Year	micrograms per cubic meter
1990	58.50614317
1995	49.43767636
2000	60.34547396
2005	52.84830506
2010	50.08807979
2011	53.47816985
2012	54.52706673
2013	47.78909296
2014	46.9526973
2015	60.48227972
2016	59.661617
2017	55.93672757
2018	54.23685549
2019	53.98926008