Year	micrograms per cubic meter
1990	51.84606211
1995	49.36244134
2000	54.88701913
2005	54.9070878
2010	54.03992338
2011	51.23693236
2012	54.18097152
2013	47.34169055
2014	48.4137269
2015	60.27252262
2016	56.2288836
2017	53.53072599
2018	53.64967602
2019	53.67033962