Year	micrograms per cubic meter
1990	15.68399331
1995	14.63948418
2000	16.61990036
2005	16.00965098
2010	15.4543969
2011	15.44989787
2012	14.92315966
2013	15.47718389
2014	15.45471741
2015	15.97528537
2016	14.43266888
2017	13.86640498
2018	13.63064322
2019	13.20059479