Year	micrograms per cubic meter
1990	14.26030327
1995	12.89488469
2000	13.6184311
2005	12.17828755
2010	11.25750375
2011	12.31364419
2012	11.21333853
2013	10.22088866
2014	9.589170072
2015	10.34568624
2016	8.943875425
2017	9.137722902
2018	9.181820028
2019	9.066401367