Year	micrograms per cubic meter
1990	45.48412352
1995	41.46454464
2000	47.6135595
2005	41.76793893
2010	41.36262362
2011	41.77270482
2012	43.41596784
2013	39.1788092
2014	39.31147497
2015	49.03490782
2016	46.48119179
2017	45.03636256
2018	46.24556121
2019	46.8584555