Year	micrograms per cubic meter
1990	21.28759264
1995	20.10142092
2000	17.30168358
2005	16.47804063
2010	15.7085395
2011	16.0961842
2012	14.50094818
2013	14.50183038
2014	12.81319702
2015	13.35116934
2016	12.30410306
2017	11.96813105
2018	12.31952644
2019	12.16821803