Year	micrograms per cubic meter
1990	36.18728766
1995	36.8562904
2000	38.80035805
2005	35.81545959
2010	36.27169287
2011	37.75459321
2012	40.28931528
2013	38.04487801
2014	37.75661455
2015	38.64241689
2016	38.48510282
2017	35.57288206
2018	36.6600241
2019	36.18491574