Year	micrograms per cubic meter
1990	20.092206834426847
1995	18.667389246329314
2000	17.441551957751464
2005	15.942349963437879
2010	15.824212210531671
2011	16.04811638838035
2012	14.360647000374287
2013	13.918851207064499
2014	12.811381597183276
2015	13.340172053723602
2016	12.143612076031811
2017	12.296339311248557
2018	12.368400465025331
2019	12.23939639452309