highly significantp = 8 × 10 −12
A paired t test on measurements at 0.8 and 2.0 m above the ground for all PDT deployments less than 5 m from the kerbside (a working definition of ‘roadside’ for local air quality assessment (Defra 2009 )) demonstrated highly significant evidence for a difference in NO 2 concentration between the two heights ( p = 8 × 10 −12 , n = 84, Table 1 ).