Year	micrograms per cubic meter
1990	26.88052783
1995	25.24915008
2000	24.24042054
2005	21.57351808
2010	20.29490103
2011	21.4270346
2012	18.84991698
2013	18.13743581
2014	17.10725238
2015	18.04119054
2016	15.75652778
2017	16.09761447
2018	16.26524128
2019	16.08828133