Abstract:
Surface ozone measurements have been made for the first time at Nainital (29.37°N,
79.45°E, 1958 m amsl), a high‐altitude site in the central Himalayas, between October 2006
and December 2008. Diurnal variations in ozone do not show the daytime photochemical
build‐up typical of urban or rural sites. The seasonal variation shows a distinct ozone
maximum in late spring (May; 67.2 ± 14.2 ppbv) with values sometimes exceeding 100 ppbv
and a minimum in the summer/monsoon season (August; 24.9 ± 8.4 ppbv). Springtime
ozone values in the central Himalayas are significantly higher than those at another highaltitude
site (Mt. Abu) in the western part of India. Seasonal variations in ozone and the
processes responsible for the springtime peak are studied using meteorological parameters,
insolation, spatial and temporal classifications of air mass trajectories, fire counts, and
simulations with a chemical transport model. Net ozone production over the Northern Indian
Subcontinent in regionally polluted air masses is estimated to be 3.2 ppbv/day in spring but
no clear build‐up is seen at other times of year. Annual average ozone values in regionally
polluted air masses (47.1 ± 16.7 ppbv) and on high insolation days (46.8 ± 17.3 ppbv)
are similar. Background ozone levels are estimated to be 30–35 ppbv. Regional pollution is
shown to have maximum contribution (16.5 ppbv) to ozone levels during May–June and
is about 7 ppbv on an annual basis, while the contribution of long‐range transport is greatest
during January–March (8–11 ppbv). The modeled stratospheric ozone contribution is
2–16 ppbv. Both the trajectory analysis and the model suggest that the stratospheric
contribution is 4–6 ppbv greater than the contribution from regional pollution. Differences
in the seasonal variation of ozone over high‐altitude sites in the central Himalayas (Nainital)
and western India (Mt. Abu) suggest diverse regional emission sources in India and
highlight the large spatial and temporal variability in ozone over the Indian region.
