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

Abstract

Simultaneous in situ measurements of ozone, CO, and NO_y have been made for the first time at a high altitude site Nainital (29.37°N, 79.45°E, 1958 m above mean sea level) in the central Himalayas during 2009–2011. CO and NO_y levels discern slight enhancements during the daytime, unlike in ozone. The diurnal patterns are attributed mainly to the dynamical processes including vertical winds and the boundary layer evolution. Springtime higher levels of ozone (57.5 ± 12.6 ppbv), CO (215.2 ± 147 ppbv), and NO_y (1918 ± 1769.3 parts per trillion by volume (pptv)) have been attributed mainly to regional pollution supplemented with northern Indian biomass burning. However, lower levels of ozone (34.4 ± 18.9 ppbv), CO (146.6 ± 71 ppbv), and NO_y (1128.6 ± 1035 pptv) during summer monsoon are shown to be associated with the arrival of air mass originated from marine regions. 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 NO_y (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-NO_y 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/NO_y value also confirms minimal influence of fresh emissions at the site. Enhancements in ozone, CO, and NO_y during high fire activity period are estimated to be 4–18%, 15–76%, and 35–51%, respectively. Despite higher CO and NO_y concentrations at Nainital, ozone levels are nearly similar to those at other global high-altitude sites.