Year	micrograms per cubic meter
1990	26.11674239
1995	24.93815019
2000	21.41269573
2005	20.87395739
2010	20.48778139
2011	20.7873617
2012	19.04740747
2013	18.71154684
2014	18.16896079
2015	17.63788699
2016	16.71289546
2017	16.73176229
2018	16.96278201
2019	16.75140093