Differential Optical Absorption Spectroscopy (DOAS) is a remote sensing technique to detect different trace gas concentrations in the atmosphere. The Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements by the Pandora instrument scan the sky at different elevation angles and main data products include near surface concentration, tropospheric column and vertical profile for different trace gases. It addresses an important gap in near surface air quality measurements that is difficult for in-situ, satellite and other remote sensing measurements to address. Different applications of the MAX-DOAS technique have been presented in this study for improving our understanding of tropospheric chemistry and near surface air quality monitoring. Formaldehyde (HCHO) concentration retrieved from the DOAS technique exhibits significant variation depending on the fitting parameters used. This systematic variation stems from different factors such as uncertainty in molecular absorption cross section measurement, temperature dependence of trace gas absorption, correlation between trace gases and combination of absorbers used in the DOAS fitting. To investigate the sensitivity and systematic uncertainty of HCHO retrieval, different fitting scenarios were created where fitting parameters like wavelength range, polynomial order, offset order and molecular absorption cross section were varied. To minimize systematic uncertainty and provide steady variability, the fitting scenario that most closely resembles the median of the range is selected and recommended as base fitting scenario. In addition, a real time analytical method to calculate HCHO near surface volume mixing ratio is presented where radiative transfer modelling is not required. The HCHO near surface volume mixing ratio calculated by MAX-DOAS is compared with surface HCHO measured by a ground in-situ instrument. The Pandora MAX-DOAS agrees very well with the ground in-situ instrument for the whole campaign (R²= 0.83, slope= 0.92) and provides excellent agreement for clear days (R²= 0.83= 0.88, slope=0.95). Additionally, a methodology is presented for detecting the mixing layer height (MLH) by using Pandora MAX-DOAS vertical water vapor distribution measurements. The wavelet method is applied to detect sharp gradients in the water vapor vertical profiles for estimation of mixing layer height. The Pandora derived mixing layer depth is compared to the estimations from the collocated Ceilometer (Vaisala CL51, EPA) measurements. Pandora MAX-DOAS agrees well with Ceilometer measurements for different time intervals during the day with a correlation coefficient of 0.68 to 0.76. Nitrogen Dioxide (NO₂) and Formaldehyde (HCHO) tropospheric columns and vertical profiles measured at the Hartsfield-Jackson Atlanta International Airport are also presented. Even though anthropogenic emissions decreased severely all over the United States due to Covid lockdown restrictions in 2020, trace gas levels at airports remained relatively same due to continuing air traffic. MAX-DOAS measurements are performed at different azimuth angles which gives a three dimensional representation of NO₂ and HCHO vertical profiles and enables to observe and distinguish air pollution at different directions. These measurements further show the potential of MAX-DOAS measurements for near surface air quality monitoring. / Doctor of Philosophy / MAX-DOAS is a ground based spectroscopic technique which can measure near surface concentration, tropospheric column and vertical distribution of different trace gases. Even though Satellite measurements can provide worldwide coverage, they generally measure only one time per day and have limited knowledge of near surface conditions. MAX-DOAS measurements performed by the NASA Pandora spectrometer systems can be used to provide near surface diurnal variation of different trace gas properties. In this work, different real-time applications of the MAX-DOAS technique are presented. At first, near surface concentration of Pandora MAX-DOAS Formaldehyde (HCHO) observations are validated by comparing with an in-situ instrument. Next, a methodology is presented for detecting the mixing layer height (MLH) by using Pandora MAX-DOAS vertical water vapor distribution measurements. Finally, MAX-DOAS measurements of Nitrogen Dioxide (NO₂) and Formaldehyde (HCHO) concentrations during the COVID-19 pandemic at The Hartsfield-Jackson Atlanta International Airport is presented. The measurements are done at different azimuth angles which produces three dimensional representations of NO₂ and HCHO vertical profiles. All these results prove that the NASA Pandora spectrometer systems have great potential for improving our understanding of tropospheric chemistry and air quality monitoring.

University, Diss., 2001--Heidelberg.

Universiẗat, Diss., 2001--Bremen.

Formaldehyde (HCHO), plays an important role in atmospheric chemistry and is an indicator of atmospheric oxidation capacity and surface ozone photo chemistry. The Pandora Spectrometer Instruments are deployed within the NASA/ESA sponsored Pandonia Global Network designed for satellite validation of various gases in atmosphere (e.g. ozone, nitrogen dioxide and formaldehyde). In addition, Pandoras are extensively used during national (e.g. DISCOVER-AQ, OWLETS, LISTOS) and international (CINDI, KORUS-AQ) field campaigns organized to better characterise air pollution and its distribution. Recently it was discovered and shown in prior research conducted by (Spinei et al. 2020), that Pandora measurements of atmospheric HCHO are impacted by HCHO produced within the telescope assembly due to temperature dependent off-gassing from the Delrin® plastic components. The purpose of the research covered in this thesis is to provide a methodology to correct total HCHO vertical column densities measured during the past field campaigns. The methodology developed through the course of this thesis is first tested on the Pandora simulated measurements derived from the surface concentration HCHO observations during KORUS-AQ (2016) field campaign. The derived correction using synthetic data shows that the proposed methodology is accurate within 30%. The second part of the thesis characterizes heat transfer processes within the telescope assembly to estimate internal temperature as a function of ambient meteorological conditions. Considering that the Pandora instruments have mostly identical design of their telescope assemblies heat transfer coefficients derived from one pandora are expected to be applicable to all Pandoras. Convective heat transfer coefficients were derived at VT wind tunnel as a function of wind speed and telescope assembly position. Internally generated power was measured for several different instruments and averaged at $2.15 pm 0.38$ W. Total long wave emissivity was calculated at 0.63. Surface absorptivities were estimated from the material properties. Semi-empirically derived model is proposed to estimate the internal temperature based on the heat transfer parameters, ambient temperature, relative humidity, solar flux, wind speed and wind direction. The correlation between the estimated and measured internal temperatures is 0.93 R^2. Finally, the methodology is applied to the actual HCHO data collected during the KORUS-AQ campaign and the results are compared to concurrent in-situ measurements made aboard DC-8 aircraft for eight days in the months of May and June 2016. / Master of Science / Formaldehyde (HCHO), is a key indicator of atmospheric health and because of this, it is an important topic for study. The Pandora Sun Photometer is a low cost instrument developed at NASA Goddard Space Flight center. It has been used in the study of HCHO in various field campaigns. During the Korea-United States Air Quality Study (KORUS-AQ), the Long Island Sound Tropospheric Ozone Study (LISTOS) and the Ozone Water-Land Environmental Transition Study (OWLETS) the Pandora instrument design included a component manufactured from Delrin® plastic. It has recently been found to produce HCHO relative to the change in temperature. Due to the location of this component inside the telescope assembly of the Pandora instrument, the HCHO produced by the plastic was incorporated into the data invalidating the results. The purpose of this thesis is to provide a methodology for analyzing this issue through quantification of the HCHO produced by the plastic. An analysis is conducted to provide the ability to quantify the temperature internal to the telescope assembly. In addition, three methods are discussed for applying this to then quantify the proportion of HCHO that had been added to the measurements. Finally, the methods are applied to data collected during the KORUS-AQ campaign and the results are compared to a reliable set of data performed by a different instrument showing the improved agreement on eight days in the months of May and June.

University, Diss., 2004--Bremen.

University, Diss., 2005--Heidelberg.

This thesis analyzes the effectiveness of spatially averaged Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements at regular azimuth angle intervals on an hourly basis to validate satellite based DOAS measurements. Off-Axis MAX-DOAS Measurements taken in Blacksburg, Virginia, between November 2021 and April 2022 with an evenly distributed set of measurements were averaged every hour and compared to Direct Sun measurements, also averaged every hour. Comparisons of the difference in average measurement from both measuring strategies, as well as the distribution standard deviations of hourly measurements suggests that the NO₂ distribution around Blacksburg is homogeneous. In order to test the effectiveness of this sampling strategy,in an inhomogeneous location, the LOTOS-EUROS high resolution (1kmx1km) chemical transport model was used to simulate profiles and vertical column densities of real measurements taken during the TROLIX'19 Field Campaign. The LOTOs-EUROS model was used to simulate vertical profiles as well as Vertical Column Densities based on real MAX-DOAS measurements as well as TROPOMI viewing geometry. While the individual ground measurements were not equal to the TROPOMI profile, the TROPOMI profile is approximately the average of the profiles of measurements made within the hour of TROPOMI overpass. / M.S. / This thesis analyzes the effectiveness of spatially averaged Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements at regular intervals of angles offset from due North on an hourly basis to validate satellite based DOAS measurements. MAX-DOAS Measurements taken relative to the position of the sun in Blacksburg, Virginia, a low NO₂ location, between November 2021 and April 2022 to determine the effectiveness of a generalized measuring strategy for satellite validation in low pollution environments. An evenly distributed set of measurements were averaged every hour and compared to measurements taken in the direction of the sun, also averaged every hour, to determine if the variability of NO₂ around Blacksburg is high enough to require a generalized sampling strategy, or if the NO₂ distribution is homogeneous enough to be accurately validated with Direct Sun measurements only.. Comparisons of the difference in average measurement from both measuring strategies, as well as the distribution of standard deviations of hourly measurements suggests that the NO₂ distribution around Blacksburg is low. In order to test the effectiveness of this sampling strategy in a higher pollution location with many sources and sinks of NO₂, the data from the LOTOS-EUROS high resolution (1kmx1km) chemical transport model run by the Royal Dutch Meteorological Institute for the TROLIX'19 Field Campaign was used to simulate vertical distributions of NO₂ and vertical column densities of measurements taken during the field campaign. The LOTOS-EUROS model was used to simulate vertical distributions of NO₂ as well as Vertical Column Densities based on real MAX-DOAS measurements as well as viewing geometry seen by the TROPOspheric Monitoring Instrument (TROPOMI) satellite-based instrument. While the individual ground measurements were not equal to the vertical distribution seen by TROPOMI, the TROPOMI vertical distribution is approximately the average of the vertical distributions of measurements made within an hour of TROPOMI passing over Rotterdam.

Propulsé sur une orbite polaire en avril 1995 par l’Agence Spatiale Européenne, l’instrument Global Ozone Monitoring Experiment (GOME) est le précurseur d’une nouvelle génération de satellites dédiés à la mesure globale de la composition atmosphérique. Ce spectromètre hyperspectral mesure, entre 240 et 790 nm, à la résolution de 0,2-0,4 nm, la radiance diffusée par l’atmosphère et réfléchie par la surface terrestre et les nuages au nadir du satellite. La technique de spectroscopie d’absorption optique différentielle (DOAS) permet d’en inverser la concentration columnaire de l’ozone et du dioxyde d’azote atmosphériques. Les travaux décrits dans cette thèse portent d’une part sur la caractérisation du contenu en information géophysique accessible par ce type de sondage atmosphérique, et d’autre part sur la mise au point des méthodes et algorithmes d’inversion propres à la mission GOME. Au cours des premiers chapitres, nous établissons les propriétés pluridimensionnelles de lissage et d’échantillonnage du champ atmosphérique associées à l’observation du rayonnement diffusé. Nous explorons ensuite les problèmes posés par le cycle diurne des oxydes d’azote, ainsi que l’effet des gradients atmosphériques interférant avec le chemin optique. Nous analysons enfin les capacités des réseaux de télédétection de l’Organisation Mondiale Météorologique (OMM) pour le diagnostic des algorithmes et données des systèmes satellitaires. Deux chapitres sont ensuite consacrés à la mise au point des méthodes et algorithmes d’inversion DOAS pour le processeur GDP, qui traite de manière opérationnelle les données radiométriques acquises par GOME. Nous abordons successivement le problème de la dépendance en température des sections efficaces d’absorption, l’évaluation du facteur d’amplification géométrique du chemin optique, l’estimation de la colonne fantôme masquée par les nuages, les effets de l’anomalie magnétique sud-atlantique des ceintures de radiation, et l’incidence du dioxyde d’azote troposphérique. Suit un diagnostic systématique de la mise au point du processeur GDP sur base des données globales fournies par les réseaux de l’OMM, ainsi qu’une critique comparée des algorithmes TOMS de la NASA. Le dernier chapitre décrit la construction de la première climatologie globale du dioxyde d’azote stratosphérique et de ses variations harmoniques. Développée pour nos études du facteur d’amplification géométrique, cette climatologie composite est issue de l’analyse conjointe des jeux de données complémentaires acquis par différents satellites, par des réseaux au sol et par des ballons stratosphériques.

University, Diss., 2002--Heidelberg.

University, Diss., 2003--Heidelberg.