Tropospheric ozone plays a key role in the air quality, atmospheric chemistry and climate
change; however, its budget and its role in these processes are not well understood mainly due to
lack of sufficient measurements. Satellite and model studies revealed elevated levels of ozone
and other pollutants over the Indo-Gangetic Plain (IGP) in the Northern India; however, in lack
of in situ measurements, the understanding of underlying physical, chemical and dynamical
processes is very limited. Moreover, since the climatic impacts of tropospheric ozone are
dependent upon its vertical distribution, the systematic and long-term measurements of ozone
distribution are needed. In view of this, weekly balloon flights have been conducted to measure
vertical distribution of ozone (EN-SCI 2ZV7 ECC Ozonesonde) and meteorological parameters
(iMet-1-RSB 403 MHz GPS Radiosonde) from a high altitude site Nainital (79.5oE, 29.4oN,
1958 m amsl) in the central Himalayas since January 2011. Additionally, surface ozone
measurements have been made at two sites, Pantnagar (79.5oE, 29.0oN, 231 m amsl) and
Dehradun (78.1oE, 30.3oN, 640 m amsl), in the IGP region since 2009. Weekly air samples are
also collected at Nainital for analysis of CO and greenhouse gases CO2 and CH4 in collaboration
with NIES, Japan. Chemical box model (NCAR Master Mechanism), a regional chemistry
transport model (WRF-Chem) and data from global models (MATCH-MPIC and MOZART) are
used to understand the observed variabilities.
Surface ozone at Pantnagar site in the IGP shows daytime build-up with levels sometimes as
high as 100 ppbv. Ozone seasonal variations exhibit highest levels during spring (39.3±18.9
ppbv in May) and lower levels in summer-monsoon (16.8±8.9 ppbv in August) and winter
(10.8±12.1 ppbv in January). This ozone seasonality is in agreement with the meteorological
parameters and satellite observations of tropospheric NO2 and CO (681 hPa). A global model
(MATCH-MPIC) captures the seasonality but overestimates the ozone levels. Model simulated
daytime H2O2/HNO3 values are higher indicating NOx-limited chemistry over this region. Box
model simulations are used to corroborate this and to estimate integrated net ozone production in
a day (72.9 ppbv). Strong positive correlation (r2 = 0.96 in May) between the daytime ozone at
IGP and nearby Himalayan site suggests transport of pollution from IGP to the cleaner
Himalayas via boundary layer evolution. Estimated 3-monthly AOT40 index, using the observed
ozone data, indicates threat for vegetations in the IGP region.
Balloon-borne observations revealed large variability in the vertical distribution of ozone and
meteorological parameters. Tropopause pressure from radiosonde observations agrees with the
satellite (AIRS and TES) and model results, but shows dramatic variability (150-250 hPa) during
winter and early spring. Lower tropospheric (2-6 km amsl) ozone shows a prominent seasonality
with spring maxima (~70-110 ppbv in May) and summer-monsoon minima (~20-50 ppbv),
which is consistent with the surface observations. Springtime ozone profiles are classified using
MODIS fire counts, and ozone levels during high fire activity periods are observed to be higher
by 19.9±4.6 ppbv in 2-4 km altitude range, as compared with low fire activity period. Signature
of ozone downward transport, noticed during winter, is corroborated with observed reduction in
relative humidity (radiosonde and AIRS satellite) and enhancement in potential vorticity (WRF).
However, model simulated ozone profiles discern enhancements at lower altitudes than
observations. A comparison of ozonesonde observations with collocated satellite (TES) retrievals
shows reasonable agreement. Tropospheric column ozone (TCO) from the ozonesonde
observations show a typical seasonality comprising of spring maxima (47.2±9.8 DU) and winter
minima (30.4±10.1 DU).
The long-term data of CO2, CH4 and CO during September 2006-December 2011, obtained from
the sample analysis, has been analyzed for the seasonal variations and long-term trends. CO2
shows a prominent seasonality over Nainital with the spring maximum (395.9±5.9 ppmv in May)
and post-monsoon/autumn minimum (374.4±5.8 ppmv in October). CO2 seasonal cycle at
Nainital is similar to that at Mauna Loa, however, the seasonal amplitude in CO2 is much larger
at Nainital (~21.5 ppmv) as compared with the Mauna Loa (~6.5 ppmv). Long-term trends in
CO2, CH4 and CO have been investigated by deseasonalizing the time series and a linear
regression fit. The estimated trend in CO2 over Nainital (1.88 ppmv per year) is consistent with
the trends over Mauna Loa (1.60 to 2.43 ppmv per year) and global marine observations (1.64 to
2.40 ppmv per year). CH4 levels discern a small positive trend of 4 ppbv per year, while, CO
levels show a negative trend of 7.3 ppbv per year.