Year	micrograms per cubic meter
1990	43.382997414855666
1995	42.94637293056721
2000	43.55311929433637
2005	43.77365159965458
2010	44.67312806112407
2011	47.041756512926725
2012	47.678910412245564
2013	50.867408314847154
2014	48.08583227201115
2015	45.31484178636296
2016	42.450558485201704
2017	40.544223999953466
2018	40.20642054196652
2019	39.11873039915321