Year	micrograms per cubic meter
1990	22.56455030549933
1995	21.743627058585126
2000	21.319640684080944
2005	19.193024736679455
2010	19.133623110496906
2011	20.235457758000027
2012	19.186864735830383
2013	19.357906150932514
2014	18.817678538885485
2015	18.597840085509134
2016	18.100532262814575
2017	17.705476837967055
2018	17.795382547468815
2019	17.71615249793444