Year	micrograms per cubic meter
1990	13.11132222
1995	12.74090733
2000	12.79177676
2005	12.05741833
2010	11.52530862
2011	12.67886893
2012	13.106628
2013	13.36533634
2014	13.71386106
2015	13.40887797
2016	13.35633595
2017	13.570691
2018	13.47597168
2019	13.46980666