Ozone production efficiency and loss of NOx in power plant plumes: Photochemical model and interpretation of measurements in Tennessee

A model for photochemical evolution in power plants has been developed and used in combination with measurements to investigate ozone production efficiency (OPE) per NO x and the rate of photochemical removal of NO x . The model is a two‐dimensional (2‐D) Lagrangian model with 1 km horizontal resolution and vertical resolution ranging from 25 m to 300 m, nested within a larger 3‐D regional model. These results are compared with measured O 3 , No x , and SO 2 along aircraft transects through power plant plumes ( Ryerson et al ., 1998). Measured flux of NO x and SO 2 in plume transects is 33%–50% lower than plume emission rates, suggesting that a fraction of plume emissions remained above the top of the convective mixed layer. The measured loss rate of SO 2 is greater than expected from photochemistry and surface deposition, suggesting the possibility of cumulus venting of the boundary layer. If model parameters are adjusted to reflect this hypothesized transport of plume air above the boundary layer, then the model NO x removal rate agrees with measurements. OPE is derived from measured fluxes of O 3 , NO x and SO 2 in the plume. The resulting OPE (1.2–2.5) is somewhat higher than the previous estimate by Ryerson et al. OPE in models is slightly higher than measurements (2–3). Higher OPE (3–4) is predicted for power plant plumes 18 hours downwind of the emission source. OPE for these far downwind plumes is comparable to OPE for other emission sources of NO x . OPE and NO x lifetime are both correlated with plume NO x concentrations. Model results suggest that OPE inferred from statistical correlations between O 3 and tracers such as SO 2 underestimate the true OPE in situations where the correlation between O 3 and SO 2 is nonlinear.