Trajectory Analysis of Summertime Sulfate Concentrations in the Northeastern United States

Abstract

This paper presents a technique for quantifying the relationships between observed concentrations of atmospheric sulfate aerosol and their corresponding upstream history of sulfur dioxide emissions, wind speed and mixing height. Using reported sulfate concentrations from several sampling sites in the northeastern United States, 72 h upstream trajectories have been computed for winds in the mixed layer of the atmosphere over the duration of their respective sampling periods. Trajectories from one site were computed for four sublayers, each 400 m thick, extending from the surface to 1600 m. The deviations in along-trajectory and cross-trajectory directions of each of the sublayers from the position of the whole mixed layer were computed. From this the functions σy(t) and σx(t) for travel times of 6 to 72 h were derived for each layer individually and collectively for the whole layer. The values of σy(t) and σx(t) for the whole mixed-layer were found to be roughly equivalent over this time period and to grow linearly in time. The growth can be described by the relationship σy(t) = 5.4t , where σy is in kilometers and t is in hours. Using these statistics to describe the potential impact from upstream sources, each trajectory was integrated over finite time steps to estimate the potential emissions loading along that trajectory as a function of time upstream. Correspondingly, estimates were made as a function of time upstream of the wind speed in the layer and the depth of the mixed layer. It was found that sulfate concentrations were insensitive to upstream mixing height, as determined in this study, but were almost always positively correlated with the inverse of wind speed occurring 24 h or more upstream of the sampling point. No consistent relationship was evident between sulfate concentrations and potential upstream SO2 emissions loading. The ratio of observed to potential sulfate generation, as determined from the total upstream sulfur dioxide input, indicates an average net efficiency of roughly 15?30% conversion of sulfate dioxide to sulfate before deposition. These values vary dramatically with increasing efficiency corresponding to increasing resultant sulfate concentration.