Changes in aerosol characteristics are examined during the transition from ‘relatively clean’ to ‘extreme’ aerosol days in the summer of 2012 at Delhi National Capital Region, India. AOD smaller than 0.54 (i.e. 12-year mean AOD −1σ) 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 (σ). Mean (±1σ) AOD increases to 1.2 ± 0.12 along with a decrease of Angstrom Exponent from 0.54 ±0.09 to 0.22 ±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 76.8 (relatively clean) to 96.8% (extreme events), while the corresponding contributions to AOD0.5 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 at the top-of-the-atmosphere increases from 21.2 (relatively clean) to 56.6 Wm−2
(extreme), while the corresponding change in surface ADRF is from −99.5 to −153.5 Wm−2. Coarse mode dust contributes 60.3% 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 AOD0.5 and 33.5% to surface ADRF during the ‘extreme’ days. The atmospheric heating rate increased by 75.1% from 1.7 to 2.96 K/day during the ‘extreme’ days.
Dr. Arun Srivastava received his Master’s degree from University of Allahabad in Applied Geology and then M. Phil and Ph. D. from Jawaharlal Nehru University, New Delhi in Environmental Sciences. Afterwards he did his postdoctoral studies from Weizmann Institute of Science, Israel and Institute for Methodologies in Environmental Analysis (Council of National Research) from Italy. His main field of expertise is air pollution; viz. indoor and outdoor air pollution, bio-aerosols, chemical mass balance model for source apportionment, and aerosol optical depth. Dr. Srivastava has published around 30 research articles in refereed journals and has supervised many M.Phil and Ph.D. scholars.