LOSS RATE OF NOY FROM A POWER-PLANT PLUME BASED ON AIRCRAFT MEASUREMENTS

This study was motivated by the recent work of Buhr et al. [1996] whic h reported losses of NOy from large power plant plumes as high as 0.25 hour(-1), much higher than generally accepted values. If true, conclu sions 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 Vall ey Authority) power plant plumes on July 7, 1995, as part of the Nashv ille/Middle Tennessee Ozone Study, a measurement program of the Southe rn Oxidants Study (SOS), whereas the results reported in this paper ar e also based on measurements made ire the same SOS study aboard anothe r instrumented aircraft (the TVA helicopter), in plumes of one of thes e 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 reduc ed bat nearly 95% by installation of scrubbers. Our data from the one 1994 day show that the ratio of excess SO2 to NOy in the plume core in creased significantly with plume age, indicating a potentially high di fferential loss rate of NOy (excess loss of NOy relative to SO2) of ab out 0.12 hour(-1). However, results based on the larger 1995 data set indicate a low differential NOy 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 NOy loss issue, the question of NOy loss rates in plumes is not c urrently resolved and additional focused field studies are needed.