Year	micrograms per cubic meter
1990	22.83611795
1995	21.38191513
2000	19.38182436
2005	18.99706651
2010	18.77805621
2011	18.1284545
2012	17.5138754
2013	17.66327367
2014	17.26757325
2015	16.65075046
2016	14.9203705
2017	14.51117826
2018	14.6057706
2019	14.53144476