5.4 Process Analysis

The CMAQ CTM predicts the spatial and temporal

distributions of ambient air pollutant concentrations affected

by important physical and chemical processes as described

above. Because CMAQ uses the fractional time step

method, the species concentrations are determined from

successive changes in concentrations due to the atmospheric

processes. In turn, changes in concentrations by each

process can be accumulated to account for the relative

contribution of the atmospheric process determining the

pollutant concentrations. This is the so-called process

analysis output. This type of information has been used to

assess how a model generates its predictions and to identify

potential sources of error in the model formulation (Pleim

[49], Jeffries and Tonnesen [50], Jang et al. [51], [52]).

Process analysis can be described in two parts: integrated

process rate (IPR) analysis for all the atmospheric processes

in the model and integrated reaction rate (IRR) analysis,

which provides detailed information specific to the chemical

reactions. IRR analysis deals with the details of the

chemical transformations that are described in the model's

chemical mechanism. This is particularly useful for

investigating mechanistic differences under different

chemical regimes (e.g., VOC- versus NOx-limited

conditions). Either analysis method can be applied

independently of the other. The current CMAQ’s

implementation of IRR analysis addresses only gas-phase

reactions. Nevertheless, the concepts should be adaptable to

the modules simulating aerosol formation and aqueous

chemistry as well. This is an area for future enhancement in

the CCTM.

23

6. ATMOSPHERIC TRANSPORT PROCESSES

6.1 Advection Processes: HADV, VADV and ADJCON

The advection process relies on the mass conservation

characteristics of the continuity equation,

 

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