EFFORTS to clean up smogs in European and North American cities will be scuppered by rising pollution from developing countries, researchers warnedthis week. Even dramatic cuts in vehicle emissions will only work in the short term, unless action is taken to clean up the global atmosphere.
The controversial finding has important implications for cities worldwide, where authorities have introduced ever tougher limits on vehicle emissions in an effort to make urban air safer to breathe. According to the new research, reductions like these soon won’t be stringent enough to meet air-quality standards.
Regulators, particularly in North America and Europe, have focused on reducing levels of nitrogen oxides (NOx) in car exhausts. They hope to minimise chemical reactions in city air that, powered by sunlight, turn NOx into ozone, the most lethal component of most city smogs.
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While ozone in the stratosphere shields the Earth from harmful ultraviolet radiation, at ground level it is a dangerous pollutant blamed for reducing crop yields and exacerbating a range of illnesses from heart disease to asthma.
But according to Arlene Fiore, an atmospheric chemist at Harvard University, clamping down on NOx emissions won’t keep ground-level ozone at bay for long. Other pollutants, she says, are raising background levels in the atmosphere.
Her computer models show that unless these other pollutants are tackled, even a 50 per cent cut in NOx emissions in the US over the next 30 years won’t be enough to prevent an upsurge in ozone smogs. “We predict worse smogs and a longer summer smog season, lasting from March to November,” she said. The same effect would be seen in European cities.
The main cause of rising background levels of ozone is methane. Most of this comes from cattle, rice paddies and leaks from gas pipelines. Emissions of the gas are expected to have risen by more than 40 per cent by 2030, mostly due to increases in the developing world.
In the past, methane has only been of concern as a greenhouse gas. But the gas also reacts slowly in the atmosphere with traces of highly reactive chemicals called hydroxyl radicals, ultimately creating ozone.
“We used to think those reactions were irrelevant for ozone smogs because they are so slow they only contribute to background ozone levels,” says Fiore. “But it is becoming clear that the rise in background levels is helping trigger smog alerts.”
While Fiore’s model suggests that reducing NOx emissions in the US over the next three decades would not prevent ozone smogs, a global attack on methane emissions might.
“Reducing methane emissions by 50 per cent nearly halves the incidence of high-ozone events in the US,” she reports in Geophysical Research Letters (DOI: 10.1029/2002GL015601).