Loss rate of NO_y from a power plant plume based on aircraft measurements

This study was motivated by the recent work of Buhr et al. [1996] which reported losses of NO_y from large power plant plumes as high as 0.25 hour^-1, much higher than generally accepted values. If true, conclusions pertaining to the efficiency of ozone and nitrate production in the lower troposphere would need major revisions. The results of Buhr et al. were based on aircraft measurements in four TVA (Tennessee Valley Authority) power plant plumes on July 7, 1995, as part of the Nashville/Middle Tennessee Ozone Study, a measurement program of the Southern Oxidants Study (SOS), whereas the results reported in this paper are also based on measurements made in the same SOS study aboard another instrumented aircraft (the TVA helicopter), in plumes of one of these power plants (the Cumberland Steam Plant in northwestern Tennessee) during five different days in 1994 and 1995. Between the 1994 and 1995 sampling periods, emissions of SO₂ at the Cumberland plant were reduced by nearly 95% by installation of scrubbers. Our data from the one 1994 day show that the ratio of excess SO₂ to NO_y in the plume core increased significantly with plume age, indicating a potentially high differential loss rate of NO_y (excess loss of NO_y relative to SO₂) of about 0.12 hour^-1. However, results based on the larger 1995 data set indicate a low differential NO_y loss rate of only 0.00 ± 0.03 hour^-1, consistent with accepted low loss rates. Because the SOS-Nashville/Middle Tennessee Ozone Study was not specifically designed to explore the NO_y loss issue, the question of NO_y loss rates in plumes is not currently resolved and additional focused field studies are needed.