Year	micrograms per cubic meter
1990	6.724221144
1995	6.677788857
2000	5.899967661
2005	6.073849362
2010	5.822674196
2011	5.495040024
2012	5.582985487
2013	5.575990242
2014	5.977969295
2015	6.029516789
2016	6.543290084
2017	6.339296397
2018	6.35907699
2019	6.300155035