Year	micrograms per cubic meter
1990	43.41338388402802
1995	42.99207524911911
2000	43.592881388338185
2005	43.812944515484304
2010	44.748208761817764
2011	47.07273505953541
2012	47.70685444767946
2013	50.879882823051325
2014	48.07188849355684
2015	45.291332277345575
2016	42.446540455570016
2017	40.51642339903715
2018	40.15511466217694
2019	39.0537560332859