LAUSR

Abstract The high-altitude regions in the Himalayas are prone to high ozone concentrations frequently resulting from diverse dynamical and transport mechanisms. Here, we report an unusual enhancement in the surface and tropospheric ozone concentrations over the central Himalayan region from ground-based and space-borne measurements in the month of December 2010. The surface ozone levels (~80 ppbv) on 18–19 December 2010 is observed to be two-fold higher relative to the seasonal average (December-January-February) of about 40–50 ppbv in the central Himalayan region. The space-borne measurements from Tropospheric Emission Spectrometer and Ozone Monitoring Instrument onboard Aqua satellite also show higher values in the tropospheric column ozone over this region. The satellite observations indicate an increase in tropopause temperature of about 5 °C and decrease in tropopause altitude about 1 km during 18–19 December 2010 resulting in the occurrence of tropopause fold facilitating the stratospheric-tropospheric exchange processes over the study region. The plausible reason for the occurrence of tropopause fold and subsequent enhancement of tropospheric and surface ozone is found to be associated with the breaking Rossby waves in the upper troposphere. The wave breaking leads to the advection of high-PV (potential vorticity) air, with magnitudes of about 3–4 PVU, towards the central Himalayan region from high-latitudes. The vertical component of PV advection also shows a deep stratospheric intrusion of high-PV air into the troposphere. The isentropic transport of ozone across the folding tropopause due to the wave breaking is clearly depicted from the satellite and reanalysis datasets. Therefore, the present study has strong implications of upper tropospheric wave dynamics to the tropospheric and surface ozone over the Himalayan regions having complex topography.