7.1. Introduction

Gaseous nitrous acid (HONO) is a highly reactive short-lived species playing a significant role in tropospheric photochemistry. The photolysis of HONO in the wavelength range of 300–400 nm is an important source of the primary hydroxyl free radical (OH) in the lower atmosphere, up to 80% of the integrated source strength [60, 61]:

$$(\text{HONO} + h\nu \left(300\text{ nm} < \lambda < 400\text{ nm}\right) \to \text{OH} + \text{NO} \tag{8}$$

quantum cascade lasers (cw-QCL) combined with a multi-pass absorption cell based on TDLAS technology were applied to the measurement of atmospheric HONO with a sub-ppb detection limit [80–85]. The advantages of using cw-QCLs in TDLAS over lead salt lasers are better mode stability, higher laser output power, and room temperature operation without the need for

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy

http://dx.doi.org/10.5772/intechopen.72948

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The developed QCL instrumental approach is depicted in Figure 27. It was based on a room temperature operation cw distributed feedback (DFB) quantum cascade laser (DQ7-M776H, Maxion Technologies, Inc.). It emitted single-mode laser power of up to 35 mW. The wavelength tuning of ~2 cm<sup>1</sup> around 1254 cm<sup>1</sup> might be achieved by ramping laser injection current and/or temperature tuning. The pre-collimated laser beam from the QCL was first coupled to a beam splitter (with 90% transmission and 10% reflection). In order to make the optical alignment easy, a visible He-Ne laser beam was adjusted to be coaxial with the invisible infrared beam from the QCL. The transmitted light was directed to a multi-pass cell with a base length of 0.8 m and a folded path length of 158 m. The emerging absorption signal from the multi-pass cell was focused onto a thermoelectrically cooled (TEC) photovoltaic VIGO detector (detector 1: PVI-4TE-10.6). The reflected beam was directed to a homemade Fabry-Perot etalon with a free spectral

2: PVMI-10.6) and used for relative wavelength metrology. The pressure in the multi-pass cell was

Figure 27. Schematic diagram of the experimental setup. Lens: f = 50 mm. PM (parabolic mirror): f = 25 mm. M: Mirror.

. The optical fringe signal was recorded with another VIGO detector (detector

cryogenic cooling, which facilitates long-term field measurements.

7.2. QCL-based instrumental platform

range of 0.03 cm<sup>1</sup>

The OH radical governs the oxidation and removal of most pollutants from the atmosphere and is also a key species in photochemical cycles responsible for ozone formation leading to the so-called "photochemical smog" pollution. Therefore, HONO directly affects the oxidative capacity of the troposphere and indirectly contributes to production of secondary pollutants via the oxidation. Knowledge of atmospheric HONO concentration is very important for precise estimation of the OH radical budget and hence precise prediction of the impact on climate and air quality [62, 63]. In the lower atmosphere, the following formation pathways of gaseous HONO are commonly considered: (1) homogenous reaction [64, 65], (2) direct emission (i.e., by traffic) [66], and (3) heterogeneous conversion of NO2 to HONO on the ground and other surfaces [67–71]. Homogeneous reaction and direct emissions have been identified, but these two sources are not sufficient to explain the observed atmospheric concentrations of HONO. At present, it is generally considered that HONO is mainly produced from heterogeneous process, namely, the heterogeneous reactions of NO2 on wet surfaces as well as on surface of reducing substances such as carbon black aerosol surface [72–74]. Despite a large amount of research, the sources and the formation mechanisms of HONO in the atmosphere are still not well understood and identified due to the lack of accurate local measurements [75].

Good understanding of HONO sources and sinks requires instruments capable of performing high sensitivity, high precision, high specificity, high spatial resolution, and fast in situ measurements. Among various analytical instruments developed for field HONO monitoring [76–78], spectroscopic detection techniques capable of performing in situ measurements without any sample preparation have been increasingly developed since last decade as an attractive alternative for quantitative assessments of HONO in the atmosphere. Methods such as DOAS, incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS), and the long-path absorption photometer (LOPAP) used in the ultraviolet region usually can get several hundred ppt-level detection limits, but the integration time of several minutes is long and cannot satisfy the requirement of fast measurements [6, 79]. In the mid-infrared region, the continuous-wave quantum cascade lasers (cw-QCL) combined with a multi-pass absorption cell based on TDLAS technology were applied to the measurement of atmospheric HONO with a sub-ppb detection limit [80–85]. The advantages of using cw-QCLs in TDLAS over lead salt lasers are better mode stability, higher laser output power, and room temperature operation without the need for cryogenic cooling, which facilitates long-term field measurements.
