if you want the coolest air available the experiments are clear: the hottest air is near the dark sun heated pavement and the coolest air is high up. the difference changes with the season but can be 20 degrees even in winter
many highway transportation departments monitor this
http://rwis.kytc.ky.gov/maps/tempmap2.htmthere are similar results for the cleanliness of the air versus height above pavement
as with other small changes less than 5%
only a two vehicle "convoy" style test
called a SAE/TMC Type IV
can reliably detect air intake differences
