Year	micrograms per cubic meter
1990	49.74982033
1995	44.07335811
2000	50.10416571
2005	51.32188604
2010	52.82073801
2011	54.42324069
2012	54.8566288
2013	52.4865762
2014	46.72140545
2015	52.80306653
2016	51.51145712
2017	53.48836405
2018	53.77109049
2019	54.67729786