### 1 Table 7

- 1. Summary of Qualitative Uncertainty Analysis of Key El ements Affecting the AAPI form of the NOx/SOx Standards

.

Source Description Potential influence of uncertainty in element

Knowledge

Base uncertainty Comments Direction (negative implies less

relative protection) Magnitude 

### Major elements (and sub

- models) of the ecological effects to ambient concentration framework

Biological/ecosystem

response to
acidification Clear associations between aquatic acidification (pH, elevated Al) and adv erse ecosystem effects (fish mortality, decreased species diversity) Both Low Low (regionally) The ecosystem level responses are well studied at regional levels. The uncertainty increases at larger scales due to an increasing number of factors influencing the patterns (e.g. latitudinal species gradient, specie

- area relationships, etc.).

Linkage between

direct acidification species and ecological indicator (ANC) The relationships across ANC, pH and dissolved Al are controlled by well defined aquatic equi librium chemistry Both Low Low ANC is the preferred ecosystem indicator as it has a direct relationship with pH and the deposition species relevant to the

NOx/SOx standard.

Linkage between ecological indictor

and adverse ecological effects Direct non linear associations

between ANC and fish mortality and species diversity Both Low

medium Low Although the pH dependency on ANC is nonlinear, it is always directionally consistent. In extremely low and high ANC environments the relationship is of minimal value as catchments are in relatively “less sensitive” regimes due to natural conditions or extreme anthropogenic influence (i.e., acid mine drainage). In sensitive areas of concern the relationship essentially is similar to the relationships between

di rect acidification species and adverse effects.

Deposition to ANC linkage through

Critical Load approach Mass

- balance Steady State critical load model is applied

to determine critical load

values. MAGIC model is used to validate steady State

model. The Steady State

critical load model

formulation is used as the foundation for deriving the AAPI equation. Both Low Low The model formulation is well conceived and based on a substantial amount of research and applications available in the peer reviewed liter ature. There is greater uncertainty associated with the availability of data to support certain model components.

### 2 Source Description Potential influence of uncertainty in element

Knowledge

Base uncertainty Comments Direction (negative implies less

relative protection) Magnitude

Atmospheric

concentrations to

d eposition Deposition is a direct function of ambient concentration, influenced by several processes, and han dled in the

AAPI through air quality modeling. Both Low Low The model design is appropriate given the spatial and temporal complexities that influence deposition velocity, as well as the variety of atmospheric species that generally are not measured.

Gre ater uncertainty resides in the information (e,g,, ammonia emissions) driving these calculations and availability of

observations to evaluate model behavior.

Ecological indicator to changes in the value of ecosystem

services Definitions of p ublic welfare may include economic considerations, based on the tradeoffs people would make to avoid the negative impacts of acidification, through

effects on the values of ecosystem services. Empirical estimates of

valuation for limited ecosystem service categories

a re used to inform the discussions of adversity

associated with alternative

ANC levels. Negative Medium

high Low

- medium There are many studies that estimate the value of increasing services that may be affected by changes in acidification and eutrophication . However, few of these studies focus on the particular impact of acidification and eutrophication on the quality of these services and preferences for avoiding these impacts.

Those studies that do are often limited to analyzing the impacts on a narrow p opulation or particular change in environmental quality. The monetized benefits to fishers and to New York residents for ecosystem improvements in the Adirondacks associated with improvements to the ecological indicator are significant underestimates of t he total benefits in the U.S. This is because those living outside New York would value improvements to the Adirondacks and similar natural

environments elsewhere.

The methodologies used in the studies that underlie the estimates of the value of changes in ecosystem services in the Adirondacks region are sound and have been subject to peer review. The method of aligning the improvements valued in the Banzhaf et al. study with estimates of eliminating current damages leads to may lead to an over or underes timate of the benefits. The range of this difference is difficult to know a priori, but the total improvements in the share of lakes that improve above an ANC threshold of 20 μeq/L are consistent.

### 3 Source Description Potential influence of uncertainty in element

Knowledge

Base uncertainty Comments Direction (negative implies less

relative protection) Magnitude


### Sub

- components and data of individual models
Atmospheric Components

Dep SOx Annual deposition of sulfur mass from dry deposition of (SO 2 and SO 4 ) and wet SO 4 derived from CMAQ 12km horizontal grid resolution averaged over 5 years both low l ow The treatment of SOx deposition in EPA air quality models

has evolv ed over the last two decades. There is general

consensus that the overall mass balance of S is treated well with difficulties in spatial pairing of observations and modeled results of wet deposition. This spatial pairing has improved

with the more recen t PRISM adjustments.

Dep NOy Annual deposition of oxidized nitrogen mass from dry deposition of (all NOy species) and wet NO 3 derived from CMAQ 12 km horizontal grid resolution averaged over 5 years both low l ow

- medium The treatment of oxidized nitroge n deposition in EPA air quality models has evolved over the last two decades. There is general consensus that the overall mass balance of oxidized N is treated well. However, the broad range of deposition velocities across NOy species, and especially u ncertainties regarding the deposition of significant species such as NO 2 pose ongoing challenges. Similarly, a shortage of NOy species measurements as well a lack of techniques to directly measure dry deposition impede progress on improving parameteriza tion of N dry deposition.

Dep NHx Annual deposition of reduced nitrogen mass from dry deposition of (NH 3 and SO 4 )

and wet NH 4 derived from CMAQ 12km horizontal grid resolution averaged over 5

years both low medium NHx deposition also is quantified through CMAQ applications. The well dispersed nature of agricultural based emissions that are influenced strongly by meteorological and surface /soil characteristics continues to challenge

characterization of ammonia emissions. Recent incorporation of a bi

- directional flux process in CMAQ improves consistency with available scientific understanding and yields improved time and space pairing of limited

observations with model results. A lack of both ammonia and ammonium ambient observations continues to comp romise our ability to characterize uncertainty in our treatment of NHx. As with all dry deposition estimates, technologies for direct measurements are not available routinely. Both NHx deposition and NOx deposition are assigned low values of magnitude based on a general dominating role of sulfur

### 4 Source Description Potential influence of uncertainty in element

Knowledge

Base uncertainty Comments Direction (negative implies less

relative protection) Magnitude deposition.

Wet deposition (generically

– N and S species) Wet component of total deposition as described in the Dep terms, above both low low Wet deposition remains an attribute of relatively high

confide nce based on the ability to directly measure chemical components in precipitation samples. However, given the stochastic nature of precipitation, models have a difficult time in matching observations. The use of 5 year averages and
post

- processing PRIS M adjustments have reduced uncertainty in spatial pairing of observations and modeled estimates.

Dry deposition (generically

– N and S species) Dry component of total deposition as described in the Dep terms, above both medium Medium

- high The absence of direct dry deposition measurements combined with the significant variability in the parameters that influence dry deposition velocity reduces the confidence level in dry deposition relative to wet deposition.

Deposition

Transference Ratios CMAQ derived ra tio of total oxidized deposition to concentration averaged over

one year both low unknown Transference ratios enable the connection between deposition and the policy relevant ambient air indicators, NOy and (SO 2 + SO 4 ). They are strictly a model construc t and cannot be

evaluated in a traditional model to observation context. The low sensitivity of these ratios to emission changes and inter annual meteorology combined with low spatial variability indicate that these ratios are necessarily stable.

C NOy Ambient concentrations of NOy through observations. negative low Low

- medium Adequate spatial coverage of NOy observations does not exist, but will be addressed in the proposed rule. The monitoring technology only over the last 5 years has been perceiv ed as “routine” based on incorporation in the NCore network. However FRM status for NOy instruments currently is not available. The negative bias direction is a standard caveat to any instrument relying on internal air stream conversion of atmospheric species prior to detection.

C SOx Ambient concentrations of NOy through observations. both low Low A lack of adequate spatial coverage is the primary concern for

SO2 + SO4 observations. FRM status is not available for SO4; although the long track rec ord of accurate and precise CASTNET FP measurements indicates that achieving FRM

status is a low hurdle. Ecosystem Components

### 5 Source Description Potential influence of uncertainty in element

Knowledge

Base uncertainty Comments Direction (negative implies less

relative protection) Magnitude

BC 0 * Pre

- industrial base cation

concentrations negative Medium

high high Both the F

- factor approach and process based MAGIC modeling were used to generate BC 0 *

. Excellent agreem ent between both approaches was established in the Shenandoah streams. The more comprehensive data requirements of MAGIC limit its widespread use to the Adirondacks, although for consistency the F

- fa ctor approach was applied nationwide. The analyses also illustrated greater divergence at higher critical loads, or areas with greater acid buffering capacity and high bas cation levels. These conditions often are screened out of our population distrib ution analyses, and when included

do not affect the location within the distribution of the more sensitive water bodies. Since MAGIC (the preferred approach) tends to overestimate BC 0 * relative to the F factor approach, and the F

- factor is more widely ap plied nationally,

the BC 0 * estimates are viewed as conservative leading to a slight positive bias in estimating critical loads. Although we have many modeled estimates of BC 0 * , there is a lack of direct measurements of BC weathering rates.

Neco pos itive low medium The term Neco, as defined, has a relatively medium confidence level and is a direct function of the uncertainty inherent in the deposition estimates from CMAQ and surface measurements

of NO 3 . However, this “measurement” difference approa ch reflects the average of all influencing processes (dinitrification, uptake, immobilization) over the time period of measurements. Consequently, there is an inherent assumption of a relatively static system (Neco is applied in a steady state model) th at generally is not tested. In concept, a true steady state vision of Neco would be based on a mature forested ecosystem. The relative bias of Neco is related, largely, to the relative productivity of the forest. The challenge in determining any po tential bias in Neco is to determine the relative “maturation age” of an ecosystem which requires knowledge of future land use activities. In areas of

### 6 Source Description Potential influence of uncertainty in element

Knowledge

Base uncertainty Comments Direction (negative implies less

relative protection) Magnitude high land use restrictions of a recovering forest, Neco would

be assumed to be overestimated. The rel ative magnitude of Neco often is mitigated by the dominance of SOx in controlling acidification processes in many systems. Furthermore, it is unclear to what extent any stored N will be released back into the system, which is assumed to not occur

in the linked system model.

Q Annual runoff rate (distance/time) for a

catchement. both low high Data used to calculate Q was compiled in 1985. Streamflow data were collected at over 12,000 gauging stations during

1951

- 80; 5,951 stations were selected for the analysis. See Gebert and others (1987) for a complete description of how the runoff was determined from the streamflow data. Appropriate maps of the data can show the geographical distribution of runoff in tributary streams for the years 1951

- 80 and can describe the magnitudes and variations of runoff nationwide. The data was prepared to reflect the runoff of tributary streams rather than in major rivers in order to represent more accurately the local or small scale variation in runoff with precipitat ion and other geographical characteristics.

t, W.A., Graczyk, D.J., and Krug, W.R., 1987, Average annual runoff in the United States, 1951

- 80: U.S. Geological Survey Hydrologic Investigations Atlas HA

- 710, scale 1:7,500,000.

DOC Surface water dissolve d organic carbon negative low medium Water bodies with high DOC levels (> 10mg/l) were screened out of the critical load calculations in order to avoid naturally acidic systems. However, the inherent assumpt ion of ANC = ∑strong CA

- ∑strong AN does not explicitly account for contributions of weak organic acids. Consequently, a small positive bias pervades the critical load calculations (i.e., the CL estimates are high). The knowledge base value of M

re flects a general shortage of DOC data.