Year	micrograms per cubic meter
1990	18.55297943
1995	17.22733302
2000	16.07573118
2005	14.89539999
2010	15.07698542
2011	14.99131566
2012	13.68529611
2013	13.61154039
2014	11.58971968
2015	12.30792454
2016	11.4918237
2017	11.43000879
2018	11.53704439
2019	11.40575038