Nearby events may interfere with this simple ratio-based comparison. To assess the potential impact of multiple emission events, sensitivity analyses were performed in which additional large (1000 kg/h) and smaller (100 kg/h) emission events were placed randomly at locations within 25 km of each of the three primary locations with simulated emission events. One large event and three smaller events were placed within each 25 km radius. Enhancements in total column concentrations due to these nearby events, at the location of the primary event, were calculated (ERnearby, Eq. 2).