Evaluation of ensemble approach for O₃ and PM_2.5 simulation

Inter-comparison of chemical transport models (CTMs) was conducted among four modeling research groups. Model performance of the ensemble approach to O₃ and PM_2.5 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 PM_2.5 concentrations by a factor of 1.4-2, and did not reproduce the PM_2.5 temporal variation at two suburban sites (r=~0.2). The ensemble average improved the simulation of SO₄^2-, 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 PM_2.5 (36-39%) was large, so improvement of the OA simulation model is essential to improve the PM_2.5 simulation.