Year	micrograms per cubic meter
1990	7.742750421
1995	7.754227815
2000	10.99824794
2005	11.098569
2010	10.93396761
2011	10.98057046
2012	10.99967408
2013	11.14022253
2014	10.73278375
2015	10.62224564
2016	11.08714633
2017	11.06543703
2018	10.98446945
2019	10.9932094