Year	micrograms per cubic meter
1990	27.24195361
1995	26.73191036
2000	22.72320586
2005	22.19434728
2010	21.47373959
2011	22.60332523
2012	20.33063119
2013	19.63711495
2014	19.3125724
2015	19.11555836
2016	17.8694289
2017	18.2454102
2018	18.49542534
2019	18.36521464