Year	micrograms per cubic meter
1990	12.72396527
1995	11.89778993
2000	11.0279213
2005	10.4600038
2010	9.902931522
2011	10.06939194
2012	9.157206628
2013	8.786223954
2014	8.522230711
2015	8.381681865
2016	7.838263466
2017	7.664546609
2018	7.916850375
2019	7.854094218