Year	micrograms per cubic meter
1990	10.28738981
1995	10.52681081
2000	10.60395119
2005	11.02061012
2010	12.10848854
2011	11.69813051
2012	11.76206355
2013	12.07044801
2014	12.31045976
2015	12.49008643
2016	12.81538467
2017	12.32052248
2018	12.75980934
2019	12.89495904