Ron Evans/RTP/USEPA/US

04/30/2008 09:01 PM

	

To

"Lee, Amanda I." <Amanda_I._Lee@omb.eop.gov>

cc

Brian Heninger, darryl weatherhead, fann.neal@epa.gov, "King, Heidi R."
<Heidi_R._King@omb.eop.gov>, Kathy Kaufman/RTP/USEPA/US@epa, Lydia
Wegman, simon.nathalie@epa.gov, Tricia Crabtree/RTP/USEPA/US@epa,
walton.tom@epa.gov

Subject

Re: response to chapter 2 comments









Here are my responses to your comments on chapter 2

page 4

Historical studies show that Gaussian dispersion models, such as ASPEN,
typically agree with monitoring data within a factor of 2 most of the
time.  In the case of Pb in the NATA assessment, model estimates at
monitor locations were generally lower than the monitor averages for Pb
[by how much, if possible?], 

I will ask Kathy to determine if this is a readily available statistic
which we can include.

page 5

Because of the systematic under-estimation of the dispersion models, the
likely benefits estimates are likely to be [over-estimated?
Under-estimated? No effect? And why]

This discussion is meant to talk about the availability of air quality
data for ambient Pb.   We go on in Chapter 3 to indicate that we do not
use air quality models in this analysis.  We need to clarify here this
fact.  I think this removes the need to directly address your question. 
 See the addition in bold to the following piece of chapter 2 where you
raised the question.

"Historical studies show that Gaussian dispersion models, such as ASPEN,
typically agree with monitoring data within a factor of 2 most of the
time.  In the case of Pb in the NATA assessment, model estimates at
monitor locations were generally lower than the monitor averages for Pb
[by how much, if possible?], suggesting that the modeling system (i.e.,
emissions estimates, spatial allocation estimates, dispersion modeling)
may be systematically underestimating ambient concentrations.  This may
be particularly true for Pb as metals tend to deposit rapidly with
distance from the source according to their particle size and weight. 
The model-to-monitor analysis is described in detail at
http://www.epa.gov/ttn/atw/nata1999/99compare.html.  The modeling system
underestimation may also be due in part to a lack of accounting for
emissions re-entrainment (these "re-entrained" particles may be observed
by the monitors, but they are not accounted for in the emissions
inventory, and thus would not contribute to the model estimate).  For
more details on the limitations of the 1999 NATA national scale
assessment, see http://www.epa.gov/ttn/atw/nata1999/limitations.html.

For more information on Pb modeling, see section 2.4 of the OAQPS Staff
Paper for the Pb NAAQS.   For reasons discussed in chapter 3 (section
3.1.1), we did not use an air quality model for this analysis."

page 8

Currently, tetraethyl lead (TEL) is still added to aviation gasoline
(avgas) which is used in most piston-engine powered aircraft

OK

page 8

This estimate is based on all leaded avgas used in the U.S. and does not
account for the fact that some lead is emitted in the local area of an
airport facility and some lead is emitted at altitude.  The focus of
this analysis is the lead emitted around [airport facilities? Airport
facilities + altitude?]  EPA’s method for estimating airport-specific
lead inventories is discussed in detail elsewhere.

The estimate is all leaded avgas used in the US.  In the following I
have tried to clarify this.

This estimate is based on all leaded avgas used in the U.S. This
estimate does not take into account for the fact that some lead is
emitted in the local area of an airport facility and some lead is
emitted at altitude.  EPA’s method for estimating airport-specific
lead inventories is discussed in detail elsewhere.

Ron Evans

Leader, Air Benefit & Cost Group

HEID/OAQPS/OAR/EPA

Mail Drop C-439-02

919-541-5488

919-541-0839 fax

