Seasonal asymmetry in diurnal variation of aerosol optical characteristics over Pune, western India

Abstract

There is a large day‐to‐day variability in aerosol optical properties due to varied meteorological conditions that exhibit different diurnal asymmetry in different synoptic seasons. Aerosol optical depth (AOD) and Ångstrom exponent (AE) measured for a period of 3 years at Pune, an urban site in western India, were analyzed to study the diurnal variation and its seasonal dependence. Mean AOD at 500 nm in the premonsoon (March–May) period was about 0.42, while in the winter (December, January, and February) it was ∼0.38. AOD departures from the daily mean showed ±20% variation (which is equivalent to AOD of 0.08) in both seasons. A systematic seasonal asymmetry in diurnal variation was found. AOD is higher in the morning and lower in the afternoon during winter, mainly because of higher relative humidities, calm winds, and associated ground‐based inversions that are conducive for haze, which persists till noon. However, in premonsoon, AODs are lower in the forenoon hours and higher in the afternoon hours because of higher air temperatures, strong convection, and stronger winds raising more dust locally and transporting it from Saudi Arabia and Afghanistan regions. The difference between Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua derived AOD showed similar seasonal asymmetry in forenoon‐afternoon difference in aerosol loading as observed at the experimental site.