Observing the Transition From NOx-Limited to NOx-Saturated O3 Production

1
Observing the Transition From NOx-Limited to
NOx-Saturated O3 Production J. A. Thornton1, P.
J. Wooldridge1, R. C. Cohen1, M. Martinez2, H.
Harder2, W. H. Brune2, E. J. Williams3, S. R.
Hall4, R. E. Shetter4, B. P. Wert4, B. Henry4, A.
Fried4, F. E. Fehsenfeld3 1 Department of
Chemistry University of California, Berkeley
Berkeley, CA 94720 2 Department of Meteorology
Pennsylvania State University 3 Aeronomy
Laboratory, NOAA Boulder, CO 4 Atmospheric
Chemistry Division, NCAR Boulder CO
I. Introduction
V. The Dependence of PO3 on Primary Radical
Production and NO
VII. Chemical Coordinate Analysis
Tropospheric O3 concentrations are functions of
the chain lengths of NOx (NOx ? NO NO2) and HOx
(HOx ? OH HO2 RO2) radical catalytic cycles.
For a fixed HOx source at low NOx concentrations,
kinetic models indicate the rate of O3 production
increases linearly with increases in NOx
concentrations (NOx-limited). At higher NOx
concentrations, kinetic models predict ozone
production rates decrease with increasing NOx
(NOx-saturated). We present observations of NO,
NO2, O3, OH, HO2, H2CO, actinic flux, and
temperature obtained during the 1999 Southern
Oxidant Study from June 15 July 15, 1999 at
Cornelia Fort Airpark, Nashville, TN. The
observations are used to evaluate the
instantaneous ozone production rate (PO3) as a
function of NO abundances and the primary HOx
production rate (PHOx). These observations
provide quantitative evidence for the response of
PO3 to NOx. For high PHOx (0.5 lt PHOx lt 0.7
ppt/s), O3 production at this site increases
linearly with NO to 500 ppt. PO3 levels out in
the range 500-1000 ppt NO, and decreases for NO
above 1000 ppt. An analysis along chemical
coordinates indicates that models of chemistry
controlling peroxy radical abundances, and
consequently PO3, have a large error in the rate
or product yield of the RO2 HO2 reaction for
the classes of RO2 that predominate in Nashville.
Photochemical models and our measurements can be
forced into agreement if the product of the
branching ratio and rate constant for organic
peroxide formation, via RO2 HO2 ? ROOH O2, is
reduced by a factor of 3-12. Alternatively, these
peroxides could be rapidly photolyzed under
atmospheric conditions making them at best a
temporary HOx reservoir. This result implies that
O3 production in or near urban areas with similar
hydrocarbon reactivity and HOx production rates
may be NOx-saturated more often than current
models suggest.
Examine the balance between PHOx and LHOx
The dependence of PO3 on NOx arises from the
competition between two categories of chain
termination reactions
Eqn (3)
Eqn (4)
II. Measurements and Site Description
Eqn (5)
Species Method Total Uncertainty
NO2 Laser-Induced Fluorescence 10
NO Chemiluminescence 10
O3 UV Absorbance lt10
OH Laser-Induced Fluorescence 20
HO2 Titration to OH by NO Followed by Laser-Induced Fluorescence 5
H2CO Tunable Diode Laser Absorption Spectroscopy 10
Solar Actinic Flux Scanning Actinic Flux Spectral Radiometer 10
Relative Humidity and Temperature Commercial Probe 5
The role of the HOx production rate, PHOx, on PO3
for constant NOx
Figure 4 The observationally constrained ratio
PHOx/LHOx is plotted versus PHOx (left), the
fraction of HOx loss due to ROOH formation
(center), and the fraction of HOx loss due to
HNO3 formation. In this initial model, we assume
alkyl nitrate formation is negligible and set ?
0. From this analysis, it is apparent that the
chemistry describing ROOH formation is in error
by nearly a factor of 10.
We calculate PHOx using observations and Equation
6.
Includes both accuracy and precision
VI. NOx-limited versus NOx-saturated O3
Production at CFA
The extensive suite of measurements made at
Cornelia Fort Airpark (CFA) over the period June
15 July 15, 1999 as part of the Southern
Oxidant Study (SOS 99) provides one of the most
detailed characterizations of an urban
environment to date. CFA is located 8 km, NE of
downtown Nashville, TN in the flood plain of the
Cumberland River. The measured species used here,
the methods used to measure them, and the
reported uncertainties are shown in the table
above. For the purposes of this study, all
species were averaged to 1-minute intervals and
none of the measurements required interpolation
to this time base.
Figure 5 The panels show PHOx/LHOx with a new
model of HOx loss plotted versus the same
chemical coordinates as in Figure 4. Reducing
ROOH formation by a factor of 10 and including
3 organic nitrate formation improves the radical
budget and removes most of the trends of
PHOx/LHOx versus chemical coordinates observed in
Figure 4. We use this improved model to calculate
the crossover point between NOx-limited and
NOx-saturated O3 production below in Figure 6.
III. Photochemical O3 Production
Figure 6 (right) shows the two fractions,
HHLoss/LHOx and NHLoss/LHOx plotted versus NO
where we have reduced the organic peroxide
formation rate by a factor of 10, and assumed a
3 organic nitrate yield. As opposed to the
results shown in the lower right panel of Figure
3, the new model suggests a crossover at
approximately 500-600 ppt NO. This result is more
consistent with the PO3 derived from observations.
Figure 1 (left) shows 1-minute averaged PO3
calcuated using Eqn. 2 plotted versus Hour of
Day. All of the data obtained at CFA is shown.
VIII. Conclusions and Implications
1
3
Figure 2 (left) shows PO3 for three consecutive
days. While there is a clear diurnal trend with
rates peaking near 12pm, there is evidence of
significant day-to-day variation and variation on
the time scale of minutes to hours as well.
Eqn (1)
Eqn (2)
JNO2 is the photolysis rate constant for NO2
derived from solar actinic flux measurements.
Reaction rate constants are taken from DeMore, et
al, 2000, DeMore, et al., 1997, and Atkinson,
1994.
O3 are the integral of PO3 (and LO3)
2
Figure 3 The upper panels, illustrate the
predictive behavior of this chemical system in a
simple model. The Left panel shows PO3 versus NO
for three different regimes of PHOx The right
panel shows PO3 (red), and the rates of HHLoss
(blue) and NHLoss relative to the total plotted
versus NO. A wide range of parameters and
conditions were used in the model. The point
where the two fractions HHLoss/LHOx and
NHLoss/LHOx were equal always occurred at an NO
concentration that was 25 less than that where
the peak in PO3 occurred, however, the model is
not expected to reproduce the observations in an
absolute sense. The lower panels show the same
quantities as the upper panels but now derived
from the observations. PO3 derived from
observations exhibits, qualitatively, the
expected behavior as a function of PHOx and NO
(lower left). However, the two fractions
HHLoss/LHOx and NHLoss/LHOx (lower right) are
equal at approximately 900-1000 ppt NO, a higher
concentration than where PO3 peaks. This
conflicts with the expected behavior shown in the
upper right panel.
old
new
IV. The Photo-Stationary State Assumption
Is the atmosphere in steady state? The closest
large NOx sources were 15 minutes away for
typical wind speeds of 4-5 m/s. This is several
e-folds in the NOx intra-conversion lifetime (
100 sec). The effect of potential surface
emissions of NO on the stationary state were
estimated by subtracting typical nighttime values
of NO (20-50 ppt) from the observed daytime
concentrations. NO emissions lead to a potential
bias of at most 5-10 in the PO3 calculated from
the steady state assumption.
Is the PSS-calculated PO3 accurate and precise?
The experiment at CFA provided some of the most
accurate measurements of NO2. The reported total
uncertainty of the measurements combine to give a
total uncertainty in PO3 of ? 34. However, we
note that for examining trends PO3 and our model
of the crossover point, precision is more
important than the total uncertainty or accuracy,
and the precision of the calculated PO3 is
approximately 10.
Acknowledgements NASA Earth Systems Science
Graduate Fellowship NOAA Office of Global Programs
These conflicting results imply that the rate of
peroxide formation (HHLoss) is either too fast or
the rate of nitrate formation (NHLoss) is too
slow, or some combination of the two.
Our new model of HOx loss predicts that O3
production will be NOx-saturated more often than
current models predict.