Year	micrograms per cubic meter
1990	18.11745994
1995	18.1303641
2000	18.17867165
2005	16.5773788
2010	16.89122391
2011	16.08669375
2012	16.78599453
2013	16.94753609
2014	16.48732491
2015	18.14588457
2016	17.6733477
2017	17.18390999
2018	17.5653142
2019	17.59200058