Summary of Suggested Options for the Elements of the C ombined NOx and SOx S econdary NAAQS

INDICATOR

NOx: NOy (total oxidized nitrogen, including NO 2 + NO + HNO 3 + PAN +2N 2 O 5 + HONO + NO 3 + organic nitrates + par ticulate

NO 3 )

SOx: SO 2 + SO 4

AVERAGING TIME 3 to 5 year averaging time (3 to 5 year averages of annual average NOy and SO2+SO4) selected based on interannual variability in deposition as reflected in components the form (see below).

FORM

The form for the secondary standards incorporates multiple design decisions, including treatment of combined effects of NOx and SOx, selection of an ecological indicator of effects, treatment of reduced forms of nitrogen, and treatment of natural ecological conditions that affect sensitivity to acidification.

**•** Treatment of combined effects of NOx and SOx: Conversion to acid deposition units (in meq/m 2 /year) through application of “transference ratios” based on modeled ratios of atmospheric concentrations to depositio n.

**•** Ecological Indicator: Acid Neutralizing Capacity

**•** Treatment of reduced forms of nitrogen: Incorporated into the form as an adjustment to the nitrogen balance of the ecosystem

**•** Treatment of natural ecological conditions: In order to focus on sites which are acidic due to atmospheric deposition of NOx and SOx, those sites which are acidic due to low weathering rates of base cations as indicat ed by modeled pre

- industrial base cation
weathering 

## [ **]** *

O BC , high organic inputs based on dissolved organic carbon (DOC) concentrations and acid mine drainage based on sulfate concentrations were removed from the critical loads dataset. Of the sites included in further analysis, nitrogen upta ke and retention (Neco) are used to account for natural abilit y of ecosystems to neutralize acid deposition.

The result of these design decisions is a form for the ambient air quality standard that ties ANC to deposition and depositio n to ambient air concentrations, incorporating ecological conditions and the cont ribution of reduced nitrogen. To incorporate all of these aspects, wedeveloped an index that would provide a consistent number nationally that is directly expressed in terms of concentrations of NOx and

SOx. This index is called the Atmospheric Acidif ic ation Protection Index (AAPI).

The formula for the APPI is derived from the critical load equation for a single catchment (eq 2, section 5.3.2) for a selected target value of ANC (ANC lim ) : 

## ( **)** 

## [ **] [**  **]** Neco Q ANC BC S N CL O ANC **− −**  **= +**  ) ( lim *

lim

The AAPI is derived by rearranging Equa tion 2 to solve for ANC lim , and replacing the values of 

## [ **]** *

O BC , Q, and Neco with the values for waterbodies representing specific percentiles of the distribution of critical loads across a population of catchments.



## [ **]** 

# [ **] [**  **]**

SOx SOx NOy NOy

eco Total

NHx

eco average

eco eco O C T C T Q Dep Q Neco Q BC AAPI

**⋅ +** 

**⋅ ⋅**  **− ⋅**  **− ⋅**  **+ =** 

% % % % * 1 1 1

The AAPI is essentially a function that determines whether ambient NOy and SO 2 + SO 4 are expected to achieve a target ANC limit.

The AAPI value selected as the level of the standard will be b ased on the target ANC limit, given uncertainties in the pa rameters used to

calculate the AAPI , and weighing other factors such as time to recovery for ecosystems

. (eq 17, section 5.3.4)

For a selected level of AAPI and specific values of the 

## [ **]** 

# [ **]**

eco O BC % *

, eco Q % , average Neco , and Total

NHx Dep , the combinations of NOy and
SO2+SO4 levels that result in attainment of the AAPI can also be expressed as a tradeoff curve. T he level of the AAPI will be one value
that applies to the entire nation, however the degree to which the values input to parameterize the equation (e.g. 

## [ **]** 

# [ **]**

eco O BC % *

, eco Q % ,

average Neco , and Total

NHx Dep ) will vary according to which options of spatial aggregation are chosen (see below). These options will also

affect the variability in the resultant tradeoff curves.Spatial aggregation options for the components of the AAPI equation
Op#1

No
subdivision

of the U.S. Op#2a Binary categorization Op#2b

5

Cluster Op#2c

1 sensitive
categ ory and

individual

sensitive eco

regions Op#2d

All

individual ecoregions O ther options

The critical loads ( CL anclim ( N+S )) from individual catchments will be aggregated to form a population. From the distribution of CL values in the population, a percentil e of the

distribution will be selected as the deposition metric ( DL %eco (i))

. The population to which an individual site belongs varies among the spatial aggregation options. The following terms are those associated

with the critical load that represents th e selected

DL %eco (i)

.

Pre

- industrial base cation weathering



## [ **]** 

# [ **]**

eco O BC % * is the value from the individual critical

load that is selected as the DL %eco (i) .

Runoff Q %eco is the value from the individual critical

load that is selected as the DL %eco (i). X X X X X

N removed by an ecosystem ( average Neco )

These are calculated from N deposition values averaged over a 3

- 5 year time period. Staff suggest the spatial area of aggregation should be consistent

with DL %eco (i) X X X X X

Reduced N deposition ( Total

NHx Dep )are modeled and averaged over a 3

- 5 yr time period, options for spatial aggregation are listed on the table X Model averaged to match areas of spatial homogeneity (See figure 5

- 16) , so that each ecoregi ons would possibly have multiple NHx values

Transference ratio s ( T SOx and T NOy ) are the X Averaged over the areas representedaggregated effective deposition velocities. These values are modeled and averaged over a 3

- 5 yr time period, options for spatial aggregation are

listed on the table by the monitors for SOx C and

NOy C

Air concentrations

SOx C

NOy C Monitored values to represent air

concentration relevant to acid deposition for sensitive water bodies (see previous discussion of air quality indicators) Target ANC limit

20 μeq/L: Expected to protect against significant losses due to fish mortality in many sensitive lakes, but will place less weight on p rotection against losses in aquatic diversity, and will be less protective against acidification episodes

50 μeq/L: Expe cted to protect against significant mortality in aquatic organisms and loss of fish health and biodiversity in sensitive lake s and streams, including losses due to acidification episodes, and will give weight to considerations of uncertainties in the time to recovery of aquatic ecosystems

>50 μeq/L: May provide additional protection beyond 50 μeq/L against declines in fitness of sensitive species (e.g. brook trout, zooplan kton), however, overall heath of aquatic communities may not be impacted

Target for the percentage of water bodies to protect

Options for selecting a percentile of the waterbodies to protect are informed by comparing levels of protection at alternate ANC levels given spatial aggregation options # 1, 2a , 2b and 2 d of the form. T he number of lakes that would be protecte d to and ANC 50 and 20 μeq/L are quantified , and the spatial distribution of those lakes that would not receive protection is illustrated , given t he 90 th , 75 th and 50 t h percentile of the distribution of the critical loads for each population (as defined by the options)

.