# Advances in High-Precision NO2 Measurement by Quantum Cascade Laser Absorption Spectroscopy
## 1 Introduction  
Nitrogen dioxide (NO2) is an important atmospheric gas-phase pollutant because of its role in O3 formation [1, 2] and its influence on the oxidation capacity of the atmosphere, e.g., by reaction with the hydroxyl radical (OH) and formation of the nitrate radical (NO3) [3, 4]. It also has a direct impact on human health and on the biosphere [5, 6, 7, 8, 9]. Furthermore, the oxidation products of NO2 strongly influence the chemistry of the atmosphere, e.g., oxidation capacity, aerosol particles formation/aging, long-range pollutants transport and nitrates deposition [10, 11, 12, 13, 14]. The amount fraction of NO2 in tropospheric air can rise up to many tens of part-per-billion by volume (ppbv, equivalent to mmol/mol) in densely populated and industrial areas, while, in the free troposphere, its level is usually in thepart-per-trillion by volume (pptv, equivalent to pmol/mol) range. The Global Atmospheric Watch program (GAW) of the World Meteorological Organization (WMO) has defined guidelines in terms of ambient NO2 measurement precision and uncertainties in order to establish an optimal monitoring [15]. These guidelines state that the instrumental limit of detection (LOD) in remote environments, i.e., very weakly impacted by anthropogenic activities, should be as low as 5 pptv (3 \(\sigma\)) with an uncertainty below 5%. There is only a very limited number of instruments capable of reaching such a high precision in field conditions. The most widely used techniques for in-situ measurement of trace amounts of NO2 are based on indirect detection by chemiluminescence (CLD) or on direct detection by laser absorption spectroscopy (LAS) in the visible range (VIS). The CLD technique relies on the reduction of NO2 to nitric oxide (NO) using a molybdenum or a photolytic converter [16; 17]. This reaction is followed by the detection of NO by using O3 as a reactant to form excited NO2, which deactivates by light emission (chemiluminescence). This method has several drawbacks, including interferences from other nitrogen oxides present in the atmosphere that release NO2 (positive bias) during the reduction process [18; 19] and quenching of the excited NO2 by water (negative bias) [20]. The precision of the CLD technique is limited by the fact that the detected luminescence signal observed after the NO2 conversion accounts for the sum of ambient NO plus NO2 (NO2). This requires subtracting the ambient NO concentration (obtained by bypassing the NO2 converter) from the total measured NO2 to obtain the NO2 concentration, thus decreasing the precision of the NO2 measurements. Nevertheless, LODs between 3 and 15 pptv have been reported [21; 22]. Among the techniques using laser absorption in the visible range, the most established ones are based on cavity enhanced spectroscopy (CES), such as cavity ring down spectroscopy (CRDS) or cavity attenuated phase shift (CAPS) spectroscopy [23; 24; 25; 26]. While these methods offer in general a similar LOD than the CLD technique, they are still subject to interferences from water due to Rayleigh scattering effects [27; 28; 29] and potentially from dicarbonyl compounds (mostly glyoxal and methylglyoxal) by light absorption [27]. To avoid post-corrections on ambient air measurements in humid conditions, modern commercial instruments often include driers in their inlets. This approach, however, still requires applying a correction for the artificial increase in the NO2 signal due to the removal of water from the sample. Furthermore, long-term measurements might suffer from changes in the drier efficiency and in its reactivity toward NO2. In parallel to the development of spectroscopic techniques in the visible range, mid-infrared (MIR) detection of NO2 has also been demonstrated using lead-salt tunable diode laser absorption spectroscopy (TDLAS) [30; 31], and then further developed using quantum cascade laser absorption spectroscopy (QCLAS) [32]. Currently, these techniques allow reaching a very high precision, especially in combination with astigmatic multipass absorption cells.