As a part of the Indian Ocean Experiment (INDOEX) programme, ground-based measurements of trace gases (SO2, NO2, NH3 and O3) and Aitken Nuclei (AN) were carried out at Pune during March-April 1998. Also, measurements of surface ozone and AN were made during 5-10 January 1998, the inter comparison campaign of various instruments at Thiruvananthapuram. The mean concentrations of SO2, NO2, NH3 and O3 at Pune were 3.75, 5.81, 3.21 and 57.8 μg/m3 respectively and that of O3 at Thiruvananthapuram was 44 μg/m3. The average concentration of AN was about five times higher at Pune (19,000/cm3) than that at Thiruvananthapuram (4,400/cm3), indicating more pollution at Pune than at Thiruvananthapuram. The diurnal variation of surface ozone showed high concentrations during the day and low concentrations during the night at both the locations. Diurnal variation of AN at Pune showed a daytime maximum and a night-time minimum. Whereas, at Thiruvananthapuram night-time values were higher than the daytime values which can be attributed to the influence of the land and sea breeze
