Environmental Engineering Reference
In-Depth Information
Chapter 2
Relating Small-Scale Emission
and Concentration Variability
in Air Quality Models
Stefano Galmarini, Jean-François Vinuesa, and Alberto Martilli
Abstract
A novel approach to account for the spatial variability of the small-scale
emission in air quality models is proposed. This approach includes a formulation for
the sub-grid variability of pollutant concentrations and relates it to the spatial het-
erogeneity of the emissions. The parameterization is implemented in a 3D transport
model and tested against large eddy simulations of convective atmospheric bound-
ary layers.
2.1 Introduction
The atmospheric motion, evolution and scalar concentrations can be described by a
system of non-linear partial differential equations. These equations are derived from
thermodynamics and fluid mechanics (Navier-Stokes equations). They describe the
characteristics of a given air mass and are the base of all geophysical models. They
link large and small scale motions due to the wide range of temporal and spatial
scales of geophysical processes and the energy feedback from smaller scales into the
larger scale motions. Since the equations are simply too complicated to be exactly
solved for any practical atmospheric modelling, statistical representations of the
complete physical description is required. Currently, the Navier-Stokes equations
are resolved by performing scale decomposition. Geophysical models are in fact an
adaptation of these equations to a spatial grid and discrete steps in time. It is there-
fore important to realize that features smaller than the dimension of one grid cell
cannot be resolved by the model. These are referred to as subgrid scale processes.
Urban and suburban environments are characterized by heterogeneous emissions.
These are not reflected in emission inventories except as an average value and so
this is not accounted for in meso/limited area air quality models. The emissions are
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