Year	micrograms per cubic meter
1990	35.10106433
1995	33.84334506
2000	36.71562148
2005	39.28993221
2010	39.72804014
2011	38.04491799
2012	44.30762611
2013	42.70851915
2014	48.53903271
2015	44.4829243
2016	43.64904826
2017	40.16357364
2018	40.41256497
2019	40.29859817