Inter-comparison of chemical transport models (CTMs) was conducted among four modeling research groups. Model performance of the ensemble approach to O3 and PM2.5 simulation was evaluated by using observational data with a time resolution of 1 or 6 hours at four sites in the Kanto area, Japan, in summer 2007. All groups applied the Community Multiscale Air Quality model. The ensemble average of the four CTMs reproduced well the temporal variation of O3 (r=0.65-0.85) and the daily maximum O3 concentration within a factor of 1.3. By contrast, it underestimated PM2.5 concentrations by a factor of 1.4-2, and did not reproduce the PM2.5 temporal variation at two suburban sites (r=~0.2). The ensemble average improved the simulation of SO42-, NO3 -, and NH4 +, whose production pathways are well known. In particular, the ensemble approach effectively simulated NO3 -, despite the large variability among CTMs (up to a factor of 10). However, the ensemble average did not improve the simulation of organic aerosols (OAs), underestimating their concentrations by a factor of 5. The contribution of OAs to PM2.5 (36-39%) was large, so improvement of the OA simulation model is essential to improve the PM2.5 simulation.
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