Numerical modeling of gas deposition and bidirectional surface

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Mesoscale Atmospheric Dispersion, 2001, Ed. Z. Boybeyi, WIT
Publications, Southampton, UK, Advances in Air Pollution, Vol 9, p. 424.

Chapter 9
Numerical modeling of gas deposition and bi-
directional surfaceatmosphere exchanges in
mesoscale air pollution systems
Devdutta S. Niyogi & Sethu Raman
North Carolina State University
Raleigh, NC 276957236, USA

Accurate representation of surface processes such as vegetation has a significant
role in air pollution models. In a variety of situations, the surface acts as a sink
for the pollutants. Using pristine relations developed on fluid mechanical
concepts, different formulations are discussed in this chapter to develop
deposition flux estimates in air pollution models. An interesting scenario also
develops when the soil and vegetation, in particular, acts as a source, in addition
to being a sink for the gaseous material. Hence as a generalized framework in air
pollution systems, the ability of the surface to generate bi-directional fluxes
needs to be represented. Accordingly, different modeling techniques are
presented ranging from regression equations, to modifications in the resistance
pathways, and detailed eco-physiological leaf scaling approach. Finally, of
particular relevance to mesoscale applications is the area averaging and regional
mapping of the bi-directional fluxes. Accordingly different methods based on
combination of surface measurements, remote sensing and model
parameterizations are discussed.
Mesoscale Atmospheric Dispersion, 2001, Ed. Z. Boybeyi, WIT
Publications, Southampton, UK, Advances in Air Pollution, Vol 9, p. 424.

1 Introduction
One of the important considerations in designing an air pollution
modeling system is the accurate and efficient representation of the surface
processes. Mathematically, surface processes form a boundary condition
in the atmospheric analysis thus becoming a pivotal component of the
modeling system for both surface energy balance as well as mass transfer.
As discussed in Niyogi and Raman [1], surface processes manifest
changes through soil-vegetation-atmosphere-transfer (SVAT) processes
in the boundary layer. At the local scale, surface features modulate
humidity, temperature, and surface energy balance. At a mesoscale,
heterogeneity in surface fluxes leads to mesoscale circulation, which
affects transport and diffusion characteristics of the atmosphere. Presence
of vegetation thus leads to humidity exchange potential [2]. In addition to
its impact on the local as well as regional scale thermodynamic structure,
vegetation has a dominant role in air pollution models principally as a
sink or a depositing surface [3], [4]. Till mid 1970s most deposition
research assumed gases deposit to bare ground with only a marginal
impact due to vegetation [5], [6]. Subsequent studies however, have
shown that vegetative processes are a dominant pathway for surface
atmosphere exchanges [4], [7], [8], [9], [10].
      In this chapter, we will discuss the issues pertaining to
mathematically representing surface effects with particular emphasis on
pollutant deposition. In the classical sense, such exchange processes
have been evaluated principally considering homogeneity. It is being
increasingly recognized that one of the largest sources of uncertainty lie
in representing surface couplings within the air pollution simulation
framework. Though in general, gaseous pollutants are deposited to the
surface, in some instances, the vegetation interface can form a source for
the biogenic emissions. Hence the biosphericatmospheric interactions
have a dominant role in the atmospheric environmental mass balance, for
a variety of applications ranging from regulatory purposes to ecosystem
synthesis (see Mooney [11]).
      Accordingly, this chapter is structured in the following manner. In
the following section, the role of multimedia couplings is discussed both
at the micro, and the regional scale.             Different gas deposition
parameterizations are discussed with particular emphasis on mesoscale
Mesoscale Atmospheric Dispersion, 2001, Ed. Z. Boybeyi, WIT
Publications, Southampton, UK, Advances in Air Pollution, Vol 9, p. 424.

models. In Section 3, the landatmosphere coupling is discussed with
particular emphasis on the bi-directional (source as well as sink)
exchanges. Here both the modifications in traditional resistance approach
as well as more mechanistic plant physiological relations are discussed.
One of the pertinent issues for mesoscale models is scaling the
parameterizations and the model results to a regional scale. Section 4
therefore outlines the procedures and approaches for regional analysis.
Finally, Section 5 presents the conclusions and a discussion for future
research in this area.

1.1 Surface processes in environmental analysis

Recent years have witnessed phenomenal growth in the computational
resources. Numerical models are being run with higher and higher grid
resolution. With increasing horizontal grid resolution, the impact of
surface processes becomes increasingly dominant in numerical
simulations (see for example, Pielke [12]). Hence, there is a growing
concern regarding a realistic representation of the continuum between the
surfaceatmosphere exchanges [13], [14], [15]. However, most models
that consider the coupling between landairwater, treat the continuum
more for energy balance than for mass balance. Hence parameterizations
need to be developed for air pollution models and environmental
assessment systems in which mass exchange or more correctly gas
exchange is explicitly accounted for to provide source/sink
representation. We will discuss formulations and development of such
parameterizations in this chapter.
      In creating a continuum, it is important to develop the coupling by
linking the heterogeneous media dynamically. As a general approach, it
will be beneficial in developing such couplings so as to integrate different
parameterizations and then link it via different conservation equations.
Such a system can thus accommodate more than one process and media.
This not only allows representation of processes other than linear
relations but also provides opportunity for checks and validations of the
exchanges. It can also replicate the reality as much as possible in terms
of what dominates the pathway and the exchange. Ideally, as discussed
earlier, these conservation equations would involve explicit energy as
well as mass exchanges. It is important that the system has to be
designed with interactive couplings. Also, different processes involved in
Mesoscale Atmospheric Dispersion, 2001, Ed. Z. Boybeyi, WIT
Publications, Southampton, UK, Advances in Air Pollution, Vol 9, p. 424.

the coupling should be sensitive to direct as well as secondorder or
indirect changes and thus show variations for reaction as well as the
stimuli. Consequently, the system has to be generalized so as to show
valid spatial and temporal variations. Hence the equations that form
coupling interfaces in the system need to have prognostic variables and
have some form of landscape or regional factors embedded in their
specification. Finally the coupled system has to be tested with
observations made under different conditions for verification.
      Consider an example of a coastal watershed. They are important
regions of socio-economic activities for which the environmental issues
(air and water quality) have significant implications. The multi-media
(land, air water) exchanges form critical pathways for pollutant as well as
nutrient transfer. The pollutants released on the land would be either
detained on the soil surface, or deposited over the vegetation. This
material can subsequently transport through runoff or percolate into the
soil and roots and affect the ground water. Portion of the pollutant, which
is not detained on the landmasses, is passed over the water body. The
terrestrial loading can thus induce changes in the water nutrient status and
affect flora and fauna. An additional feature of such a watershed is the
mesoscale variability in the surface, subsurface and the meteorological
features which can affect the hydro-meteorological exchanges. These
features need to be resolved through a mesoscale model with the
multimedia systems coupled to it.

1.2 Developing the mathematical framework

In developing the simulation framework the first principle approach can
be adopted. As that, pristine laws related to the laminar and turbulent
fluid flows are modified to represent the boundary conditions. Typical
examples of the extension of such principles are the wind profile `laws',
development of the mixing length (see Holt and Raman [16]), turbulent
kinetic energy closure schemes, and the atmospheric boundary layer
similarity theories (see Stull [17]). Such fundamental fluid assumptions
have been extended in almost every numerical as well as analytical model
developed for environmental simulations. The equations and models for
simulating the atmospheric or oceanic phenomena have some
fundamental assumptions at their core. One such assumption is surface
homogeneity. However in recent years, there have been significant
Mesoscale Atmospheric Dispersion, 2001, Ed. Z. Boybeyi, WIT
Publications, Southampton, UK, Advances in Air Pollution, Vol 9, p. 424.

improvements in the modeling strategies. More and more realistic
features are being incorporated in the model equations, with emphasis on
linking causal behavior within heterogeneous systems.
     Interfacing heterogeneous media in its simplest form involves
development of equations, which pass on the base state at System-1
boundary condition as the initial state for the System-2 boundary.
However, this transition is not easy as the two systems cannot be treated
in a homogeneous state. There has to be a buffer or interface layer that
can act as a moderator for the two boundary conditions to be in
congruence with the prevailing environments (without shocking or
destabilizing the local equilibrium). This intrinsic concept will be
discussed for applications involving water vapor and other gas exchanges
and deposition over vegetation (canopy, leaf, stomata and intercellular
conduits) and atmospheric boundary.
     In the following section, we review some of the underlying
principles and assumptions of gas exchange for differential media,
including the approximations made in experimental field studies for toxic
deposition and exchange. Then, we discuss the techniques, and
approaches for vapor exchange at vegetationatmosphere interface.
Finally, we present a discussion summary and conclusions regarding
some of the limitations and developments for next generation models.

2 Developing gas deposition relations
Understanding the transport and fate of gaseous pollutants is important
for diverse applications. For example, it is now widely recognized plants
emit volatile organic compounds (VOCs) such as monoterpene and
isoprene (cf. Arey [18]). These biogenic emissions can undergo
photolysis and other chemical transformation and generate pollutants such
as ozone in the lower troposphere [19]. Results from both special
observational campaigns (e.g., Monson and Fall [20]) as well as
numerical modeling studies (e.g., Langford and Fehsenfeld [21]; Gunther
et al. [22]) confirm this feature at diverse scales. For instance, Fitzgerald
[23] estimated that, of the 6000 to 7500 tons of mercury emitted into the
atmosphere, 2550 % is released from natural surfaces.
      In addition to being a source of biogenic emission and other primary
as well as secondary pollutants, the terrestrial biosphere is known to be a
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