Long-Term Trends in Surface Ozone
A comparison of surface ozone observations at Ahmedabad during 1954-55 and 1991-93 show increase of 1.45 % year 1 (Naja and Lal 1996). These observations also show change in seasonal variations with maximum ozone in winter-spring during 1950s, while maximum ozone has been in autumn-winter during 1990s. Surface ozone observations made at the National Physical Laboratory, Delhi, show positive trends in daily maximum and daytime (1000-1700 h) ozone levels with a rate of about 1.7 (±0.7) and 1.3 (±0.6) ppbv year-1, during 1997-2004, respectively. Observations of surface ozone at Dayalbagh educational site, Agra, show a tendency of ozone increase during 2009-2014, and some possibilities of decrease in average ozone (all data) are seen at Nainital. But data set at Nainital and Agra are for a short period to conclude on their trends.
Long-Term Trends in NOx
Nitrogen dioxide (NO2) plays a key role in the chemistry of the atmosphere and is emitted mainly by combustion processes. These emissions have been increasing over India over the past few years due to rapid economic growth. The GOME (Global Ozone Monitoring Experiment) and SCIAMACHY data show an increasing trend for the NO2 tropospheric column for some of the Indian regions considered, but sometimes not as rapid as is expected from the urban growth rate in those cities. Delhi and Ahmedabad show very high growth rates of 11.3 and 10 %
Fig. 9 Trends in tropospheric NO2 column from OMI and SCIAMACHY over India (Adapted from Ghude et al. 2013)
per year, respectively, during 1996-2006 period (Sheel et al. 2010). However, Bengaluru and Pune show much lower growth rates of 3.5 and 1.5 %, respectively. Gurjar et al. (2008) have shown that transport emissions are responsible for most of the NOx. The number of vehicles is increasing very fast all over India. The vehicular population in Delhi rose from 1.9 million vehicles in 1990 to nearly 3.6 million in the year 2001 (an increase of nearly 8.7 % per annum). Satellite (OMI and SCIAMACHY) data of total column NO2 for India show an increasing trend of 1.65 % in anthropogenic NO2 emission for the same period (Fig. 9) (Ghude et al.
2013). These observed trends are comparable to the growth rate in NOx emission estimated in bottom-up inventories and can be attributed to the growth in power, transportation, and industrial sectors in India. Coal and oil consumption in India has also increased significantly in the last decade.