"The Great Wall may not, after all, be visible from space – but Chinese air pollution is. My team of researchers at Yale and Columbia Universities, along with Battelle Memorial Institute, recently used satellite readings to produce data on fine particulate concentrations in Chinese provinces. While these satellite measurements are not perfect, they provide the first estimates of ground-level annual average concentrations of the pollutant PM 2.5 for all of China over the last decade." (Quoted from article)