Year	micrograms per cubic meter
1990	38.35866875
1995	36.11607713
2000	34.53332391
2005	35.32246182
2010	35.4185359
2011	29.91406001
2012	31.34154258
2013	33.7865692
2014	29.43028859
2015	28.04174447
2016	30.69239101
2017	26.93362982
2018	27.48165939
2019	27.40271356