Year	micrograms per cubic meter
1990	41.473630163395576
1995	39.62467074272815
2000	42.3986941692503
2005	41.6292473924623
2010	42.55650753648631
2011	43.923013491793036
2012	46.31267444751961
2013	44.49735431538934
2014	44.57341585348229
2015	46.84952404583708
2016	45.5115251463003
2017	43.22530358721612
2018	43.274675142136644
2019	43.048088432773916