Summary of Suggested Options for the Elements of the Combined NOx and SOx Secondary NAAQS

INDICATOR

NOx: NOy (total oxidized nitrogen, including NO2 + NO + HNO3 + PAN +2N2O5 + HONO + NO3 + organic nitrates + particulate 

NO3)

SOx: SO2 + SO4

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/m2

/year) through application of 

“transference ratios” based on modeled ratios of atmospheric concentrations to deposition.

• 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 indicated by modeled pre-industrial base cation 

weathering [ ]

* BC O , 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 uptake and 

retention (Neco) are used to account for natural ability 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 deposition to ambient 

air concentrations, incorporating ecological conditions and the contribution of reduced nitrogen. To incorporate all of these aspects, we 
developed 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 Acidification 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 (ANClim):

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

lim

The AAPI is derived by rearranging Equation 2 to solve for ANClim, and replacing the values of [ ]

* BC O , Q, and Neco with the values for 

waterbodies representing specific percentiles of the distribution of critical loads across a population of catchments. 

[[ ] ] [ ] NOy NOy SOx SOx

eco

Total

NHx

eco

average

eco

O eco T C T C

Q Dep Q

Neco

Q

AAPI = BC + ⋅ − ⋅ − ⋅ ⋅ + ⋅

% % %

%

* 1 1 1

The AAPI is essentially a function that determines whether ambient NOy and SO2 + SO4 are expected to achieve a target ANC limit. 

The AAPI value selected as the level of the standard will be based on the target ANC limit, given uncertainties in the parameters 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 [[ ] ] BC O %eco

* , Q%eco , Necoaverage , and Total DepNHx , the combinations of NOy and 

SO2+SO4 levels that result in attainment of the AAPI can also be expressed as a tradeoff curve. The 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. [[ ] ] BC O %eco

* , Q%eco , 

Necoaverage , and Total DepNHx ) 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 

category and 

individual 

sensitive ecoregions

Op#2d

All 

individual 

ecoregions

Other options

The critical loads (CLanclim(N+S))

from individual catchments will be aggregated to 

form a population. From the distribution of CL 

values in the population, a percentile 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 the selected 

DL%eco(i).

Pre-industrial base cation weathering 

[[ ] ] BC O %eco

* 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( Necoaverage )

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 DepNHx )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 ecoregions would 

possibly have multiple NHx values

Transference ratios (TSOx and TNOy) are the X Averaged over the areas represented 
aggregated 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 CSOx and 

CNOy

Air concentrations

CSOx

CNOy

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 protection against losses 

in aquatic diversity, and will be less protective against acidification episodes

50 μeq/L: Expected to protect against significant mortality in aquatic organisms and loss of fish health and biodiversity in sensitive lakes 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, zooplankton), 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 2d of the form. The number of lakes that would be protected 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 the 90th, 75th and 50th percentile of the distribution of the critical loads for each population (as 

defined by the options).