Year	micrograms per cubic meter
1990	32.60394534
1995	33.23779288
2000	29.91257847
2005	31.16942288
2010	35.54506403
2011	32.78115955
2012	31.68059462
2013	33.17491042
2014	32.65425575
2015	33.99185965
2016	31.86869856
2017	31.4019564
2018	31.37040945
2019	30.95362503