Changes in surface ozone amount and its diurnal and seasonal patterns, from 1954–55 to 1991–93, measured at Ahmedabad (23 N), India

A comparative study of surface ozone observed at Ahmedabad during 1954–55 and 1991–93 periods has been made. The 1954–55 measurements were made at 0600, 1000, 1400, 1800, and 2100 hours (IST) by an electro‐chemical method, whereas 1991–93 measurements are made at every 15 minutes interval by an UV absorption technique. Higher ozone concentrations including larger amplitudes of diurnal and seasonal variations have been observed during 1991–93 as compared to the measurements made during 1954–55. The observed seasonal maximum during 1954–55 is in spring months whereas during 1991–93, it is in winter months. Increase in the concentration of ozone from 0600 hrs in the morning to 1400 hrs in the noon time is higher in summer and October–November months for both the periods and lowest during monsoon season. Furthermore, this change is higher during 1991–93 period as compared to 1954–55 suggesting higher levels of ozone precursors. The average ozone concentration increased from 14.7 ppbv during 1954–55 to 25.3 ppbv during 1991–93, resulting in a linear increase of 1.45% per year. The increase in background ozone concentrations, for 2100 and 0600 hrs during monsoon months when there is low photochemical ozone production even during day time, is 0.49% per year.