Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 1999, 38 (11), 1634 - 1637
The Latest about Smog
More chemical compounds than previously expected
are formed in summer smog
Isoprene is a simple hydrocarbon whose derivatives play an important role in the metabolism of plants. Oaks, above all, release isoprene into the environment - but so do all other plants, right down to algae. Worldwide, up to 450 million tonnes of this compound are emitted into the atmosphere every year. As a team of chemists working with Karlheinz Ballschmiter at the University of Ulm has now discovered, isoprene - like other hydrocarbons as well - is altered by interaction with vehicle emissions on sunny days. This forms chemical compounds, some of which have only now been detected for the first time.
These "new" isoprene derivatives are nitrates and hydroxy nitrates of isoprene. They form at night in a reaction between isoprene and the brew of ozone and nitrogen oxides produced mostly by the exhaust of cars and trucks in strong, persistent sunlight during the day. This is the well known summer smog.
These new trace components have so far clearly slipped through the fingers of the environmental analysts. Ballschmiter and his co-workers discovered this when they reproduced the isoprene derivatives, which are formed in summer smog, in the lab. for the purpose of comparison. They were eventually also able to confirm the presence of several of these compounds in the atmosphere by using ingenious analytical methods - on the oak-lined campus of the University of Ulm, next to a car park. However, some of the compounds decompose in the sensitive instruments before they can even cause a needle to jump on the display. The Ulm researchers are now searching for these with refined techniques.
In the end, these isoprene nitrates won't cause any students in Ulm to attend their lectures with a breathing mask. The concentrations of these "new" air pollutants register at merely a few billionths of a gram per cubic meter of air. Nevertheless, Ballschmiter's research shows that the "chemical menagerie" in our air is not fully catalogued. Plants do not only release isoprene into the environment: for example, among the substances also emitted in trace quantities are the hundreds of scents, terpenes. Their fate in smog conditions has also not yet been investigated.