Year	micrograms per cubic meter
1990	10.09239222
1995	9.768846476
2000	9.200371951
2005	9.020255828
2010	7.884019371
2011	7.340076929
2012	7.600495484
2013	7.275026619
2014	7.492005176
2015	7.279802012
2016	6.205823409
2017	7.281874712
2018	7.07877626
2019	7.100156005