The aerosol size distribution, particularly the number and mass distributions, plays a crucial role in understanding changes in optical properties due to hygroscopic growth, which affects visibility and radiative forcing on a regional scale. The Indo-Gangetic Plain (IGP), including National Capital Region (NCR) of Delhi, experiences severe fog and haze with reduced visibility during the post-monsoon to winter months (October-February) every year. This study reports aerosol mass size distribution over Delhi during a winter fog campaign (December 15, 2015-February 15, 2016) using a ground-based optical particle counter. The fine and coarse mode aerosols were contributed to similar to 85% and 15% to the total aerosol mass concentration during the campaign period. The characteristic changes in aerosol size distribution, effective radius, and the influence of meteorological factors, particularly relative humidity (RH) and temperature, under three visibility conditions: Vis-1 (1200 m) were investigated. Fine-mode aerosols accounted for similar to 85 % of the total aerosol mass, with their concentration increasing by a factor of 3.7 during Vis-1 and 2.3 during Vis-2 compared to Vis-3, when the effective radius of aerosol was lowest (R-eff: 0.44 mu m). Fine particle concentrations showed a positive correlation with RH (R = 0.35) and a negative correlation with visibility (R = -0.65), suggesting that the high RH and fine-mode aerosols contribute to fog formation and reduced visibility in Delhi-NCR.

The fine-mode aerosol absorption optical depth (AAOD) retrieved from the Aerosol Robotic Network (AERONET) has been used in previous studies to calculate the radiative forcing of black carbon (BC) aerosol, assuming that the absorption by fine-mode aerosols (diameter >= 1 mu m) is primarily from BC while the absorption by larger particles (diameter > 1 mu m) is principally from dust. In the present study, the Community Earth System Model was used to simulate and quantify the contribution of fine-mode dust to fine-mode AAOD in eastern China (29-41 degrees N, 104-122 degrees E)-an area where concentrations of BC are high. The simulated fine-mode dust concentrations were constrained by observations from nine sites belonging to the Chinese Meteorological Administration Atmosphere Watch Network. Averaged over eastern China, the simulated annual mean fine-mode dust AAOD was 3.6 x 10(-3), with the maximum AAOD in spring and the minimum value in winter. The contribution of fine-mode dust to the total fine-mode AAOD (sum of fine-mode dust, BC, and organic carbon) in winter, spring, summer, and autumn was 3.4%, 25.2%, 12.5%, and 14.9%, respectively, with an annual mean value of 15.1%. The results indicate the importance of removing fine dust AAOD when the AERONET fine-mode AAOD is used for calculating the radiative forcing of BC in eastern China.

The aerosols in the Indo-Gangetic Basin (IGB) are a mixture of sulfate, dust, black carbon, and other soluble and insoluble components. It is a challenge not only to identify these various aerosol types, but also to assess the optical and radiative implications of these components. In the present study, appropriate thresholds for fine-mode fraction and single-scattering albedo have been used to first identify the aerosol types over IGB. Four major aerosol types may be identified as polluted dust (PD), polluted continental (PC), black carbon-enriched (BCE), and organic carbon-enriched (OCE). Further, the implications of these different types of aerosols on optical properties and radiative forcing have been studied. The aerosol products derived from CIMEL sun/sky radiometer measurements, deployed under Aerosol Robotic Network program of NASA, USA were used from four different sites Karachi, Lahore, Jaipur, and Kanpur, spread over Pakistan and Northern India. PD is the most dominant aerosol type at Karachi and Jaipur, contributing more than 50 % of all the aerosol types. OCE, on the other hand, contributes only about 12-15 % at all the stations except at Kanpur where its contribution is similar to 38 %. The spectral dependence of AOD was relatively low for PD aerosol type, with the lowest AE values (1.0). SSA was found to be the highest for OCE (>0.9) and the lowest for BCE (<0.9) type aerosols, with drastically different spectral variability. The direct aerosol radiative forcing at the surface and in the atmosphere was found to be the maximum at Lahore among all the four stations in the IGB.

Advection of anthropogenic aerosols from the Indo-Gangetic Plain (IGP) and dust aerosols from distant deserts towards a high-altitude station Merak, in the trans-Himalayan region are reported during June -July 2011. In order to differentiate the advection event, aerosol optical properties were examined during aged background conditions at the site. During the aged background conditions, aerosol optical depth (AOD at 500 nm) and Angstrom exponent (alpha) at the station were similar to 0.06 and 1.36, respectively which were increased to 0.13 and 1.62, respectively during the advection event. Further, a strong signature of fine-mode aerosol volume size distribution, dominated by absorbing aerosols, was observed during the advection event. The average atmospheric forcing during the aged background condition was found to be 0.57 Wm(-2) (with corresponding heating rate of 0.05 Kday(-1)) and these results were enhanced to 2.58 Wm(-2) (with corresponding heating rate of 0.22 Kday(-1)) during the advection event. The present study reveals that during the advection event, heating rate in the atmosphere was increased by about four times than the aged background condition. Such atmospheric warming in the region may influence the melting of the Himalayan glaciers and consequently it may effect the local atmospheric circulation. (C) 2014 Elsevier Ltd. All rights reserved.