The representation of chemical interactions among

atmospheric constituents is an essential element of the

AQMs. Although many atmospheric pollutants and their

precursors are emitted as gases and react primarily in the

gaseous phase, environmental problems such as acid

deposition and the formation of aerosols also involve

pollutant interactions among the gas, liquid and solid phases.

Thus, a multi-pollutant, multi-phase, one-atmosphere

modeling approach is often needed for a comprehensive

representation of chemical processes taking place in the


The CMAQ modeling system was designed to facilitate such

an approach. For purposes of computational efficiency,

however, the treatments of gas-phase chemistry, aqueous

chemistry, and heterogeneous processes involving the

formation of aerosols are divided into three separate

modules. This section describes the treatment of gas-phase

chemistry and the linkages added to simulate the interactions

of gas-phase constituents with those in other phases. The

methods used to simulate aqueous chemistry and aerosol

formation are described in subsequent sections.

7.1 Photolysis Rates

Simulation accuracy of the CMAQ chemical system is

highly dependent upon the accuracy of photolysis rates,

which influence the primary sources of radicals in the

troposphere. The photolysis rate (J-value) for a

photodissociation reaction (i) is computed by

where Ji (min-1) is the photolysis rate, F(λ) the actinic flux

(photons cm-2 min-1 nm-1), σ

i (λ ) the absorption cross

section for the molecule undergoing photodissociation

(cm2 molecule-1), φ

i (λ ) the quantum yield of the photolysis

reaction (molecules photon-1), and λ the wavelength (nm).

Absorption cross sections and quantum yields are functions

of wavelength, and may also be functions of temperature



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