Tropospheric ozone (O3) and aerosols have been regarded as major global concerns owing to their adverse effects on the environment, vegetation and human health when present at elevated levels in the atmosphere. Of late, the levels of these pollutants in the atmosphere have witnessed a steady increase. Tropospheric ozone, a greenhouse gas, a strong oxidant and a secondary pollutant has emerged as a ‘new age pollutant’ for tropical and industrializing countries like India owing to the fast-paced urbanisation and economic growth. Delhi, for instance, has not only observed an increasing trend in the levels of day-time tropospheric ozone in the atmosphere, but the levels of night-time or post sunset ozone has also increased over the past few decades. Simultaneously, atmospheric aerosols have been reported to increase several folds in urban settings like Delhi. Atmospheric aerosols play a crucial role in driving the atmospheric chemistry therefore strongly influencing the weather and climatic condition of an area. Also, aerosols can absorb and scatter solar radiation, hence interferes with the ozone formation and influences the overall accumulation of tropospheric ozone in urban environment. Studies conducted in past commonly report the seasonal and diurnal patterns of tropospheric ozone, but relatively lesser research has gone into examining the post sunset tropospheric ozone build-up. Also, the interactions of ozone, aerosols and other meteorological parameters like temperature, relative humidity, wind speed, wind direction and atmospheric pressure at tropospheric levels haven’t been much explored. Considering this, the present study aims to bridge the existing research gaps in the field of tropospheric interactions of these parameters in an urban city i.e. Delhi. The capital city of India situated in northern part experience high air pollution episodes throughout the year. The study seeks to focus on the post sunset ozone climatology, the phenomenon which contributes to ozone accumulation during post sunset hours, the synergetic impacts and interactions of tropospheric ozone with aerosol particles on agricultural crops. Tropospheric ozone has been reported as a threat to agricultural crops like wheat, risking the overall agricultural productivity of the country and specifically held threat to northern India with wheat laden agricultural fields.
The research thesis had three main objectives; to study climatology of tropospheric ozone over Delhi: to understand the relationship between tropospheric ozone and aerosols: to examine the effect of tropospheric ozone and aerosols on wheat crop. The study adopts multi-level approach using statistical tools, real time monitoring of the pollutants and developing an experimental set-up to evaluate the impacts of tropospheric ozone and aerosols on agricultural food crop, wheat in this case. At first the long-term existing trend of tropospheric ozone was studied using secondary data which further was subjected to projections in near future. Later the possible contributors to evening traffic rush hour or evening ozone (EO) accumulation were identified based on the real time monitoring experiment at traffic intersections. Secondly, the possible relationship of tropospheric ozone with precursor pollutants and meteorological variables was established using long- term satellite observations across the study site. Finally, the cumulative impact of elevated exposure of aerosols and ozone on wheat crop was estimated using a controlled experimental setup in ambient settings.
The results of the study highlight that Delhi city has witnessed an increasing trend in the accumulation of tropospheric ozone during both daytime as well as night time between 1998 and 2012. The Auto Regressive Integrated Moving Average (ARIMA) model forecast has also suggested that this trend is expected to increase for both day time as well as for night time till 2025. The increased night time ozone trend laid the baseline to examine the possible contributors to it using real time monitoring technique at different traffic intersections. The interactions between CO, aerosols and EO concentrations at the traffic intersections, was used as the stepping stone to assess the relationship between aerosols and tropospheric ozone by evaluating the satellite observations over a period of 10 years, i.e., 2008 and 2018. The results highlighted that long term analysis of satellite dataset can be helpful in understanding the behaviour of different pollutants, specifically tropospheric ozone. Considering the increasing trend of tropospheric ozone accumulation, it is anticipated that it will produce varying impacts on human health and food crops. The present study specifically focussed on assessing the cumulative impact of tropospheric ozone and aerosols on wheat crop under ambient conditions. The results of the controlled experiment highlighted that exposure to ozone and aerosols produce negative impacts on the growth and yield parameters of wheat crop. A decline of ~14% in the harvest index of the wheat crops grown in fully exposed setup with elevated levels of both tropospheric ozone and aerosols was observed in comparison to the controlled setup. The decline in the growth, biochemical and yield parameters was highly pronounced when the crops are simultaneously exposed to aerosols and ozone.
Key Words of the Research: Tropospheric ozone, aerosols, satellite data, crop yield reduction, Delhi.