Development and application of UV-visible and mid-IR differential absorption spectroscopy techniques for pollutant trace gas monitoring

Spatial representativeness is an important quality criterion in trace gas monitoring, especially if measurements are intended for regulatory and model validation purposes. Open-path absorption spectroscopy techniques meet the representativeness requirement by providing concentrations averaged over atmospheric paths ranging from some hundred meters to some kilometers. This research concerns the characterization and application of a UV-visible differential optical absorption spectroscopy (DOAS) system, and the development and demonstration of a trace gas detection technique based on tunable, mid-infrared (mid-IR) quantum-cascade lasers (QCL). The spectral accuracy, stability and resolution of the DOAS spectrometer, and the efficiency of its PMT1-based light detection system were characterized and tested in laboratory and field conditions. Additional laboratory experiments included exploratory test of a photodiode array (PDA) as multichannel analyzer, and spectral analysis of the arc of high-pressure Xe lamps. PDA measurements of NO2 in the 360-530 nm range show an effective lower detectable fractional absorbance of 8·10-4 (1-sigma) for a 3-s integration time, overpassing the detection capabilities of the scanner/PMT system. DOAS measurements of SO2, NO2, and O3 were extensively validated through laboratory calibration and intercalibration, and field intercomparison with conventional point monitors and EPA2-certified DOAS systems, showing good agreement among the different instruments. A nitrous acid (HONO) generator was developed and used for characterizing the DOAS detection capability. Laboratory measurements indicate a lower detection limit (LDL) of ~0.2 ppm·m HONO (2-sigma) for a 30-min integration time. Concerning formaldehyde (HCHO), a field intercomparison with HPLC3-analyzed DNPH4 samples shows a non-aleatory, high degree of correlation (r2 = 97%) between the two techniques, but unexplained, significant deviation from the 1:1 line (which is nevertheless within the 85% confidence interval of the correlation curve). A mathematical method was developed for the estimation of detection limits through the analysis of sequential lamp spectra. LDL estimations with this method are in very good agreement with operational detection limits. Other data processing and concentration retrieval algorithms were developed and used for analyzing laboratory and field measurements (12 campaigns). The analysis and interpretation of observations in two of these field studies are reported in detail. The first study concerns the dynamics and photochemical production regime of HCHO in the Grenoble region, France. DOAS measurements at a suburban location of Grenoble allowed estimating the sources of formaldehyde, and served at validating the results of a mesoscale photochemical grid model. Measurement-derived emission and photochemical production ratios are in good agreement with literature values, the emission inventory, and model calculations. A large fraction (>80%) of the HCHO observed at the measurement location is photochemically produced, and its production is VOC-limited / NOx-saturated. This investigation demonstrates also that DOAS is a well-suited technique for model validation purposes. The second study concerns the emission of monocyclic aromatic hydrocarbons (MAH) from a wastewater treatment plant in Lausanne (Switzerland) using DOAS and GC5/FID6-analyzed grab samples. Combined analysis of the vertical wind speed and the MAH concentration time series suggest a quasi-diurnal cycle involving accumulation of fugitive emissions during the stagnant early morning hours followed by rapid upward convective dispersion from midday on. This study indicates that the wastewater treatment plant is a potentially significant source of MAH in the Lausanne area. Measurements carried out in a high altitude (~2500 m ASL7) Andean valley (Sogamoso Valley, Colombia) are also presented and interpreted. A combined analysis of meteorological and O3 measurements performed at ground level (including DOAS) and airborne (with a homemade tethered balloon), along with model calculations, provide evidence for mesoscale inflow to the Sogamoso Valley of clean air masses from the Colombian Eastern Savanna (~200 m ASL). This quasi-diurnal anabatic flow plays an important role in the ventilation of the Sogamoso valley, particularly during the dry season. Despite of its high selectivity and sensitivity, and good temporal resolution, the UV-visible DOAS technique is restricted to a limited number species that display highly structured electronic bands. The advent of the quantum-cascade laser (QCL) in 1994, and its rapid development thereafter, offers to open-path absorption spectroscopy a promising doorway to the mid-IR. Quantitive detection of O3 at ambient pressure with a 9.6 µm pulsed-operated, single-mode DFB8 QCL was demonstrated in laboratory conditions. QCL transmission spectra in the 1044-1050 cm-1 range were obtained by tuning the laser temperature. O3 column densities retrieved from the mid-IR spectra are in good agreement with simultaneous DOAS measurements but the detection limit attained (~25 ppm·m) is still too high for immediate test in open path conditions. Currently ongoing improvements on the QCL pulse acquisition system should allow achieving detection limits at the level of commercial DOAS systems (~2 ppm·m) in the very near future. These results demonstrate the applicability of the differential absorption method to QCL absorption spectroscopy at ambient pressure and encourage its use for open path detection. -------------------------------------------------- 1 PMT: PhotoMultiplier Tube 2 EPA: US Environmental Protection Agency 3 HPLC: High Performance Liquid Chromatography 4 DNPH: 2,4-DiNitroPhenylHydrazine 5 GC: Gas Chromatography 6 FID: Flame Ionization Detector 7 ASL: Above the Sea Level 8 DFB: Distributed FeedBack