Aerosol Features during Drought and Normal Monsoon Years: A Study Undertaken with Multi-Platform Measurements over a Tropical Urban Site

Abstract

This paper reports the results of aerosol optical characteristics over a tropical urban station at Pune, India, during drought and normal monsoon years, based on ground-based and satellite observations for the period 2008-2010. Ground-based data from MICROTOPS-II and AERONET, and satellite data products from Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI) sensors, were utilized in the study. Detailed analysis of this data revealed that the maximum values of aerosol optical depth (AOD) and minimum values of precipitable water content (PWC) were observed during a drought year (2009) compared to normal monsoon years (2008 and 2010). In order to characterize aerosols further, the Angstrom parameters α and β were evaluated. Using the least squares method, α is calculated in the spectral interval of 380-1020 nm, along with the coefficients a1 and a2 of the second-order polynomial fitted to the plotted logarithm of AOD versus the logarithm of wavelength. Meteorological parameters, such as temperature and relative humidity, have been measured during the course of the study, as well as the variations in monthly rainfall over the experimental site. The high surface temperatures and low rainfall amounts during the pre-monsoon and monsoon seasons of 2009 lead to an increase in AOD as compared to those in 2008 and 2010. The ground-based observations from MICROTOPS-II reveal good correlation with satellite observations of the MODIS and OMI sensors, and also correlate with the AERONET observations, corroborating the results. Discrimination of aerosol types from radiometric measurements and the tropospheric circulation obtained from the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis were also discussed, and found to be consistent.