Year	micrograms per cubic meter
1990	21.6841061
1995	21.53427898
2000	21.81550263
2005	19.66107509
2010	19.82709385
2011	19.10315605
2012	20.0647388
2013	20.08742225
2014	20.26503477
2015	21.94013568
2016	21.17807748
2017	21.00966255
2018	21.38302419
2019	21.48591655