Year	micrograms per cubic meter
1990	39.49893211
1995	39.35947828
2000	39.65498549
2005	35.92843565
2010	35.88052514
2011	36.98630498
2012	39.0123562
2013	38.18559351
2014	36.66375068
2015	40.27107846
2016	37.9286139
2017	35.59581848
2018	36.47254408
2019	35.86088793