dependency becomes important, the CMAQ version of CB4

uses pressure-dependent forms.

RADM2 mechanism

RADM2 is a lumped species mechanism that uses a

reactivity-based weighting scheme to account for lumping

chemical compounds into surrogate species [77]. The base

mechanism implemented in the CMAQ system contains 57

model species and 158 reactions, 21 of which are photolytic.

Other than the methane reaction change, only one other

change was made to the mechanism as described in the

original publication. The reaction of the hydroxyl radical

with CSL (cresol and other hydroxyl substituted aromatics)

was restructured to eliminate the negative product

coefficient. This change does not alter the gas-phase

chemistry predictions.

SAPRC99 mechanism

The SAPRC99 chemical mechanism included in CMAQ is a

condensed, fixed parameter version of a detailed variable

lumped parameter mechanism [44]. The base mechanism

contains 72 model species and 214 reactions, of which 30

are photolytic. The parameters for the lumped species were

derived from the ambient mixture data used in the reactivity

simulations of Carter [78,79]. The only non-methane related

change made to the condensed mechanism described in

reference [44] is the addition of the reactions of formic,

acetic and higher organic acids with the hydroxyl radical.

These reactions were taken from the detailed SAPRC99


These reactions were taken from the detailed SAPRC99

mechanism that forms the basis for the condensed CMAQ


Isoprene extensions

Over the past few years, the importance of isoprene in

contributing to the ozone formation in the troposphere has

become more fully understood. Its representation in earlier

gas-phase mechanisms was substantially condensed,

partially because of computational resource considerations

and partially due to uncertainties about its degradation

products and their subsequent reaction pathways. Results of

recent mechanistic and environmental chamber studies have

led to a greater understanding of isoprene chemistry, and

thus to improved mechanism representations [80]. In the

CMAQ system, two variants of the RADM2 mechanism

have been created that have the original RADM2 isoprene

chemistry replaced with different levels of more detailed

isoprene chemistry. The two levels of detail are referred to

as the one-product and the four-product Carter isoprene

mechanisms [81]. Both are condensed from a more detailed


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