PARTICLES in the air we breathe are blamed by some for growing health problems in developed countries. But the processes responsible for creating them are poorly understood. Now it turns out that the particles form much more easily than had been thought 鈥� from the cocktail of traffic fumes and industrial pollution found in urban areas.
Renyi Zhang and his colleagues from Texas A&M University in College Station simulated levels of urban pollution by varying the amount of aromatic acid and sulphuric acid vapours in an enclosed vessel. Sulphuric acid is produced by burning coal, while aromatic acids are associated with vehicle emissions. The team found that the formation of particles increased as much as tenfold in the presence of both aromatic acids and sulphuric acid (Science, vol 304, p 1487).
The researchers believe that the interaction between these acids reduces the amount of energy needed to form particulate matter. Their idea is supported by calculations that suggest the substances combine to form an intermediate stable aromatic acid-sulphuric acid complex which acts as a stepping stone to the formation of mixed organic sulphate particles.
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Suspensions of particles in the atmosphere, known as aerosols, also affect the global climate by scattering incoming radiation and creating a cooling effect. More locally they can affect weather systems by changing the way clouds form and altering the chances of rain or snow falling.