  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 PM2.5. To understand to which extent the aerosol processes modeled in
Polair3D impact simulated inorganic PM2.5, Polair3D is run with different
options in the aerosol module, e.g. with/without heterogeneous reactions. To
quantify the impact of processes outside the aerosol module, simulations are
also done with another CTM (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 PM2.5. 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.
