Year	micrograms per cubic meter
1990	22.96583352
1995	21.81013588
2000	24.17892698
2005	24.07469414
2010	24.69380238
2011	21.56928542
2012	22.42414797
2013	22.99523942
2014	20.96900691
2015	18.97034499
2016	20.7396319
2017	19.92492371
2018	19.69446106
2019	19.96439581