Year	micrograms per cubic meter
1990	6.745468652
1995	6.628074959
2000	6.321862093
2005	6.361755729
2010	6.028100954
2011	5.785407287
2012	6.13427501
2013	6.082991842
2014	6.378015917
2015	6.226964273
2016	6.472452957
2017	6.616395462
2018	6.515998783
2019	6.462977097