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. Copyright © 2010 Royal Meteorological Society