Year	micrograms per cubic meter
1990	21.56645306
1995	21.54509333
2000	21.85873031
2005	19.86449855
2010	19.89151671
2011	19.11755404
2012	20.05991754
2013	20.17665798
2014	20.14066609
2015	21.86217654
2016	21.04710708
2017	20.75716759
2018	21.15799134
2019	21.22131742