Year	micrograms per cubic meter
1990	20.24931985
1995	20.1194563
2000	21.46914658
2005	20.93139471
2010	23.04711653
2011	24.33177624
2012	22.43318785
2013	22.52866596
2014	21.94482038
2015	22.22732307
2016	21.83009872
2017	21.45612493
2018	21.67255829
2019	21.74698878