11/7/2011

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MEMORANDUM

SUBJECT: The Estimated Contribution of Ambient Lead (Pb) to Class I Area Visibility Impairment

FROM: Mark Schmidt, OAR/OAQPS/AQAD

TO: File

Overview:

This memorandum provides rough estimates of Pb-associated visibility impairment levels in 

mandatory Class I Federal areas, and the corresponding proportions of those estimated levels to associated

total visibility impairment. Pertinent background information, underlying analysis assumptions, and 

descriptions of the implemented analysis tasks are summarized in bullet form below:

The Clean Air Act establishes a Regional Haze Program which is intended to improve and protect 

visibility in mandatory Class I Federal areas. Mandatory Class I Federal areas include certain 

national parks (larger than 6000 acres), wilderness areas (larger than 5000 acres), national 

memorial parks (over 500 acres) , and other areas of special national and cultural significance.

The National Park Service operates an ambient air monitoring program called “IMPROVE” 

(Interagency Monitoring of Protected Visual Environments) with a primary objective of 

characterizing visibility impairment in Class 1 and other similar areas.

The IMPROVE program has developed assorted equations for estimating visibility impairment in 

mostly non-urban environments. The original, and still widely accepted IMPROVE formula for 

estimating visibility impairment, calculates “24-hour average total light extinction” (in units of 

inverse megameters) from inputs of 1) 24-hour average concentrations of select speciated PM2.5 

components (some of which are used in reported form in the equation and others which are 

transformed to the explicit formula variables via additional simple equations; 2) 24-hour average 

concentration estimates of PM10-2.5 (a.k.a., “PM-coarse”), and 3) estimates of the hygroscopic 

impacts of relative humidity on associated ambient inputs, specifically sulfate and nitrate. The 

calculated light extinction formula is:

 P M ext:

= (3 S if(RH)i) + (3 N if(RH)i) + (4 OMi) (10 ) + (FSi) (0.6 Mc) 10

i

b ulfate itrate ECi P

Where:

PM bext: i = PM-related light extinction in Mm-1for day i; and

Sulfate i = ammonium sulfate (reported concentration of sulfate ion * 1.375) for day i; and 

Nitrate i = ammonium nitrate (reported concentration of nitrate ion * 1.29) for day i; and 

OM i = organic mass (blank-adjusted reported concentration of organic carbon * 1.4) for day i; and 

EC i = the reported concentration of elemental carbon for day i; and 

FS i = fine soil (reported concentration of Al * 2.20 + reported concentration of Si * 2.49 + 

reported concentration of Ca * 1.63 + reported concentration of Fe * 2.42 + reported 

concentration of Ti * 1.94) for day i; and 

PMc i = PM_coarse (reported PM10 concentration – reported PM2.5 concentration) for day i; and

10 = the “Rayleigh” factor which is assumed light reflection from gasses; and

f(RH) i = the RH hygroscopic growth factor as determined from the EPA climatological f(RH) database 

corresponding to the month of day i for the grid point closest in distance to the monitoring site.
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In this analysis, the above formula was used to estimate 24-hour average total light extinction at 

all IMPROVE sites for days (containing all formula components) in years 2007-2009. The 

formula was also used to estimate 24-hour average total light extinction at all Chemical Speciation 

Network (CSN) sites for the same time-frame (for days containing all formula components). As 

opposed to the IMPROVE network which encompasses monitors in predominately non-urban

areas, the CSN network is mostly urban-based. CSN sites were used in addition to IMPROVE 

sites in order to obtain more light extinction estimates (and make the anticipated conclusion more 

robust), but because sulfate, nitrate, and carbon concentrations are typically much higher in urban 

areas than in non-urban areas and because those light extinction formula components generally 

constitute the majority share of light extinction estimates (as opposed to the fine soil and PMcoarse concentrations), the light extinction estimates calculated at CSN sites are thought to be

upper bound estimates of the absolute levels expected in Class 1 areas.

To estimate the portion of light extinction associated with Pb, only two of the IMPROVE formula 

components were used as shown below. 24-hour average concentrations of PM2.5_Pb were used 

as the fine soil factor, and 24-hour average concentrations of PM10-2.5_Pb were used as the 

PM_coarse factor. Because PM10_Pb is not monitored at many sites, but the larger size-cut 

TSP_Pb is (since the Pb NAAQS is based on that indicator), TSP_Pb 24-hour average 

concentrations were used as a conservative surrogate for 24-hour average PM10_Pb in the 

computation of site-day 24-hour average PM10-2.5_Pb. Only site-days (2007-2009) with both 

PM2.5_Pb and PM10-2.5_Pb estimates were used in the analysis. 

PM_Pb = (FS ) (0.6 M_coarse) ext: i

i

b P

Where:

PM_Pb bext: i = PM_Pb-related light extinction in Mm-1for day i; and

FS i = fine soil (reported concentration of PM2.5_Pb) for day i; and 

PMc i = PM_Pb_coarse (reported PM10_Pb concentration – reported PM2.5_Pb concentration) for

 day i. If 

As with the processing of calculated total light extinction, the above equation for calculated Pbassociated light extinction was applied to data for both IMPROVE and CSN sites. As with the 

total extinction estimates, the CSN Pb-associated extinction estimates are thought to be an upper 

bound as to what would be expected in Class I areas.

The site-day estimates of total light extinction and Pb-associated light extinction were then

matched by site-day and only site-days with both estimates present (total and Pb-associated) were 

used in the subsequent summarizations. For each site-day in the matched file, the ratio of Pbassociated light extinction to total light extinction was computed.

Consistent with the Regional Haze Program, the implemented summarizations focused on the 20 

percent best and worst visibility days (as determined with regard to site-level total calculated PMlight extinction). For each site in the matched file, the 20th and 80th percentiles for total calculated

PM- light extinction were determined. No data completeness cutoff was imposed. The daily 

extinction estimates and corresponding ratio were then averaged by site-category (i.e., those where 

the total extinction was >= the 80th percentile, those where the total extinction was <= the 20th

percentile, and those where the total extinction was in between the 20th and 80th percentiles). The 

site-category averages for total extinction and for Pb-associated extinction were then averaged 

nationally by network-category. 
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Results and Conclusion:

The results of the described analysis are shown below. Although the CSN estimates for total light 

extinction and Pb-associated light extinction are significantly higher than the corresponding 

estimates at IMPROVE sites, the ratios of the two extinction estimates were quite comparable for 

both networks. For the high extinction days (best 20% based on site-level total extinction),

ambient Pb contributed 0.01 percent, on average, to total light extinction in both networks. For 

the low extinction days (worst 20% based on site-level total extinction), ambient Pb contributed 

0.03 percent, on average, to total light extinction in both networks. For the “other” days (where 

total extinction was between the 20th and 80th percentiles), ambient Pb contributed around 0.02 

percent, on average, to total light extinction in both networks.

In conclusion, it appears that the Pb-related visibility effects (in Class I and other areas) are 

insignificant in comparison to those associated with sulfate, nitrate, and carbon PM.

IMPROVE: 164 sites (with all components)

Bext24_total Bext24_Pb

ratio of 

Bext24_Pb 

to 

Bext24_total

59.10879 0.00783 0.00013

44.10702 0.00551 0.00011

average of high days 

median of high days

Bext24_total Bext24_Pb

ratio of 

Bext24_Pb 

to 

Bext24_total

10.42402 0.00255 0.00026

7.20448 0.00162 0.00023

average of low days 

median of low days

Bext24_total Bext24_Pb

ratio of 

Bext24_Pb 

to 

Bext24_total

25.45386 0.00472 0.00020

18.49656 0.00350 0.00018

average of other days 

median of other days 

CSN: 117 sites (with all components)

Bext24_total Bext24_Pb

ratio of 

Bext24_Pb 

to 

Bext24_total

68.02410 0.01185 0.00017

68.00097 0.00876 0.00014

average of other days 

median of other days 

Bext24_total Bext24_Pb

ratio of 

Bext24_Pb 

to 

Bext24_total

32.14398 0.00820 0.00027

32.25639 0.00756 0.00025

average of low days 

median of low days

Bext24_total Bext24_Pb

ratio of 

Bext24_Pb 

to 

Bext24_total

140.93796 0.01998 0.00013

135.79025 0.01216 0.00010

average of high days 

median of high days