Mesoscale wetfall chemistry around Philadelphia during frontal storms

Preliminary results from a mesoscale "acid rain" study are presented. The study concentrates on precipitation-event sampling at a network of ∼40 stations located between 20 km and 60 km from the city of Philadelphia, Pennsylvania, with the goal of establishing the contribution of local emissions to the total ionic wet deposition load and of determining the spatial variability of this deposition on the suburban-rural scale. Two storms associated with weak warm fronts and displaying similar meteorological features were sampled successfully. For the first, urban emissions appear to have significantly affected the deposition of several inorganic ionic species, most notably NO3−; no effect was evident for the second storm. A possible explanation of the apparent contrast is given in terms of the weekday-weekend emissions variability. A sampling density of approximately one collector per 200 to 300 km² appears adequate for both storms.