Year	micrograms per cubic meter
1990	25.37778737
1995	25.51417334
2000	24.84005617
2005	23.11401387
2010	23.88204643
2011	23.07978384
2012	23.42327297
2013	23.37482286
2014	23.10280385
2015	23.45888757
2016	22.08084937
2017	23.41488293
2018	23.18486991
2019	23.34521196