Some features of columnar aerosol optical depth, ozone and precipitable water content observed over land during the INDOEX-IFP99

Columnar aerosol optical depth (AOD), ozone and precipitable water content measurements have been made under different atmospheric conditions on a total of 46 days at the Indian Institute of Tropical Meteorology (IITM), Pune during the Intensive Field Phase (IFP) of the Indian Ocean Experiment INDOEX (January-March 1999). These observations have been obtained using two compact, on-line, multi-band solar radiometers (MICROTOPS-II) simultaneously. The results indicate higher aerosol optical depth (more than double) on hazy days as compared to clear stable days. Further increase in aerosol optical depth and markable changes in the size spectrum are also noticed during the occasions of smoke particles' emission from combustion processes in proximity to the experimental site and high water content in the atmosphere. Size spectra of aerosols exhibit characteristic bimodal distribution with mode radii between 0.1 and 1.0 mum on the days associated with hazy-sky conditions and combustion activity. However, the mean aerosol optical depth over the entire IFP shows almost exponential decrease with increase in wavelength, and corresponding size spectrum portrays power law distribution, which may be due to the averaging effect of time variations such as forenoon, afternoon and day-to-day. Moreover, the daily values of columnar aerosol optical depth and water content show almost similar variations. Besides an increasing. trend in the total colunm ozone variations throughout the period of study, it shows opposite variation with those observed in AOD and water content. These interesting features reveal the influence of water content on the growth processes of aerosol particles, and relationship between the increase in aerosol extinction and reduction in ozone amount and vice versa. The columnar aerosol optical depth or content or loading during the IFP99 exhibit higher values as compared to those during the First Field Phase (FFP98).