Satellite data reveal the true scope of China’s pollution problem
Feb 11th 2012 | from the print edition
“PM2.5” seems an odd and wonkish term for the blogosphere to take up, but that is precisely what has happened in China in recent weeks. It refers to the smallest solid particles in the atmosphere—those less than 2½ microns across. Such dust can get deep into people’s lungs; far deeper than that rated as PM10. Yet until recently China’s authorities have revealed measurements only for PM10. When people realised this, an online revolt broke out. Such was the public pressure that the government caved in and PM2.5 data are now being published for Beijing and a handful of other cities.
But what of the rest of China? At the moment, only PM10 data are available. However, officialdom’s hand may soon be forced here, too. Though pollution data are best collected near the ground, a plausible estimate may be made from the vantage-point of a satellite by measuring how much light is blocked by particles, and estimating from those particles’ chemical composition the likely distribution of their sizes. And a report prepared for The Economist by researchers at Yale and Columbia universities, and Battelle Memorial Institute, under the auspices of Angel Hsu of Yale, does just that. It draws on data from American satellites to map out PM2.5 pollution across the entire country.
World Health Organisation guidelines suggest that PM2.5 levels above ten micrograms per cubic metre are unsafe. Ms Hsu has found (as the map shows) that almost every Chinese province has levels above that. Indeed, much of the country’s population endures air so foul that it registers above 30 micrograms per cubic metre. Beijing averages 35. Shandong and Henan top 50. And because these readings reflect the average pollution that a typical resident in a province is likely to endure during a given year, they underplay the sharp spikes in pollution seen on particularly dirty days, when spot readings go much higher.
Ms Hsu’s approach is not perfect. Satellites are not great at taking readings over bright surfaces like snow and deserts, and cannot easily distinguish particles high up in the atmosphere from those closer to the ground. And the data also have to be adjusted to take account of the fact that pollution and people tend to coincide, otherwise the uninhabited areas in a province would drag the overall figure down below the real experience of someone living in the region.
Such caveats aside, however, this study shows how far China still needs to go in cleaning up its act. Pollution and development have always marched hand in hand, and the former may even be regarded as tolerable as long as it is only a temporary blip on the road to prosperity. What is intolerable is that it is left to outsiders to lift the lid on what is happening.