Year	micrograms per cubic meter
1990	17.51943295
1995	16.43451936
2000	14.35994595
2005	14.05574145
2010	14.70448134
2011	12.99478901
2012	13.0260807
2013	13.0277502
2014	13.3329535
2015	11.80372346
2016	10.78161948
2017	10.19686116
2018	10.48238618
2019	10.37350411