Year	micrograms per cubic meter
1990	8.801565223
1995	8.138244898
2000	7.505004878
2005	7.114175404
2010	7.205869088
2011	7.018955644
2012	6.363062222
2013	5.983816655
2014	6.373112438
2015	5.62877844
2016	5.589432223
2017	5.257788389
2018	5.596788665
2019	5.565330197