Year	micrograms per cubic meter
1990	28.40140577
1995	31.27209259
2000	29.1775103
2005	33.33893938
2010	35.89120302
2011	33.72917392
2012	33.23273929
2013	35.54875145
2014	33.92842262
2015	35.43994788
2016	32.03573347
2017	32.5078793
2018	31.45267441
2019	30.64558855