Chimie atmosphérique | EPFL Graph Search

La chimie atmosphérique est une branche des sciences de l'atmosphère étudiant la chimie de l'atmosphère terrestre et des autres planètes. Il s'agit d'un champ de recherche pluridisciplinaire impliquant entre autres la chimie environnementale, la physique, la météorologie, la modélisation informatique, l'océanographie, la géologie et la volcanologie. Son champ de recherche est à l'heure actuelle de plus en plus rapproché à celui de la climatologie. La composition et la chimie de l'atmosphère sont importantes pour de nombreuses raisons, la principale étant les interactions entre l'atmosphère et les organismes vivants. La composition de l'atmosphère de la Terre change du fait de processus naturels comme les émissions volcaniques, la foudre, ou encore le bombardement de particules venant de la couronne solaire. Elle change également du fait de l'activité humaine, et certains de ces changements sont nocifs pour la santé humaine, les cultures et les écosystèmes. Parmi les problèmes abordés

A laboratory study of heterogeneous reactions relevant to the atmospheric boundary layer

Dominik Stadler

The present work deals with two subjects. The interaction of NO2 and HONO with different types of soot are examined in the first part whereas in the second part an experimental set-up is presented which has been built in order to measure the kinetics of the degradation of organic compounds by OH radicals. Both soot particles as well as NO2 are mainly produced by fossil fuel and biomass burning. The two species are therefore ubiquitous in the atmospheric boundary layer where they may react with each other. It has been shown that one possible product of this heterogeneous interaction is nitrous acid (HONO). HONO is an important trace gas in atmospheric chemistry because it is easily photolysed resulting in OH radical and NO. Thus, the photolysis of HONO significantly enhances photooxidation processes early in the morning. In the present study two different types of decane soot have been produced. It has been shown that the air/fuel ratio of the diffusion flame is a key parameter influencing the reactivity of soot towards NO2. Whereas soot from a rich flame ("grey" decane soot) leads to HONO with yields up to 100% upon interaction with NO2, only small amounts of HONO, but significant amounts of NO are formed in the presence of soot originating from a lean flame ("black" decane soot). A reaction mechanism has been developed showing that NO2 is initially converted to HONO by a redox reaction. HONO produced in this way is subsequently either quantitatively desorbed into the gas phase on "grey" decane soot or it decomposes to a large extent in a disproportionation reaction on "black" decane soot. The products of disproportionation are NO, which is instantaneously desorbed into the gas phase and NO2, which undergoes secondary reactions. In addition to the HONO producing pathway there is a reaction channel that leads to adsorption of NO2 for both types of soot which is irreversible on the time scale of our standard experiments. Saturation effects that are occurring on the time scale of our experiments are primarily caused by saturation of adsorption sites and not by the depletion of the reducing agent. It seems that the fraction of NO2 that is irreversibly adsorbed is blocking part of the surface sites where NO2 is initially adsorbed. Uptake coefficients γ take initial values of up to 0.1 for both types of soot. However, with increasing uptake of NO2 γ is decreasing. After a NO2 consumption of approximately 8×1013 molecule cm-2, which corresponds to 13% of a formal monolayer, γ drops to values of 3×10-7 for "grey" decane soot and 6×10-7 for "black" decane soot, respectively. Furthermore, our results show that the rate of initial uptake is the rate limiting step of the NO2/soot interaction mechanism. Experiments where "black" decane soot has been exposed to a flow of HONO revealed that it readily decomposes into NO and NO2. For HONO concentrations above 3.3×1011 molecule cm-3 a total product yield of 50%, whereof 40% NO and 10% NO2, has been observed. The initial uptake coefficient ( γ0) for HONO on "black" decane soot did not show clear saturation effects but varied randomly in the concentration range from 1.2×1012 to 5.9×1012 molecule cm-3. The average value measured at these concentrations was equal to 0.027. The strong interaction of HONO with "black" decane soot shows that soot is not only a potential source, but also a potential sink of HONO. On the other hand, no significant uptake could be observed when "grey" decane soot was exposed to HONO. This is consistent with the observation that NO2 is converted to HONO at yields of up to 100%. Acetylene soot is an additional type of soot that has been examined. The results for the initial uptake coefficients were basically the same as for "grey" and "black" decane soot, that is maximum values of γ0 of 0.1 for NO2 uptake decreasing with increasing concentrations of NO2. This similarity holds only for the first few seconds of interaction because acetylene soot saturates much faster than "grey" and "black" decane soot with continuous exposure. The acetylene samples are generally almost completely saturated after only three minutes of exposure to NO2. Soxhlet extractions of "grey" decane soot which have been performed using tetrahydrofuran as a solvent also produced HONO upon interaction with NO2. This shows that the active reactants are not part of the graphitic soot backbone but are extractable, rather polar compounds of the organic fraction of soot. This assumption is supported by the observation that the corresponding benzene extraction is not reactive in relation to NO2 uptake and HONO formation. In the second part of the present work an experimental set-up is described which has been built in order to measure the rate constants of the reaction of organic compounds with the OH radical in the gas phase using a relative rate technique at atmospheric pressure. The system may be called a "mini smog chamber" as the reaction cell has a volume of only 480 cm3. The system was validated by measuring the relative rate constants of the reactant pair cyclohexane/toluene. These measurements have been performed for temperatures ranging from 300 to 360 K and led to results which are in excellent agreement with literature data. However, the experimental set-up also had some important limitations. Biphenyl which has a boiling point of 529 K was the compound with the lowest volatility that could be examined in our reaction cell. Compounds with lower vapor pressures or compounds with polar groups adsorbed to a large extent onto the glass walls of the reaction cell even at temperatures of up to 373 K. Additional problems occurred in the closed ion source of the mass spectrometer leading to non linear and/or unstable MS signals. These phenomena were most probably caused by secondary ion-molecule reactions in the closed ion source.

EPFL2000

Development of the Jungfraujoch UV DIAL Lidar to Observe the Vertical Ozone Distribution in the Context of Stratosphere Troposphere Exchange and Long Range Transport

Marcel Bartlome

Tropospheric ozone is a climate relevant greenhouse gas, as well as an atmospheric pollutant. Its abundance in the troposphere is mainly governed by the influx of stratospheric air masses and the photochemical production due to anthropogenic and biogenic ozone precursors. Precise model simulations and measurements are needed to assess the exact impact of these sources onto the total tropospheric ozone content. In the latter case, vertical tropospheric ozone profiles are commonly obtained from in-situ balloon-borne soundings. These routine observations are performed with a maximum frequency of several measurements per week. Therefore, they are not suited to resolve the rather fast changes in the tropospheric ozone concentration. However, accurate vertical ozone profiles with temporal and spatial resolutions suitable for studying those changes have been produced for decades using remote sensing by means of the ozone UV differential absorption lidar (DIAL) technique. The main goals of this thesis aimed to address some of the unresolved problems of the ozone origin and evolution in the upper troposphere and lower stratosphere and were: to develop an UV DIAL for the observation of the vertical ozone distribution in the free troposphere and at the tropopause region. to perform experimental measurements of vertical ozone profiles in the free troposphere and the tropopause in order to assess the ability of the ozone UV DIAL to observe fast variations in vertical ozone distribution caused by stratosphere-troposphere exchange (STE), long range transport, and advection from the boundary layer. to assess the feasibility of systematic ozone profile retrieval by the ozone UV DIAL technique from the High Altitude Research Station Jungfraujoch (HARSJ, 3580 m ASL, 7°59'2''E, 46°32'53''N). The HARSJ was chosen for the experiment because of its location, which allows ozone UV DIAL measurements of the tropopause region without perturbation of the lower troposphere. Furthermore, previous atmospheric studies have shown that because of the station's location, the upper tropospheric data taken at the HARSJ can be regarded as representative for central and western Europe. During the first two and a half years of the thesis project the ozone UV DIAL was designed and built. The ozone UV DIAL transmitter is based on a commercial, fourth harmonic Nd:YAG laser. The on- (284 nm) and off- (304 nm) ozone UV DIAL wavelengths are produced by stimulated Raman scattering in high-pressure nitrogen. The receiver uses the existing astronomical Cassegrain telescope (76 cm in diameter) at the HARSJ. Spectral separation of the backscattered ozone UV DIAL wavelengths is carried out by a polychromator based on an concave imaging diffraction grating. The entire ozone UV DIAL detection unit is integrated into the existing long-range multi-wavelength polychromator box optically coupled at the Cassegrain telescope's rear end. With the current design, the ozone UV DIAL system provides hourly averaged ozone profiles reaching from 6 km to 12 km ASL with a vertical resolution better than 400 m at 6 km ASL and 1000 m at 12 km ASL. The relative statistical error of the profiles is 10% at 12 km ASL. The experimental measurements started in the spring of 2008 but were interrupted several times because of serious laser failures. As a first step of the experiment, the ozone UV DIAL profiles were compared to balloon borne ozone profiles taken by electrochemical concentration cell (ECC) sondes. The sondes were launched by the Swiss Meteorological Institute – Payerne (SMI, 46°49'N, 6°56'E, 491 m ASL) situated 80 km in northwestern direction of the HARSJ. The ozone UV DIAL and the sonde measurements were taken quasi simultaneously. The relative differences between the ozone UV DIAL and the sonde profiles were found to be below ±25% in a horizontally homogeneous atmosphere. The intercomparison has shown that the ozone UV DIAL is capable to accurately reproduce the vertical ozone distribution. Intercomparison with vertical ozone profiles taken in the vicinity of the HARSJ by an airplane-borne UV-photometer confirmed the performance of the ozone UV DIAL. Time series with duration of up to 21 hours were taken in order to study the time evolution of tropospheric events characterized by elevated ozone concentrations. As a result of these measurements three events, demonstrating different processes have been identified. In the first case an air layer with elevated ozone concentration was observed in the altitude range between 6 and 7 km ASL. The lower than 10% relative humidity and the time evolution of this layer allowed to determine its stratospheric origin. The observations are in a good agreement with the EURopean Air Pollution Dispersion (EURAD) model. The model predictions also agree with the elevated ozone concentration measured by the UV-photometer of the National Air Pollution Monitoring Network (NABEL) operated at the HARSJ. The evolution of an ozone-enriched layer in the tropopause region was followed for 21 hours during the second case study. The time evolution, the ECC data, and the EURAD model predictions suggest the possibility of a short filamentation process developing at the tropopause region. The measurements of the third event were made according to a prediction of STE forecasted by a dynamical model developed at the ETHZ. A short living enhancement of the ozone concentration has been observed at an altitude of about 9.5 km ASL. There are two possible explanations for this event. The first one could be a short living reversible STE event. A second explanation is a possible long-range transport of ozone and ozone precursor rich air from the forest fires in the Athens region suggested by the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) backward trajectories. However, the available information does not allow to make a definite conclusion. The ozone UV DIAL has been built successfully and has shown its ability to observe fast variation in vertical ozone distribution with high accuracy and precision. In combination with additional water vapor and temperature lidar observations and supported by model forecasting, this system could become a powerful tool for the improving of the understanding of upper troposphere and lower stratosphere ozone related processes. However, to allow future systematic observations the ozone UV DIAL will need an essential upgrade to resolve the problems related to the long preparation time, and the limited by technical reasons time of observation.

EPFL2010

Development of a high spatial and temporal resolution Raman lidar for turbulent observations

Pablo Roberto Ristori

Water vapor plays an important role in weather, global climate processes and atmospheric chemistry. It is the most significant greenhouse gas and affects the planet's radiative and non-radiative energy balance. The distribution of water vapor in the atmosphere is quite variable both horizontally and vertically and has a significant influence on the atmospheric circulation and temperature structure. Accurate data about water vapor and temperature is needed for weather forecasting, weather and climate research, boundary layer and cloud process studies, and atmospheric chemistry. Despite the need for these data, obtaining accurate water vapor and temperature measurements with high temporal and spatial resolution has remained an only partially solved problem. The Raman lidar technique for water vapor and temperature measurements is a straightforward method and is based on well-known physical principles. It can supply accurate data with high spatial and temporal resolution for weather, climate, atmospheric boundary layer (ABL) studies and other atmospheric research. One of the two goals of this thesis is to develop and construct a Raman lidar instrument with high spatial (1.5 m) and temporal (1 s) resolution and an operational range of 15 – 500 m for systematic observations of water vapor profiles. The measured profiles together with a new generation of Large Eddy Simulations (LES) will be used to attain an improved understanding of the complex linkages between the land surface and the overlying atmospheric boundary layer. The second goal is to develop and test a new method for the determination of the atmospheric transmission correction factor for non solar-blind water vapor Raman lidars based exclusively on the use of Raman lidar signals without needing a priori information about the aerosol optical properties.

EPFL2008