Year	micrograms per cubic meter
1990	38.84386173851974
1995	38.45079438052494
2000	38.98428158705404
2005	38.52903300885296
2010	39.55493489747366
2011	41.910400063249796
2012	42.02884676982632
2013	44.284095128881816
2014	42.23929660383486
2015	40.24564205446514
2016	37.75110254395102
2017	36.37510374063703
2018	36.08076459274185
2019	35.19506828134704