 μm. The experimental set-up is schematically depicted in Fig. 1. A water-cooled continuous-wave EC-QCL (Daylight Solutions, Model CW-MHF 41000), mode-hop free tunable in the infrared spectral region from 1223 to 1263 cm1, was used to probe broadband absorption of N2O5 of the ν12 band near 8 μm. The EC-QCL beam was collimated and injected into a home-made multipass cell (MPC) installed inside the atmospheric simulation chamber (ASC) in open-path configuration. MPC was formed with two spherical mirrors separated by about 1 m resulting in an effective optical pathlength of Leff = 70 m. The laser beam emerging from the MPC was focused onto a VIGO detector (PVI-4TE-4). The direct absorption signal of N2O5 from the VIGO detector was digitalized with a data acquisition card (NI-6036E) and processed with a Labview-based program associated with a laptop computer. The developed EC-QCL-based N2O5 sensing platform was evaluated by real-time tracking N2O5 concentration in its most important nocturnal tropospheric chemical reaction of NO3 + NO2 ↔ N2O5 (Eq.1) in an ASC operating at atmospheric pressure and room temperature (293.2 ± 0.5 K) under dry conditions (RH < 1%).

A specific algorithm was developed for the precise retrieval of N2O5 concentration (Fig. 2. left), which allowed us to significantly eliminate the unavoidable intrinsic etalon fringes of the EC-QCL and spectral interference lines of H2O vapor absorption, allowing us to improve detection sensitivity of the EC-QCL instrument by a factor of 10 and to eliminate bias error of ~21% caused by the etalon effects in the retrieved N2O5 concentration. Using a Leff = 70 m, a minimum detection limit (DL) of 15 ppbv was achieved with a 25 s integration time (Fig. 2. right).

Fig. 2. left: Flow chart illustrating the developed N2O5 concentration retrieval algorithm, in comparison with a traditional method. right: Retrieval of N2O5 concentration. (a) Measured (black) and fitted (red) N2O5 absorption spectra including H2O vapor, etalon fringe and baseline fluctuation. (b) Decomposed 206 ppb N2O5 absorption spectrum (black) and the corresponding fit (red) from (a). (c) Fit residual.

The equilibrium rate constant Keq in Eq.1 was determined with the help of the direct concentrations measurements using the developed EC-QCL sensing platform. The result agreed well with the theoretical value deduced from a referenced empirical formula under well controlled experimental conditions. This work demonstrates the potential and the unique advantage of using a modern EC-QCL for applications in direct quantitative measurement of broadband absorption of key climate-change related molecular species.

Hongming Yi1,*, Tao Wu1,2, Amélie Lauraguais1, Vladimir Semenov3, Cecile Coeur1, Andy Cassez1, Eric Fertein1, Xiaoming Gao4, Weidong Chen1
1Laboratoire de Physicochimie de l’Atmosphère, Université du Littoral Côte d’Opale, 59140 Dunkerque, France
2Key Laboratory of Nondestructive Test, Nanchang Hangkong University, Nanchang 330063, China
3A.M. Prokhorov General Physics Institute, Russian academy of sciences, Moscow, Russia
4Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
*now with Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, USA

Publication

High-accuracy and high-sensitivity spectroscopic measurement of dinitrogen pentoxide (N2O5) in an atmospheric simulation chamber using a quantum cascade laser.
Yi H, Wu T, Lauraguais A, Semenov V, Coeur C, Cassez A, Fertein E, Gao X, Chen W
Analyst. 2017 Dec 4

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