Year	micrograms per cubic meter
1990	37.4439452
1995	34.43226535
2000	41.34572294
2005	39.49592692
2010	39.24576366
2011	41.0035213
2012	41.19727621
2013	40.22177745
2014	35.86344781
2015	40.60479868
2016	37.94147452
2017	38.66290029
2018	37.97705763
2019	37.55277786