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Simultaneous measurements of aerosol optical depth and incoming global solar flux were carried out with a MICROTOPS-II Sunphotometer and Eppley Precision Spectral Pyranometer over Nowrosjee Wadia College (NWC, Pune) as the nodal centre, and at Pune University (PU) and IUCAA Girwali Observatory (IGO), in a field campaign mode during December 2010-May 2011. Data was analyzed to determine the surface short-wave (SW) aerosol direct radiative forcing (ADRF, ∆F per unit 0.1 AOD) and to study the characteristics of the aerosols. The results indicate that ADRF shows significant day-to-day variability and co-varies with AOD. The cause of the day-to-day variation in ADRF is an anticorrelation between ADRF/AOD, and differences in the daily maximum minus minimum RH and temperature. At NWC, ADRF ranges between -37.7 W/m 2 (highest) and -5.9 W/m 2 (lowest). For 500 nm, ADRF takes values in the range -17.3 ± 7.1 W/m 2 to -54.2 ± 5.5 W/m 2 at PU, whereas the corresponding values at IGO are -15.1 ± 2.1 W/m 2 and -36.6 ± 6.4 W/m 2 . Monthly ADRF is at the minimum level in winter and maximum during the pre-monsoon period. The magnitude of AOD shows significant diurnal variability. In winter, the mean AOD diurnal percentage departure at 500 nm is positive in the morning and negative during the afternoon, and this is reversed in the pre-monsoon period. The diurnal cycle of AOD is related to the prevalent meteorological conditions, surface-based nocturnal temperature inversion in the atmospheric boundary layer (ABL), and influx of aerosols from different source regions.

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The present study evaluates the temporal variation of aerosol optical depth (AOD 500 nm ) and the Ångström parameters [viz., Ångström exponent (AE, α), Ångström turbidity coefficient (β) and second order Ångström exponent (α′)] at a tropical observing site, Pune (18°32′N; 73°49′E, 559 m AMSL) during 2008-15. Six-year means for winter and premonsoon seasons together are found to be 0.534 ± 0.13, 1.054 ± 0.27, 0.254 ± 0.08 and 0.167 ± 1.33 for AOD 500 nm , AE, β and α′ respectively. Average month-to-month variability of AOD 500 nm , AE, β and α′ during 2008-15 depicts seasonal cycle with strong departures with respect to long-term means. Frequency distributions for AOD, AE and β are positively skewed (skewness = 0.77, 0.32 and 1.14 respectively) while it is negatively skewed for α′ (skewness = -0.18). Analysis of AE difference, curvature parameter difference (α 2 -α 1 ) and AOD 500 nm -AE 440-870 nm contour density map reveals that the aerosol ensemble at Pune consists of four aerosol types viz., UI (urban/industrial), CM (clear maritime), DD (desert dust) and MT (mixed type). Their relative magnitudes, however, differ during winter and pre-monsoon seasons. Thus, the contour density map shows dominance of UI and relatively less occurrence of MT type aerosols during winter. In pre-monsoon, however, the aerosol scenario is driven by MT type aerosol although UI and DD type aerosols show their remarkable existence.

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