On 6-7 July 2002 a nearly stationary upper level low over Maine produced an extended fetch of northerly flow that transported smoke into the mid-Atlantic and New England regions from wildfires 1500 kilometers north of Philadelphia, PA, where an air quality study was being conducted. Continuous measurements of trace gas concentrations (CO, SO2, O3, NO/NO2/NOX), fine particle mass from three ambient samplers (TEOM, CAMM, and RAMS), EC/OC, b scattering coefficients of total particulates and dry PM2.5, and conventional meteorological data were collected at the surface, while a tethered balloon and RASS profiler were used to document aloft distributions of PM2.5, virtual temperature, and wind velocity.

The smoke plume radically altered the local atmospheric and air quality conditions around Philadelphia. While trace gas concentrations remained relatively low (e.g., O3 < 50 ppbv; SO2 < 5 ppbv on 6 July) under steady, brisk northerly flow from Canada and reduced solar irradiance, CO concentrations, total scattering coefficients, and fine particle mass increased rapidly by factors of 5-10 with a concurrent decrease in visibility to one kilometer within an hour after the onset of the event. Contrary to forecasts calling for a period of dry continental air, relative humidity remained elevated at 60% even as temperatures on both days peaked at 32 C. The increased moisture and high concentrations of fine particles resulted in aloft concentrations of PM2.5 nearly an order of magnitude greater (0.6 mg/m3) than those typically observed for a day with deleterious air quality (0.06 ? 0.2 mg/m3).

Preliminary results are presented that provide key insights into the degradation of an otherwise clean, dry, continental air mass due to the injection of smoke from wildfires. The 6-7 July smoke event provides an interesting study of the effects of particles on the degradation of air quality and visibility without being contaminated by the influence of high criteria gas concentrations and their precursors.