Little is currently known regarding the nature and consequences of interactions between photoactive surfaces, including mineral dust and ‘urban film’, and gas-phase pollutants in urban environments. In order to address this knowledge gap, this thesis explores the photochemical reactivity of these environmental surfaces in controlled laboratory settings. The photoenhanced ozonation of pyrene, a toxic product of incomplete combustion, proceeds at different rates and via different mechanisms at three model ‘urban film’ surfaces. These results are important because they suggest that the reactivity of a molecule on simplified surfaces may not accurately reflect its reactivity in the real environment. The photooxidation of isopropanol at the surface of TiO2, here used as a proxy for the photoactive component of mineral dust, yields gas-phase acetone. This chemistry is amplified by nitrate, a major surficial component of atmospherically processed dust. These results suggest that dust has the potential to convert non-absorbing species to photochemically active species, and thereby serve as a source of reactive organic radicals for further gas- or surface-phase chemistry. Oxalic acid, the most atmospherically abundant dicarboxylic acid, is efficiently oxidized to gas-phase CO2 at the surface of Mauritanian sand and Icelandic volcanic ash. These experiments indicate that the lifetime of oxalic acid may be limited in arid regions by Fe and Ti-catalyzed aerosol-phase photochemistry. Fluorotelomer alcohols (FTOHs), a class of industrial chemicals used in the production of surface coatings, undergo photooxidation at the surface of sand and ash to yield toxic and persistent perfluorinated carboxylic acids (PFCAs). These results provide the first evidence that the metal-catalyzed heterogeneous oxidation of FTOHs may act as a local source of aerosol-phase PFCAs. Illumination of Nigerien sand in the presence of gas-phase SO2 leads to the formation of surface-sorbed sulfate. This chemistry proceeds more efficiently on fine sand than on coarse sand. In chamber experiments, the illumination of SO2 in the presence of realistically produced dust aerosol results in new particle formation. Together, these results suggest that SO2 photochemistry at the dust surface has the potential to change not only dust hygroscopicity but also the net scattering potential of dust-containing air masses.

La contamination des milieux aquatiques urbains se caractérise aujourd'hui par un mélange de micropolluants d'origines et de types variés. Les conséquences de ce type de contamination, multiples mais faibles et chroniques, sur les écosystèmes aquatiques sont difficiles à évaluer. En effet, il est difficile d'une part de dresser un état des lieux précis de la contamination et d'autre part, de relier cette contamination à la réponse biologique des organismes exposés puisqu'elle n'engendre pas nécessairement des effets visibles et immédiats. Cette étude a pour objectif de développer une méthodologie permettant d'utiliser le périphyton pour évaluer l'impact d'une contamination urbaine typique. Les biofilms sont des communautés aquatiques complexes jouant un rôle primordial dans le fonctionnement des écosystèmes aquatiques. Leur réponse biologique à une perturbation peut s'évaluer au niveau structurel comme au niveau fonctionnel. Une première étape de ce travail a consisté à développer d'une part un test de toxicité aiguë permettant d'évaluer la tolérance de la composante hétérotrophe des biofilms selon le concept PICT, et d'autre part l'utilisation de la technique d'empreinte génétique ARISA pour évaluer des modifications de structure des communautés bactériennes et eucaryotes des biofilms. Les méthodologies développées ont ensuite été testées au laboratoire par des expositions de périphyton à des métaux à des concentrations environnementales. Par la suite, elles ont pu être déployées in situ : d'abord dans un gradient de contamination multi-métallique sur la Seine avec des prélèvements ponctuels de biofilms naturels ainsi qu'une expérience de transposition (amont/aval de l'agglomération parisienne). Enfin, le lien entre la tolérance du périphyton et la contamination métallique globale du milieu a été exploré à l'échelle d'un bassin versant. Les résultats sont prometteurs quant à l'utilisation du périphyton pour évaluer l'impact d'une contamination urbaine / Urban aquatic ecosystems are these days contaminated by mixtures of micropollutants of diverse types and origins. Such contaminations, which are multiple, chronic and non-lethal, raise increasing concern among the scientific community. Indeed, a precise evaluation of exposure levels is difficult to obtain. Moreover, establishing a causal link between contamination and the induced biological response of aquatic organisms is complex as no direct and visible effects can be observed. The aim of this study is to develop a methodology for the use of periphyton as a biomarker of urban pollution. Biofilms are complex aquatic communities and play an important role in aquatic ecosystems. Their response to a disruption can be assessed both in terms of structure or functional modifications. Our first step was to develop both a PICT approach with a new short-term toxicity test to measure the tolerance to metal of the heterotrophic component of biofilms and the use of a fingerprinting technique (ARISA) to reveal structure modifications of both bacterial and eukaryotic communities. The methodologies thus developed were tested in a microcosm study by exposing periphyton to metals at environmental levels. They were then used in field studies: first in a multi-metallic gradient along the Seine river by punctual sampling of natural periphyton and also in a translocation study (upstream/downstream the Paris urban area). Furthermore, the link between biofilm tolerance levels and the global metallic contamination was explored at the watershed scale. It is hoped that the future use of this approach will help to understand the impacts of human activities, especially toxic pressure, on aquatic ecosystems