Recent extremes in total ozone content over the northern parts of India in view of the Montreal Protocol

The Montreal Protocol on substances that deplete the ozone layer entered into force on 1 January 1989. One of the major questions is whether or not the atmosphere is on the path to ozone recovery due to the implementation of the Montreal Protocol and its amendments. In view of this, the frequency of the low/high ozone days and lowest /highest total ozone content (TOC) values have been analysed over the northern parts of India in the winter season for the time periods 1979-1988 and 1997-2005. Daily station total ozone data from TOMS on the Nimbus-7 (1979-1988) and Earth Probe (1997-2005) satellites of the National Aeronautics and Space Administration (NASA)/Goddard Space Flight Center (GSFC) during the period 1979-2005 are used in the present analysis. Stations located to the north of 20°N latitude regions of India have been chosen for the study. To compare the recent changes with extremes prior to the implementation of the Montreal Protocol, statistical percentile thresholds are computed based on the daily ozone data during winter months (January and February) for the period 1979-1988, and using these percentile threshold values extremes in TOC are computed for both time periods (1979-1988 and 1997-2005) to determine whether the day is a low, high or normal ozone day. Trends in extremes in TOC over northern India during the recent period (1997-2005) compared to the period prior to the implementation of the Montreal Protocol (1979-1988) in general indicate recovery of TOC either in terms of opposite trends or similar trends that are less in magnitude. However, most of the trends are statistically insignificant, probably due to the dynamic variability of the atmosphere