Year	micrograms per cubic meter
1990	22.35879537
1995	21.87502321
2000	22.68514994
2005	22.00218626
2010	23.25243023
2011	23.61494743
2012	23.59782449
2013	23.70516014
2014	23.55656662
2015	24.01223447
2016	23.12560737
2017	22.13970265
2018	21.54200034
2019	20.77406654