Surface ozone is damaging wheat and rice crops across the country


Surface ozone, a plant-damaging pollutant could be destroying up to 15% wheat crop and up to 6% rice yield every year across India, a first-of-its kind study based on ground observations of ozone, has revealed.

A multi-institute study comprising 14 scientists and led by the Physical Research Laboratory (PRL), Ahmedabad, under the Department of Space, has estimated that every year, surface ozone is likely to wipe out 4.0–14.2 million tonnes (MT) of the total 95 MT of wheat produced in India, and 0.3–6.7 MT of the total 105 MT rice grown.

Surface ozone – also called a secondary pollutant – is generated through chemical reactions between primary pollutants such as oxides of nitrogen and volatile organic compounds, in the presence of sunlight. The sources of these primary pollutants are power plants, vehicles, industries, and bio-mass burning.

With a projected rise in man-made pollution including surface ozone, researchers said the findings are important since a decrease in crop yield in India, which is the second populous country, will have a serious impact on its food security and economic growth.

“Wheat and rice are important crops for India since they form a staple diet,” said professor Shyam Lal, principal investigator, PRL. “Photosynthesis takes place during the day. When ozone molecules, which are generated in the presence of sunlight, get into the plant cells, the plant gets affected and can get damaged.”

Previous studies have calculated the annual loss of wheat crop in the range of 5% to 30%, and between 2.1% and 16 % for rice yields. However, these high estimates were based on either limited observations – restricted to some states only – or regional and global chemistry transport models.

“Ozone is a very new phenomenon in India now, and ozone levels are rising. The findings are important, as we now know from different sources of the impact of surface ozone on vegetation and crops, in addition to health. This means that pollution is ultimately reaching food security,” said Anumita Roychowdhury, executive director, research and advocacy and head of the air pollution and clean transportation programme, Centre for Science and Environment, Delhi, who was not involved in the study. “Ozone reduction strategies have to become an important policy focus now.”

The present study, drawn from data on surface ozone recorded at 17 locations across India, has found that general pattern of ozone variability shows higher ozone in April–May, lower in August and again increasing in October– November.

This is because high pollution levels and strong sunlight especially between March and May results in high levels of surface ozone damaging crops. Low surface ozone levels recorded during winter months when pollution levels are high but sunlight is low will either not damage crops or the loss is less.

Though there is a permissible human exposure level for surface ozone set by the Central Pollution Control Board, there are no safe levels prescribed for plants.

For the study, researchers used two internationally adopted methods to assess the exposure of crops to surface ozone – average ozone levels for seven hours during the day for three-month cropping period and second, accumulated ozone levels that exceed 40 parts per billion by volume (ppbv) over the same period.

[S2] In the present study, exposure to wheat in the north and west-central regions of India crossed the average surface ozone 40 ppbv threshold between January and March. The North Indian region includes Uttar Pradesh, Punjab, Haryana, Rajasthan and Uttarakhand, while west-central regions comprise Madhya Pradesh, Gujarat and Maharashtra.

Average ozone levels during the wheat crop duration of three months were highest in the west/central region at about 47 ppbv followed by north India at about 41 ppbv. The corresponding accumulated ozone levels recorded in the west was 16,300 ppbv*hr and it was about 7,200 ppbv*hr in the north. However, a major part of the wheat crop is grown in north India only.

The damage to rice though lower was also pronounced in northern and western-central parts of the country. The seasonal average ozone levels in the north and west-central were 33 ppbv and 27 ppbv – and the corresponding accumulated ozone levels in both these regions were 4730 ppbv*hr and 3400 ppbv*hr.

Researchers said as compared to wheat, crop loss for rice is less because surface ozone levels are lower as the main harvesting period is soon after the summer monsoon. Also average surface ozone levels are lower in the eastern and southern regions of India where large quantity of rice is grown.

India has two cropping seasons – Kharif and Rabi. Sowing for rice, a kharif crop, takes place between June and July, and it’s harvested from September to October. Wheat is a rabi crop that is sown between November and December, and harvested from February to April.

North India logged the highest annual loss of about 9 MT of wheat. For rice, the maximum loss of about 2.6 MT every year was recorded from the country’s eastern region of Bihar, West Bengal, Odisha, Chattisgarh, Assam, and Jharkhand.

With pollution levels set to rise further including levels of surface ozone, international studies have predicted higher losses for wheat and rice for India by 2030.

“Ozone is a very reactive gas, and while it is very harmful for health it also has a huge impact on vegetation. Ozone will increase if emissions of gases from combustion of sources such as vehicles and industries increase. If you have to control ozone, then need to control emissions from nitrogen oxides, hydrocarbons,” said Roychowdhury.

Based on the study findings, researchers have suggested that wheat, 73% of which is cultivated in north India alone, should be sown as early as possible starting November so that it is grown in the period when ozone is relatively lower.

“Wheat and rice cultivars, which have resistance to ozone damage, need to be developed. We would like to suggest policy makers to work out plans to reduce air pollution, particularly in the northern India. This will take care not only of the losses of the crops but also health of the public,” read the study.


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