Effects of NO_xcontrol and plume mixing on nighttime chemical processing of plumes from coal-fired power plants

[1] Coal-fired electric power plants produce a large fraction of total U.S. NO_x emissions, but NO_x from this sector has been declining in the last decade owing to installation of control technology. Nighttime aircraft intercepts of plumes from two different Texas power plants (Oklaunion near Wichita Falls and W. A. Parish near Houston) with different control technologies demonstrate the effect of these reductions on nighttime NO_xoxidation rates. The analysis shows that the spatial extent of nighttime-emitted plumes to be quite limited and that mixing of highly concentrated plume NO_x with ambient ozone is a determining factor for its nighttime oxidation. The plume from the uncontrolled plant had full titration of ozone through 74 km/2.4 h of downwind transport that suppressed nighttime oxidation of NO₂ to higher oxides of nitrogen across the majority of the plume. The plume from the controlled plant did not have sufficient NO_x to titrate background ozone, which led to rapid nighttime oxidation of NO₂ during downwind transport. A plume model that includes horizontal mixing and nighttime chemistry reproduces the observed structures of the nitrogen species in the plumes from the two plants. The model shows that NO_x controls not only reduce the emissions directly but also lead to an additional overnight NO_x loss of 36–44% on average. The maximum reduction for 12 h of transport in darkness was 73%. The results imply that power plant NO_xemissions controls may produce a larger than linear reduction in next-day, downwind ozone production following nighttime transport.