Nascent, nasty and lethal, ozone has risen to dangerous levels in Delhi, and is increasing in various cities -- damaging human health and affecting agriculture
going by the limited data that the Central Pollution Control Board ( cpcb ) is willing to part with -- based on monitoring of ozone at a single site in Delhi -- there is no cause for alarm. But according to a report published in September this year by another governmental agency, the Central Road Research Institute ( crri ), ozone had already crossed the danger mark in 1993. cpcb 's claim flies in the face of logic and contradicts its own findings on the magnitude of air pollution in the Capital.
cpcb has been monitoring ozone at the ito crossing for the past eight months. But cpcb officials remain tightlipped about their findings. All that they are willing to say is that ozone levels in the city have been "hovering around 40-45 parts per billion by volume (ppbv), much lower than the World Health Organisation limit of 50-60 ppbv". The cpcb attitude is a slight improvement over its previous stand. In an interview with a researcher from the Centre for Science and Environment in November 1996, cpcb chairperson D K Biswas had altogether dismissed the issue, saying that ozone is "not a problem". While it now recognises the need to monitor ozone levels in the city, it has yet to set up a comprehensive system.
Moreover, cpcb 's claim is in sharp contrast to the evidence presented by the crri . The crri study is based on tests conducted at seven locations in Delhi in the winter of 1993 -- Parliament Street, Daryaganj, Paharganj, Ashram chowk, Karol Bagh, Maharani Bagh and Vasantkunj. Average levels of ozone at five sites over eight daytime hours exceeded the who mean standard of 50-60 ppbv by 10 to 40 per cent (see graph: Ozone rising ).
Even more alarming, the peak levels observed in areas with heavy traffic and congestion like Karol Bagh, Daryaganj, and Parliament Street were as high as 119.0 ppbv, 125.8 ppbv, 123.9 ppbv, respectively. In most cases, the maximum concentration of ozone exceeded the upper who limit by two times.
Scientists point out that even short-term exposure to high levels of ozone can be deadly -- talking about mean or average levels does not help understand the true impact of ozone on health. Studies in other countries have shown that exposure to high ambient ozone over a short period can adversely affect the lungs. It causes inflammation of the airways (bronchus and bronchioles) and leads to typical respiratory problems -- shortness of breath, wheezing and chest pain (see box: Erroneous ozone ). Keeping this in mind, the who has set two standards for ozone concentration. The first, an average measured over eight hours, is 50-60 ppbv. The second, for peak concentrations over one hour, is 76-100 ppbv.
The scope of the crri study was restricted to monitoring ozone for a few months in Delhi and, therefore, did not indicate the seasonal trend in ozone formation. Since the objective of the study was to see the effect of atmospheric inversion on the concentration of pollutants during winter, ozone was monitored along with other pollutants. But Anil Singh of crri emphasises that the trend in ozone formation during summer should be investigated separately: bright sunlight catalyses photochemical reactions in the atmosphere, aiding the formation of ozone.
Scientists had, in fact, sounded the alarm some six to seven years ago. Studies conducted by the School of Environmental Studies, Jawaharlal Nehru University ( jnu ) and the Tata Energy Research Institute ( teri ) in Delhi had shown that peak levels of ozone in the city had touched the 128 ppbv mark in 1989-90 and 114 ppbv in 1990-91, crossing the who safety standard (based on an hourly average) of 76-100 ppbv.
Ozone is not emitted directly into the atmosphere. High atmospheric concentrations of ozone are a result of a complex set of reactions in the atmosphere which involves emissions of nitrogen oxides and certain reactive hydrocarbons. Most of the primary pollutants are generated by motor vehicles and industries in cities and towns. Scientists point at the dramatic rise in motor vehicles in Indian cities as primarily responsible for emission of ozone-forming gases like no x and reactive hydrocarbons.
crri scientists had observed a close correlation between peak traffic flow and peak ozone levels in their study. Interestingly, the study indicated that ozone concentrations in winter peaked before noon and in the evening -- closely following the peak traffic trend. Though ozone levels peak at noon in a normal diurnal pattern, the actual pattern showed discrepancies in winter. According to the scientists, this could be due to slow dispersion and trapping of ozone owing to inversion of temperature in the lower atmosphere during evenings and at night. Higher wind speeds at noon could also help in dispersion of ozone.
According to C K Varshney, professor at the School of Environmental Sciences, jnu , ozone formation accelerates during summer due to higher solar radiation. The same factor is responsible for higher ozone levels at noon time, when the sun is brightest. Seasonal variations were noted between 1989 and 1991 in Delhi by Varshney and his team. The maximum concentration was in June 1990, with the monthly mean between March and July ranging from 76 to 81 microgramme per cubic metre (one microgramme is one millionth of a gramme). The mean decreased between September and January to 55-59 microgramme/cu m, but increased between March and July 1991 to 83.45-88.94 microgramme/cu m. Between September 1991 and January 1992 it ranged from 61.85 to 64.11 microgramme/cu m.
Sonia Thimmiah of teri , who has put together these findings in a study, says that ozone at the ground level in Delhi has often far exceeded the who limit (see table: All across the country ). Both crri and jnu scientists feel that ozone is bound to rise in Delhi with increase in emissions of no x , carbon monoxide, methane and other hydrocarbons. This will be facilitated by climatic and weather conditions such as high temperature, intense sunshine, and stable wind.
|All across the country
Ozone levels in select cities in India
|City||Year of monitoring||Ozone levels||Status with respect to standards|
|9-128 parts of billion by volume(ppbv) (hourly concentration)||Upper level exceeds World Health Organisation(WHO) standard as well as the one used by the US National Crop Loss Assement Network(NCLAN)|
|Ahmedabad||1991||9-55ppbv(15 minute average)||
Exceeds NCLAN standard
|29-57ppbv(monthly average based daily mean)||
Exceeds NCLAN standard
|10-76ppbv(two-hour average)||Close to WHO standard|
|1-34ppbv(hourly mean)||Dose not exceed any standard|
|Source: Sonia Thimmiah, 1996, Air pollution in India with respect to deleterious impact on agriculture, Tata Energy Research Institute, New Delhi|
Estimated crop losses due to ozone in India
|Cities||Loss in yield
|Loss in yield
|Source: M R Ashmore, Nigel Bell and others, Estimating crop losses in the field, Imperial College, Centre for Environmental Technology, London|
Interestingly, ozone does not affect some plants and cultivars. Scientific evidence from field studies in Egypt show that stomata in a turnip variety almost 'waive off' ozone penetration, and in a variety of radish stomata open readily to allow ozone to pass through. Moreover, a combination of plant enzymes determines the level of resistance of a variety to ozone. Scientists feel that this knowledge could be useful in planning cultivation of crops in areas with high levels of ozone.
Overall, however, the bulk of evidence suggests that the problem of ozone impact on agriculture is no longer restricted to North America and Western Europe. It cannot, therefore, be dismissed as a problem of the North. Ozone has had a severe impact on regions and localities in countries of the South. The evidence is further reinforced by the steady increase in global and regional no x emissions, primarily from vehicles in urban areas.
Agricultural scientists are worried about the limited or poorly coordinated monitoring of air pollution in rural areas in Asia, Africa and South America. As of now, it is only possible to draw on isolated studies in a few countries that indicate the gravity of the problem. Urges Bell: "It is important to combine data on pollutants and agricultural yield in a compatible form in developing risk assessment models." He suggests appropriate use of geographical information systems in landuse planning and pollution control to minimise the adverse impact of ozone on crops.
With inputs from Priti Kumar and Shefali Verma.
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