Aircraft flights made in June 1990 substantiate the expected variability of ozone and reactive nitrogen constituents in the lower atmosphere over Mississippi and Alabama due to the variety and proximity of sources and the photochemical production and loss of ozone. Urban, biomass burn, electrical power plant, and paper mill plumes are examples of sources encountered during these flights, and some of their characteristics are described. For example, in the urban plume from Mobile, Alabama, an upper limit to the ozone production efficiency was 6--7 parts per billion by volume (ppbv) of O3 per ppbv of NOx oxidized. Emissions from several biomass fires had lower efficiencies of 2--4 ppbv ppbv-1, but the average rate of production of ozone was as high as 58 ppbv hr-1 for one fire where the plume was isolated from mixing to the surface. Close encounters with paper mill and power plant emissions showed chemical consumption of O3, while far-field encounters with power plant plumes showed enhanced O3 due to photochemical production. Observations made in early morning, below a nocturnal inversion provide indirect evidence of significant nighttime oxidation of freshly emitted NOx to reservoir constituents. Although statistics for the region covered by these flights clearly depend on precisely where the aircraft flew, the mean and median temporal and vertical behaviors of constituent mixing ratios are summarized. ¿ 1998 American Geophysical Union