Analysis
of
NOx­
only
Impacts
to
Support
Response
to
Comments
In
Section
III.
B.
1.
a
of
the
Preamble
and
in
Chapter
III
of
the
Response
to
Comments
document
for
the
CAIR
final
rule,
EPA
addresses
the
comment
that
the
Agency
should
require
regional
reductions
in
VOC
as
well
as
NOx
emissions
as
part
of
this
rulemaking.
As
noted
in
the
Preamble,
EPA
continues
to
assert,
based
on
previous
regional
ozone
analyses
(
e.
g.,
OTAG
and
NARSTO)
and
the
modeling
completed
as
part
of
the
analysis
for
this
rule,
that
NOx
emissions
are
chiefly
responsible
for
regional
ozone
transport.
This
assertion
was
confirmed
by
other
commenters.
EPA
agrees,
however,
that
VOCs
from
some
upwind
States
do
indeed
have
an
impact
in
nearby
downwind
states,
particularly
over
short
transport
distances.
The
Agency
expects
that
States
will
need
to
examine
the
extent
to
which
VOC
emissions
affect
ozone
pollution
levels
across
state
lines,
and
identify
areas
where
multi­
state
VOC
strategies
might
assist
in
meeting
the
8­
hour
standard,
in
planning
for
attainment.
However,
this
does
not
alter
the
basis
for
the
CAIR
NOx
requirements
in
this
rule.
EPA's
modeling
supports
the
conclusion
that
NOx
emissions
from
upwind
States
will
contribute
significantly
to
downwind
nonattainment
and
also
interfere
with
maintenance
of
the
8­
hour
ozone
standard.

The
purpose
of
this
document
is
to
present
the
results
from
a
modeling
analyses
in
which
only
the
impacts
of
NOx
emissions
from
upwind
States
on
downwind
receptors
were
used
to
establish
significant
air
quality
linkages.
The
process
whereby
upwind
States
are
identified
as
significant
contributors
to
residual
nonattainment
areas
is
described
in
detail
in
the
Air
Quality
Modeling
Technical
Support
Document
for
this
rule.
In
general,
EPA
uses
two
modeling
techniques
(
source
apportionment
and
zero­
out
modeling)
and
three
factors
(
magnitude,
frequency,
and
relative
amount)
to
calculate
and
assess
the
significance
of
contributions
from
a
State's
emissions
on
a
downwind
receptor
county.
All
of
the
State's
manmade
VOC
and
NOx
emissions
are
considered
in
the
CAIR
contribution
determinations.
As
a
practical
matter,
one
would
have
to
redo
the
zero
out
runs
to
conduct
the
full
significance
analysis
separately
for
VOC
and
NOx.
However,
because
the
source
apportionment
technique
allows
one
to
quantify
the
impacts
of
a
State's
emissions
by
pollutant,
we
have
used
that
technique
on
its
own
to
gauge
the
potential
effects
of
the
VOC
impacts
on
the
significance
determinations.

There
are
two
screening
criteria
for
determining
potentially
significant
upwind
State
to
downwind
nonattainment
linkages:

a)
a
maximum
contribution
of
greater
than
or
equal
to
2
ppb,
and
b)
a
percent
contribution
to
total
nonattainment
greater
than
or
equal
to
1%
(
after
rounding
to
nearest
integer
percentage).

There
were
448
State/
receptor
linkages
that
passed
the
screening
criteria
based
on
the
source
apportionment
technique.
Of
these
443
(
98.9%)
pass
the
same
screening
criteria
when
only
the
NOx
emissions
impacts
were
considered.

The
five
linkages
that
passed
the
screening
test
(
i.
e.,
were
potentially
significant)
only
because
the
VOC
impacts
were
included
are:
Increase
in
max
ppb
contribution
when
VOC
impacts
are
added
55.86%

35.99%

4.24%
2.46%
0.76%
0.25%
0.25%
0.17%

0%
10%
20%
30%
40%
50%
60%

no
change
0.1
to
0.4
0.5
to
0.9
1.0
to
1.9
2.0
to
2.9
3.0
to
3.9
4.0
to
4.9
5.0
to
5.3
MI
on
Arlington
VA
(
2.0
ppb
max
goes
to
1.9
ppb
max
w/
NOx
only),
MI
on
Fairfax
VA
(
2.0
ppb
max
goes
to
1.9
ppb
max
w/
NOx
only),
WI
on
Bucks
PA
(
2.0
ppb
max
goes
to
1.8
ppb
max
w/
NOx
only),
IN
on
Monmouth
NJ
(
2.0
ppb
max
goes
to
1.9
ppb
max
w/
NOx
only),
and
MA
on
Middlesex
CT
(
0.5
%
total
goes
to
0.4%
total
w/
NOx
only).

As
can
be
seen,
the
only
linkages
that
were
lost
were
those
that
most
narrowly
passed
the
screening
test
when
both
VOC
and
NOx
were
considered.
Further,
the
first
four
linkages
from
that
list
were
eventually
deemed
not
to
be
significant
because
the
zero
out
technique
showed
very
small
impacts.
While
the
Massachusetts
/
Middlesex
Co.,
CT
linkage
was
concluded
to
be
significant,
there
was
an
additional
county
impacted
by
NOx­
only
emissions
from
Massachusetts
(
Kent
Co.,
RI).

Figure
1.1
is
a
histogram
displaying
the
change
in
maximum
contribution
(
ppb)
from
NOxonly
impacts
to
combined
NOx/
VOC
impacts.
As
can
be
seen,
in
over
90
percent
of
the
cases,
the
difference
in
maximum
impact
is
less
than
0.5
ppb.

Figure
1.1.
Changes
in
maximum
ozone
contribution
between
the
NOx­
only
case
and
the
combined
NOx
+
VOC
case
for
all
State/
receptor
linkages.

Figure
1.2
shows
the
maximum
NOx­
only
contribution
by
State
to
any
downwind
residual
nonattainment
receptor
county.
For
the
25
States
with
significant
linkages
in
CAIR
for
ozone,
the
smallest
NOx­
only
max
impact
for
a
significant
State
is
4.5
ppb
(
Arkansas
on
Jefferson
TX).
Based
on
these
modeling
results,
EPA
concludes
that
it
is
likely
that
the
same
25
States
would
be
covered
by
this
rule
if
only
NOx
impacts
had
been
considered.
EPA
thus
continues
to
find
that
NOx
emissions
are
chiefly
responsible
for
regional
ozone
transport,
and
that
NOx
reductions
are
therefore
essential
for
its
control.
Max
NOx
Only
Source
Apportionment
Impact
(
ppb):
Upwind
State
on
Downwind
Receptor
51.5
50.2
50.1
44.0
38.9
28.9
26.6
25.5
22.5
21.8
18.1
17.5
11.0
11.0
8.6
8.5
8.4
8.2
7.7
6.0
5.3
5.3
5.2
4.9
4.5
0.0
10.0
20.0
30.0
40.0
50.0
60.0
Figure
1.2.
Maximum
ozone
contributions
by
upwind
State
considering
only
NOx
emissions
impacts.
