Aerosol optical depth and fine mode fraction variations deduced from Moderate Resolution Imaging Spectroradiometer (MODIS) over four urban areas in India

Abstract

Interannual and intra-annual variations in aerosol optical depths (AOD) and fine mode fraction (FMF) analyzed from Moderate Resolution Imaging Spectroradiometer (MODIS) during 2001-2005 over four urban areas in India, Chennai, Mumbai, Kolkata and New Delhi, are investigated. MODIS derived AODs and FMFs are found to compare well with in situ Aerosol Robotic Network (AERONET) Sun photometer results over Kanpur. Monthly mean 550 nin AODs are higher than 0.30 in all the four locations and show prominent seasonal variations of summer high and winter low. FMF values are low during premonsoon and summer over Chennai, Mumbai and New Delhi, while they are 0.9 or higher over Kolkata during the entire year. AODs exhibit a positive correlation with rainfall. A strong convection in summer accompanied by a deeper boundary layer and hygroscopic growth of fine mode water soluble aerosols result in higher AODs during summer monsoon. The 5-year mean AOD is 0.4 in Chennai and Mumbai while in Kolkata and New Delhi it is >0.55. AODs <0.4 contribute 50% or more in Chennai and Mumbai, while in Kolkata and New Delhi AODs in the 0.4-0.8 range contribute higher than 50%. Variation in annual mean FMF is the lowest in Kolkata and highest in New Delhi, consistent with seasonal variations in FMF values in these locations. In Chennai, Mumbai and Kolkata, FMF values >0.6 contribute higher than 75%, while over Kolkata FMF values in the 0.8-1.0 range contribute 94 which indicates the dominance of fine mode aerosols in Kolkata throughout the year. Copyright 2007 by the American Geophysical Union.