The Tragedy of the Commons has long been seen to apply to air pollution. Many nations freely dispose of their emissions, but others — downwind, and maybe far away — will be most affected by the consequences.

In this context, the paths of emissions from Asia across the Pacific Ocean, and their potential effects on air quality in the United States, have been well investigated. Reginald Newell and Mathew Evans of the Massachusetts Institute of Technology have taken a different perspective, as they report in Geophysical Research Letters (16, 2509–2512; 2000). They have investigated the question of how much pollution reaches Asia from sources elsewhere.

Newell and Evans set up particles along an imaginary wall, extending meridionally from north of Bangkok to Siberia. Data on wind strengths and directions allowed them to reconstruct the particles' possible routes through the atmosphere. The picture here shows the resulting ‘spaghetti plot’, with each line representing a path that would have taken an air parcel, starting at an altitude of 2.5 km or more, to the imaginary wall within five days.

A detailed analysis of the complete data set reveals that in January and February, 30–40% of the air that arrives at longitude 100° E has passed over Europe. Considering European emission levels, and the tendency of winter storms to swirl pollution up through the atmosphere, European air up to a height of about 10 km is likely to be far from pristine. So, in winter, Europe may contribute significantly to Asian pollution.

So much for the dynamics. At this point, the researchers hand over to the atmospheric chemists, who will have to measure the composition of the atmosphere above Asia to confirm or disprove the point.