Scales of sulfur concentrations and deposition from the perspective of the receptor

Median source-receptor separation distances and atmospheric transport times for atmospheric [SO2] and [SO4[2−] and wet and dry deposition of S are calculated from the perspective of the receptor. The temporal and spatial separations are weighted according to the relative contribution of the source to the total atmospheric concentration or deposition at the receptor as calculated with the Advanced Statistical Trajectory Regional Air Pollution (ASTRAP) model; thus, the scales are functions not just of climatological conditions and processes of atmospheric chemistry (as resolved and treated by the regional model) but also of the emission distribution. Calculations are made for a grid of receptor points covering the United States, Canada, and northern Mexico. The minimum spatial and temporal scale lengths occur in the principal emission region, the Ohio Valley, and near isolated western sources; maximum scale lengths occur in remote regions without significant local emissions. The spatial scale of [SO42−] is typically almost twice as large as that of [SO2] or dry deposition of S in the eastern United States, and the spatial scale of wet deposition of S is slightly greater than that for [SO42−]. The temporal scales of [SO42−] and wet deposition of S are two to three times those of [SO2] and dry deposition of S in the eastern United States. Weighted-mean spatial separations are larger than median separations by roughly 25% except in areas remote from sources where they are about equal. Over most of the region scales for wet deposition of S are shortest in summer and longest in winter.