Abstract:
Simultaneous in situ measurements of ozone, CO, and NOy 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 NOy 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 NOy (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
NOy (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 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%, espectively. Despite
higher CO and NOy concentrations at Nainital, ozone levels are nearly similar to those at other global high-
altitude sites.
