Review
of
memorandum
"
Recommended
revision
of
the
fraction
of
diesel
particulate
emissions
mass
less
than
2.5
microns
in
size"

I
have
read
this
memorandum
with
interest.
The
memorandum
contains
the
recommendation
for
a
change
in
how
the
diesel
particulate
emissions
are
considered
as
a
fraction
of
the
PM2.5
particle
size
fraction.
The
current
fraction
is
specified
as
92%
and
it
is
recommended
that,
on
the
basis
of
two
recent
experimental
studies,
that
fraction
should
be
adjusted
upwards
to
97%.
Based
on
the
limited
scenario
presented,
the
proposed
change
seems
reasonable.
However,
for
the
argument
to
be
truly
persuasive,
the
memorandum
should
address
the
following
questions:­

1.
The
memorandum
contains
no
information
on
the
rationale
for
the
specified
fraction.
Does
it
relate
to
a
standard
that
is
used
by
EPA
for
regulating
emissions
from
diesel
engines,
specifically
to
a
method
of
making
valid
measurements
and
interpretation
of
such
emissions?
The
memorandum
would
be
stronger
if
it
were
to
contain
a
short
introductory
paragraph
that
outlines
the
context
for
what
is
proposed,
include
a
description
of
the
purpose
and
scope
of
EPA
MOBILE.
In
addition,
some
indication
should
be
given,
for
the
record,
of
the
practical
significance
or
impact
of
what
is
being
proposed.

2.
For
completeness,
it
should
be
stated
that
the
particle
sizes
mentioned
in
the
text
are
defined
in
terms
of
particle
aerodynamic
diameter.

3.
In
the
first
sentence
that
describes
PM2.5
as
the
fraction
less
than
2.5
µ
m,
it
should
be
clearly
stated
that
it
is
the
mass
fraction
that
is
being
referred
to.
In
addition,
since
the
term
"
less
than"
is
used,
it
should
be
clarified
whether
this
indicates
a
truly
sharp
cut,
or
a
selection
curve
(
e.
g.,
as
is
the
case
for
PM10).

4.
It
should
be
stated
clearly
that
the
mass
fraction
in
question
refers
only
to
the
diesel
emission
itself,
and
not
diesel
fume
in
environments
where
other
aerosol
may
be
present.

5.
For
the
two
studies
on
which
this
recommendation
is
based,
a
few
words
of
description
should
be
given.
I
assume
these
were
carried
out
in
the
laboratory.
To
what
extent
were
the
engines
and
their
operating
conditions
`
typical'
of
what
might
be
found
in
the
real
world
(
i.
e.,
to
which
EPA
MOBILE
is
directed)?

6.
The
two
reports
on
which
the
recommendation
is
based
appear
to
have
been
internal
to
the
respective
organizations
that
sponsored
the
studies
in
question.
These
do
not
appear
to
have
been
peer
reviewed.
Although
this
does
not
necessarily
disqualify
them
for
applications
like
the
one
proposed,
it
does
mean
that
they
should
be
viewed
very
critically.
Or
the
work
should
be
offered
for
publication.
In
this
regard,
I
do
note
that
at
least
one
of
the
reports
has
one
or
more
authors
from
academia.
Perhaps
the
reports
themselves
should
undergo
third­
party
peer
review,
if
they
have
not
already
done
so.
In
summary,
the
memorandum
needs
to
be
clarified
by
the
inclusions
of
some
information
about
the
aims,
scope
and
impact
of
what
is
being
proposed,
and
by
attention
to
some
of
the
details
mentioned
above.
Based
on
the
evidence
presented
in
the
memorandum,
the
conclusions
certainly
seem
reasonable.
However
I
am
not
entirely
convinced
that
the
available
evidence
is
sufficient
to
be
taken
entirely
at
face
value,
especially
if
the
implications
of
the
change
may
be
significant.
It
would
be
good
if
available
information
could
be
enriched
by
the
inclusion
of
data
from
other
sources,
including
from
overseas
laboratories.
If
such
information
cannot
be
found,
and
the
implications
are
indeed
significant,
then
independent
peer
review
of
the
two
reports
may
provide
the
level
of
`
comfort'
that
is
desired.
