Analysis
of
Substitute
Data
for
SO2
The
Acid
Rain
Program
relies
on
the
complete
and
accurate
accounting
of
sulfur
dioxide
(
SO2)
emissions
to
support
the
implementation
of
the
trading
mechanism
used
to
reduce
the
overall
amount
of
SO2
emissions.
The
provisions
of
40
CFR
part
75,
regulate
how
emissions
data
are
to
be
collected
through
effective
emissions
monitoring
techniques.
To
ensure
a
complete
accounting
of
emissions,
part
75
includes
provisions
for
substitute
data
to
be
reported
during
periods
where
real
measured
data
is
missing,
or
is
not
adequately
quality­
assured.

The
substitute
data
provisions
are
designed
to
encourage
and
promote
a
high
percent
monitor
availability
(
PMA)
which
is
the
percent
of
operating
hours
where
quality
assured
data
are
recorded
as
compared
to
the
last
8760
hours
of
operation.
Typical
PMA's
exceed
98
percent.
There
are
four
basic
"
tiers"
of
substitute
data
for
SO2.

The
first
tier
is
reserved
for
units
that
have
achieved
and
maintained
a
PMA
of
95
percent
or
higher.
In
this
tier,
if
the
duration
of
the
missing
data
event
is
less
than
24
consecutive
operating
hours
then
the
average
of
the
quality
assured
data
from
the
hour
before
and
hour
after
the
missing
data
period
is
used
to
substitute
for
the
missing
data.
This
form
of
substitute
data
is
not
intended
to
be
conservative,
rather
it
is
designed
to
fill
the
data
gap
that
would
otherwise
exist
for
the
occasional,
acceptable
level
of
data
loss
that
is
expected.
If
the
missing
data
period
is
greater
than
24
consecutive
operating
hours,
then
the
appropriate
substitute
data
value
is
either
the
90
th
percentile
value
from
the
previous
720
hours
of
real
measured
quality
assured
data,
or
the
hour
before/
hour
after
average,
whichever
is
greater.
The
90
th
percentile
value
is
usually
higher
than
the
average
hour
before/
hour
after
value,
and
is
intended
to
be
a
conservative
estimate
of
emissions
during
a
prolonged
missing
data
period.

The
second
tier
is
for
units
that
have
a
PMA
of
90
percent
or
more,
but
less
than
95
percent.
This
tier
is
similar
to
the
first,
except
that
the
allowable
duration
to
use
the
hour
before/
hour
after
value
is
8
consecutive
operating
hours
and
not
24.
If
the
missing
data
period
exceeds
8
consecutive
operating
hours,
then
the
appropriate
substitute
data
value
is
either
the
95
th
percentile
value
from
the
previous
720
hours
of
real
measured
quality
assured
data,
or
the
hour
before/
hour
after
average,
whichever
is
greater.
Again,
the
95
th
percentile
value
is
usually
higher
than
the
average
hour
before/
hour
after
value,
and
is
intended
to
be
a
more
conservative
estimate
of
emissions
during
a
prolonged
missing
data
period
to
encourage
good
monitoring
practices
that
minimize
loss
of
real
quality
assured
data.

The
third
tier
is
for
units
that
have
a
PMA
of
80
percent
or
more,
but
less
than
90
percent.
This
tier
requires
that
the
unit
substitute
the
maximum­
recorded
value
from
the
previous
720
hours
of
available
measured
quality
assured
data.
This
level
of
substitute
data
is
intended
to
be
somewhat
conservative,
so
as
to
discourage
sources
from
having
a
relatively
low
PMA
and
large
amounts
of
missing
data.
Units
that
have
add
on
controls
may
petition
to
use
a
maximum
expected
controlled
value
instead
of
the
maximum
recorded
value
for
any
hour
where
the
controls
are
shown
by
parametric
data
to
be
operating
properly.

The
fourth
and
last
tier
of
substitute
data
is
reserved
for
when
the
PMA
for
a
monitoring
system
falls
below
80
percent.
At
this
level,
the
source
has
failed
to
achieve
an
acceptable
PMA
and
has
an
unacceptable
amount
of
missing
data
over
the
last
8760
hours
of
operation.
In
such
cases,
the
unit
must
substitute
the
maximum
potential
value
for
each
hour
of
missing
data.

Analysis
of
recent
monitoring
data
shows
that
less
than
one
percent
(
0.69%)
of
the
hourly
SO2
data
reported
in
the
first
three
calendar
quarters
of
2004
is
substitute
data.
More
than
three­
quarters
(
76%)
of
the
substitute
data
used
was
the
non­
conservative
average
of
the
quality
assured
data
from
the
hour
before
and
hour
after
each
missing
data
period.
The
maximum
potential
value
only
made
up
three
percent
of
all
substitute
data
(
less
than
0.02
percent
of
all
data
reported).
EPA's
recent
policy
change
regarding
the
implementation
of
substitute
data
will
further
minimize
the
amount
of
maximum
potential
data
that
will
be
required
in
the
future.
