Year	micrograms per cubic meter
1990	19.00295967
1995	19.19401083
2000	17.21162364
2005	16.51679403
2010	17.80213302
2011	19.82880145
2012	20.13867477
2013	19.58915924
2014	19.38130856
2015	18.48419633
2016	17.68579608
2017	18.02704398
2018	17.81426399
2019	17.85586499