Trends in total ozone column over India: 1979–2008

Time-series decomposition analysis was performed on, (1) Multi Sensor Reanalysis (MSR) Total Ozone Column (TOC) monthly mean time-series data-set [1979–2008], and (2) Total Ozone Mapping Spectrometer (TOMS) Version 8 Overpass monthly mean time-series data-set from Nimbus 7 satellite, (TOMS N7) [1979–1993] to estimate long-term linear trends in the data to assess a scale of surface UV changes over India. Long-term trend estimation, subsequent to the removal of annual cyclic variations, for MSR TOC data-set was done over Indian region covering latitude spread 0° N – 40° N, and Longitude spread 67.5° E to 97.5° E. Trend estimates for TOMS Overpass data-sets, treated on similar lines, was done for fifteen locations over India (1°19′N to 34°04′N). Statistically significant declining trends ranging from (−) 0.8 – (−) 1.5 percent/decade were seen over Indian region above 25° N latitude in MSR TOC data-set (1979–2008). In case of TOMS N7 data-set (1979–1993), statistically significant declining trends were estimated over New Delhi (28°40′N) and Srinagar (34°04′N) with a value of (−) 2.5 and (−) 3.6 percent/decade respectively. Observed TOC decline covered 40% of total geographical area of Indian region, however rest of the Indian region (peninsular) did not show statistically any significant trend.

Research highlights
► Statistically significant declining trends ranging from (−) 0.8 – (−) 1.5 percent/decade were observed over Indian region above 25° N latitude.
► The decline in TOC covers about 40% of geographical area of India.
► No significant decline in TOC was found below 25°N (peninsular region).
► Quantum of decline between 1979 and 2008 was smaller when compared with that estimated from 1979 to 1993.
► Area of the region over which TOC decline was observed increased from 30% to 40% of total geographical area of India, between 1979–1993 and 1979–2008.