Year	micrograms per cubic meter
1990	37.86580591
1995	36.98057787
2000	38.54485822
2005	36.89651998
2010	38.03920932
2011	38.52385349
2012	39.05647597
2013	41.03303244
2014	38.07507474
2015	41.60345611
2016	40.36513118
2017	38.36895785
2018	39.50958584
2019	39.30341272