Changes in aerosol characteristics (spectral aerosol optical depth, AOD and composition) are examined during the transition from 'relatively clean' to 'extreme' aerosol days in the summer of 2012 at Delhi National Capital Region (NCR), India. AOD smaller than 054 (i.e. 12-year mean AOD - 1 sigma) represents 'relatively clean' days in Delhi during the summer. 'Extreme' days are defined by the condition when AOD(0.5) exceeds 12-year mean AOD + 1 standard deviation (sigma). Mean (+/- 1 sigma) AOD increases to 1.2 +/- 0.12 along with a decrease of Angstrom Exponent from 0.54 +/- 0.09 to 022 +/- 0.12 during the 'extreme' days. Aerosol composition is inferred by fixing the number concentrations of various individual species through iterative tweaking when simulated (following Mie theory) AOD spectrum matches with the measured one. Contribution of coarse mode dust to aerosol mass increased from 763% (relatively clean) to 96.8% (extreme events), while the corresponding contributions to AOD(0.5) increased from 35.0% to 70.8%. Spectrally increasing single scattering albedo (SSA) and CALIPSO aerosol sub-type information support the dominant presence of dust during the 'extreme' aerosol days. Aerosol direct radiative forcing (ADRF) at the Lop-of-the-atmosphere increases from 212 W m(-2) (relatively clean) to 56.6 W m(-2) (extreme), while the corresponding change in surface ADRF is from -995 W m(-2) to 153.5 W m(-2). Coarse mode dust contributes 603% of the observed surface ADRF during the 'extreme' days. On the contrary, 0.4% mass fraction of black carbon (BC) translates into 13.1% contribution to AOD(0.5) and 33.5% to surface ADRF during the 'extreme' days. The atmospheric heating rate increased by 75.1% from 1.7 K/day to 2.96 K/day during the 'extreme' days. (C) 2016 Elsevier B.V. All rights reserved.
