Year	micrograms per cubic meter
1990	17.97383325
1995	16.70703907
2000	14.88087098
2005	13.06616084
2010	13.15189172
2011	13.81779708
2012	12.71146517
2013	12.47195588
2014	10.60883998
2015	11.59075488
2016	10.60281759
2017	9.854778198
2018	10.15005926
2019	9.902709701