Dominant aerosol processes during high-pollution episodes over Greater Tokyo

This paper studies two high-pollution episodes over Greater Tokyo: 9 and 10 December 1999 and 31 July and 1 August 2001. Results obtained with the chemistry transport model (CTM) Polair3D are compared to measurements of inorganic particulate matter (PM_2.5). To understand to which extent the aerosol processes modeled in Polair3D impact simulated inorganic PM_2.5, Polair3D is run with different options in the aerosol module, i.e., with/without heterogeneous reactions. To quantify the impact of processes outside the aerosol module, simulations are also done with another CTM (community multiscale air quality, CMAQ). In the winter episode, sulfate is mostly impacted by condensation, coagulation, long-range transport, and deposition to a lesser extent. In the summer episode, the effect of long-range transport largely dominates. The impact of condensation /evaporation is dominant for ammonium, nitrate, and chloride in both episodes. However, the impact of the thermodynamic equilibrium assumption is limited. The impact of heterogeneous reactions is large for nitrate and ammonium and, taking heterogeneous reactions into account, appears to be crucial in predicting the peaks of nitrate and ammonium. The impact of deposition is the same for all inorganic PM_2.5. It is small compared to the impact of other processes although it is not negligible. The impact of nucleation is negligible in the summer episode and small in the winter episode. The impact of coagulation is larger in the winter episode than in the summer episode because the number of small particles is higher in the winter episode as a consequence of nucleation.