Year	micrograms per cubic meter
1990	26.11057713
1995	25.65266755
2000	23.65186739
2005	23.07265013
2010	21.21532951
2011	23.24147685
2012	20.57402212
2013	20.04200652
2014	19.54244679
2015	19.94360466
2016	17.72824296
2017	18.34290048
2018	18.62479372
2019	18.46495986