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Acid rain - foe or friend?

Research suggests smog offsets greenhouse gas

CLIMATE change resulting from greenhouse gases in countries like China and India is being offset by acid rain, according to research from MMU’s Earth Systems Science group.

Curiously, the smog currently plaguing Asian cities appears to be mitigating one of the worst greenhouse emissions, that of methane which is 21 times more powerful as a greenhouse gas than CO2, and is a by-product of the Asian rice paddy field.

The British team of MMU's Professor Nancy Dise and Dr Vincent Gauci of the Open University added sulphate to laboratory rice paddies in an effort to mimic the effect of acid rain on Asia’s most common food crop.

The acid rain surrogate reduced methane emissions by up to a quarter in the research funded by the Natural Environment Research Council.


"The reduction in pollution happens during a stage of the life cycle when the rice plant is producing grain. This period is normally associated with around half of all methane emissions from rice and we found that simulated acid rain pollution reduced this emission by 24 per cent," say Dise and Gauci.

To test the effects of acid rain, the researchers added frequent small doses of sulphate, which simulate acid rain experienced in polluted areas of China.

"We had similar results when exposing natural wetlands to simulated acid rain but this could be more important since natural wetlands are mostly located far from major pollution sources, whereas for rice agriculture, the methane source and the largest source of acid rain coincide in more urban environments," said the researchers.

And they said more research was vital: "One line of investigation we'd like to confirm is that the sulphate component of acid rain may actually boost rice yields. This might, paradoxically, have the effect of reducing a source of food for the methane producing microorganisms that live in the soil.


"There is also likely to be competition between these microorganisms and sulphate-reducing bacteria. Normally in these conditions sulphate-reducers win which results in less methane."

But they added a note of caution to the results. "Acid rain is one of several pollution problems in Asia that need solving but we need to appreciate the potential consequences of that clean-up, one of which could be an increase in methane emissions as the effect of the acid rain wears off."

The paper Suppression of rice methane emission by sulphate deposition in simulated acid rain is published in the Journal of Geophysical Research, volume 113. The authors are Vincent Gauci (lead author) Nancy Dise - Manchester Metropolitan University (grant holder), Graham Howell - Open University, and Meaghan Jenkins - Open University and University of New South Wales.

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