Back-trajectory-based source apportionment of airborne sulfur and nitrogen concentrations at Rocky Mountain National Park, Colorado, USA

► New ensemble method for air quality source apportionment receptor model discussed. ► Trajectory Mass Balance used to attribute nitrogen and sulfur concentrations. ► Half ammonia at Rocky Mountain National Park, CO, USA estimated from within state. ► Third of sulfate at Rocky Mountain National Park, Colorado, USA from within state. ► Trajectory Mass Balance insensitive to many parameters if all sources included.