Year	micrograms per cubic meter
1990	23.014996534364744
1995	22.111510103383658
2000	21.507454987714777
2005	19.226149223326953
2010	19.033432140349483
2011	20.150601740371766
2012	19.114333765166986
2013	19.24029197888644
2014	18.691357199640812
2015	18.373123068455445
2016	17.847730840875325
2017	17.496748036075243
2018	17.562083705228687
2019	17.47894326047695