Surface reactions produce pollutants

Your walls, floors and ceilings could be factories for harmful pollutants, scientists have found.

  Conventional wisdom held that hydrogen chloride and nitrogen oxides, abundant byproducts of combustion reactions, were locked-up for good when deposited on surfaces. But this view is now being challenged.

  The researchers made nitrosyl chloride, ClNO, and nitryl chloride, ClNO2, from nitrogen oxides adsorbed onto wet surfaces using UV radiation and HCl (PNAS doi:10.1073/ pnas.0904195106).

  In most settings, the chlorine species produced by these reactions will not be produced in sufficient quantity to pose a health risk. But they break down in UV light to free chlorine atoms, which can feed into radical chain reactions that produce ozone, a harmful pollutant and greenhouse gas.

  They integrated this chemistry with computer models of air pollution in southern California, which returned, in some regions, a 20% increase in ozone.

  ‘The unique aspect of this chemistry is that water is essential for these reactions to take place – without it, you don’t form ClNO or ClNO2,’ says author Barbara Finlayson-Pitts, from the University of California, Irvine, US. ‘Any surface will do, as long as it contains a thin film of adsorbed water.’

  HCl can be produced by natural and industrial processes, such as the burning of biomass and waste. It is also found in cigarette smoke and can be generated by chlorine-containing cleaning products.

  Paul Monks, an atmospheric chemist at the University of Leicester, UK, says that to combat this effect in the indoor environment we need materials that resist the adsorption of nitrogen oxides. We also need to reconsider the chemicals we routinely use indoors. Limonene, for example, is used in many cleaning products to give a lemon smell, but it is a surprisingly reactive compound. Monks asks: ‘Do we really need that smell?’

 The climate change debate has drawn some attention away from the wider issues of air quality, he adds. But, atmospheric processes are interwoven, and we need to consider them together, or we risk exacerbating one problem as we try to fix another.