The key algorithms simulating aerosol processes in the

CCTM include: (1) aerosol removal by size-dependent dry

deposition; (2) aerosol-cloud droplet interaction and removal

by precipitation; (3) new particle formation by binary

homogeneous nucleation in a sulfuric acid/water vapor

system; (4) the production of an organic aerosol component

from gas-phase precursors; and (5) particle coagulation and

condensational growth.

Two important aerosol processes included in the CCTM

involve the heterogeneous conversion of N2O5 to HNO3 and

the condensation of semi-volatile compounds resulting from

the atmospheric oxidation of some organic compounds. In

CMAQ, the heterogeneous conversion of N2O5 to HNO3 is

based on the method of Dentener and Crutzen [97], in which

the loss of gaseous N2O5 is treated as a pseudo first-order

process. The rate coefficient for this process is expressed as

a function of the gas-phase diffusion coefficient of N2O5, the

radius of the aerosol particle, the aerosol surface area, and

the reaction probability. Dentener and Crutzen used a value

0.1 for the reaction probability, but CMAQ incorporates the

method used by Riemer et al. [98] in which the reaction

probability is tied to the nitrate composition of the aerosol

and varies between 0.02 and 0.002. It is based on

experimental results suggesting that the heterogeneous N2O5

conversion rate is inhibited by the presence of nitrates in the

aerosol phase [99]. At the end of the aerosol time step, the

amount of HNO3 retained in the aerosol phase is determined

from thermodynamic equilibrium considerations. The

portion of N2O5 that is not converted is assumed to remain in

the gas-phase.

The formation of secondary aerosols



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