Year	micrograms per cubic meter
1990	22.69528563
1995	26.19512957
2000	24.5397947
2005	23.1507772
2010	25.08164307
2011	27.00590351
2012	28.76775881
2013	28.23862769
2014	27.48496809
2015	26.06618737
2016	24.38549474
2017	25.0632901
2018	25.13594199
2019	25.4950546