Table 2 Ratio and Spearman correlation coefficient (r) between pollutants measured at three (urban background, kerbside and rural) monitoring stations in Copenhagen

	Kerbside/urban background (22 September 2004 to 26 December 2005), n = 105		Rural/urban background (22 April 2005 to 11 September 2005), n = 47
	Ratio*	r	Ratio	r
NC†_tot	4.6	0.62	0.7	0.80
NC_a12	25.2	0.25	1.0	0.22
NC_a23	7.6	0.63	0.2	0.48
NC_a57	2.9	0.75	0.8	0.91
NC_a212	1.0	0.95	0.9	0.96
PM₁₀	1.7	0.65	1.0	0.86
PM_2.5	1.2	0.86	–	–

*Ratio of pollutant number (NC_tot, NC_a12, NC_a23, NC_a57, NC_a212) or mass (PM_10, PM_2.5) concentrations at two measuring stations.
†NC, number concentration; tot, total (all particles 6–700 nm in diameter); a12, size mode with median diameter of 12 nm; a23, size mode with median diameter of 23 nm; a57, size median with mean diameter of 57 nm; a212, size mode with median diameter of 212 nm.