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Formaldehyde is a primary and secondary air pollutant. As an intermediate product of hydrocarbon photooxidn., it contributes to the formation of photochem. pollution in urban areas. In addn., chronic or acute exposure to HCHO (a possible carcinogen) constitutes an important health hazard. Differential optical absorption spectroscopy (DOAS) is one of the best-suited techniques for monitoring formaldehyde. Within the frame of a European Union project, a field measurement campaign (PIPAPO) was carried out from May to June, 1998, in the Milan, Italy, area. During PIPAPO, a large no. of air pollutants, including formaldehyde, were monitored at several sites from Milan to the Swiss Alps foothills. Located at Seregno (25 km north of downtown Milan), EPFL (the Ecole Polytechnique Federale de Lausanne) carried out formaldehyde measurements by DOAS and performed intercomparison DNPH (2,4-dinitrophenylhydrazine) samples, which were analyzed at JRC (the Joint Research Center). Despite having different spatial and time resoln., DOAS and DNPH measurements of formaldehyde compare fairly well. The formaldehyde concn. time series retrieved from DOAS were analyzed with respect to the meteorol. conditions, particularly regarding heterogeneous removal processes, and the concn. of other air pollutants assocd. with formaldehyde emission and prodn. Pollutant dynamics calcns., made with a 3-dimensional photochem. grid (Eulerian) model over a wide spatial domain, were compared to the measurements and provided a basis for the interpretation of the formaldehyde measurements performed during one of PIPAPO's intensive observation periods (IOP).

The Grenoble metropolitan area located in the French Alps with more than 400 thousand inhabitants suffers from periods of high ozone (O3) concns. in summertime. Grenoble is moreover located in the Y shaped convergence of three deep valleys with surrounding mountains up to 3,000 m above sea level (asl) leading to a very complex wind pattern. During summer 1999, a major field campaign GRENOble PHOTochem. (GRENOPHOT) involving std. ground based stations, LIDAR, aircraft, DOAS, microwave windprofilers was carried out in order to obtain the measurements needed for model validation. The air quality model METeorol. PHOtochem. MODel (METPHOMOD) developed at the University of Bern and at the Swiss Federal Institute of Technol. at Lausanne, was used to investigate the dynamic characteristics of air pollution in the Grenoble area during GRENOPHOT. The lumped chem. mechanism Regional Atm. Chem. Mechanism (RACM) was used for the calcn. A one-way nesting technique was used in which the large grid is a squared 198 km one with a grid resoln. of 6 * 6 km and the small one is a rectangular 78 * 68 km with a grid resoln. of 2 * 2 km. There are 24 vertical levels up to 8,000 m asl. The meteorol. and chem. simulations were validated with ground and vertical profiles measurements performed during the Intensive Observation Period (IOP) July, 25-27, when the O3 concns. reached up to 95 ppb. Both the spatial as well as the temporal variabilities of the simulated O3 concns. correspond reasonably well to the measured values. They caught the O3 plume south of Grenoble. The model results indicate a lower Planet Boundary Layer (PBL) height than the LIDAR obsd. The PBL calcd. by the model is about 2,100 m, whereas the LIDAR indicates about 2,400 m. The results show that the PBL in mountain terrain is higher than those found over flat terrain. A specific vertical stratification of the atm. at day and night time is obsd. during typical photochem. episodes. Highest O3 values appear in the south at 25 km downwind the city center, and the results show that about 32 ppb of fresh O3 are generated in the Grenoble plume.

A new lidar for the measurement of tropospheric ozone is proposed, based on the differential analysis of the Raman backscattered signals on nitrogen and oxygen, which is far less sensitive to the aerosols than the classical DIAL system. Using a third Raman channel, the system is able to measure the water vapor mixing ratio simultaneously. The transmitting section of the instrument is composed of a single wavelength at 266 nm, generated by a quadrupled Nd:YAG laser, while the receiving section is the combination of a 20 cm Newton telescope, a polychromator, custom made band pass filters and miniature photomultiplier, giving a compact and efficient optical layout. The cross-talk between the different channels, and the rejection of the 266 nm wavelength have been measured in detail and will be presented. Time series of ozone and water vapor vertical profile during some days have been performed in the early spring 99.

Recent research works show that wastewater treatment plants may contribute significantly to the total emission of volatile org. compds. (VOCs) in urban areas. In order to investigate this contribution, a measurement campaign was carried out during Sept., 1999, in the Lausanne, Switzerland, area. The concns. of monocyclic arom. hydrocarbons (MAHs) over a wastewater treatment plant at Lausanne were monitored by differential optical absorption spectroscopy (DOAS) and gas chromatog. with a flame ionization detector (GC-FID). An FID was used for monitoring the overall concn. of nonmethane hydrocarbons (NMHCs). The local meteorol. conditions were also measured. The concns. of benzene, toluene, ethylbenzene, xylenes, 1,2,4-trimethylbenzene, and phenol over the wastewater treatment plant as measured by DOAS and GC-FID are compared. In addn., based upon the results of a recent field expt., the impact of the oxygen interference on DOAS measurements of arom. compds. is addressed. The MAH concn. time series and its relation to that of NMHCs, as well as to the meteorol. conditions, are presented. The VOC release and prodn. phenomena involved in the wastewater and sludge treatment processes are analyzed using complementary measurements of arom. compds. in wastewater and gas processing streams.

Applied Physics B, 78 (2), ISSN:0946-2171, ISSN:1432-0649, ISSN:0721-7269, ISSN:0340-3793

The temporal and spatial retrieve of ozone (O3) concentration and water vapor (H2O) mixing ratio in the troposphere is of essential interest. Contrary to the stratospheric case, the tropospheric ozone can have a harmful impact, with its toxic effect, on humans and vegetation, accelerating the degradation of the minerals and participating in the green-house problem. Concerning the water vapor, knowledge of its highly variable concentration is essential to both the chemistry of the troposphere (O(1D) + H2O —> 2 OH) where it participates, among others, in the generation of the hydroxyl radical (OH) and to the meteorology. Water vapor is the dominant green-house gas, it plays an important role in the atmospheric chemistry. The conversion and transport of water in the atmosphere is the essential point in the earth's radiation budget. Due to the complexity and the non-linearity of the air pollution system including emissions, chemistry, thermal radiation, transport and deposition, pollution abatement strategies can only be designed rightly by the use of a three-dimensional mesoscale Eulerian photochemical transport model. To check such models, measurement campaigns are undertaken, in which many physical (wind, temperature, H2O, etc.) and chemical parameters (emissions and imissions) are measured at different parts of the atmosphere. LIDAR (LIght Detection And Ranging), which is a real-time method for measuring air pollutants in situ, is one of the best tools to make 3-D measurements of gases concentrations like O3, H2O and others. Contrary to the ground based measurements that are highly sensitive to the very local conditions, lidar sensitivity and resolution in space and time is optimal to obtain measurements and to compare or give some input data for the models. During the last thirty years, (elastic backscatter) Differential Absorption Lidar (DIAL) has been established as a convenient tool for the monitoring of the three dimensional real time concentrations of air pollutants [Measures, 1992], [Schoulepnikoff et al., 1998]. But the DIAL apparatus has shown limitations: — the operation in layers with high aerosol loading like in the Planetary Boundary Layer (PBL) where they are highly variable — the simultaneous detection of several atmospheric components or pollutants is impossible [Bösenberg, 1996] — the detection at short range is difficult due to the high dynamics. Furthermore, due to its spectrum and the strong influence from other elements, the water vapor can not be easily measured in the UV with classical DIAL systems. The goal of this work was to develop a method to simultaneously measure the ozone absolute concentration and the water vapor mixing ratio in the PBL. Experiments with Nd : YAG and KrF lasers were made and utilization of both analog and photon counting techniques, increasing the dynamic range, were investigated. To retrieve the ozone concentration profile, we take advantage of the simultaneous spontaneous Raman backscattering on the molecules of nitrogen (N2) and oxygen (O2) that have different ozone absorption cross-sections. Thus with a modified DIAL technique, the ozone concentration can be measured without most of the interference from poorly known backscatter by particles. Water vapor mixing ratio profile can also be obtained with a set of three Raman backscattered signals, simultaneously detected, from the molecules of H2O, N2 and O2. The main advantage of this Raman system is its essential independence to the wavelength dependent backscatter problems as induced by aerosols, and the fact that the N2 and O2 concentrations are well known as well as the Raman cross-sections of interest. Although the Raman cross-sections are two or three orders of magnitude lower than the elastic backscattering cross-sections, they are compensated by the proportionally much higher concentrations of O2, N2 and H2O compared to trace gases like O3. The development of the Raman — DIAL method for atmospheric measurements in the PBL presents several challenges. One is the development of highly-sensitive lidar systems, in particular the optical receiver, the spectrometer and the signal acquisition for the Raman part of the Raman-DIAL system. Also the data processing procedure for simultaneous evaluation of the ozone and the water vapor profiles. Both of these challenges present a number of issues, theoretical and practical, that are investigated in the frame of this work.

Climate changes and global warming are generally associated with the enhanced greenhouse effect, but aerosols can induce a cooling effect and thus regionally mask this warming effect. Unfortunately, the strong variability both in space and in time of the aerosols and thus the difficulty to characterize their global basic properties induce large uncertainties in the predictions of the numerical models. Those uncertainties are as high as the absolute level of the enhanced greenhouse forcing. To solve this problem it is necessary to improve the set of well-calibrated instruments (both in situ and remote sensing) with the ability to measure the changes in stratospheric and tropospheric aerosols amounts and their radiative properties, changes in atmospheric water vapor and temperature distributions, and changes in clouds cover and cloud radiative properties. The quantity used to assess the importance of one compound (greenhouse gases, aerosols) to the variation of the radiative budget of the Earth is the radiative forcing. One of those forcings is the direct aerosol radiative forcing and it depends on the optical depths and the upscatter fraction of the aerosols. Those two parameters depend on the chemical composition and size distribution of the aerosols. Thus the key parameters of this radiative forcing are the chemical composition through its refractive index and the size distribution of the aerosols. This thesis deals with the design and the implementation of one multi-wavelength lidar system at the Jungfraujoch Alpine Research Station (Alt. 3580m asl). This lidar system is a combination of one standard backscatter lidar and one Raman lidar. Its design have been supported by a ray tracing analysis of the receiver part. The laser transmitter is based on a tripled Nd:YAG laser and the backscattered light is collected by one Newtonian telescope for the tropospheric measurements and by one Cassegrain telescope for the future stratospheric measurements. The received wavelengths for each telescope include three elastically scattered wavelengths (355, 532 and 1064nm), two spontaneous Raman signals from nitrogen (387 and 607nm) and one spontaneous Raman signal from the water vapor (408nm). The optical signals received by each of the telescopes are separated spectrally by two filter polychromators. They are build up around a set of beamsplitters and custom design thin band pass filters with high out-of-band rejection. On the visible channel, the adds of a Wollaston prism separates the parallel polarized backscattered signal (532(p)nm) of the perpendicular polarized one (532(c)nm). Photomultiplier tubes perform the detection of the signals for the UV and visible wavelengths and by Si-avalanche photodiodes for the near-infrared signal. The acquisition of the signals is performed by seven transient recorders in analog and in photon counting modes. Within the frame of the EARLINET (European Aerosol Research Lidar Network), hardware and software intercomparisons have been done. The software intercomparison has been divided into the validation of the elastic algorithm and the Raman algorithm. Those intercomparisons of the inversions of the lidar signals have been performed using synthetic data for a number of situations of different complexity. The hardware intercomparison have been achieved with the mobile micro-lidar of the Observatoire Cantonal de Neuchâtel. The present lidar system provides independent aerosol extinction and backscatter profiles, depolarization ratio and water vapor mixing ratio up to the tropopause. Their uncertainties could be smaller than 20% and thus make possible the retrieval of the microphysical aerosol parameters like the volume concentration distribution and the mean and integral parameters of the particle size distribution, (effective radius, total surface-area concentration, total volume concentration and number concentration of particles). This retrieval is performed by one algorithm of the Institute of Mathematic of the University of Postdam based on the hybrid regularization method. The first results of the retrieval of the volume concentration distribution with three backscatter (355, 532 and 1064nm) and one extinction (355nm) profiles has demonstrated promising results. Future upgrades of the system will add ozone concentration and temperature profile up to the stratopause.

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