Year	micrograms per cubic meter
1990	26.41036352
1995	26.04456945
2000	25.38939064
2005	25.8689361
2010	26.37801788
2011	21.75888724
2012	22.99309691
2013	24.66969698
2014	21.54907826
2015	20.78189635
2016	23.40354416
2017	19.26088509
2018	20.50574635
2019	20.54733513