Year	micrograms per cubic meter
1990	35.76193715
1995	35.12029354
2000	33.39348749
2005	36.57941224
2010	37.52067534
2011	31.16283822
2012	34.9152645
2013	36.13453786
2014	32.27396916
2015	31.18146951
2016	32.41346793
2017	29.10398763
2018	29.57682003
2019	29.43314277