Year	micrograms per cubic meter
1990	24.69631748
1995	27.11934085
2000	28.73172483
2005	21.93514106
2010	26.09846929
2011	32.27881291
2012	30.77259876
2013	30.55977858
2014	30.61966084
2015	28.62109508
2016	27.51227497
2017	27.12775327
2018	26.54690284
2019	26.19766251