The CMAQ modeling system simulates various chemical and physical processes that are

thought to be important for understanding atmospheric trace gas transformations and

distributions. The CMAQ modeling system contains three types of modeling

components (Models-3): a meteorological modeling system for the description of

atmospheric states and motions, emission models for man-made and natural emissions

that are injected into the atmosphere, and a chemistry-transport modeling system for

simulation of the chemical transformation and fate. The CMAQ chemical transport

model includes the following process modules: horizontal advection, vertical advection,

mass conservation adjustments for advection processes, horizontal diffusion, vertical

diffusion, gas-phase chemical reactions and solvers, photolytic rate computation,

aqueous-phase reactions and cloud mixing, aerosol dynamics, size distributions and

chemistry, plume chemistry effects, and gas and aerosol deposition velocity estimation.

This paper describes the Models-3 CMAQ system, its governing equations



Aj area of vegetation type j

Cvd specific heat capacity for dry air at

constant volume,

Ef emissions factor

Ei emission rate for species i

F(λ) actinic flux

Fij environmental factor


x , Fqi

y , Fqi

z turbulent flux components in Cartesian





, Fqi



side-wall deposition flux terms

Fˆ qi

k turbulent flux component in a generalized


s horizontal forcing vector

qi horizontal turbulent flux vector of trace


f Coriolis factor

fc cloud fraction

fsc scattering phase function asymmetry


g gravity

Hi Henry’s Law coefficient for species i

h height

hAGL height above the ground level

hdep thickness of lowest model layer

i trace species index

Jabove above-cloud photolysis rate

Jbelow below-cloud photolysis rate

Jclear clear-



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