The researchers investigated nitrogen oxide emissions from a major roadway by measuring NO and NO2 levels at different distances from the road. They found that NOx concentration decreases rapidly with distance due to dispersion and conversion to other compounds. Close to the road, most NOx is NO, which converts to NO2 further away. NO2 levels also decrease with distance, possibly due to reactions with ozone. Emissions from individual vehicles near the analyzer intake caused variations in NO and NOx levels close to the road. The CALINE4 model accurately predicted NO2 concentrations at different distances from the road, with a sensitivity of ± 5 ppb.