Year	micrograms per cubic meter
1990	23.58068506
1995	23.01906852
2000	22.6202028
2005	21.00124164
2010	18.69962284
2011	21.04575637
2012	19.6942048
2013	22.42094081
2014	21.65038865
2015	22.05209141
2016	21.08657207
2017	20.95634663
2018	21.26808522
2019	21.17639652