Year	micrograms per cubic meter
1990	27.276857984985053
1995	26.42313160280171
2000	23.386303020091326
2005	22.446528130325778
2010	22.379877450365782
2011	22.87640718481992
2012	20.76868384991052
2013	19.902040518524593
2014	19.962891649045158
2015	19.2058018292805
2016	18.12853519110921
2017	18.689235810944336
2018	18.759687814237516
2019	18.639740834345567