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Dumka U.C.,Aryabhatta Research Institute of Observational science | Kaskaoutis D.G.,Shiv Nadar University | Srivastava M.K.,Banaras Hindu University | Devara P.C.S.,Amity University
Atmospheric Chemistry and Physics | Year: 2015

Light scattering and absorption properties of atmospheric aerosols are of vital importance for evaluating their types, sources and radiative forcing. This is of particular interest over the Gangetic-Himalayan (GH) region due to uplift of aerosol from the plains to the Himalayan range, causing serious effects on atmospheric heating, glaciology and monsoon circulation. In this respect, the Ganges Valley Aerosol Experiment (GVAX) was initiated in Nainital from June 2011 to March 2012 with the aim of examining the aerosol properties, source regions, uplift mechanisms and aerosol-radiation-cloud interactions. The present study examines the temporal (diurnal, monthly, seasonal) evolution of scattering (σsp) and absorption (σap) coefficients, their wavelength dependence, and the role of the Indo-Gangetic plains (IGP), boundary-layer dynamics (BLD) and long-range transport (LRT) in aerosol evolution via the Atmospheric Radiation Measurement Mobile Facility. The analysis is separated for particles 10μm and <1 μm in diameter in order to examine the influence of particle size on optical properties. The σsp and σap exhibit a pronounced seasonal variation between the monsoon low and post-monsoon (November) high, while the scattering wavelength exponent exhibits higher values during the monsoon, in contrast to the absorption Ångström exponent which maximizes in December-March. The elevated-background measuring site provides the advantage of examining the LRT of natural and anthropogenic aerosols from the IGP and southwest Asia and the role of BLD in the aerosol lifting processes. The results reveal higher aerosol concentrations at noontime along with an increase in mixing height, suggesting influence from IGP. The locally emitted aerosols present higher wavelength dependence of the absorption in October-March compared to the rather well-mixed and aged transported aerosols. Monsoon rainfall and seasonally changing air masses contribute to the alteration of the extensive and intensive aerosol properties.

Dumka U.C.,Aryabhatta Research Institute of Observational science | Kaskaoutis D.G.,Shiv Nadar University
Atmospheric Environment | Year: 2014

Regular near-surface measurements of aerosol scattering and absorbing coefficients, along with other aerosol and meteorological parameters, were performed during June 2011 to March 2012 at Nainital, central Himalayas in the frameworks of Ganges Valley Aerosol Experiment (GVAX). The spectral scattering (0.45, 0.55 and 0.70μm) and absorption (0.467, 0.53 and 0.66μm) coefficients exhibit increased values in November, mostly affected by the biomass-burning aerosols in Indo-Gangetic Plains. Both parameters are considered for fine (D<1μm) and sub-10μm particles (D<10μm) revealing the influence of particle size in aerosol properties. Furthermore, estimations of spectral (0.467, 0.55 and 0.66μm) single scattering albedo (SSA) and aerosol radiative forcing efficiency (ARFE) at 0.55μm were performed focussing on determination of the role of particle size in spectral SSA and climate implications. The results show relatively high SSA values ranging from 0.90 (±0.09) to 0.95 (±0.01) for D<10μm, and from 0.87 (±0.10) to 0.93 (±0.02) for D<1μm particles, on monthly basis, suggesting large heterogeneity in the aerosol sources. The SSA for the sub-micron aerosols decreases with wavelength in the majority of the cases, in contrast to the increase for the super-micron particles suggesting different source apportionment for the particle groups. The ARFE at the top of the atmosphere is found to range from -3 to -20Wm-2 with a mean of ~-17Wm-2 for both particle-size groups; however, during the June-October period, the ARFE for the sub-10μm particles is found to be more negative than that for the fine aerosols. © 2014 Elsevier Ltd.

Medhi B.J.,Aryabhatta Research Institute of Observational science | Tamura M.,Japan National Astronomical Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2013

Interstellar polarimetric data of the six open clusters Hogg 15, NGC 6611, NGC 5606, NGC 6231, NGC 5749 and NGC 6250 have been used to estimate the membership probability for the stars within them. For proper-motion member stars, the membership probability estimated using the polarimetric data is in good agreement with the proper-motion cluster membership probability. However, for proper-motion non-member stars, the membership probability estimated by the polarimetric method is in total disagreement with the proper-motion cluster membership probability. The inconsistencies in the determined memberships may be because of the fundamental differences between the two methods of determination: one is based on stellar proper motion in space and the other is based on selective extinction of the stellar output by the asymmetric aligned dust grains present in the interstellar medium. The results and analysis suggest that the scatter of the Stokes vectors q (per cent) and u (per cent) for the proper-motion member stars depends on the interstellar and intracluster differential reddening in the open cluster. It is found that this method could be used to estimate the cluster membership probability if we have additional polarimetric and photometric information for a star to identify it as a probable member/non-member of a particular cluster, such as the maximum wavelength value (λmax), the unit weight error of the fit (σ1), the dispersion in the polarimetric position angles (∈̄), reddening (E(B-V)) or the differential intracluster reddening (ΔE(B-V)). This method could also be used to estimate the membership probability of known member stars having no membership probability as well as to resolve disagreements about membership among different proper-motion surveys. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Pandey J.C.,Aryabhatta Research Institute of Observational science | Singh K.P.,Tata Institute of Fundamental Research
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present an analysis of seven flares detected from five RSCVn-type binaries (UZLib, σGem, λAnd, V711Tau and EIEri) observed with XMM-Newton observatory. The quiescent state X-ray luminosities in the energy band of 0.3-10.0keV of these stars were found to be 1030.7-30.9ergs-1. The exponential decay time in all the sample of flares range from ∼1 to 8h. The luminosity at peak of the flares in the energy band of 0.3-10.0keV was found to be in the range of 1030.8-1031.8ergs-1. The great sensitivity of the XMM-European Photon Imaging Camera instrument allowed us to perform time resolved spectral analysis during the flares and also in the subsequent quiescent phases. The derived metal abundances of coronal plasma were found to vary during the flares observed from σGem, V771Tau and EIEri. In these flares elemental abundances found to be enhanced by factors of ∼1.3-1.5 to the quiescent states. In most of the flares, the peak temperature was found to be more than 100MK, whereas emission measure increased by factors of 1.5-5.5. Significant sustained heating was present in the majority of flares. The loop lengths (L) derived for flaring structure were found to be in the order of 1010-11cm and are smaller than the stellar radii (R{black star}), i.e. L/R{black star}≲ 1. The flare from σGem showed a high and variable absorption column density during the flare. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Kumar R.,Aryabhatta Research Institute of Observational science | Naja M.,Aryabhatta Research Institute of Observational science | Venkataramani S.,Physical Research Laboratory | Wild O.,Lancaster University
Journal of Geophysical Research: Atmospheres | Year: 2010

Surface ozone measurements have been made for the first time at Nainital (29.37°N, 79.45°E, 1958 m amsl), a high-altitude site in the central Himalayas, between October 2006 and December 2008. Diurnal variations in ozone do not show the daytime photochemical build-up typical of urban or rural sites. The seasonal variation shows a distinct ozone maximum in late spring (May; 67.2 ± 14.2 ppbv) with values sometimes exceeding 100 ppbv and a minimum in the summer/monsoon season (August; 24.9 ± 8.4 ppbv). Springtime ozone values in the central Himalayas are significantly higher than those at another high-altitude site (Mt. Abu) in the western part of India. Seasonal variations in ozone and the processes responsible for the springtime peak are studied using meteorological parameters, insolation, spatial and temporal classifications of air mass trajectories, fire counts, and simulations with a chemical transport model. Net ozone production over the Northern Indian Subcontinent in regionally polluted air masses is estimated to be 3.2 ppbv/day in spring but no clear build-up is seen at other times of year. Annual average ozone values in regionally polluted air masses (47.1 ± 16.7 ppbv) and on high insolation days (46.8 ± 17.3 ppbv) are similar. Background ozone levels are estimated to be 30-35 ppbv. Regional pollution is shown to have maximum contribution (16.5 ppbv) to ozone levels during May-June and is about 7 ppbv on an annual basis, while the contribution of long-range transport is greatest during January-March (8-11 ppbv). The modeled stratospheric ozone contribution is 2-16 ppbv. Both the trajectory analysis and the model suggest that the stratospheric contribution is 4-6 ppbv greater than the contribution from regional pollution. Differences in the seasonal variation of ozone over high-altitude sites in the central Himalayas (Nainital) and western India (Mt. Abu) suggest diverse regional emission sources in India and highlight the large spatial and temporal variability in ozone over the Indian region. Copyright 2010 by the American Geophysical Union.

Srivastava A.K.,Indian Institute of Tropical Meteorology | Singh S.,National Physical Laboratory India | Pant P.,Aryabhatta Research Institute of Observational science | Dumka U.C.,Aryabhatta Research Institute of Observational science
Atmospheric Science Letters | Year: 2012

The characteristics of aerosol black carbon (BC) were studied at two different climatic regimes, i.e. Delhi and Manora Peak during winter and spring of 2007. Spring BC was found to be ~59% lower at Delhi and ~23% higher at Manora Peak than their corresponding winter BC. Diurnal BC variation showed two enhanced peaks at Delhi (morning and night) whereas a single late afternoon peak at Manora Peak. Delhi BC showed a clear correlation with prevailing winds whereas no correlation was observed at Manora Peak. The major contribution of BC at Manora Peak can be expected from biomass burning while at Delhi fossil fuel dominates. © 2012 Royal Meteorological Society.

Kumar R.,U.S. National Center for Atmospheric Research | Barth M.C.,U.S. National Center for Atmospheric Research | Pfister G.G.,U.S. National Center for Atmospheric Research | Naja M.,Aryabhatta Research Institute of Observational science | And 2 more authors.
Atmospheric Chemistry and Physics | Year: 2014

The impact of a typical pre-monsoon season (April-June) dust storm event on the regional aerosol optical properties and radiation budget in northern India is analyzed. The dust storm event lasted from 17 to 22 April 2010 and the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) estimated total dust emissions of 7.5 Tg over the model domain. Both in situ (AERONET-Aerosol Robotic Network) and satellite observations show significant increase (> 50%) in local to regional scale aerosol optical depth (AOD) and decrease (> 70%) in the Ångström exponent (α) during this period. Amongst the AERONET sites in this region, Kanpur was influenced the most, where the AOD reached up to 2.1 and the α decreased to-0.09 during the dust storm period. The WRF-Chem model reproduced the spatial and temporal distributions of dust plumes and aerosol optical properties but generally underestimated the AOD. The average MODIS and WRF-Chem AOD (550 nm) values in a subregion (70-80 E, 25-30 N) affected the most by the dust storm are estimated as 0.80 ± 0.30 and 0.68 ± 0.28, respectively. Model results show that dust particles cool the surface and the top of the atmosphere, but warm the atmosphere itself. The radiative perturbation due to dust aerosols averaged over the subregion is estimated as-2.9 ± 3.1 W m-2 at the top of the atmosphere, 5.1 ± 3.3 W m-2 in the atmosphere and-8.0 ± 3.3 W m-2 at the surface. The simulated instantaneous cooling under the dust plume was much higher and reached-227 and-70 W m-2 at the surface and the top of the atmosphere, respectively. The impact of these radiative perturbations on the surface energy budget is estimated to be small on a regional scale but significant locally. © 2014 Author (s).

Sharma M.,Raman Research Institute | Nath B.B.,Raman Research Institute | Chand H.,Aryabhatta Research Institute of Observational science
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2013

We report a correlation between velocity offset (β = v/c) of strong MgII absorption systems and the bolometric luminosity (Lbol) of quasars in SDSS-DR7. We find that β shows a powerlaw increase with Lbol, with a slope 1/4. We find that such a relation of - with Lbol is expected for outflows driven by scattering of black hole radiation by dust grains, and which are launched from the innermost dust survival radius. Our results indicate that a significant fraction of the strong MgII absorbers, in the range of β = 0-0.4, may be associated with the quasars themselves. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Bose S.,Aryabhatta Research Institute of Observational science | Kumar B.,Aryabhatta Research Institute of Observational science
Astrophysical Journal | Year: 2014

Type IIP supernovae (SNe) are recognized as independent extragalactic distance indicators; however, keeping in mind the diverse nature of their observed properties as well as the availability of good quality data, more and newer events need to be tested for their applicability as reliable distance indicators. We use early photometric and spectroscopic data of eight Type IIP SNe to derive distances to their host galaxies by using the expanding photosphere method (EPM). For five of these, the EPM is applied for the first time. In this work, we improved EPM application by using SYNOW estimated velocities and by semi-deconvolving the broadband filter responses while deriving color temperatures and blackbody angular radii. We find that the derived EPM distances are consistent with that derived using other redshift-independent methods. © 2014. The American Astronomical Society. All rights reserved.

Sinha K.,Aryabhatta Research Institute of Observational science
Solar Physics | Year: 2015

Following our predictions on detectability, very weak lines of the SH molecules have been reported as identified in the photospheric spectrum of the Sun. This could be the first detection of a mercapto radical in the solar spectrum, thus placing confidence in the solar abundance of sulfur. The observation additionally tested the theoretical band oscillator strength that was used. Because sunspots are cooler than the photosphere, they host a large number of molecular species. However, owing to photospheric radiation scattered into the observed umbra, no lines of SH were detected in the sunspot spectrum, where they are expected to show equivalent widths larger than those in the photosphere. Detection of weak lines in the photosphere coupled with a non-detection of the relatively strong same lines in the sunspot spectrum might raise doubts about the detection itself. We investigated the choice of photospheric and sunspot models and the microturbulence values that indicate that the predicted sunspot lines are about half as intense as reported before, and the lines remain detectable. © 2015, Springer Science+Business Media Dordrecht.

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