Year	micrograms per cubic meter
1990	45.94933521518175
1995	44.24407936998841
2000	46.31821831208301
2005	45.97711906420822
2010	48.22747111322471
2011	50.06755386580439
2012	52.107380056126935
2013	53.36765928697996
2014	53.72557310677579
2015	52.21745865803125
2016	51.466477585188315
2017	47.96806968309511
2018	48.208169399826026
2019	47.86036846222433