Dear teachers
I am investigating persistent discrepancies in CMAQv5.3 simulations of diurnal variation in NO2/NO concentration over Beijing urban area during January-March 2024 . The simulated NO₂ and NO consistently underestimate observations in the 00:00-08:00 BJT period, while showing a overestimation during early evening (17:00-22:00 BJT). Notably, simulated diurnal variation in Ox (NO₂ + O₃) maintains good agreement with observations throughout the day.
The bias characteristics in NO/NO2/Ox diurnal variations you are noting likely result from a combination of the diurnal variation in NOx emission profiles and the evolution of the planetary boundary layer. During the day, when the PBL is high dilution of emissions through the depth of the PBL may lead to underestimation of grid averaged NOx relative to an urban monitor. During the evening hours when NOx emissions could be high (conceivably from traffic) a premature collapse of the model PBL could lead to overestimation. An evaluation of oxidized nitrogen compounds across the contiguous US presented in https://doi.org/10.1525/elementa.2020.00158 may be of interest to you.
Also, since the representation of the titration of O3 (NO+O3->NO2) is influenced by both the variations in NOx emissions and dilution through the grid volume, in general Ox would show better agreeement with observations than either NO2 or O3 in urban areas.