Year	micrograms per cubic meter
1990	40.86085275588554
1995	39.508601102781185
2000	41.312633399055095
2005	41.036829663197736
2010	42.97451561425862
2011	44.6673038134341
2012	46.33396856328058
2013	47.423606168396326
2014	47.65394643987676
2015	46.47201422395311
2016	45.704441042051265
2017	42.837750504668385
2018	43.07753416196032
2019	42.80284066528663