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FIREWORKS on Independence Day and Guy Fawkes produce a lot of smoke when they
burn, but they could also create pollution in an unexpected way. The bright
flashes they emit may mimic the effect of sunlight and generate ozone in the air
we breathe.

Arun Attri and his colleagues from Nehru University in New Delhi, India,
looked at levels of nitrogen oxides and ozone in the air in Delhi during the
Hindu festival Diwali in 1999. Diwali, the 鈥渇estival of lights鈥�, marks the
beginning of the Hindu New Year and is celebrated by lighting lamps and
exploding firecrackers.

At ground level, ozone is normally created when sunlight breaks down nitrogen
oxides. The oxygen radicals released react with oxygen molecules in the air to
make ozone (O₃). So levels of nitrogen oxides go down as ozone goes up,
and the process halts at night.

But at the height of the festival, the researchers found that ozone levels
jumped by 9 parts per billion during the night, while nitrogen oxide levels
stayed constant. Lab tests with sparklers confirmed that fireworks can have this
effect (Nature, vol 411, p 1015).

Ozone can be produced directly from oxygen without nitrogen oxides, when
ultraviolet light splits apart oxygen molecules. This normally happens only in
the upper atmosphere, but Attri thinks that the fireworks emit enough
ultraviolet light to produce ozone this way.

Colin Johnson of the Hadley Centre for Climate Prediction and Research in
Bracknell, Berkshire, is sceptical. 鈥淚 find it surprising that fireworks would
have that intensity of light,鈥� he says. But Alan Williams of Leeds University
thinks the heat of the fireworks could be to blame.