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Abstract: Every year during the winter and pre-monsoon periods most of the Indian peninsula, Bay of Bengal and the adjoining Indian Ocean are affected by a polluted regional haze, known as the South Asian Winter Haze (SAWH). This article is the first of a series of three papers collectively anticipated at the central idea of retrieving aerosol component type and effective radius of the SAWH by incorporating experimental data of ground-based solar radiometer into Mie theory, and validation by collocated in-situ measurements. The work presented in this paper provides an approach to retrieve optical depths and their spectral curvature for SAWH at a tropical coastal station, Visakhapatnam (17.7°N, 83.3°E). A dataset comprised of 92 wintertime atmospheric optical depths are obtained from spectral extinction measurements of direct solar flux by means of a Multi-Wavelength Solar Radiometer during the period of October 2002 to February 2004. The inter-channel correlations established a linear relation and the airmass conditions specific to this dataset suggested a two-component linear model; the stable and variable components of which are analysed to obtain the mentioned optical parameters. Results indicate that the spectral curvature is best represented by a Lorentz model with a central wavelength of 0.40425±0.02945μm, a width of 0.40382±0.10253μm, a constant of 0.92623±0.33793, and a wavelength independent component of 0.95778±0.15678. Resonance-like structure of the obtained spectral curvature resembles wavelength dependent extinction characteristic of absorbing soot particles; thereby supports the key findings from INDOEX campaign. [Copyright &y& Elsevier]

Abstract: Aerosol Robotic Network (AERONET) aerosol optical depth (AOD) and almucantar retrievals (single scattering albedo (SSA) and aerosol size distribution) from 2005–2009 in Beijing and Kanpur are used to analyze differences and similarities in aerosol optical properties over these two regions. The examination of monthly mean AOD (440 nm) shows that maximum and minimum values occurred in summer and winter, respectively, for Beijing, while the range in AOD in Kanpur was lower. Precipitation in both Beijing and Kanpur peaked in summer; however, the columnar water vapor (CWV) exhibited a high correlation with AOD in Beijing (R 2 = 0.79) but had a weak relationship with AOD in Kanpur (R 2 = 0.13). The Angstrom exponent (α, 440–870 nm) generally increased linearly as the fine mode fraction (FMF) of AOD (500 nm) increased for FMF < 90% in both regions, with a high correlation of R 2 > 0.96. However a clear decrease in α for FMF > 90% found in Beijing is not shown distinctly in Kanpur, and is mainly due to the higher aerosol loading in this FMF bin at Beijing (AOD at 440 nm > 2.2) which results in a stronger coagulation of fine mode particles. Bimodal seasonally-averaged size distributions reveals similar aerosol mixtures composed of fine pollution particles and coarse dust particles in both regions. The analysis of spectral SSA as a function of α is emphasized in this paper. The average SSA at 440 nm in both regions shows a similar low dynamic range of ∼0.03 for α < 1.4. The obvious increase in SSA at 440 nm for α > 1.4 in both regions can be attributed to a higher FMF leading to fine mode coagulation. However, the distinctly smaller increase in Kanpur suggests that fine mode aerosols at Beijing are less absorbing than those at Kanpur. The visibly lower SSA at 675 nm at Kanpur compared to that at Beijing for α > 0.4 is due to a larger find-coarse mode separation radius of ∼0.76 μm in Beijing versus a value lower than ∼0.58 μm in Kanpur. Another reason lies in the weaker absorption by fine mode aerosols in Beijing. The distinctly lower near-infrared SSA in Kanpur when α < 1.4, compared to Beijing, may be attributed to lower aerosol concentrations in all α bins, resulting in less aggregation of absorbing black carbon particles on coarse particles in Kanpur. The classification of aerosol properties shows that the AOD accumulation mode in all seasons, even including spring, in Beijing and in post-monsoon and winter seasons in Kanpur can be attributed to fine particle coagulation or hygroscopic growth; during pre-monsoon and monsoon seasons in Kanpur, it is due to coarse mode particle accumulation or cloud contamination. [Copyright &y& Elsevier]

Recent analysis of trends in global short-wave radiation measured with pyranometers in major cities in India support a decrease in solar radiation in many of those cities since 1990. Since direct and diffuse radiation measurements include cloud effects, spring and summer dust and the variable summer monsoon rains, we concentrate in this paper on wintertime (November–February) aerosol optical depth measurements. The aerosol optical depth is derived from cloud-free turbidity measurements beginning in the 1960s and more recent sun photometer direct aerosol optical depth measurements. We compare the sun photometer derived trends with the pyranometer-derived trends using a radiative transfer model. These results are then compared to total ozone mapping spectrometer (TOMS) satellite-derived regional aerosol optical depths from 1980 to 2000. The results show that inclusion of the earlier turbidity measurements helps to establish an increasing regional turbidity trend. However, most of the increasing trend is confined to the larger cities in the Ganges River Basin of India (mainly Calcutta and New Delhi) with other cities showing a much less increase. Regional satellite data show that there is an increasing trend in aerosol off the coast of India and over the Ganges River Basin. The increase over the Ganges River Basin is consistent with population trends over the region during 1980–2000. [Copyright &y& Elsevier]