If there is one gas that plays a double role that beats any actor hollow, it is ozone. High up in the atmosphere, it plays the good guy who saves the world by blocking the dangerous ultraviolet rays in sunlight. Down at the ground level, it is Face-off time as ozone plays blackguard!

Scientists have suspected this since a long time but conclusive proof came recently. In a study done by UK's Centre for Ecology and Hydrology and the University of Exeter, it was seen that high levels of ozone at low levels hamper the ability of plants to absorb carbon dioxide.

Low level ozone occurs naturally but what has increased the levels dangerously is once again the human hand. Transport emissions and industry have caused a doubling of ozone since 1850, says the report in Nature.
The presence of ozone was found to cause plants to close their stomato (tiny pores through which exchange of gases takes place) in a defence mechanism that blocks out carbon di-oxide as well! Carbon dioxide is a vital ingredient in the photosynthesis process that plants use to make food.

Using a global model, the team which included NERC Director Peter Cox, found that gross primary productivity or the effectiveness of plants to conduct photosynthesis will decrease by 14-23 per cent due to this ozone effect!
This aspect has not so far been included in climate projections.

By increasing ozone levels, we have also thus aided the increase of carbon dioxide levels. And carbon dioxide is one of the villains in global warming being a greenhouse gas that stays for as long as 100 years in the atmosphere!

Ozone is also a greenhouse gas but its natural occurrence was hitherto minimal. The phenomenal addition of vehicles to the roads laid on the planet has contributed significantly. This is especially so in tropical countries where sunlight is available in plenty. The vehicle emissions of nitrous oxides are converted by sunlight into ozone.

This has implications for agricultural production. With winds blowing polluted air across cities to surrounding regions, it is obvious that agricultural lands will suffer. These areas are more exposed to sunlight and the conversion to ozone will be more here than in cities where high-rises and flyovers give some cover!

At ground level, not only is ozone bad for plants but also for humans. The WHO air quality guidelines note that high levels of ozone can cause respiratory problems and asthma attacks. It gives a 100 micrograms per cubic metre standard for an 8-hour average.

In India, we have no standards whatsoever for ozone. But early next year, 22 stations will be set up across the country to monitor ozone levels, primarily for research work.