IT SEEMS obvious: cutting pollution should make our air cleaner. In fact, it鈥檚 not quite that simple. While particulates from diesel engines, industry and burning biomass make the atmosphere dirtier, it turns out that they also help prevent the dangerous build-up of ozone in the air we breathe.
Common pollutants such as methane and carbon monoxide are cleansed from the atmosphere by highly reactive ions such as hydroxyl, which is manufactured by reactions that are driven by ultraviolet light. Because particulates, also known as aerosols, absorb hydroxyl ions onto their surfaces, they leave fewer free for cleaning duties so levels of the pollutants rise.
When Randall Martin, an atmospheric chemist at Harvard University, modelled the effect of particulates on hydroxyl ions he found that they halve levels of the ions in the air over much of Europe during the summer. This leads to a build-up of the polluting gases that would otherwise be broken down by the ions.
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But particulates also had a surprising beneficial effect on pollution. Hydroxyl ions also take part in the reaction between nitrogen oxides and hydrocarbons that produce ozone, which is harmful when present in the air near the ground, and Martin鈥檚 model showed that by stripping hydroxyl ions out of the atmosphere the particulates caused ozone production to slump. The particulates also hinder the ozone-producing reactions by absorbing the ultraviolet light needed to drive them. As a result, surface ozone levels are 25 per cent lower than they might be over Europe in summer, and up to 10 per cent lower over the US.
The discovery puts regulators in a bind, says Martin. 鈥淭ough limits on particulate matter will increase surface ozone over Europe and other industrial regions,鈥� he says. Particulates from smoke are responsible for most the annual 60,000 deaths from air pollution in the US. Ozone is also implicated, and triggers asthma attacks.
鈥淕iven the choice, I would prefer to avoid aerosols than ozone,鈥� says Martin. 鈥淭he trick will be to cut the gases that produce ozone at the same time as cutting aerosols.鈥�