Long-term trends and variability in measured multi-spectral aerosol optical depth over a tropical urban station in India

Sun photometer-derived multi-spectral aerosol optical depth (AOD) measurements made at Pune (18°32'N, 73°51'E), India, during the period May 1998-December 2007 have been used to examine possible long-term trends and variability. Long-term mean AOD in the spectral range 380-1020 nm ranged between 0.593 ± 0.11 and 0.220 ± 0.04 with a coefficient of variation (COV) of 34-49. Day-to-day variations in AOD at all the wavelengths in the range 380-870 nm showed significant long-term increasing trends; the increasing trend at 380 nm being 45 per decade. AODs are higher at shorter wavelengths in winter (December-February) and at longer wavelengths in the pre-monsoon months (March-May) which implies that relatively smaller-sized particles are more in winter and larger sized particles of soil dust origin are more during the pre-monsoon season. Overall mean value of Angstrom-size exponent (α) at this urban location is 0.55, and the mean turbidity coefficient (β) is 0.76. Value of α is higher in winter season compared to the pre-monsoon season. Both α and β show long-term increasing trends of 25.3 and 8.4 per decade, respectively, which points out that with increasing urbanisation and human activity, more and more smaller-sized aerosols are being added to the atmosphere over the urban regions