Year	micrograms per cubic meter
1990	17.53520585
1995	17.19593342
2000	17.29704116
2005	16.77624805
2010	17.23567916
2011	16.40831114
2012	17.20652719
2013	16.13110733
2014	16.18184039
2015	15.77724688
2016	16.95211573
2017	15.69097484
2018	15.51584391
2019	15.65130893