Year	micrograms per cubic meter
1990	49.28239771
1995	48.5648929
2000	51.55453373
2005	46.15192895
2010	51.82178913
2011	56.24722763
2012	54.70023262
2013	58.79165857
2014	61.86532919
2015	60.59701482
2016	57.20370424
2017	53.35536322
2018	52.80462038
2019	52.41704109