Ozone is a secondary pollutant and greenhouse gas that is formed from molecular oxygen in the presence of sunlight and nitrogen oxides. The extent of production also depends on the presence of volatile hydrocarbons, carbon monoxide and methane1,2,3,4,5,6. But we have discovered a surprising source of ozone which is generated in spontaneous bursts even in the absence of sunlight and nitrogen oxides — namely, the exuberant mass of colour-emitting sparklers that are lit during the Diwali festivities, which take place every year during October and November in Delhi, India. The underlying process of ozone formation resembles that induced by ultraviolet radiation in the stratosphere7,8.
We undertook a routine, real-time monitoring of the concentrations of NOx (NO and NO2), O3 and other microclimatic factors at a known pollutant-receptor site9 in Delhi in order to determine the effects of burning unprecedented numbers of fireworks on the local environment during the festive period in November 1999. We found that during the festival period the ozone concentration peaked at around noon and fell to negligible levels after sunset.
On Diwali night (7 November), a small build-up of O3 (9 ± 1 parts per billion by volume) was detected between 20:40 and 02:30 hours (results not shown). During this period, no correlation was found between NOx concentration and O3 formation, indicating that the ozone was unlikely to have been generated in reactions involving ambient NOx. This observation was surprisingly different from night-time O3 measurements obtained on the other dates, when no O3 formation was detected.
Further experiments carried out under different climatic conditions showed that there is a linear regression between the total amount of inflammable material present in sparklers and the cumulative O3 formed (correlation, 0.993). There was no change in ambient NOx concentration before, during or after these experiments (Fig. 1).
Sparklers depend on a combination of different metal salts to generate their colour and sparkle10 — these include potassium perchlorate, sulphur, strontium nitrate, barium nitrate, sodium oxalate, calomel, aluminium and manganese. When burnt, a significant proportion of the light emitted by these constituents has a wavelength below 240 nm (ref. 11). The radiative energy of these emissions is sufficient to dissociate atmospheric molecular oxygen into atomic oxygen, enabling the reaction O2 + O → O3 to take place. This proposed mechanism could explain the formation of bursts of O3 without the participation of NOx, and is therefore similar to the process of ultraviolet-radiation-induced formation of O3 in the stratosphere7,8.
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