11/7/2011

1

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 provide s 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 i mplemented analysis tasks are summarized in bullet form below:

T he Clean Air Act establishes a Regional Haze Program which is intended to improve and pro tect visibility in mandatory Class I Federal area s. Mandatory Class I Federal areas include certai n 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 c alled “ IMPROVE ” (Interagency Monitoring of Protected Visual Environments) with a primar y objective of
characteriz ing 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 . T he original, and still widely accepted IMPROVE formula for estimating visibility impairment

, calc ulates “ 24

- hour average total light extinction ” ( in units of
inverse mega meters ) from inputs of 1) 24

- hour average concentration s of select speciated PM 2.5 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 PM 10

- 2.5 ( a.k.a., “ PM

- coarse ” ), and 3 ) estimates of the hygroscopic impact s of relative humidity on associated ambient inputs, spe cifically sulfate and nitrate. The
calcula ted light extinction formu l a is:

Wh ere:

PM b ext: i = PM

- related light extinction in Mm - 1 for 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 PM 10 concentration

– reported P M 2.5 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 EP A climatological f(RH) database corresponding to the month of day i for the grid point closest i n distance to the monitoring site

.11/7/2011

2

In this analysis, t he 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 formul a 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 use d 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 major ity share of light extinction estimates (as opposed to the fine soil and PM

coarse concentrations) , the light extinction estimates calculated at CSN site s are thought to be upper bound estimate s 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 PM 2.5 _Pb were u sed as the fine soil factor, and 24

- hour average concentrations of PM 10

- 2.5 _Pb were used as the PM_coarse factor. Because PM 10 _Pb is not monitored a t many sites, but the larger size

- cut TSP_Pb is ( s ince the Pb NAAQS is based on that indicator), TSP_Pb 24

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

- hour average PM 10 _Pb in the computation of site

- day 24

- hour average PM 10

- 2.5 _Pb. Only site

- day s (2007

- 2009) with both
PM 2.5 _Pb and PM 10

- 2.5 _Pb estimates were used in the analysis.

Where:

PM _Pb b ext: i = PM _Pb

- related light extinction in Mm - 1 for day i ; and
FS i = fine soil (reported concentration of PM2.5_Pb) for day i ; and
PMc i = PM _Pb _coarse (reported PM 10 _Pb concentration

– reported PM 2.5 _Pb concentration) for

day i . If

As with the processing of calculated total light extinction, the above equation for calculated Pb

associated 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 Pb

associated light extinction to total light extinction was comp uted. Consistent with the Regional Haze Program, the implemented summarization s focus ed on the 20 percent best and worst visibility days ( as determined with regard to site

- level total calculated PM

light extinction )

. For each site in the matched file, the 20 th and 80 th 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

- categ ory (i.e., those where
the total extinction was >= the 80 th percentile, those where the total extinction was <= the 20 th percentile, and those where the total extinction was in between the 20 th and 80 th percentiles). The
site

- category averages for total e xtinction and for Pb

- associated extinction were then averaged nationally by network

- category.11/7/2011

3

Results and Conclusion :

The results of the described analysis are shown below

. Although the CSN estimates for total light
extinction and Pb

- associated ligh t extinction are significantly higher th a n 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 20 th and 80 th 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 are as) are insignificant in comparison to those associated with sulfate, nitrate, and carbon PM.