Year	micrograms per cubic meter
1990	45.2320172
1995	44.42833063
2000	45.264541
2005	41.7241462
2010	43.8549236
2011	46.92326876
2012	47.17214767
2013	44.7263164
2014	42.4180555
2015	48.22091955
2016	45.52426043
2017	44.3882319
2018	46.10881594
2019	46.3767539