Observation and modeling of the evolution of Texas power plant plumes

During the second Texas Air Quality Study 2006 (TexAQS II), a full range of pollutants was measured by air- craft in eastern Texas during successive transects of power plant plumes (PPPs). A regional photochemical model is applied to simulate the physical and chemical evolution of the plumes. The observations reveal that SO2 and NOy were rapidly removed from PPPs on a cloudy day but not on the cloud-free days, indicating efficient aqueous processing of these compounds in clouds. The model reasonably represents observed NOx oxidation and PAN formation in the plumes, but fails to capture the rapid loss of SO2 (0.37 h 1 ) and NOy (0.24 h 1 ) in some plumes on the cloudy day. Adjustments to the cloud liquid water content (QC) and the default metal concentrations in the cloud module could explain some of the SO2 loss. However, NOy in the model was insensitive to QC. These findings highlight cloud processing as a ma- jor challenge to atmospheric models. Model-based estimates of ozone production efficiency (OPE) in PPPs are 20-50 % lower than observation-based estimates for the cloudy day.