Year	micrograms per cubic meter
1990	21.75745986
1995	22.09834847
2000	25.47181528
2005	23.2584201
2010	26.8091823
2011	28.78168174
2012	29.07804791
2013	28.08814029
2014	27.25581094
2015	28.08947284
2016	26.88967903
2017	25.06403775
2018	23.28569035
2019	21.61458718