Year	micrograms per cubic meter
1990	41.71304485978722
1995	40.02516855612292
2000	42.38001008274379
2005	41.82500290624148
2010	42.91275469561925
2011	44.13173799516952
2012	46.823870918527355
2013	45.06808097704782
2014	45.256265954279364
2015	46.978270109032536
2016	45.595633272242395
2017	43.34850600610331
2018	43.434526522323296
2019	43.185326373305614