Year	micrograms per cubic meter
1990	41.0299561
1995	39.4431899
2000	40.4530048
2005	40.67379917
2010	38.71368093
2011	44.5453414
2012	45.41253543
2013	49.82200792
2014	49.15322338
2015	47.15602971
2016	42.7322875
2017	42.74423807
2018	44.30760137
2019	44.32957478