Inorganic synthesis of Fe–Ca–Mg carbonates at low temperature     

Christopher S. Romanek  Concepcin Jimnez-Lpez  Alejandro Rodriguez Navarro  Monica Snchez-Romn  Nita Sahai  Max Coleman 《Geochimica et cosmochimica acta》2009,73(18):5361-5376

A set of free-drift experiments was undertaken to synthesize carbonates of mixed cation content (Fe, Ca, Mg) from solution at 25 and 70 °C to better understand the relationship between the mineralogy and composition of these phases and the solutions from which they precipitate. Metastable solid solutions formed at 25 °C which are not predicted from the extrapolation of higher temperature equilibrium assemblages; instead, solids formed that were intermediary in chemical composition to known magnesite–siderite and dolomite solid solutions. A calcite–siderite solid solution precipitated at 25 °C, with the percentage of CaCO₃ in the solid being proportional to the aqueous Ca/Fe ratio of the solution, while Mg was excluded from the crystal structure except at relatively high aqueous Mg/Ca and Mg/Fe ratios and a low Ca content. Alternatively, at 70 °C Mg was the predominant cation of the solid solutions. These results are compatible with the hypothesis that the relative dehydration energies of Fe, Ca and Mg play an important role in the formation of mixed cation carbonates in nature.  相似文献   

Thermospheric Meridional Wind Control on Equatorial Scintillations and the Role of the Evening F-Region Height Rise, E?×?B Drift Velocities and F2-Peak Density Gradients     

M. T. A. H. Muella  E. R. de Paula  P. R. Fagundes  J. A. Bittencourt  Y. Sahai 《Surveys in Geophysics》2010,31(5):509-530

The possible role, on L-band scintillation activity, played by the nighttime magnetic meridional component of the thermospheric horizontal neutral winds, the post-sunset F-layer base height, the electrical field pre-reversal enhancement (PRE) and the latitudinal gradients of the F2-layer peak density is analyzed, considering different cases of scintillation occurrence (and their latitudinal extent) during August and September 2002. The meridional winds were derived over low-latitudes from a modified form of the nonlinear time-dependent servo-model. A chain of two scintillation monitors and three digital ionosondes was operational in Brazil and used to collect, respectively, global positioning system signal amplitude scintillation and ionospheric height (h′F; hpF2) and frequency (foF2) parameters. From the overall behavior in the 2 months analyzed, the results suggest that high near sunset upward vertical plasma drifts are conducive for the generation of spread-F irregularities, whereas large poleward meridional winds tend to suppress the development of plasma bubble irregularities and the occurrence of their associated scintillations. Even when generated, a reduced fountain effect, due to weak electric field PRE, acts for the bubbles to be expanded less effectively to higher latitudes. The results also reveal that high F-layer base and peak heights (at equatorial and off-equatorial latitudes), and intense gradients in the F2-peak density between the dip equator and the equatorial anomaly crests, are favorable conditions for the generation of F-region irregularities and increased scintillation activity. Other distinct features of the controlling factors in the cases of occurrence and non-occurrence of equatorial scintillations are presented and discussed.  相似文献   

Longitudinal variability of the equatorial counter electrojet during the solar cycle 24     

Soares  Gabriel  Yamazaki  Yosuke  Matzka  Jürgen  Pinheiro  Katia  Stolle  Claudia  Alken  Patrick  Yoshikawa  Akimasa  Uozumi  Teiji  Fujimoto  Akiko  Kulkarni  Atul 《Studia Geophysica et Geodaetica》2019,63(2):304-319

Studia Geophysica et Geodaetica - Ground and space-based geomagnetic data were used in the investigation of the longitudinal, seasonal and lunar phase dependence of the equatorial counter...  相似文献   

An engineering geological appraisal of the Lakhwar Dam, Garhwal Himalaya, India     

Sanjeev Sharma  Tarun Raghuvanshi  Atul Sahai 《Engineering Geology》1999,53(3-4):381-398

A 204 m high solid concrete gravity dam is proposed across the River Yamuna in Garhwal Himalaya, India. It will be located on dolerite rocks which have been intruded into the slates of Chandpur Formation. The present study includes the evaluation of the dam foundation by means of drifts, drill holes, water pressure tests and abutment slope stability studies. The water pressure test indicate the necessity of providing a grout curtain below the dam foundation. The analysis of the dam abutments for stability using the Limit equilibrium method indicates that the right abutment slope is kinematically unstable for plane failure mode. The plane failure analysis of the right abutment slope was carried out by modifying the Hoek and Bray (1981, Rock Slope Engineering, 3rd ed., Institute of Mining and Metallurgy, London) technique of plane failure analysis. The analysis reveals that right abutment slope may become unstable during the stripping operation. Based upon the analysis a safe cut slope design for the abutments have been suggested. Subsurface exploration by means of cross drift and drill holes has indicated a sheared contact of slate and dolerite in the foundation area. To avoid the settlement of the dam along this shear zone precautionary measures are suggested.  相似文献   

Climate Change: A Case Study Over India   总被引：1，自引：0，他引：1  

A. K. Sahai 《Theoretical and Applied Climatology》1998,61(1-2):9-18

Summary A brief account of various causes of climate change in recent decades and climate change trends in the Indian region is presented. It is of great importance to determine the influence of human activities on the likely climate change during recent decades. Local temperature is one of the major climatic elements to record the changes in the atmospheric environment caused by industrialization and urbanization. It is mentioned in the literature that there is either a cooling tendency or cessation of warming after the late 1950s at most of the Indian industrial cities. A case study of Nagpur, a centrally located city in India, is done to understand and the possible cause of cooling. Nagpur is the only city in India for which a long-term record of temperature, for urban (Mayo Hospital) and relatively suburban (Sonegaon Airport) area, is available. The study of the diurnal asymmetry in maximum and minimum temperatures indicates that the role of suspended particulate matter dominates over that of increasing greenhouse gases. Received April 15, 1998  相似文献   

Nightglow OH (8, 3) band intensities and rotational temperature at 23°S     

H. Takahashi  B.R. Clemesha  Y. Sahai 《Planetary and Space Science》1974,22(9):1323-1329

The OH (8, 3) band airglow emission has been observed over 1 year at a latitude of 23°S. The average band intensity observed was 385 Rayleighs with a nocturnal range typically less than 100 R. The nocturnal variation in rotational temperature was usually less than 10°K, and the mean temperature was 179°K. The nocturnal variation of intensity is usually uncorrelated with that of the rotational temperature. Time average values of these parameters do, however, show some correlation. On some occasions large post-twilight and pre-dawn intensity enhancements are observed.  相似文献   

Recent changes in the spatio-temporal characteristics of monsoon intraseasonal oscillations     

Joseph  Susmitha  Sahai  A. K.  Shabu  Hindhiya  Chattopadhyay  R.  Kaur  Manpreet 《Theoretical and Applied Climatology》2022,147(1-2):251-264

Theoretical and Applied Climatology - Monsoon intraseasonal oscillations (MISOs) define a significant proportion of intraseasonal variability of the Indian summer monsoon. In the backdrop of...  相似文献   

North Indian heavy rainfall event during June 2013: diagnostics and extended range prediction     

Susmitha Joseph  A. K. Sahai  S. Sharmila  S. Abhilash  N. Borah  R. Chattopadhyay  P. A. Pillai  M. Rajeevan  Arun Kumar 《Climate Dynamics》2015,44(7-8):2049-2065

  相似文献   

Comparison of results of BIS and GSI guidelines on macrolevel landslide hazard zonation — A case study along highway from Bhalukpong to Bomdila,West Kameng district,Arunachal Pradesh     

C. D. Singh  Atul Kohli  Pankaj Kumar 《Journal of the Geological Society of India》2014,83(6):688-696

This paper compares the findings of macrolevel landslide hazard zonation carried out along the highway from Bhalukpong to Bomdila, West Kameng district, Arunachal Pradesh following GSI and BIS guidelines. The map resulted from the GSI guideline shows that 69.31% of the faceted area falls under the Low Hazard Zone (LHZ) while 17.69%, 7.31%, 5.03% and 0.65% of the area are in Moderate Hazard Zone (MHZ), High Hazard Zone (HHZ), Very Low Hazard Zone (VLHZ) and Very High Hazard Zone (VHHZ) respectively. Correlation between the landslide incidences and different hazard zones reveals that maximum failure percentage is in VHHZ and it is followed by HHZ, MHZ and LHZ. The second map resulting from BIS guideline reveals that 45.77% of the faceted area falls under MHZ while 41.39%, 11.52% and 1.29% of the area are in HHZ, LHZ and VHHZ respectively. Not a single facet falls in VLHZ.With regard to failure percentage VHHZ experiences 50%, while that of HHZ, MMH and LHZ is roughly 11.5% each. In the study area, the landslide hazard zonation map resulting from GSI guideline broadly conforms to field condition. It may be due to the fact that the study area is along the road corridor where slope cutting and landslides are very common and GSI guideline considers both the slope cutting and landslide parameters, while it is not so in the case of BIS guidelines. However, a final conclusion can be drawn after carrying out such studies in different geological settings.  相似文献   

Black carbon and chemical characteristics of PM<Subscript>10</Subscript> and PM<Subscript>2.5</Subscript> at an urban site of North India     

Suresh Tiwari  Atul K. Srivastava  Deewan S. Bisht  Tarannum Bano  Sachchidanand Singh  Sudhamayee Behura  Manoj K. Srivastava  D. M. Chate  B. Padmanabhamurty 《Journal of Atmospheric Chemistry》2009,62(3):193-209

The concentrations of PM₁₀, PM_2.5 and their water-soluble ionic species were determined for the samples collected during January to December, 2007 at New Delhi (28.63° N, 77.18° E), India. The annual mean PM₁₀ and PM_2.5 concentrations (± standard deviation) were about 219 (± 84) and 97 (±56) μgm⁻³ respectively, about twice the prescribed Indian National Ambient Air Quality Standards values. The monthly average ratio of PM_2.5/PM₁₀ varied between 0.18 (June) and 0.86 (February) with an annual mean of ∼0.48 (±0.2), suggesting the dominance of coarser in summer and fine size particles in winter. The difference between the concentrations of PM₁₀ and PM_2.5, is deemed as the contribution of the coarse fraction (PM_10−2.5). The analyzed coarse fractions mainly composed of secondary inorganic aerosols species (16.0 μgm⁻³, 13.07%), mineral matter (12.32 μgm⁻³, 10.06%) and salt particles (4.92 μgm⁻³, 4.02%). PM_2.5 are mainly made up of undetermined fractions (39.46 μgm⁻³, 40.9%), secondary inorganic aerosols (26.15 μgm⁻³, 27.1%), salt aerosols (22.48 μgm⁻³, 23.3%) and mineral matter (8.41 μgm⁻³, 8.7%). The black carbon aerosols concentrations measured at a nearby (∼300 m) location to aerosol sampling site, registered an annual mean of ∼14 (±12) μgm⁻³, which is significantly large compared to those observed at other locations in India. The source identifications are made for the ionic species in PM₁₀ and PM_2.5. The results are discussed by way of correlations and factor analyses. The significant correlations of Cl⁻, SO₄²⁻, K⁺, Na⁺, Ca²⁺, NO₃⁻ and Mg²⁺ with PM_2.5 on one hand and Mg²⁺ with PM₁₀ on the other suggest the dominance of anthropogenic and soil origin aerosols in Delhi.  相似文献