As part of the Northeast oxidant and particle study (NE-OPS) field campaign in the summers of 1999 and 2001, NO/NO_y, CO, and O₃ were measured in the surface layer at the Baxter field site in Northeast Philadelphia. These data will be compared to a climatology of aged air pollutants in a rural setting developed from similar measurements made at the Harvard Forest site in central Massachusetts since 1990.

We address three major questions using this subset of the NE-OPS data. The covariance of NO_y and CO in ambient air yields an estimate of the primary emission ratios for urban pollution sources, and provides a test of emission inventories. Apparent depletion of NO_y relative to CO provides a measure of pollutant aging. Background ozone levels can be estimated by selecting data with the lowest levels of primary pollutants NO_y and CO. By background, we mean the concentration of O₃ in air not significantly impacted by recent addition of pollutants.

We infer an emission ratio of 0.18 molecule NO_y per molecule of CO in the Philadelphia source region. Periods of intense NO_y removal are evident in the data. These periods indicate significant contribution from aged pollutants or very rapid processing of local emissions. An episode of maximum NO_y loss stands out as a period having the highest O₃ concentrations (30-min. average O₃ up to 150 ppb).. The O₃ concentration during mid-day periods with the lowest 10% of CO or NO_y concentrations is 47 ppb at the Baxter site. The corresponding value for Harvard Forest is 41 ppb.