Year	micrograms per cubic meter
1990	60.38782104
1995	59.87209076
2000	59.47831452
2005	55.3530761
2010	54.96105771
2011	56.9350963
2012	62.12654192
2013	51.51717091
2014	53.61781493
2015	61.80443087
2016	57.52369277
2017	61.41990384
2018	58.00293314
2019	58.08072376