'Acid soot' worsens smog

Researchers in the US say soot particles in the atmosphere combine with other pollutants to pick up an acid coating that may worsen their influence on local smog and global warming. 

Many aerosol particles suspended in the atmosphere have a cooling effect, since they scatter sunlight away from the Earth and also act as seeds for cloud formation. But soot aerosols, or ’black carbon’ - which make up 10 to 50 per cent of the particles in the lowest portion of Earth’s atmosphere - absorb sunlight, contributing to global warming. And when associated with other atmospheric pollutants, such as sulfate, soot’s absorbing power seems greater.

Researchers at Texas A&M University mimicked tropospheric conditions in the laboratory to discover what happens to soot particles as they age in the Earth’s atmosphere. ’We found that sulfuric acid accumulates on soot aerosols and gives them a coating that significantly enhances their atmospheric effects,’ says Renyi Zhang, who led the research team. ’The implications of this study are profound - on climate, air pollution, human health and cloud formation.’ 

The potent ’acid soot’ forms when sulfur dioxide, which is responsible for acid rain, comes into contact with particles of carbon produced by fossil fuel burning. The acidified soot particles are much larger and trap twice as much heat as normal soot. This increased size is likely to cause more health problems, Zhang says, indicating that the particles are probably a large contributor to the ill-effects of smog. 

Soot detectives

The small size of particles in soot aerosols has made them hard to study in the lab, says Zhang. His team used a battery of analytical techniques, including a transmission electron microscope and several specialised particle analysers. ’There exist several atmospheric measurements supporting the coating of soot and the presence of sulfate, but the previous studies provide little insights into the mechanism of soot transformation. This is the first time that anyone has been able to mimic the behaviour of soot in the atmosphere and analyse its properties,’ Zhang notes. 

The team hopes their research will aid calculations into the global warming dangers of soot and sulphur dioxide emissions - particularly relevant for countries such as China, where new coal-burning power plants are being built rapidly, but only a minority are fitted with desulfurisation systems to reduce sulfur dioxide emissions.

The latest Intergovernmental Panel for Climate Change report estimates that absorption by black carbon could be responsible for a large global warming effect - comparable to around 10 per cent of all greenhouse gas emissions. But it is not well understood how much cooling black carbon provides by helping clouds to form. And there are many other atmospheric aerosol interactions to consider - with nitrates, volatile organic carbon compounds, sea-salt and mineral dust. 

’Emissions of man-made aerosols are thought to offset some of the global warming by greenhouse gases,’ says Jim Haywood, an expert in atmospheric chemistry at the UK Met Office. ’So developing a greater understanding of the effects of these aerosols is vital for better predictions of the magnitude of global warming in the future.’

Lewis Brindley

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