Year	micrograms per cubic meter
1990	7.6809684
1995	7.580038751
2000	6.817300222
2005	6.214736156
2010	6.296361103
2011	5.57145349
2012	6.242564048
2013	5.995986
2014	6.131923431
2015	6.720400954
2016	6.772591917
2017	6.74257415
2018	6.841077504
2019	6.85168132