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

Abstract

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.

Comparison of Aerosol Products Retrieved from AERONET, MICROTOPS and MODIS over a Tropical Urban City, Pune, India

“…However, in pre-monsoon, a reversal in the SSA variation is observed (SSA is 0.87 ± 0.02 at 440 nm; and 0.89 ± 0.04 at 1020 nm). This suggests that the dominance of coarse particles (mostly dust) during premonsoon, is either due to local activities like urbanization, industrialization and construction activities or wind-blown dust due to strong surface heating (Devara et al, 2002;Devara et al, 2005;Ramachandran and Rajesh, 2007;Bhawar, 2008;Babu et al, 2010;Dani et al, 2010;Kumar et al, 2011). Additionally, long-range transport of dust may also play a role in this regard, for example, aerosols get transported from Saudi Arabia and Afghanistan to the study region (Pandithurai et al, 2007).…”

“…Pandithurai et al (2007) observed seasonal diurnal asymmetry in aerosol optical properties over Western India. Dani et al (2010Dani et al ( ) studied long-term (1998Dani et al ( -2007 AOD variations over Pune and found significant long-term increasing trend. They observed dominance of smaller size particles during winter and coarse-mode particles during pre-monsoon.Over Indore, Gupta et al (2003) also found low AOD during winter and high AOD during pre-monsoon.…”

Analysis of variation trends and causes of aerosol optical depth in Shaanxi Province using MODIS data

“…Koukouli et al [3] showed a negative trend in AOD over the Southern Balkan/Eastern Mediterranean region, where the highest annual decreasing rate is 4.09% for the summer months and the lowest 2.55% for the winter months. Dani et al [4] observed an increasing AOD trend of 380 nm at 45% per decade at Pune, India, during May 1998-December 2007. In addition, long-term increasing trends in Angstrom wavelength exponent (Alpha) and mean turbidity coefficients of 25.3% and 8.4% per decade is revealed respectively, which indicates that fine-mode aerosols have been increasingly added to the atmosphere over urban regions due to increases in urbanization and human activity.…”

Satellite-based study of physico-optical properties of aerosols over a westernmost location of Brahmaputra valley

“…Similarly, higher value of Ångström exponent in winter season compared to the pre-monsoon season with increasing trends of 25.3% per decade has been reported over Pune, India [22] . Trajectory (NOAA-HYSPLIT) model [23] , 5-day isentropic air mass back concentration weighted trajectories are computed to find out the probable flow paths and source regions of air flow with time over Bongaigaon.…”