Year	micrograms per cubic meter
1990	21.02891948
1995	20.87261133
2000	22.20915288
2005	21.10676982
2010	21.13580447
2011	22.4347061
2012	21.42944248
2013	22.96134961
2014	22.30595213
2015	23.55853654
2016	23.76052341
2017	21.93712901
2018	22.72046987
2019	22.76585719