Year	micrograms per cubic meter
1990	33.56379431
1995	35.28282568
2000	40.96379882
2005	32.78807708
2010	34.51170734
2011	46.03697934
2012	41.23014791
2013	42.28231885
2014	39.63461873
2015	38.97318659
2016	38.13679678
2017	38.4793437
2018	38.14076689
2019	38.31858602