High spatial resolution measurements of NO₂ applying Topographic Target Light scattering-Differential Optical Absorption Spectroscopy (ToTaL-DOAS)

International audienceTomographic Target Light scattering ? Differential Optical Absorption Spectroscopy (ToTaL-DOAS), also called Target-DOAS, is a novel experimental procedure to retrieve trace gas concentrations present in the low atmosphere. Scattered sunlight (partially or totally) reflected from natural or artificial targets of similar albedo located at different distances is analyzed to retrieve the concentration of different trace gases like NO2, SO2 and others. We report high spatial resolution measurements of NO2 mixing ratios in the city of Montevideo (Uruguay) observing three buildings as targets with a Mini-DOAS instrument. Our instrument was 146 m apart from the first building, 196 m from the second and 286 m from the third one. All three buildings are located along a main Avenue. We obtain temporal variation of NO2 mixing ratios between 30 ppb and 65 ppb (±2 ppb). Our measurements demonstrate that ToTaL-DOAS measurements can be made over very short distances. In polluted air masses, the retrieved absorption signal was found to be strong enough to allow measurements over distances in the range of several ten meters, and achieve a spatial resolution of 50 m approximately

Tomographic MAX-DOAS observations of sun illuminated targets: a new technique providing well defined absorption paths in the boundary layer

A novel experimental procedure to measure the surface-near distribution of atmospheric trace gases using passive Multi-Axis-Differential Absorption Optical Spectroscopy (MAX-DOAS) is proposed. The idea consists of pointing the receiving telescope of the spectrometer to non-reflecting surfaces or to ‘bright’ targets placed at known distances from the measuring device, which are illuminated by sunlight. We show that the partial trace gas absorptions between the top of the atmosphere and the target can be easily removed from the measured total absorption. Thus it is possible to derive the average concentration of trace gases like e.g. NO2, HCHO, SO2, H2O, Glyoxal, BrO and others along the line of sight between the instrument and the target like for the well-known long-path DOAS observations (but with much less expense). If tomographic arrangements are used, even two- or three-dimensional trace gas distributions can be retrieved. The basic assumptions of the proposed method are confirmed by test measurements across the city of Heidelberg

Composition law for polarizers

The polarization process when polarizers act on an optical field is studied. We give examples for two kinds of polarizers. The first kind presents an anisotropic absorption - as in a polaroid film - and the second one is based on total reflection at the interface with a birefringent medium. Using the Stokes vector representation, we determine explicitly the trajectories of the wave light polarization during the polarization process. We find that such trajectories are not always geodesics of the Poincar\'e sphere as it is usually thought. Using the analogy between light polarization and special relativity, we find that the action of successive polarizers on the light wave polarization is equivalent to the action of a single resulting polarizer followed by a rotation achieved for example by a device with optical activity. We find a composition law for polarizers similar to the composition law for noncollinear velocities in special relativity. We define an angle equivalent to the relativistic Wigner angle which can be used to quantify the quality of two composed polarizers.Comment: 23 pages, 9 figures, accepted for publication in Physical Review

Optical current and voltage sensor using differential spectroscopy

A novel optical system that combines a polarimetric current sensor based on the Faraday effect, and a voltage sensor based on the Pockels effect, is described. The proposed device uses polarizers that possess high polarization ratios in specific spectral ranges. This allows the simultaneous codification of current and voltage information in different spectral regions, which, in turn, enables independent measurements of both magnitudes. Validation experiments are presented. © 2009 Society of Photo-Optical Instrumentation Engineers

Determination of NOx emissions from Frankfurt Airport by optical spectroscopy (DOAS) – A feasibility study

Standard methods like in-situ measurements can hardly register NOx (= NO + NO2) emissions from aircrafts during take-off, when engines run at high load and thus an important amount of fuel is consumed and most of the harmful emissions are produced . The goal of this work is to show that it is possible to measure aircraft emissions generated during take-off (and initial part of the climb) by a remote spectroscopic method like automobile – based – Differential Optical Absorption Spectroscopy (Mobile-DOAS), which uses scattered solar radiation in the blue spectral range (around 445 nm). In order to test its feasibility, total column measurements of NO2 encircling Frankfurt Airport were carried out on 23 February 2012 using Mobile-DOAS. Also, NOx fluxes were derived from the NO2 observations. Unlike standard mobile-DOAS measures using a spectrometer looking at zenith, the measurements were performed looking at 22° elevation angle leading to a roughly two to three times higher sensitivity compared to zenith observations. The origin of the observed NO2 is discussed and the total NOx fluxes are calculated. As result of three round-trips encircling the Frankfurt Airport, the mean NOx flux was found to correlate with the number of aircrafts taking-off. Our results demonstrate that mobile-DOAS method is suitable for quantifying emissions from airports and to study their impact in the planetary boundary layer, which is most relevant concerning the impact on the environment and the human health

Edge enhancement and image equalization by unsharp masking using self-adaptive photochromic filters

A new method for real-time edge enhancement and image equalization using photochromic filters is presented. The reversible self-adaptive capacity of photochromic materials is used for creating an unsharp mask of the original image. This unsharp mask produces a kind of self filtering of the original image. Unlike the usual Fourier (coherent) image processing, the technique we propose can also be used with incoherent illumination. Validation experiments with Bacteriorhodopsin and photochromic glass are presented. " 2009 Optical Society of America.",,,,,,"10.1364/AO.48.003570",,,"http://hdl.handle.net/20.500.12104/40870","http://www.scopus.com/inward/record.url?eid=2-s2.0-67749132668&partnerID=40&md5=15f7e8d2b31390ac1a96eeb3cb283e43",,,,,,"19",,"Applied Optics",,"357

Optical current sensor by self-compensating the Faraday effect

A novel optical system that combines a polarimetric current sensor based on the Faraday effect, and a voltage sensor based on the Pockels effect, is described. The proposed device uses polarizers that possess high polarization ratios in specific spectral ranges. This allows the simultaneous codification of current and voltage information in different spectral regions, which, in turn, enables independent measurements of both magnitudes. Validation experiments are presented. " 2009 Society of Photo-Optical Instrumentation Engineers.",,,,,,"10.1117/1.3139299",,,"http://hdl.handle.net/20.500.12104/43379","http://www.scopus.com/inward/record.url?eid=2-s2.0-78149485161&partnerID=40&md5=0e2b3473a53e45282e34abca1c41e63d",,,,,,"5",,"Optical Engineering",,,,"48",,"Scopus",,,,,,"Faraday effect; optical current sensor; optical voltage sensor; Pockels cell",,,,,,"Optical current and voltage sensor using differential spectroscopy",,"Article" "45159","123456789/35008",,"Beltran, H.C., Electronic Engineering Department, University of Guadalajara, Av. Revolución #1500, CP. 44840, Guadalajara, Jal., Mexico; Flores, J.L., Electronic Engineering Department, University of Guadalajara, Av. Revolución #1500, CP. 44840, Guadalajara, Jal., Mexico; Ferrari, J.A., Instituto de Física, Facultad de Ingeniería, J. Herrera y Reissig 565, 11300 Montevideo, Uruguay; García-Torales, G., Electronic Engineering Department, University of Guadalajara, Av. Revolución #1500, CP. 44840, Guadalajara, Jal., Mexico; Cabrera, J., Electronic Engineering Department, University of Guadalajara, Av. Revolución #1500, CP. 44840, Guadalajara, Jal., Mexico",,"Beltran, H.C