Air pollutant concentrations were studied in Highway 71, an arterial highway, dominated by passenger vehicles located in Austin, TX. The air pollutants studied include ultrafine particles (UFP, < 100 nm dia), CO, NOx, VOC, several particle-bound organics (PAH, alkanes, hopanes), several carbonyl species, and PM2.5 mass and carbon content. The chemical interactions associated with the observed fall-off measurements for gas phase pollutants and particle-bound organic species were studied. Mobile measurements for gas phase pollutants showed that pollutants exhibit exponential decay characteristics. CO concentrations decreased comparably to total NOx concentrations with increasing distance from the roadway. NO and NO2 concentrations indicated that NO was converting to NO2 via reaction with O3 pn the length scale of total NOx falloff. Both NO and total NOx decayed exponentially, but NO2 decayed less rapidly because it was chemically generated. Near Highway 71, formaldehyde showed decreasing concentrations with increasing distance downwind of the roadway. Acetaldehyde increased in concentration at the far downwind site. This is an abstract of a paper presented at the 101st AWMA Annual Conference and Exhibition (Portland, OR 6/24-27/2008).