Year	micrograms per cubic meter
1990	26.67318543
1995	25.86689287
2000	25.66637392
2005	26.45818252
2010	27.10947116
2011	23.78443184
2012	23.62616281
2013	25.2377872
2014	22.34041158
2015	21.82366181
2016	22.54560976
2017	20.47951266
2018	20.5326508
2019	20.3898157