Author Steven Q. Andrews analyzes the discrepancy between reported blue sky days in Beijing and published monitoring station data in his report, "Inconsistencies in air quality metrics: 'Blue Sky' days and PM10 concentrations in Beijing." This report, published in Environmental Research Letters, indicated that although nine continuous years of air quality improvement has been reported in Beijing between 1998 and 2007, these improvements, as indicated by the annual number of `Blue Sky' days, are due to irregularities in the monitoring and reporting of air quality and not to less polluted air.