Correlation of particulate matter 2.5 (PM2.5) concentrations from 2006 to 2011 using Spearman’s analysis
| Year | 2006 | 2007 | 2008 | 2009 | 2010 | 2011 |
| 2006 | 1.00 | 0.97* | 0.96* | 0.96* | 0.95* | 0.94* |
| 2007 | – | 1.00 | 0.98* | 0.97* | 0.97* | 0.95* |
| 2008 | – | – | 1.00 | 0.98* | 0.96* | 0.94* |
| 2009 | – | – | – | 1.00 | 0.98* | 0.96* |
| 2010 | – | – | – | – | 1.00 | 0.96* |
| 2011 | – | – | – | – | – | 1.00 |
*P<0.05.