A RECEPTOR-ORIENTED METHODOLOGY FOR DETERMINING SOURCE REGIONS OF PARTICULATE SULFATE OBSERVED AT DORSET, ONTARIO

A statistical receptor-oriented model was developed for long-range tra nsport of atmospheric sulfate to Dorset (elevation 320 mk, latitude 45 13'26'' N and longitude of 7855'52'' W), Ontario. This model computes the potential for sources within 1 latitude by 1 longitude grid cells across North America that contribute to the airborne concentrations me asured at the ground station at Dorset. Airborne concentration data an d air parcel backward trajectories were incorporated explicitly in the model calculation to identify the geographical areas of potential con tributing sources. The present model is qualitative in nature; however , it provides a reasonable receptor-oriented approach to examine the l ong-range transport of atmospheric species. In order to fully understa nd the methodology and in a hope to optimize it, several aspects of th e PSCF methodology have been examined in detailed in this study. Resul ts of this study are presented that suggest interpolation of trajector y endpoints to increase the counting statistics for the potential sour ce contribution function (PSCF) values is not reliable. The average co ncentration provides a reasonable criterion value; however, using the fiftieth percentile value as the criterion point provides an opportuni ty for identifying source areas that cannot be previously found by usi ng the average concentration. The fiftieth percentile value may be a b etter choice for the particulate sulfate data in this case since Dorse t is a relatively clean background site. Using the seventy-fifth perce ntile, which is generally larger than the average, may not be suitable because it reduces the number of degrees of freedom. This could rende r the model to behave like a regular trajectory analysis model that ha s been used commonly for analyzing episodic pollution events. Separati on of data into summer and winter periods is useful to illustrate the effects of photochemistry and meteorology on the PSCF results. Invokin g the total probability concept and examining the trajectory arrival a t different heights directly above the sampling site, the total PSCF w as computed. This resultant function thus provides a time-integrated g eographical map useful for identifying sources of airborne particulate sulfate in a receptor-oriented manner.