Year	micrograms per cubic meter
1990	47.412578946610054
1995	45.574485544129324
2000	47.734720067780486
2005	47.31533367369279
2010	49.52848573372494
2011	51.47658490102483
2012	53.62557818135363
2013	54.94115085602534
2014	55.296307803802875
2015	53.73617469337188
2016	52.94854561979118
2017	49.31441256884977
2018	49.54477577169691
2019	49.16596278366577