Year	micrograms per cubic meter
1990	22.75636946228621
1995	21.911877006139775
2000	21.447828400209723
2005	19.292762248054938
2010	19.242395389786147
2011	20.3595523592103
2012	19.273580464658643
2013	19.44655305918608
2014	18.882060205807306
2015	18.641977025091833
2016	18.143423481094796
2017	17.75510213926461
2018	17.846589454671047
2019	17.77390081997376