Year	micrograms per cubic meter
1990	45.60135732
1995	44.04414579
2000	41.70419101
2005	37.15307677
2010	35.8443433
2011	41.05690857
2012	36.79534047
2013	33.90486411
2014	34.79278398
2015	32.55975286
2016	30.49533977
2017	31.43565921
2018	30.88528202
2019	30.26931856