Year	micrograms per cubic meter
1990	21.207683231456336
1995	21.26901270754577
2000	22.385555626158258
2005	22.039417392773363
2010	22.409681251319753
2011	22.816971276732808
2012	23.529692938481002
2013	22.769441886629917
2014	22.358028699084112
2015	23.62573635819771
2016	22.63339601835202
2017	22.516082516075038
2018	22.43506312491186
2019	22.279812670616042