First simultaneous measurements of ozone, CO, and NOy at a high-altitude regional representative site in the central Himalayas

Downward transport from higher altitudes is estimated to enhance surface ozone levels over Nainital by 6.1-18.8 ppbv. The classification based on air mass residence time, altitude variations along trajectory, and boundary layer shows higher levels of ozone (57 +/- 14 ppbv), CO (206 +/- 125 ppbv), and NOy (1856 +/- 1596 pptv) in the continental air masses when compared with their respective values (28 +/- 13 ppbv, 142 +/- 47 ppbv, and 226 +/- 165 pptv) in the regional background air masses.

In general, positive interspecies correlations are observed which suggest the transport of air mass from common source regions (except during winter). Ozone-CO and ozone-NOy slope values are found to be lower in comparison to those at other global sites, which clearly indicates incomplete in situ photochemistry and greater role of transport processes in this region. The higher CO/NOy value also confirms minimal influence of fresh emissions at the site.

Enhancements in ozone, CO, and NOy during high fire activity period are estimated to be 4-18%, 15-76%, and 35-51%, respectively. Despite higher CO and NOy concentrations at Nainital, ozone levels are nearly similar to those at other global high-altitude sites.