A new portable open-path instrument for ambient NH3 and on-road emission measurements

•A portable open-path instrument based on DOAS was developed for mobile measurement of ambient NH3.•The on-road NH3 and NO concentrations were measured for the arterial highway of Shanghai.•The measured NH3/NO was associated to the proportions of light gasoline vehicles and heavy-duty diesel vehicles in the fleet.•The mileage-based NH3 emission factor was estimated to be 17.9 ± 6.3 mg/km per vehicle based on the tunnel measurement.