A back-trajectory and air mass climatology for the Northern Shenandoah Valley, USA

Synoptic-scale air quality climatologies can often be categorized into trajectory methods based on air parcel movement or air mass approaches that are primarily dictated by thermal and moisture conditions. Here, we examine the relationship between these two essentially independent methods of synoptic-scale climate classification: 72-h Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model back-trajectories and the Spatial Synoptic Classification (SSC) of air masses for Martinsburg, West Virginia, located at the northern end of the Shenandoah Valley, from 1997 to 2006. Statistically significant differences exist between trajectory groups across the six SSC air mass types within each season and between seasons within each air mass type. Further examination of the trajectory distributions reveals a high degree of overlap between trajectories associated with different air mass types. In some cases, up to 50% of one air mass's trajectories are statistically closer to the trajectories of a different air mass type. Accordingly, air mass alone cannot be used to identify specific source regions of pollutants. This suggests that, while an air mass classification system can be used to identify general flow patterns, the integration of both thermodynamic and air-flow variables in a synoptic classification should provide information that is not discernible from either method independently. The inclusion of both back-trajectory and air mass information represents the suite of environmental conditions that may enhance or suppress pollutant concentrations in the Shenandoah Valley better than using either measure independently.