Airborne measurements were performed in the plume of the Cumberland Power Plant during August 1998 using a highly sensitive SO 2 instrument. The measurements confirmed previous suggestions that NO y species are removed from the plume at a faster rate than SO 2 . The differential removal rate (the difference between loss rate of NO y and that of SO 2 ) was estimated to be 0.06h - 1 . This value implies that the NO y loss rate is in the range of 0.09-0.14h - 1 . The application of a mathematical argument, based on the convolution integral, enabled improved synchronization of the data from the SO 2 and NO y instruments. Examination of the synchronized data revealed that the concentration ratio of SO 2 and NO y varies across the plume. Near the source it is higher at the wings of the plume, while in the core of the plume it is similar to the ratio at the release point. Two possible explanations of the observations are discussed: conversion to non-measurable NO y species, and in-plume loss of NO y (as HNO 3 ) via dry deposition.