Precipitation chemistry data collected on an event basis at Ithaca, New York, were interpreted using the ARL trajectory model and meteorological maps. Forty-five events were separated according to trajectory directions and were categorized by using four compass sectors around the collection site. It was shown that the acid precipitation (pH<5.6) came predominantly from the SW sector, where the largest anthropogenic sources of acid-forming gases exist. The NW sector also showed high acidity. The method described in this paper will prove useful in the future for identifying general sources of acid precipitation as increased amounts of fossil fuel are burned.