Prediction of deleterious air quality episodes along the Northeast corridor from Baltimore-Washington to Boston, MA can be confounded by the frequent occurrence of local and regional scale circulations induced by coastal-to-piedmont gradients and complicated by the convoluted coastline. During a nine-week field campaign in 1999 and a month-long intensive in summer 2001, an extensive suite of surface and aloft instrument platforms deployed near Philadelphia, PA as part of the North American Research Strategy for Tropospheric Ozone-Northeast Oxidant and Particle Study (NARSTO-NE-OPS) documented several circulations that significantly modified the air mass and the air chemistry. Sea and Bay convergence zones are capable of increasing wind speed, water vapor mixing ratio, and particle scattering, while reducing ozone concentration and other criteria pollutants all by a factor of two in less than 10 minutes. These 100 meter-deep layers occur in the late afternoon and stabilize the surface layer and modify the daytime boundary layer to suppress convection in the marine air mass while enhancing it along the meso-front. Outflow boundaries from distant convective systems can have a similar effect. Numerical models often fail to capture the strength and timing of such events.

The nocturnal period is often dominated by low-level jets that can transport significant quantities of pollutants hundreds of kilometers along the Northeast corridor. The jets are a consequence of daytime differential heating and cooling between the coast and the Appalachian Mountains and form when the inertial adjustment of the ageostrophic wind commences with the rapid cessation of turbulence attending the developing nocturnal inversion. When the jet reaches its maximum after midnight, the wind can attain speeds nearly twice that supported by the mass field (~ 15-18 m s-1) creating vertical shear instabilities that drive downward momentum and pollutant transport to the surface creating periodic "bursting" events accompanied by rapid increases in particle and trace gas concentrations. Numerical models still have difficulty simulating the strength and vertical profile of low-level jets.

This paper reports on several local and regional circulation events observed during the NARSTO-NE-OPS 1999-2001 field campaigns and their influence on surface and aloft ozone and fine particle concentrations. Measurements of meteorological variables, ozone, and PM2.5 obtained using Raman lidar, tethered balloon sounding systems, aircraft, and a suite of surface instruments are used to characterize these events and provide input and validation for numerical models.