Year	micrograms per cubic meter
1990	24.9474816
1995	24.12933961
2000	24.30286936
2005	22.16948971
2010	21.63144957
2011	23.414815
2012	21.55597679
2013	20.05028087
2014	19.94217042
2015	19.54397528
2016	17.81730527
2017	19.01906113
2018	18.82011853
2019	18.63878032