Let’s Abandon
the “High NO
x
” and “Low NO
x
” Terminology
Paul O. Wennberg
*
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https://doi.org/10.1021/acsestair.3c00055
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Recommendations
“High-NO
x
” and
“low-NO
x
” are
used
ubiquitously
in the
atmospheric
chemistry
community
as shorthand
terms
meant
to describe
the
chemical
end-member
photochemical
con-
ditions
that
span
from
“urban”/“anthropogenically-impacted”
to “remote”/“pristine”.
They
do not,
however,
have
precise
or
accepted
definitions.
Following
a rather
heated
discussion
at
the 2012
Atmospheric
Chemistry
Mechanism
conference
in
Davis,
California,
I was
tasked
with
suggesting
appropriate
definitions.
I’ve
come
to the opinion
that
these
terms
cause
more
confusion
than
they
do insight
and
we should
abandon
them
entirely.
Within
the air quality
community,
“high-NO
x
” is often
used
to describe
an environment
which
is “NO
x
-saturated”
with
respect
to the
production
of oxidants,
in particular,
ozone
(O
3
). In such
environments,
where
NO
x
concentrations
are
measured
in 10s of ppb
or more,
the production
rate
of O
3
is
either
independent
of or decreases
with
additional
NO
x
. This
dependence
results
from
the titration
of O
3
by NO
and
by a
slowing
of the rate
of oxidation
of volatile
organic
chemicals
(VOCs)
by OH
(the
reduction
in OH
results
from
loss
in its
reaction
with
NO
2
).
Within
the
community
interested
in the
atmospheric
photochemical
oxidation
of organic
molecules,
the
term
“high-NO
x
” has
generally
been
used
to refer
to conditions
where
the fate of peroxy
radicals
formed
from
the OH-initiated
oxidation
of any
number
of hydrocarbons
is exclusively
reaction
with
NO.
Such
NO-dominated
peroxy
radical
chemistry
occurs
in the
atmosphere
(and
many
laboratory
studies)
when
NO
concentrations
are typically
greater
than
2
×
10
10
molecules
cm
−
3
(>
∼
1
ppb
at 1 atm),
e.g.,
more
than
an
order
of magnitude
lower
than
the “NO
x
-saturated”
conditions
described
above.
Even
here,
however,
the term
is used
rather
loosely
and
often
just shorthand
for laboratory
(e.g.
chamber)
experiments
with
initially
large
concentrations
of NO
(100s
of
ppb).
Even
with
these
extreme
NO
levels,
the
“high-NO
x
”
terminology
can
be confusing.
Consider
an atmospheric
chamber
experiment
probing
the OH
oxidation
of an alkane
that
begins
with
100
ppb
of NO
and 200
ppb
of alkane.
As the
photochemistry
proceeds,
the reaction
of NO
with
both
the
organic
peroxy
radicals
and
any
O
3
produced
in the system
rapidly
converts
the NO
and
NO
2
. NO
x
(the
sum
of NO
and
NO
2
) may
decrease
only
slowly
as organic
nitrates,
PAN-type
compounds,
and HNO
3
are formed.
Even
with
very
high
initial
concentrations
of NO,
the chemistry
may
transition
over
the
course
of a single
experiment
to NO-starved
conditions
where
the peroxy
radicals
react
with
HO
2
(and
other
peroxy
radicals),
producing
organic
hydroperoxides
(and
alcohols),
compounds
Received:
October
12, 2023
Accepted:
October
19, 2023
Published:
November
29,
2023
Figure
1.
Topological
confusion.
Original
artwork
by Jeff Jennings
and
Robin
Strelow
from
IGAC
News.
Viewpoint
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© 2023
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Chemical
Society
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that,
as described
below,
are generally
considered
“low-NO
x
”
products.
As another
example
of the ambiguity,
consider
the
formation
of secondary
organic
aerosol
(SOA)
from
isoprene
in “high-NO
x
” environments.
The
SOA
forms
as a result
of the
oxidation
of a third-generation
isoprene
oxidation
product,
MPAN,
by OH
radicals.
The
lifetime
of MPAN,
and
thus
the
amount
formed
that
can
be oxidized
to produce
SOA
is,
however,
controlled
by the
ratio
NO
2
/NO.
Thus,
SOA
formation
may
be efficient
in a NO
2
-rich
“high-NO
x
”
environment
(high
ozone
and
low
photolysis
rates,
e.g.,
a
warm
but
cloudy
day
in Atlanta),
while
almost
no isoprene
SOA
will
form
in a NO-rich
“high-NO
x
” environment
(low
ozone,
high
photolysis,
e.g.,
a tropical
coastal
city).
Clearly,
the
“high-NO
x
” terminology
is insufficient
to describe
the richness
of the “urban
impacted”
chemistry.
Beyond
the simple
tautological
definition,
“low-NO
x
” also
does
not
define
a single
chemical
regime.
As for its obvious
meaning,
even
here
there
is little
agreement.
At the
2012
Atmospheric
Chemistry
Mechanisms
conference,
for example,
a participant
tongue-in-cheek-suggested
that
“low-NO
x
”
conditions
could
be defined
as when
the NO
concentration
was
too
small
to be
measured
with
a commercial
chemiluminescence
NO
x
sensor.
Analytical
challenges
aside,
the relationship
between
a “pristine”
atmosphere
and
a “low-
NO
x
” one
is certainly
not
unique:
laboratory
experiments
characterized
as “low-NO
x
” may
or may
not
be relevant
to
pristine
conditions
found
in the atmosphere.
The
diversity
of “low-NO
x
” chemical
regimes
results
from
the diversity
of the chemistry.
When
NO
is absent,
peroxy
radicals
react
with
other
peroxy
radicals
(including
HO
2
) or
can
undergo
unimolecular
processes
such
as isomerization,
photolysis,
or heterogeneous
uptake.
The
fraction
of peroxy
radicals
that
follow
each
pathway
varies
by system
and depends
on environmental
conditions.
In the laboratory
a “low-NO
x
”
environment
occurs
where
the
peroxy
radicals
may
be
dominated
by self-reaction,
while
in the
atmosphere,
what
may
be thought
of as the same
chemistry
(e.g.,
oxidation
of an
alkene
by OH
in the absence
of NO)
may
proceed
entirely
via
a reaction
with
HO
2
to form
hydroperoxides
and
none
of the
alcohols
and aldehydes
produced
in the laboratory
experiment.
One
possible
solution
to the terminological
confusion
might
be to add
additional
end-members
to our
vocabulary
(e.g.
“HO
2
-dominated”,
“isomerization-dominated”,
etc.).
But given
that
richness
of the NO-free
chemistry,
such
a solution
is not
likely
to produce
any
efficiency
beyond
that
of simply
describing
the chemical
state.
In summary,
rather
than
helping
to clarify
and
systematize,
the “low-NO
x
”/“high-NO
x
” terminology
we employ
as short-
hand
to describe
the photochemical
conditions
in the lab and
in the field
often
leads
to confusion
and
muddled
thinking.
These
photochemical
conditions
encompass
a topology
that
is
not
a line
between
two
unique
end
members,
but
rather
a
continuum
of photochemical
states
of which
only
a small
fraction
can
be found
in the atmosphere.
In reporting
both
laboratory
and
field
studies,
rather
than
characterize
the
conditions
as either
low
or high
NO
x
, let us provide
a
description
of the fate
of the peroxy
radicals
(along
with
the
necessary
estimate
of the
uncertainty).
In this
way,
the
comparability
among
laboratory
studies
and between
them
and
the field
will be made
more
explicit
and
transparent.
■
AUTHOR
INFORMATION
Corresponding
Author
Paul
O. Wennberg
−
Division
of Engineering
and Applied
Science
and Division
of Geological
and Planetary
Sciences,
California
Institute
of Technology,
Pasadena,
California
91125,
United
States;
orcid.org/0000-0002-6126-3854;
Email:
wennberg@caltech.edu
Complete
contact
information
is available
at:
https://pubs.acs.org/10.1021/acsestair.3c00055
Notes
This
essay
was
originally
published
in issue
50 of IGAC
News
(July,
2013).
A decade
later,
I feel it is still relevant.
I advocate
that
as authors,
reviewers,
and editors
we work
to weed
out the
high
and
low
NO
x
terminology
from
our
manuscripts.
Reprinted
with
permission
of IGAC
News.
Views
expressed
in this viewpoint
are those
of the author
and
not necessarily
the views
of the ACS.
The
author
declares
no competing
financial
interest.
Biography
Paul
Wennberg
is the R. Stanton
Avery
Professor
of Atmospheric
Chemistry
and
Environmental
Science
and
Engineering
at Caltech.
He studies
the composition
of the atmosphere
of Earth.
He is trained
as a physical
chemist
and most
of his investigations
begin
with
remote
or in situ
measurements
of atmospheric
trace
gases
made
via remote
sensing
or in the laboratory
or in the field.
■
NOTE ADDED
AFTER ASAP PUBLICATION
This
paper
was
published
ASAP
on November
29,
2023,
without
the correct
graphics
and without
the author
photo
and
biography.
The
corrected
version
was
posted
on December
8,
2023.
ACS ES&T Air
pubs.acs.org/estair
Viewpoint
https://doi.org/10.1021/acsestair.3c00055
ACS EST Air
XXXX,
XXX,
XXX
−
XXX
B