Year	micrograms per cubic meter
1990	32.68134958
1995	31.75915858
2000	30.40916722
2005	23.14446696
2010	21.61912278
2011	24.71743397
2012	22.81352392
2013	23.60777306
2014	22.03917349
2015	22.11412862
2016	21.27024268
2017	20.4160723
2018	20.39335761
2019	20.12401203