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&lt;1μm) and sub-10μm particles (D&lt;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&lt;10μm, and from 0.87 (±0.10) to 0.93 (±0.02) for D&lt;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.

Dumka U.C.

Kaskaoutis D.G.

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Shiv Nadar University

Atmospheric Environment

This study investigates the modification of the clear-sky spectral diffuse-direct irradiance ratio (DDR) as a function of solar zenith angle (SZA), spectral aerosol optical depth (AOD) and single scattering albedo (SSA). The solar spectrum under various atmospheric conditions is derived with Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS) radiative transfer code, using the urban and continental aerosol models as inputs. The spectral DDR can be simulated with great accuracy by an exponentially decreasing curve, while the aerosol optical properties strongly affect the scattering processes in the atmosphere, thus modifying the DDR especially in the ultraviolet (UV) spectrum. Furthermore, the correlation between spectral DDR and spectral AOD can be represented precisely by an exponential function and can give valuable information about the dominance of specific aerosol types. The influence of aerosols on spectral DDR increases with increasing SZA, while the simulations using the urban aerosol model as input in SMARTS are closer to the measurements taken in the Athens urban environment. The SMARTS simulations are interrelated with spectral measurements and can be used for indirect estimations of SSA. Overall, the current work provides some theoretical approximations and functions that help in understanding the dependence of DDR on astronomical and atmospheric parameters. © 2016 Elsevier B.V..

Atmospheric Research

Dependence of the spectral diffuse-direct irradiance ratio on aerosol spectral distribution and single scattering albedo

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 &lt;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.

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Atmospheric Chemistry and Physics

Scattering and absorption properties of near-surface aerosol over Gangetic-Himalayan region: The role of boundary-layer dynamics and long-range transport

Atmospheric aerosols are key elements in cloud microphysics, the hydrological cycle and climate by serving as cloud condensation nuclei (CCN). The present work analyzes simultaneous measurements of number concentration of CCN (NCCN) and condensation nuclei (NCN) obtained at Nainital, in the Gangetic-Himalayan (GH) region, during the frameworks of Ganges Valley Aerosol Experiment (GVAX), June 2011 to March 2012. The NCCN, NCN and activation (AR=NCCN/NCN) at 0.31-0.33\% S (supersaturation ratio), exhibit significant daily, monthly and seasonal variations within a range of 684-2065cm-3 for NCCN, 1606-4124cm-3 for NCN, and 0.38-0.60 for AR, suggesting large inhomogeneity in aerosol properties, types and sources, which control the degree of aerosol potential activation. Thus, transported aerosols from the Ganges valley and abroad, the boundary-layer dynamics and atmospheric modification processes play an important role in aerosol-cloud interactions over the GH region. The NCN and NCCN show monthly-dependent diurnal variations with afternoon maxima due to transported aerosols from the Ganges valley up to the Himalayan foothills, while the AR is lower during these hours implying lower hygroscopicities or smaller sizes of the transported aerosols. The dependence of NCCN on S is highest during Dec-Mar and lowest during monsoon (Jun-Sep), suggesting different aerosol chemical composition. Comparison between Nainital and Kanpur shows that NCN and NCCN are much lower at Nainital, while the similarity in AR suggests aerosols of similar type, source and chemical composition uplifted from the Ganges valley to the Himalayan foothills. © 2014 Elsevier B.V.