*2.1. MLS Data*

Earth Observing System (EOS) MLS is one of the four instruments aboard NASA's Aura satellite. The Aura MLS gives around 3500 vertical profiles per day and it crosses the Equator at ∼01:40 and ∼13:40 local time [36]. In the present work, version 4.2 Aura MLS data of O3, CO and WV were used. The vertical resolution of the WV is in the range 2.0 to 3.7 km from 316 to 0.22 hPa and the along-track horizontal resolution varies from 210 to 360 km for pressure greater than 4.6 hPa. For ozone, vertical resolution is ∼2.5 km and the along-track horizontal resolution varies between 300 and 450 km. CO is retrieved from radiance measurements of two bands in the MLS 240 GHz radiometer. Vertical resolution for CO is in the range 3.5–5 km from the upper troposphere to the lower mesosphere and the useful range is 215–0.0046 hPa. The horizontal resolution for CO is about 460 km at 100 hPa and 690 km at 215 hPa. The precision and systematic uncertainty for WV and O3 are ±10–40%, ±10–25% and ±0.02–0.04 ppmv, ±0.02–0.05 ppmv ±5–10%, respectively. The accuracy of CO at 100 hPa is ±19 ppbv and ±30%. The data was collected for the period from January 2006 to December 2017. Profiles of O3, CO and WV were used to construct a monthly mean 2.5◦ × 2.5◦ degree gridded data set for the study period. We used data from 2006 to 2014 for background climatology for each individual month. To get zonal mean of trace gases in each month during the reporting period, all the available MLS profiles within the latitude bands that is, for 10◦N–10◦S. More details about MLS version 4 level 2 data can be found in Livesey et al. [34].
