Year	micrograms per cubic meter
1990	22.41327571
1995	22.71361857
2000	26.43412436
2005	22.24598196
2010	21.69833304
2011	20.95555676
2012	22.32520255
2013	21.66481921
2014	21.30120727
2015	22.92175634
2016	22.33097583
2017	21.43446346
2018	20.8255222
2019	20.10090382