Year	micrograms per cubic meter
1990	48.0301721131652
1995	46.22342086326031
2000	48.365378042882824
2005	48.07604717576197
2010	50.449701408018235
2011	52.45400118766793
2012	54.7380432046257
2013	56.33707648094427
2014	56.89531799965504
2015	54.79716785716239
2016	54.1567669992031
2017	50.26054876266563
2018	50.52609077571745
2019	50.15594178516976