Multi-Year Analysis of Aerosol Properties Retrieved from the Ångström Parameters for Different Spectral Ranges over Pune

The present study evaluates the temporal variation of aerosol optical depth (AOD500 nm) and the Angstrom parameters [viz., Angstrom exponent (AE, α), Angstrom turbidity coefficient (β) and second order Angstrom 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 pre-monsoon seasons together are found to be 0.534 ± 0.13, 1.054 ± 0.27, 0.254 ± 0.08 and 0.167 ± 1.33 for AOD500 nm, AE, β and α′ respectively. Average month-to-month variability of AOD500 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 (α2–α1) and AOD500 nm–AE440-870 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.