5.3.1.2 Tropospheric Ozone
Although a majority of the ozone is concentrated in the
stratosphere, tropospheric ozone plays a vital role in atmospheric chemistry, determining the oxidative capacity of the
atmosphere through the production of the hydroxyl radical
(OH), and can also act as a pollutant affecting human health
and crop productivity. Efforts have been taken toward estimating trends in surface and tropospheric column ozone.
The estimated amplitude of trend varies with season and
location (urban and rural). The annual mean trends in tropospheric column ozone derived by integrating the vertical
profiles of ozonesonde data (in situ observations on balloon
platforms) over Delhi, Pune, and Trivandrum (1972–2001)
show an increasing trend of 2.7 ± 2.3%, 0.9 ± 1.8%, and
0.3 ± 2.6% year
−1 , respectively. These values of trends are
in close agreement with that obtained from TOMS data
(Saraf and Beig 2004). Nimbus-7 and Earth Probe satellite—
Total Ozone Mapping Spectrometer (TOMS) data for the
period of 1979–2005 show positive trends 0.7–0.9% year
−1
over South Asia. Also, the trends estimated from the
MOZART model are in agreement with observations over
the Bay of Bengal region (0.4 ± 0.29–0.6 ± 0.43% year
−1 ,
Beig and Singh 2007). The regressed tropospheric ozone
residual (TOR) data shows an annual trend of
*0.4 ± 0.25 1r% per year over the northeastern Gangetic
region (Lal et al. 2012). Similar estimates of trends in ozone
at various stations in India are reported (Lal et al. 2013,
2014, 2017).
The multi-model ensemble-mean from CMIP5 historical
simulations (2009–2000) shows an increase in tropospheric
column ozone *25–35 DU (decadal mean) over the Indian
region. Future projections (2090–2100) are tabulated in
Table 5.1. ACCMIP multi-model simulations for relative
changes of tropospheric ozone between 2000 and 2030
(2100) for the different RCPs show decrease of −5% (−22%)
in RCP 2.6, 3% (−8%) in RCP 4.5, 0% (−9%) and increase
in 15 RCP 6.0, and 5% (15%) in RCP 8.5 (Young et al.
2013). However, there are large uncertainties in model
simulations over the Indian region due to uncertainties in
emission inventories, model parameterization, chemistry
representation, etc. (Fadnavis et al. 2015).
Surface ozone observations have shown an increasing
trend at various locations around India, which are attributed
to increasing anthropogenic activity. Naja and Lal (1996)
reported increasing ozone by 14.7 ppbv during 1954–1955
and 25.3 ppbv during 1991–1993, which results in linearly
increasing trends of 1.45% year
−1 . The seasonal trends also
show a significant increase, e.g., winter *1.91% year
−1 and
summer 0.86% year
−1 at Ahmedabad (23° N, 75.6° E). At
southern peninsula station, Thiruvananthapuram (8.542° N,
76.858° E), surface ozone measurements were obtained
during 1973–1975, 1983–1985, 1997–1998, and 2004–2014
(Nair et al. 2018). These measurements show a slow increase
of *0.1 ppb year
−1 during 1973 to 1997 and faster growth
of 0.4 ppb year
−1 afterward till 2009 after which it showed a
steady-state till 2012 followed by a minor decrease. The
above studies show that trends in surface and tropospheric
column ozone are positive but have low confidence.
Profiles of ozone show variations in the vertical structure
of ozone. From ozonesonde observations, Saraf and Beig,
(2004) reported long-term trends in ozone at Trivandrum,
Pune, and Delhi. The observed trend at Delhi was increasing
between 1.5% year
−1 and 7.3% year
−1 during 1972–2001,
but negative trends with low confidence (statically
insignificant) *−0.5 to −3% year
−1 were observed at Pune
and Trivandrum. Seasonal trends in tropospheric ozone
are positive, particularly around 500 hPa and 200–300 hPa
during months of January–March. The role of biomass
burning and stratosphere–troposphere exchange is evident in
these two layers. There is also a prominent increasing trend
in ozone near the tropopause (*100 hPa) during June and
July (monsoon month) (Fadnavis et al. 2014a). The
GEOS-Chem model simulations also show a positive trend
of 0.19 ± 0.07 ppb year
−1 (p-value < 0.01) (an annual
mean) in the lower troposphere between 1990 and 2010 (Lu
et al. 2018).
5.3.1.3 Ozone Trends in the Upper Troposphere
and Stratosphere
It is important to understand ozone variability and trends in
the UTLS since a small amount of variation of ozone in the
UTLS has a large impact on radiative forcing and climate
change (Forster and Shine 1997). Fadnavis et al. (2014a)
reported an increasing ozone trend between 0.6 ± 0.65 and
2.35 ± 1.3 year
−1 in the upper troposphere and in the lower
stratosphere from multiple satellite data sets and model
simulations. The estimated trends are slightly positive up to
30 hPa and then negative between 30 and 10 hPa. Seasonal
mean
trends
vary
between
−0.04 ± 0.3
and
3.48 ± 2% year
−1 (low confidence). In the stratosphere (20–
50 km), ozone shows a decreasing trend of (medium confidence) *−0.4 ± 0.1% year
−1 near 16–20 km while trends
values are positive near 24–30 km (0.05 ± 0.04 to
0.1 ± 0.9% year
−1 ) (low confidence) during 1993–2015
(Raj et al. 2018).
A decrease in abundance of ozone-depleting substances
(ODSs) under the compliance of the Montreal Protocol was
the start of the recovery of stratospheric ozone. Since the
atmospheric burden of ozone-depleting substances is
declining, changes in CO 2 , N 2 O, and CH 4 will have an
increasing influence on the ozone layer (WMO 2019b).
Ozone layer changes in the latter half of this century will be
complicated, with projected increase or decrease in different
regions. Eyring et al. (2013) reported the evolution of
stratospheric ozone over the CMIP5 historical period (1960
to 2005) and the sensitivity of ozone to future GHGs (2006–
5 Atmospheric Aerosols and Trace Gases
105
Although a majority of the ozone is concentrated in the
stratosphere, tropospheric ozone plays a vital role in atmospheric chemistry, determining the oxidative capacity of the
atmosphere through the production of the hydroxyl radical
(OH), and can also act as a pollutant affecting human health
and crop productivity. Efforts have been taken toward estimating trends in surface and tropospheric column ozone.
The estimated amplitude of trend varies with season and
location (urban and rural). The annual mean trends in tropospheric column ozone derived by integrating the vertical
profiles of ozonesonde data (in situ observations on balloon
platforms) over Delhi, Pune, and Trivandrum (1972–2001)
show an increasing trend of 2.7 ± 2.3%, 0.9 ± 1.8%, and
0.3 ± 2.6% year
−1 , respectively. These values of trends are
in close agreement with that obtained from TOMS data
(Saraf and Beig 2004). Nimbus-7 and Earth Probe satellite—
Total Ozone Mapping Spectrometer (TOMS) data for the
period of 1979–2005 show positive trends 0.7–0.9% year
−1
over South Asia. Also, the trends estimated from the
MOZART model are in agreement with observations over
the Bay of Bengal region (0.4 ± 0.29–0.6 ± 0.43% year
−1 ,
Beig and Singh 2007). The regressed tropospheric ozone
residual (TOR) data shows an annual trend of
*0.4 ± 0.25 1r% per year over the northeastern Gangetic
region (Lal et al. 2012). Similar estimates of trends in ozone
at various stations in India are reported (Lal et al. 2013,
2014, 2017).
The multi-model ensemble-mean from CMIP5 historical
simulations (2009–2000) shows an increase in tropospheric
column ozone *25–35 DU (decadal mean) over the Indian
region. Future projections (2090–2100) are tabulated in
Table 5.1. ACCMIP multi-model simulations for relative
changes of tropospheric ozone between 2000 and 2030
(2100) for the different RCPs show decrease of −5% (−22%)
in RCP 2.6, 3% (−8%) in RCP 4.5, 0% (−9%) and increase
in 15 RCP 6.0, and 5% (15%) in RCP 8.5 (Young et al.
2013). However, there are large uncertainties in model
simulations over the Indian region due to uncertainties in
emission inventories, model parameterization, chemistry
representation, etc. (Fadnavis et al. 2015).
Surface ozone observations have shown an increasing
trend at various locations around India, which are attributed
to increasing anthropogenic activity. Naja and Lal (1996)
reported increasing ozone by 14.7 ppbv during 1954–1955
and 25.3 ppbv during 1991–1993, which results in linearly
increasing trends of 1.45% year
−1 . The seasonal trends also
show a significant increase, e.g., winter *1.91% year
−1 and
summer 0.86% year
−1 at Ahmedabad (23° N, 75.6° E). At
southern peninsula station, Thiruvananthapuram (8.542° N,
76.858° E), surface ozone measurements were obtained
during 1973–1975, 1983–1985, 1997–1998, and 2004–2014
(Nair et al. 2018). These measurements show a slow increase
of *0.1 ppb year
−1 during 1973 to 1997 and faster growth
of 0.4 ppb year
−1 afterward till 2009 after which it showed a
steady-state till 2012 followed by a minor decrease. The
above studies show that trends in surface and tropospheric
column ozone are positive but have low confidence.
Profiles of ozone show variations in the vertical structure
of ozone. From ozonesonde observations, Saraf and Beig,
(2004) reported long-term trends in ozone at Trivandrum,
Pune, and Delhi. The observed trend at Delhi was increasing
between 1.5% year
−1 and 7.3% year
−1 during 1972–2001,
but negative trends with low confidence (statically
insignificant) *−0.5 to −3% year
−1 were observed at Pune
and Trivandrum. Seasonal trends in tropospheric ozone
are positive, particularly around 500 hPa and 200–300 hPa
during months of January–March. The role of biomass
burning and stratosphere–troposphere exchange is evident in
these two layers. There is also a prominent increasing trend
in ozone near the tropopause (*100 hPa) during June and
July (monsoon month) (Fadnavis et al. 2014a). The
GEOS-Chem model simulations also show a positive trend
of 0.19 ± 0.07 ppb year
−1 (p-value < 0.01) (an annual
mean) in the lower troposphere between 1990 and 2010 (Lu
et al. 2018).
5.3.1.3 Ozone Trends in the Upper Troposphere
and Stratosphere
It is important to understand ozone variability and trends in
the UTLS since a small amount of variation of ozone in the
UTLS has a large impact on radiative forcing and climate
change (Forster and Shine 1997). Fadnavis et al. (2014a)
reported an increasing ozone trend between 0.6 ± 0.65 and
2.35 ± 1.3 year
−1 in the upper troposphere and in the lower
stratosphere from multiple satellite data sets and model
simulations. The estimated trends are slightly positive up to
30 hPa and then negative between 30 and 10 hPa. Seasonal
mean
trends
vary
between
−0.04 ± 0.3
and
3.48 ± 2% year
−1 (low confidence). In the stratosphere (20–
50 km), ozone shows a decreasing trend of (medium confidence) *−0.4 ± 0.1% year
−1 near 16–20 km while trends
values are positive near 24–30 km (0.05 ± 0.04 to
0.1 ± 0.9% year
−1 ) (low confidence) during 1993–2015
(Raj et al. 2018).
A decrease in abundance of ozone-depleting substances
(ODSs) under the compliance of the Montreal Protocol was
the start of the recovery of stratospheric ozone. Since the
atmospheric burden of ozone-depleting substances is
declining, changes in CO 2 , N 2 O, and CH 4 will have an
increasing influence on the ozone layer (WMO 2019b).
Ozone layer changes in the latter half of this century will be
complicated, with projected increase or decrease in different
regions. Eyring et al. (2013) reported the evolution of
stratospheric ozone over the CMIP5 historical period (1960
to 2005) and the sensitivity of ozone to future GHGs (2006–
5 Atmospheric Aerosols and Trace Gases
105