Year	micrograms per cubic meter
1990	36.72380163
1995	36.26619836
2000	38.18887373
2005	35.95591128
2010	34.36075673
2011	38.14529617
2012	34.47534031
2013	32.95117432
2014	31.60560208
2015	32.86433955
2016	28.77587802
2017	29.92213704
2018	29.93448538
2019	29.46633319