Comments by Rudolf Husar
Ch 5: Conceptual Framework
Q 10: Conceptual framework.
The Draft PA document and Ch 5 in particular represents a novel approach toward a secondary
AQ standard. The conceptual framework described in Chapter 5 is well designed and
subsequently applied to formulate the proposed expression for the standard. The assumptions and
simplifications applied for the AAPI are reasonable and the outcome is a proposed standard of
moderate complexity.
At this stage of the Draft, the Chapter is somewhat deficient in highlighting and explaining many
of the key concepts. Even well informed readers find it difficult to assimilate the big picture that
is presented. The comments below, therefore, represents well-intended constructive suggestions
on how to communicate the complex topic in a more digestible manner. The approach here relies
on the concepts of natural an artificial complexity.
Simplify and explain the natural complexity. Chapter 5 clearly represents a novel approach
toward a secondary standard that is integrative over at least three dimensions:
 Multi-media integration is included since it links air, water, and to some extent soil.
 Multi-pollutant approach combines the adverse effects of SOx and NOy (and NH4)
 Natural as well as anthropogenic forcing is incorporated.
The interactions along each of these dimensions are naturally complex and deserve clearer
representation. The conceptual diagram, Figure 1-1, represents the pollutant flow across the
media from atmospheric concentrations through deposition to ecological effects represented by
indicator. This diagram is rather generic and is applicable to virtually all standards, may that be
for human health or for multi-media, multi-pollutant, human-perturbed ecosystem protection.
The second dimension, the multi-pollutant aspect of the standard is not included in the
conceptual diagram. This omission is significant since there is no way to represent where in the
generic diagram the merging of the multiple pollutants is taking place. Evidently at some point
after deposition, but is the merging before or after the ecological effects function?
The generic diagram is also marginal in representing the linkages along the third dimension, i.e.
the role and the relationship between the controllable and the external pollutant variables, e.g.
ammonia on the air-side, and the role and the relationship of weathering and other factors on the
aquatic/terrestrial/ecological side.
For these reasons it is suggested that the generic, universally applicable conceptual diagram is
expanded. Such an expansion could be guided by the form of the proposed AAPI standard. In
other words, the conceptual diagram should have a box representing each of the parameters and
the terms in the AAPI equation. Such a diagram could show that NH4 is an indicator as NOx and
SOx. Or is it a modifying factor which is different from NOx /SOx ??. On the aquatic-terrestrial
ecological side the conceptual diagram should include representation of the g-term so that its
relationship to the ecological effects function is made clear.
Minimize the artificial complexity. Artificial complexity is introduced (by humans) during the
formulation and explanation of the proposed structures and functions. By reducing artificial
complexity one can-de-mystify the rather complicated relationships arising from natural
complexity.
First, there is clear conceptualization of the dry and wet deposition velocities in equations (1) and
(2) , p 169. In both cases, the deposition velocity is defined as the proportionality constant
between the ambient concentration and the deposition of a specific species. It is true that in the
literature deposition velocity is generally applied to gases, but it is clearly applicable to wet
deposition since, by definition it is the ratio of Dep/Conc. Even in the case of gases, deposition
velocity is not a true ‘velocity’ of anything but simply a coefficient defined by the Dep/Conc
ratio. So, it is not clear as to why definition of the wet deposition velocity is abandoned.
By sticking to the wet deposition velocity as defined in (2), the mysterious Atmospheric
Deposition Transformation Function (ADTF) becomes the average total deposition velocity for
all the species, as well stated on p 171, lines 4-5. The trick, of, course, is that the averaging has to
be clearly defined as the concentration-weighed average of all species, i. Accordingly,
V = Deptot / Ctot = Deptot / (sumi ( vidry + viwet) Ci)
Given this definition the mysterious V becomes the concentration-weighed average total
(dry+wet) deposition velocity for S and N compounds.
The term Atmospheric Deposition Transformation Function (ADTF) is unnecessary confusing.
In particular, Transformation conflicts with the same term used in defining the atmospheric
Source-Receptor Relationship which includes the Transport, (Chemical) Transformation and
Removal processes. I do not follow, what transformations are referred to in ADTF. If the
atmospheric indices are the concentrations of specific species that are being deposited, then there
is no chemical transformation.
Comments on the terminology for mass transfer across surfaces, as used in atmospheric and
engineering sciences (and as understood by this reviewer). The mass (or molar) flow rate across
an arbitrary surface Mass (or mole)/Time), e.g. the total S deposition rate over the Eastern US =
8 Tg S/yr. The flux is flow of materials through a unit surface area (Mass (or Mole)/Area*Time)
e.g. the Deposition Flux = 2 g S/m2,yr. I understand that other disciplines may use alternative
nomenclature.
The conceptual diagram, Figure 1-1 is reversed in Chapter 5, Figure 5-1. Evidently, the rationale
for flipping the diagram is to indicate that the atmospheric indices for the AAPI standard are
determined backwards from the ecological effects through the ecological effects function,
deposition metric and the ‘ADTF’. This message could be conveyed without flipping the
conceptual diagram, but having two sets of arrows in Figure 1-1 and elsewhere. The green
arrows at the top in the direction from the air to the ecosystem. A second set of the blue arrows
indicating the direction of the control for establishing the ambient concentrations from the value
of AAPI. Having both set of arrows on the same diagram would retain a consistent conceptual
diagram and it would also highlight the different direction for the causality (air concentration->
ecosystem) and regulatory response (ecosystem -> air concentration).
Finally, I would gently suggest to extend the conceptual diagrams with additional arrows that
represent the flow from sources of SOx, NOx and ammonia to the boxes representing the ambient
concentration indices. Such arrows would indicate that the ambient concentrations themselves
are the result of specific emissions. Such arrows would also allow graphical representations of
the sources and the magnitude of uncertainties in establishing the AAPI.