Year	micrograms per cubic meter
1990	43.64239799
1995	39.6555777
2000	43.15936887
2005	42.79003956
2010	41.07175478
2011	44.83063053
2012	46.57105523
2013	44.05922815
2014	40.65538072
2015	44.0639596
2016	42.27135251
2017	44.03325236
2018	44.31794214
2019	44.57247023