https://etd.iisc.ac.in/handle/2005/35 2026-04-24T05:04:12Z 2026-04-24T05:04:12Z Pramanik, Chirantan https://etd.iisc.ac.in/handle/2005/5214 2021-08-06T06:06:59Z

Ab initio Quantum Chemical Studies on Kinetic Fractionation during the analysis of Carbonates for the Clumped Isotope Thermometry Pramanik, Chirantan Stable and clumped isotopic compositions of molecules and minerals carry the signatures of temperatures and other physical and chemical conditions of the time of their formation. Isotopic compositions of such precipitate are preserved in carbonate that provides information about the climate through geological time. The carbonates formed in the aquatic environment serve as an archive for past climate and temperature reconstruction. In nature, sedimentary carbonate rocks are primarily composed of minerals calcite (CaCO3) and dolomite (CaMg(CO3)2). They comprise ~20% of the surface sedimentary rocks on Earth and are also found in other planets, with the oldest being ~3.5 Ga, almost as primitive as the Earth itself. The majority of them form in the aquatic environments that ranged from the warm, sunlit shallow seafloor to the cold, perpetually dark, deep ocean. Carbonate rocks are also traced in the terrestrial and extraterrestrial environment, which includes terrestrial spring waters, rivers and lakes, caves, soils, and meteorites from outer space. Ab initio quantum chemical simulations are used to calculate the extent of equilibrium and kinetic isotope fractionations, which provided additional theoretical references in clumped isotope paleothermometry. Ab initio quantum chemical simulations provide the inputs in terms of vibration frequencies and thermal energies of the optimized stable molecules and the transition state structures for the partition function calculations of equilibrium and kinetic fractionations. Ab initio calculations using density functional theory (DFT) are performed using 'Gaussian09' computational chemistry packages. Equilibrium constant and partition function calculations are performed using scripts in Matlab and Python. Though the clumped isotope proxy is based on the temperature dependence of 13C-18O bonding preference in the mineral lattice, which is captured in the product CO2, there is limited information on the phosphoric acid reaction mechanism and the magnitude of clumped isotopic fractionation (mass 63 in CO32- to mass 47 in CO2) during the acid digestion. We explored the reaction mechanism for phosphoric acid digestion of calcite using first-principles density functional theory. We identified the transition state structures for each protonation reaction involving different isotopologues and used the corresponding vibrational frequencies in reduced partition function theory to estimate Δ47 acid fractionation. We showed that the acid digestion reaction, which results in the formation of CO2 enriched with 13C-18O bonds, commences with the protonation of calcium carbonate in the presence of water. Our simulations yielded a relationship between Δ47 acid fractionation for calcite and reaction temperature as Δ47 acid fractionation in calcite = -0.30175 + 0.57700*105/ T2 - 0.10791* (105/ T2)2, with T varying between 298.15 K and 383.15 K. This relationship shows a higher slope (Δ47 acid fractionation vs. 1/T2 curve) than previous studies based on the H2CO3 model. The theoretical estimates from the present and earlier studies encapsulate experimental observations from both 'sealed vessel' and 'common acid bath' acid digestion methods from literature. Previous theoretical models for determining clumped isotopic fractionation in product CO2 during acid digestion of carbonates are independent of the cations present in the carbonate lattice. Hence further study is required to understand the cationic effect. We studied the acid reaction mechanism and calculated the acid fractionation factor for dolomite using partition functions and vibrational frequencies obtained for the transition state structure, and determined the effect of cations on the acid fractionation factor. Theoretically obtained acid fractionation factor for dolomite can be expressed as Δ47 acid fractionation in dolomite = -0.28563 + 0.49508*(105/ T2) - 0.08231* (105/ T2)2 for a temperature range between 278.15 K and 383.15 K. The theoretical slope of the dolomite-acid digestion curve is lower than that of the calcite-acid digestion curve obtained using the identical reaction mechanism. Our theoretical slope is consistent with the result from the common acid bath experiments but higher than the slope obtained in the experimental study using the sealed vessel and modified sealed vessel method and previous theoretical study using the H2CO3 model. Transition state structure, obtained in our study, includes the cations present in the carbonate minerals and provides distinct acid fractionation factors for calcite and dolomite. The observed gentler slope of theoretically calculated dolomite-acid digestion curve compared to calcite is expected considering the stronger Mg-O bond. In the present theoretical study, we provided the acid digestion reaction mechanism based on the protonation and determined a quantitative acid digestion correction factor for a range of reaction temperature for the experimental protocols where the product CO2 is immediately removed from the system, and there is not enough chance of post-digestion isotope exchanges. We suggest using appropriate acid digestion correction factors depending on the experimental techniques used for acid digestion of carbonates.

Sindhu, Kapil Dev https://etd.iisc.ac.in/handle/2005/937 2020-09-28T05:43:01Z 2010-11-02T00:00:00Z

Aerosol Absorption And Source Characteristics Over Different Environments Sindhu, Kapil Dev Extremely fine liquid droplets or solid particles, those remain suspended in the air, are known as aerosols. They are produced by natural sources and anthropogenic activities. Several types of aerosols produced by different processes are present in the atmosphere and every type of aerosol species exhibit different types of physical and chemical properties. Though making up only a small fraction of atmospheric mass aerosols are capable of altering Earth’s climate by scattering and absorbing incoming solar radiation and absorbing outgoing radiation. Adding to the complexity, they can act as cloud condensation nuclei and modify cloud properties. Major objective of this thesis is to study absorption due to aerosols and factors controlling the absorbing efficiency of aerosols over various environments. We have demonstrated a new method to quantify the organic carbon in terms of optical depth. Our studies demonstrate large “anomalous” absorption in the UV wavelength region over several regions. Further investigations revealed that a major part of this additional absorption is contributed by organic carbon aerosols and partly due to dust aerosols. We show that it is possible to discriminate UV absorption by dust and organic carbon by making use of the fact that dust aerosols are much larger in size compared to organic aerosols. Examination of aerosol optical depth values measured at cities south of Saharan desert indicates high short wave absorption due to coarse mode aerosols probably dust. Even at low values of Angstrom wavelength exponent, which indicates the presence of large aerosols (e.g., dust over land), absorption was found reasonably high compared to that of pure dust. On the other hand, over regions in the northern part of the Sahara close to Europe, short wave absorption was found to be lower. The enhanced short wave absorption due to coarse particles is unexpected. It appears that the deposition of anthropogenic aerosols such as black carbon over dust aerosols is likely to be responsible for this enhanced short wave absorption. This is a typical example of how anthropogenic aerosols can modify the properties of natural aerosols. We have carried out source apportionment using backward air parcel trajectories by applying k-means method of clustering and obtained various aerosol terms corresponding to each cluster. We have selected three island sites and one site in the middle of Saharan desert for this study. High aerosol radiative forcing values are observed even over remote island locations. Our study demonstrates the role of aerosols transported from the main land in influencing the aerosol environment even over remote marine regions.

2010-11-02T00:00:00Z Raju, Jampana V S https://etd.iisc.ac.in/handle/2005/437 2020-10-09T05:31:52Z 2009-03-19T11:43:50Z

Air-Sea Flux Measurements Over The Bay Of Bengal During A Summer Monsoon Raju, Jampana V S Majority of the rain producing monsoon systems in India form or intensify over the Bay of Bengal and move onto the land. We expect the air-sea interaction to be a crucial factor in the frequent genesis and intensification of monsoon systems over the Bay. Knowledge of air-sea fluxes is essential in determining the air-sea interactions. However, the Bay remains a poorly monitored ocean basin and the state of the near surface conditions during the monsoon months remains to be studied in detail. For example, we do not know yet which among the various flux formulae used in the General circulation models are appropriate over the Bay since there are no direct measurements of surface fluxes here during the peak monsoon months. The present thesis aims towards filing that gap. In this thesis fluxes were computed using the Bulk method, Inertial dissipation method and direct covariance method. The flux comparisons were reasonable during certain flow conditions which are clearly identified. When these conditions are not met the differences among the fluxes from these methods can be larger than the inherent uncertainties' in the methods. Stratification, flow distortion and averaging time are the key variables that give rise to the differences in the fluxes. It is found that there are significant differences in the surface flux estimates computed from different atmospheric General Circulation Model bulk parameterization schemes. In this thesis, the flow gradients are estimated by taking advantage of the natural pitch and roll motion of the ship. A attempt is made to gain insight into the flow distortion and its influence on the fluxes. In our analysis it is found that the displacement of the streamlines is an important component in quantifying flow distortion.

2009-03-19T11:43:50Z Guruprasad, Asha https://etd.iisc.ac.in/handle/2005/8877 2026-03-13T10:10:36Z

Analysis of tropical convection using satellite data Guruprasad, Asha Organised convection in the tropics occurs primarily in the Intertropical Convergence Zone (ITCZ). Many of the important phenomena in the tropics such as El Niño or the monsoons involve large displacements of the ITCZ. Understanding the nature of the space-time variations of the ITCZ is therefore an important problem in tropical meteorology. The work reported in this thesis is aimed at increasing our knowledge of the nature of the intraseasonal and interannual variations of the ITCZ using digitised satellite data. Major features of the intraseasonal variations of the ITCZ over the Indian longitudes during the summer monsoon have been brought out by earlier studies of satellite imagery. These are: Oscillations in the intensity of the continental ITCZ between active spells and weak spells or breaks. A bimodal distribution of convection with latitude due to the presence of an ITCZ over the equatorial Indian Ocean as well as one over the continent. Poleward propagations of the ITCZ from the equatorial Indian Ocean onto the continent at intervals of 2–6 weeks. In order to study whether these features are observed over other parts of the tropics, an objective method to delineate the ITCZ from the 2.5° daily data on outgoing longwave radiation (OLR) and albedo has been developed (Chapter 3). The method involves identification of the grid points with a large fraction of deep convective clouds by the imposition of a bispectral threshold (185 W/m² for daytime OLR and 0.5 for albedo). Such selected grid points are then filtered to retain only the large-scale convection. The variations of the ITCZ deduced by this method compare well with that obtained by the subjective analysis of satellite imagery (Sikka and Gadgil, 1980). The region of convection delineated is very similar to that obtained by the analysis of the equivalent blackbody temperature data on pixel-scale (Murakami, 1983). The intraseasonal variation of the ITCZ over different parts of the tropics is analysed in Chapter 4. It is found that oscillation between active and weak spells is a basic feature of the ITCZ. Bimodality is found only over the Asian monsoon zone. Poleward propagations are seen over the Asian winter monsoon regions only in some years. The interannual variation of the ITCZ has been studied in Chapter 5. Since rainfall is known to be well correlated with the presence of highly reflective clouds in the tropics, we expected the variation in the number of convective days in a month (counted as the number of days on which the threshold criteria for OLR and albedo are satisfied) to be related to the variation in the rainfall over the Indian region. However, the interannual variation of number of convective days during the period 1974–1985 was found to be not completely consistent with that of rainfall. Therefore, the variations of a more basic variable, viz., monthly mean OLR were analysed. This analysis of the variation of the OLR and rainfall over the Indian region suggests the possibility of a systematic bias in the OLR (both daytime as well as daily-averaged) with consistently lower values during 1982–85 as compared with 1974–78. It is also shown that the bias can be removed by an empirical formula akin to the one used in earlier corrections.