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.

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.

Water pollution is a major concern in vast countries such as India and other developing nations. Several methods of water purification have been practiced since many decades, Semiconductor photocatalysis is a promising technique, for photodegradation of various hazardous chemicals that are encountered in waste waters. The great significance of this technique is that, it can degrade (detoxify) various complex organic chemicals, which has not been addressed by several other methods of purification. This unique advantage made this field of research to attract many investigators particularly in latter eighties and after. This thesis incorporates the studies on the various semiconductor photocatalysts that have been employed for the detoxification purposes. The fundamental principles involved in the photoelectrochemistry, reactions at the interface (solid - liquid or solid - gas) and photocatalytic reactions on fine particles are briefed. General nature and size quantization in semiconductor particles, photocatalytically active semiconductors, TiCh and ABO3 systems, chemical systems and modifications for solar energy conversions are brought out in the introduction chapter besides giving brief description about photocatalytic mineralization of water pollutants with mechanism involved, formation of reactive species and the factors influencing photomineralization reactions. Scope of the present work is given at the end of the first chapter. Second chapter deals with the materials used for the preparation of photocatalyst, preparative techniques, methods of analysis, instruments employed for the photodegradation experiments and a brief description of material characterization methods such as X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, differential thermal analysis, optical absorption spectro photometry, Electron paramagnetic resonance (EPR), and gas chromatograph - mass spectroscopy (GC - MS). Various preparative routes such as wet chemical and hydrothermal methods for obtaining TiO2 (both rutile and anatase forms), BaTiOs and SrTiO3 fine particles and the chemical analysis of their constituents have been described in brief. Third chapter presents the results of materials characterization. T1O2 (rutile and anatase), BaTiO3 and SrTiO3 have been characterized separately using various techniques. Different routes of obtaining the photocatalyst fine particles, heat treatment at various temperature ranges, experimental procedures and the results of characterization are brought out in this chapter. Fourth and fifth chapters present the details of degradation studies carried out on the photomineralization of chlorophenol, trichloroethylene and formaldehyde. Studies include photodegradation of the pollutants with different catalysts varying experimental conditions to check the effects of change in concentration of pollutants, oxidizer, pH, surface hydroxylation, etc. The most favorable conditions for the complete mineralization of the pollutants have been studied. In case of TiO2, anatase form has shown greater photoactivity when compared to rutile and complete mineralization of chlorophenols has been achieved at low pollutant concentrations, neutral pH, with H2O2 and UV illumination. Retarding effects of surface hydroxylation and the formation of peroxotitanium species during photodegradation have been presented. TCE and HCHO degradation with BaTiO3/SrTiO3 has been studied. Photocatalyst heat-treated at 1100°G-1300°C is found to be highly active in combination with H2O2 as electron scavenger. HCHO is not getting degraded to its completeness in aqueous conditions owing to the strong competition in surface adsorption posed by H2O molecules. Vapour-solid phase reaction however gave good results in the detoxification of HCHO via disproportionation. Summary and conclusions are given at the end of the thesis.