Year	micrograms per cubic meter
1990	28.04612218
1995	26.69048619
2000	26.03477866
2005	23.85248605
2010	22.7674356
2011	25.5775453
2012	22.6468314
2013	21.22186088
2014	21.63806967
2015	20.32357683
2016	19.40617985
2017	19.66415885
2018	19.65003099
2019	19.36594583