Abstract. Lower tropospheric (surface to 600 hPa) ozone over India poses serious risks
to both human health and crops, and potentially affects global ozone distribution
through frequent deep convection in tropical regions. Our current
understanding of the processes controlling seasonal and long-term variations in
lower tropospheric ozone over this region is rather limited due to spatially
and temporally sparse observations. Here we present an integrated process
analysis of the seasonal cycle, interannual variability, and long-term trends
of lower tropospheric ozone over India and its linkage to the South Asian
monsoon using the Ozone Monitoring Instrument (OMI) satellite observations
for years 2006–2014 interpreted with a global chemical transport model
(GEOS-Chem) simulation for 1990–2010. OMI observed lower tropospheric ozone
over India averaged for 2006–2010, showing the highest concentrations
(54.1 ppbv) in the pre-summer monsoon season (May) and the lowest
concentrations (40.5 ppbv) in the summer monsoon season (August). Process
analyses in GEOS-Chem show that hot and dry meteorological conditions and
active biomass burning together contribute to 5.8 Tg more ozone being produced in
the lower troposphere in India in May than January. The onset of the summer
monsoon brings ozone-unfavorable meteorological conditions and strong upward
transport, which all lead to large decreases in the lower tropospheric ozone
burden. Interannually, we find that both OMI and GEOS-Chem indicate strong positive correlations (r=0.55–0.58) between ozone and
surface temperature in pre-summer monsoon seasons, with larger correlations
found in high NOx emission regions reflecting NOx-limited
production conditions. Summer monsoon seasonal mean ozone levels are strongly
controlled by monsoon strengths. Lower ozone concentrations are found in
stronger monsoon seasons mainly due to less ozone net chemical production.
Furthermore, model simulations over 1990–2010 estimate a mean annual trend
of 0.19 ± 0.07 (p value < 0.01) ppbv yr−1 in Indian lower
tropospheric ozone over this period, which are mainly driven by increases in
anthropogenic emissions with a small contribution (about 7 %) from global
methane concentration increases.
Long-Term Climate Simulations Using the IITM Earth System Model (IITM-ESMv2) With Focus on the South Asian Monsoon