Year	micrograms per cubic meter
1990	21.606939529244563
1995	20.258765632221397
2000	18.623337524108376
2005	17.172220564718206
2010	17.045457470834837
2011	17.320892855641922
2012	15.52069596364663
2013	14.976761904499192
2014	14.14549720435126
2015	14.371246656648657
2016	13.220931189987779
2017	13.479646685383353
2018	13.527681472720895
2019	13.399150604781388