Smog | EPFL Graph Search

vignette|Smog à Kuala Lumpur en 2005. Le smog, fumard ou brumée est un brouillard grisâtre urbain qui limite la visibilité dans l’atmosphère. Issu du mélange de particules fines et d'ozone, le smog est associé à plusieurs effets néfastes pour la santé et pour l'environnement. Étymologie Le terme smog est un mot-valise formé à partir des mots anglais smoke (fumée) et fog (brouillard). Ce néologisme fut inventé par Henry Antoine Des Vœux en 1905 pour décrire le mélange de brouillard et de fumée qui accablait périodiquement Londres et les autres grandes villes d'Europe. Dès le il est identifié à Londres où on utilisait le charbon pour le chauffage domestique, mais c'est au milieu du que les choses s'aggravent avec la révolution industrielle. Origine et mécanismes Le smog résulte de la condensation dans les basses couches de l'atmosphère (troposphère) de l'humidité (brouillard) sur des poussières en suspension, souvent en pré

Estimation of the total VOC reactivity with OH by pump-and-probe OH lidar to determine a new indicator for ozone production

Many of our most severe problems of environmental pollution involve the Earth's atmosphere. The adverse effects of atmospheric pollution are particularly evident in the low troposphere, especially in urban/suburban areas where many sources contribute to the atmospheric burden. Ozone (O3) is one of the major constituents of the air pollution over cities commonly called photochemical smog. It is a secondary pollutant, produced by the reaction of primary pollutants, nitrogen oxides (NOx) and hydrocarbons (VOC), in the presence of sunlight. Although hydrocarbons and NOx have long been identified as the two key precursors to photochemical O3, the development of an effective strategy for reducing ozone production in photochemical smog by controlling the anthropogenic emissions of these precursors has proved to be problematic. The objective of the present study is to present and demonstrate the robustness of a new observable "indicator" for determining the sensitivity of ozone formation to VOC and NOx (VOC- and NOx-limited regime). This indicator is based on the reactivity of VOC and NOx with respect to OH. Simulation tests of the ability of this new indicator Θ = τOHVOC/τOHNOx to distinguish between a NOx and a VOC-limited regime were performed with a box model and a 3D model applied to a specific episode of photochemical smog (Athens) and of a less polluted area (the Swiss plateau). Whereas the NOx reactivity 1/τOHNOx can easily be measured by standard methods, the total VOC reactivity 1/τOHVOC will be estimated by measuring the inverse lifetime of OH with a new Pump-and-Probe Lidar instrument. The method consists of producing high OH concentrations by the flash photolysis of ozone and the subsequent O(1D) reaction with H2O and by following the OH relaxation by laser-induced fluorescence (LIF). Numerical simulation of the experiment will show that the total VOC reactivity with OH can be obtained by measuring the inverse lifetime of the OH and its absolute initial concentration with the Pump-and-Probe experiment as well as the NOx, CO and O3 concentrations with standard trace-gas detectors. This total in-situ VOC reactivity will be used to determine the new indicator Θ but will also help to test the accuracy of the VOC-lumped chemical mechanism in the numerical atmospheric simulation. In order to demonstrate the feasibility of the Pump-and-Probe experiment, laboratory investigations have been performed in controlled atmosphere: OH absorption and fluorescence emission spectra allowed the OH transition bands and the non-resonant fluorescence wavelength to be identified. High-resolution OH absorption spectra allowed the OH effective absorption cross section to be determined for assessing the experimental sensitivity and retrieving the OH concentration. The experimental retrieval of the rate constant for the OH+CH4 and OH+NO2 reactions reveals the ability of the pump-and-probe experiment to monitor the chemical kinetics of OH at atmospheric pressure and room temperature. OH relaxation measured in different VOC/NOx/O3 gas mixtures confirm the relevance of the model predictions of the OH relaxation approximated by a first-order chemical decay and the validity of the total VOC reactivity with respect to OH estimated by such an approximation. A first in-situ OH relaxation measurement realized directly in the air of the laboratory showed that the pump-and-probe method could be performed under typical PBL conditions. Preliminary range-resolved measurements yielding vertical profiles of the OH concentrations produced by flash photolysis of ambient ozone suggest a promising future for the Lidar pump-and-probe technique. The perspectives for the Pump-and-Probe Lidar experiment are expressed in the concluding section.

EPFL2000

Ozone flux determination with a shot per shot DIAL instrument during the MEDCAPHOT field campaign

Bertrand Calpini, Hubert van den Bergh

A combined measurement of O3 concn. and wind speed by lidar is presented, which permits a flux detn. of this pollutant. Measurements were obtained using a shot per shot differential absorption lidar (DIAL) system in which each return is recorded, stored, and treated allowing the study of the temporal evolution of the signal. O3 and wind range resolved measurements are key variables of simulations used to analyze photochem. smog episodes. Results demonstrated that the usual detn. of static O3 concns. can be replaced by measurement of the dynamic variable, O3 flux, when using a shot per shot method. This represents an advance over the classical hardware averaging DIAL approach by taking advantage of the information contained in the shot to shot evolution of the lidar signals which are used simultaneously for O3 and wind profiling.

1997

Estimation of the total VOC reactivity with OH by pump-and-probe OH lidar to determine a new indicator for ozone production

EPFL2000