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G. M. Wolfe,
J. A. Thornton,
N. C. Bouvier-Brown,
A. H. Goldstein,
Park J.-H,
McKay M,
D. M. Matross, Mao J,
W. H. Brune,
B. W. LaFranchi, [......],
Min K.-E,
P. J. Wooldridge,
R. C. Cohen,
J. D. Crounse,
I. C. Faloona,
J. B. Gilman,
W. C. Kuster,
J. A. de Gouw,
Huisman A,
F. N. Keutsch
[show abstract]
[hide abstract]
ABSTRACT: In a companion paper, we introduced the Chemistry of Atmosphere-Forest Exchange (CAFE) model, a vertically-resolved 1-D chemical transport model designed to probe the details of near-surface reactive gas exchange. Here, we apply CAFE to noontime observations from the 2007 Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX-2007). In this work we evaluate the CAFE modeling approach, demonstrate the significance of in-canopy chemistry for forest-atmosphere exchange and identify key shortcomings in the current understanding of intra-canopy processes. CAFE generally reproduces BEARPEX-2007 observations but requires an enhanced radical recycling mechanism to overcome a factor of 6 underestimate of hydroxyl (OH) concentrations observed during a warm (~29 °C) period. Modeled fluxes of acyl peroxy nitrates (APN) are quite sensitive to gradients in chemical production and loss, demonstrating that chemistry may perturb forest-atmosphere exchange even when the chemical timescale is long relative to the canopy mixing timescale. The model underestimates peroxy acetyl nitrate (PAN) fluxes by 50% and the exchange velocity by nearly a factor of three under warmer conditions, suggesting that near-surface APN sinks are underestimated relative to the sources. Nitric acid typically dominates gross dry N deposition at this site, though other reactive nitrogen (NOy) species can comprise up to 28% of the N deposition budget under cooler conditions. Upward NO2 fluxes cause the net above-canopy NOy flux to be ~30% lower than the gross depositional flux. CAFE under-predicts ozone fluxes and exchange velocities by ~20%. Large uncertainty in the parameterization of cuticular and ground deposition precludes conclusive attribution of non-stomatal fluxes to chemistry or surface uptake. Model-measurement comparisons of vertical concentration gradients for several emitted species suggests that the lower canopy airspace may be only weakly coupled with the upper canopy. Future efforts to model forest-atmosphere exchange will require a more mechanistic understanding of non-stomatal deposition and a more thorough characterization of in-canopy mixing processes.
Atmospheric Chemistry and Physics. 01/2011;
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E. C. Browne,
A. E. Perring,
P. J. Wooldridge,
Apel E,
S. R. Hall,
L. G. Huey, Mao J,
K. M. Spencer,
J. M. St. Clair,
A. J. Weinheimer,
Wisthaler A,
R. C. Cohen
[show abstract]
[hide abstract]
ABSTRACT: Using measurements from the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) experiment, we show that methyl peroxy nitrate (CH3O2NO2) is present in concentrations of ~5–15 pptv in the springtime arctic upper troposphere. We investigate the regional and global effects of CH3O2NO2 by including its chemistry in the GEOS-CHEM 3-D global chemical transport model. We find that at temperatures below 240 K inclusion of CH3O2NO2 chemistry results in decreases of up to ~20% in NOx, ~20% in N2O5, ~5% in HNO3, ~2% in ozone, and increases in methyl hydrogen peroxide of up to ~14%. Larger changes are observed in biomass burning plumes lofted to high altitude. Additionally, by sequestering NOx at low temperatures, CH3O2NO2 decreases the cycling of HO2 to OH, resulting in a larger upper tropospheric HO2 to OH ratio. These results may impact some estimates of lightning NOx sources as well as help explain differences between models and measurements of upper tropospheric composition.
Atmospheric Chemistry and Physics Discussions. 01/2011;
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Ren X,
Gao H,
Zhou X,
J. D. Crounse,
P. O. Wennberg,
E. C. Browne,
B. W. LaFranchi,
R. C. Cohen,
McKay M,
A. H. Goldstein, Mao J
Atmospheric Chemistry and Physics. 01/2010;
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Mao J,
D. J. Jacob,
M J Evans,
J. R. Olson,
Ren X,
W. H. Brune,
J. M. S. Clair,
J. D. Crounse,
K. M. Spencer,
M. R. Beaver, [......],
S. R. Hall,
A. J. Weinheimer,
R. C. Cohen,
Chen G,
J. H. Crawford,
Jaeglé L,
J. A. Fisher,
R.M. Yantosca,
P. Le Sager,
Carouge C
[show abstract]
[hide abstract]
ABSTRACT: We use observations from the April~2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals (HO<sub>x</sub>≡H+OH+peroxy radicals) and their reservoirs (HO<sub>y</sub>≡HO<sub>x</sub>+peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> concentrations. Computation of HO<sub>x</sub> and HO<sub>y</sub> gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect HO<sub>2</sub> uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H<sub>2</sub>O<sub>2</sub>. Such a mechanism could involve HO<sub>2</sub> aqueous-phase reaction with sulfate (58% of the ARCTAS submicron aerosol by mass) to produce peroxymonosulfate (HSO<sub>5</sub><sup>−</sup>) that would eventually convert back to sulfate and return water. We implemented such an uptake of HO<sub>2</sub> by aerosol in the model using a standard reactive uptake coefficient parameterization with γ(HO<sub>2</sub>) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different HO<sub>x</sub> species and HO<sub>y</sub> reservoirs. HO<sub>2</sub> uptake by aerosol is then a major HO<sub>x</sub> and HO<sub>y</sub> sink, decreasing mean OH and HO<sub>2</sub> concentrations in the Arctic troposphere by 48% and 45% respectively. Circumpolar budget analysis in the model shows that transport of peroxides from northern mid-latitudes contributes 50% of the HO<sub>y</sub> source above 6 km, and cloud chemistry and deposition of H<sub>2</sub>O<sub>2</sub> account together for 40% of the HO<sub>y</sub> sink below 3 km. Better rate and product data for HO<sub>2</sub> uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.
Atmospheric Chemistry and Physics Discussions. 01/2010;
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Ren X,
Gao H,
Zhou X,
J. D. Crounse,
P. O. Wennberg,
E. C. Browne,
B. W. LaFranchi,
R. C. Cohen,
McKay M,
A. H. Goldstein, Mao J
[show abstract]
[hide abstract]
ABSTRACT: Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH) in the lower troposphere. Understanding HONO chemistry, particularly its sources and contribution to HOx (=OH+HO2) production, is very important for understanding atmospheric oxidation processes. A highly sensitive instrument for detecting atmospheric HONO based on aqueous long path absorption photometry (LOPAP) was deployed in the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at Blodgett Forest, California in late summer 2007. The median diurnal variation shows minimum HONO levels of about 20–30 pptv during the day and maximum levels of about 60–70 pptv at night, a diurnal pattern quite different from the results at various other forested sites. Measured HONO/NO2 ratios for a 24-h period ranged from 0.05 to 0.13 with a mean ratio of 0.07. Speciation of reactive nitrogen compounds (NOy) indicates that HONO accounted for only ~3% of total NOy. However, due to the fast HONO loss through photolysis, a strong HONO source (1.59 ppbv day−1) existed in this environment in order to sustain the observed HONO levels, indicating the significant role of HONO in NOy cycling. The LOPAP HONO measurements were compared to the HONO measurements made with a Chemical Ionization Mass Spectrometer (CIMS) over a three-day period. Good agreement was obtained between the measurements from the two different techniques. Using the expansive suite of photochemical and meteorological measurements, the contribution of HONO photolysis to HOx budget was calculated to be relatively small (6%) compared to results from other forested sites. The lower HONO mixing ratio and thus its smaller contribution to HOx production are attributed to the unique meteorological conditions and low acid precipitation at Blodgett Forest. Further studies of HONO in this kind of environment are needed to test this hypothesis and to improve our understanding of atmospheric oxidation and nitrogen budget.
Atmospheric Chemistry and Physics Discussions. 01/2010;
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Choi W,
I. C. Faloona,
N. C. Bouvier-Brown,
McKay M,
A. H. Goldstein, Mao J,
W. H. Brune,
B. W. LaFranchi,
R. C. Cohen,
G. M. Wolfe,
J. A. Thornton,
D. M. Sonnenfroh,
D. B. Millet
[show abstract]
[hide abstract]
ABSTRACT: To better understand the processing of biogenic VOCs (BVOCs) in the pine forests of the US Sierra Nevada, we measured HCHO at Blodgett Research Station using Quantum Cascade Laser Spectroscopy (QCLS) during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) of late summer 2007. Four days of the experiment exhibited particularly copious HCHO, with midday peaks between 15–20 ppbv, while the other days developed delayed maxima between 8–14 ppbv in the early evening. From the expansive photochemical data set, we attempt to explain the observed HCHO concentrations by quantifying the various known photochemical production and loss terms in its chemical budget. Overall, known chemistry predicts a factor of 3–5 times less HCHO than observed. By examining diurnal patterns of the various budget terms we conclude that, during the high HCHO period, local, highly reactive oxidation chemistry produces an abundance of formaldehyde at the site. The results support the hypothesis of previous work at Blodgett Forest suggesting that large quantities of oxidation products, observed directly above the ponderosa pine canopy, are evidence of profuse emissions of very reactive volatile organic compounds (VR-VOCs) from the forest. However, on the majority of days, under generally cooler and more moist conditions, lower levels of HCHO develop primarily influenced by the influx of precursors transported into the region along with the Sacramento plume.
Atmospheric Chemistry and Physics. 01/2010;
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[show abstract]
[hide abstract]
ABSTRACT: Global models of atmospheric mercury generally assume that OH and ozone are the main oxidants converting Hg0 to HgII and thus driving mercury deposition to ecosystems. However, thermodynamic considerations argue against the importance of these reactions. We demonstrate here the viability of atomic bromine (Br) as an alternative Hg0 oxidant. We conduct a global 3-D simulation with the GEOS-Chem model assuming Br to be the sole Hg0 oxidant (Hg + Br model) and compare to the previous version of the model with OH and ozone as the sole oxidants (Hg + OH/O3 model). We specify global 3-D Br concentration fields based on our best understanding of tropospheric and stratospheric Br chemistry. In both the Hg + Br and Hg + OH/O3 models, we add an aqueous photochemical reduction of HgII in cloud to impose a tropospheric lifetime for mercury of 6.5 months against deposition, as needed to reconcile observed total gaseous mercury (TGM) concentrations with current estimates of anthropogenic emissions. This added reduction would not be necessary in the Hg + Br model if we adjusted the Br oxidation kinetics downward within their range of uncertainty. We find that the Hg + Br and Hg + OH/O3 models are equally capable of reproducing the spatial distribution of TGM and its seasonal cycle at northern mid-latitudes. The Hg + Br model shows a steeper decline of TGM concentrations from the tropics to southern mid-latitudes. Only the Hg + Br model can reproduce the springtime depletion and summer rebound of TGM observed at polar sites; the snowpack component of GEOS-Chem suggests that 40% of HgII deposited to snow in the Arctic is transferred to the ocean and land reservoirs, amounting to a net deposition flux of 60 Mg a−1. Summertime events of depleted Hg0 at Antarctic sites due to subsidence are much better simulated by the Hg + Br model. Model comparisons to observed wet deposition fluxes of mercury in the US and Europe show general consistency but the Hg + Br model is unable to capture the summer maximum over the southeast US because of low subtropical Br concentrations. Vertical profiles measured from aircraft show a decline of Hg0 above the tropopause that can be captured by both the Hg + Br and Hg + OH/O3 models, except in Arctic spring where the observed decline is much steeper than simulated by either model; we speculate that oxidation by Cl species might be responsible. The Hg + Br and Hg + OH/O3 models yield similar global budgets for the cycling of mercury between the atmosphere and surface reservoirs, but the Hg + Br model results in much larger fraction of mercury deposited to the Southern Hemisphere oceans.
Atmospheric Chemistry and Physics Discussions. 01/2010;
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M. J. Alvarado,
J. A. Logan, Mao J,
Apel E,
Riemer D,
Blake D,
R. C. Cohen,
Min K.-E,
A. E. Perring,
E. C. Browne, [......],
I. B. Pollack,
P. O. Wennberg,
Kurten A,
Crounse J,
J. M. St. Clair,
Wisthaler A,
Mikoviny T,
R.M. Yantosca,
C. C. Carouge,
P. Le Sager
[show abstract]
[hide abstract]
ABSTRACT: We determine enhancement ratios for NOx, PAN, and other NOy species from boreal biomass burning using aircraft data obtained during the ARCTAS-B campaign and examine the impact of these emissions on tropospheric ozone in the Arctic. We find an initial emission factor for NOx of 1.06 g NO per kg dry matter (DM) burned, much lower than previous observations of boreal plumes, and also one third the value recommended for extratropical fires. Our analysis provides the first observational confirmation of rapid PAN formation in a boreal smoke plume, with 40% of the initial NOx emissions being converted to PAN in the first few hours after emission. We find little clear evidence for ozone formation in the boreal smoke plumes during ARCTAS-B in either aircraft or satellite observations, or in model simulations. Only a third of the smoke plumes observed by the NASA DC8 showed a correlation between ozone and CO, and ozone was depleted in the plumes as often as it was enhanced. Special observations from the Tropospheric Emission Spectrometer (TES) also show little evidence for enhanced ozone in boreal smoke plumes between 15 June and 15 July 2008. Of the 22 plumes observed by TES, only 4 showed ozone increasing within the smoke plumes, and even in those cases it was unclear that the increase was caused by fire emissions. Using the GEOS-Chem atmospheric chemistry model, we show that boreal fires during ARCTAS-B had little impact on the median ozone profile measured over Canada, and had little impact on ozone within the smoke plumes observed by TES.
Atmospheric Chemistry and Physics. 01/2010;
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Mao J,
D. J. Jacob,
M J Evans,
J. R. Olson,
Ren X,
W. H. Brune,
J. M. St. Clair,
J. D. Crounse,
K. M. Spencer,
M. R. Beaver, [......],
R. C. Cohen,
Chen G,
J. H. Crawford,
McNaughton C,
A. D. Clarke,
Jaeglé L,
J. A. Fisher,
R.M. Yantosca,
P. Le Sager,
Carouge C
[show abstract]
[hide abstract]
ABSTRACT: We use observations from the April 2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals (HO<sub>x</sub>≡H+OH+peroxy radicals) and their reservoirs (HO<sub>y</sub>≡HO<sub>x</sub>+peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> concentrations. Computation of HO<sub>x</sub> and HO<sub>y</sub> gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect HO<sub>2</sub> uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H<sub>2</sub>O<sub>2</sub>. We implemented such an uptake of HO<sub>2</sub> by aerosol in the model using a standard reactive uptake coefficient parameterization with γ(HO<sub>2</sub>) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different HO<sub>x</sub> species and HO<sub>y</sub> reservoirs. HO<sub>2</sub> uptake by aerosol is then a major HO<sub>x</sub> and HO<sub>y</sub> sink, decreasing mean OH and HO<sub>2</sub> concentrations in the Arctic troposphere by 32% and 31% respectively. Better rate and product data for HO<sub>2</sub> uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.
Atmospheric Chemistry and Physics. 01/2010;
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B. W. LaFranchi,
G. M. Wolfe,
J. A. Thornton,
S. A. Harrold,
E. C. Browne,
K. E. Min,
P. J. Wooldridge,
J. B. Gilman,
W. C. Kuster,
P. D. Goldan,
J. A. deGouw,
McKay M,
A. H. Goldstein,
Ren X, Mao J,
R. C. Cohen
[show abstract]
[hide abstract]
ABSTRACT: Acyl peroxy nitrates (APNs, also known as PANs) are formed from the oxidation of aldehydes and other oxygenated VOC (oVOC) in the presence of NO2. Formation of APNs suppresses NOx (NOx≡NO+NO2) in urban areas and enhances NOx downwind in urban plumes, increasing the rate of ozone production throughout an urban plume. APNs also redistribute NOx on global scales, enhancing NOx and thus ozone production. There are both anthropogenic and biogenic oVOC precursors to APNs, but a detailed evaluation of their chemistry against observations has proven elusive. Here we describe measurements of PAN, PPN, and MPAN along with the majority of chemicals that participate in their production and loss, including OH, HO2, numerous oVOC, and NO2. Observations were made during the Biosphere Effects on AeRosols and Photochemistry Experiment (BEARPEX 2007) in the outflow of the Sacramento urban plume. These observations are used to evaluate a detailed chemical model of APN ratios and concentrations. We find the ratios of APNs are nearly independent of the loss mechanisms and thus an especially good test of our understanding of their sources. We show that oxidation of methylvinyl ketone, methacrolein, methyl glyoxal, biacetyl and acetaldehyde are all significant sources of the PAN+peroxy acetyl (PA) radical reservoir, with methylvinyl ketone (MVK) often being the primary non-acetaldehyde source. At high temperatures, oxidation of non-acetaldehyde PA radical sources contributes over 60% to the total PA production rate. An analysis of absolute APN concentrations reveals a missing APN sink that can be resolved by increasing the PA+∑RO2 rate constant by a factor of 3.
Atmospheric Chemistry and Physics Discussions. 01/2009;
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B. W. LaFranchi,
G. M. Wolfe,
J. A. Thornton,
S. A. Harrold,
E. C. Browne,
K. E. Min,
P. J. Wooldridge,
J. B. Gilman,
W. C. Kuster,
P. D. Goldan,
J. A. de Gouw,
McKay M,
A. H. Goldstein,
Ren X, Mao J,
R. C. Cohen
[show abstract]
[hide abstract]
ABSTRACT: Acyl peroxy nitrates (APNs, also known as PANs) are formed from the oxidation of aldehydes and other oxygenated VOC (oVOC) in the presence of NO2. There are both anthropogenic and biogenic oVOC precursors to APNs, but a detailed evaluation of this chemistry against observations has proven elusive. Here we describe measurements of PAN, PPN, and MPAN along with the majority of chemicals that participate in their production and loss, including OH, HO2, numerous oVOC, and NO2. Observations were made during the Biosphere Effects on AeRosols and Photochemistry Experiment (BEARPEX 2007) in the outflow of the Sacramento urban plume. These observations are used to evaluate a detailed chemical model of APN ratios and concentrations. We find that the ratios of APNs are nearly independent of the loss mechanisms and thus an especially good test of our understanding of their sources. We show that oxidation of methylvinyl ketone, methacrolein, methyl glyoxal, biacetyl and acetaldehyde are all significant sources of the PAN+peroxy acetyl (PA) radical reservoir, accounting for 26%, 2%, 7%, 20%, and 45%, of the production rate on average during the campaign, respectively. At high temperatures, when upwind isoprene emissions are highest, oxidation of non-acetaldehyde PA radical sources contributes over 60% to the total PA production rate, with methylvinyl ketone being the most important of the isoprene-derived sources. An analysis of absolute APN concentrations reveals a missing APN sink that can be resolved by increasing the PA+∑RO2 rate constant by a factor of 3.
Atmospheric Chemistry and Physics. 01/2009;
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Mao J,
Ren X,
W. H. Brune,
J. R. Olson,
J. H. Crawford,
Fried A,
L. G. Huey,
R. C. Cohen,
Heikes B,
H. B. Singh,
D R Blake,
G. W. Sachse,
G. S. Diskin,
S. R. Hall,
R. E. Shetter
[show abstract]
[hide abstract]
ABSTRACT: The measurement of OH reactivity, the inverse of the OH lifetime, provides a powerful tool to investigate the atmospheric photochemistry. A new airborne OH reactivity instrument was designed and deployed for the first time on the NASA DC-8 aircraft during Intercontinental Chemical Transport Experiment-B (INTEX-B) campaign. The OH reactivity was measured by adding OH, generated by photolyzing water vapor with 185 nm UV light in a moveable wand, to the flow of ambient air in a flow tube and measuring the OH signal with laser induced fluorescence. As the wand was pulled back away from the OH detector, the OH signal decay was recorded; the slope of −Δln(signal)/Δtime was the OH reactivity. From the median vertical profile obtained in the second phase of INTEX-B, the measured OH reactivity (4.0±1.0 s−1) is higher than the OH reactivity calculated from assuming that OH was in steady state (3.3±0.8 s−1), and even higher than the OH reactivity that was calculated from the total measurements of all OH reactants (1.6±0.4 s−1). Model calculations show that the missing OH reactivity is consistent with the over-predicted OH and under-predicted HCHO in the boundary layer and lower troposphere. The over-predicted OH and under-predicted HCHO suggest that the missing OH sinks are most likely related to some highly reactive VOCs that have HCHO as an oxidation product.
Atmospheric Chemistry and Physics Discussions. 01/2008;
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T. R. Shirley,
W. H. Brune,
Ren X, Mao J,
Lesher R,
Cardenas B,
Volkamer R,
L. T. Molina,
M. J. Molina,
Lamb B,
Velasco E,
Jobson T,
Alexander M
[show abstract]
[hide abstract]
ABSTRACT: The Mexico City Metropolitan Area (MCMA) study in April 2003 had measurements of many atmospheric constituents, including OH and HO<sub>2</sub>. It provided the first opportunity to examine atmospheric oxidation in a megacity in a developing country that has more pollution than typical U.S. and European cities. At midday, OH typically reached 0.35 pptv (~7×10<sup>6</sup> cm<sup>−3</sup>), comparable to amounts observed in U.S. cities, but HO<sub>2</sub> reached 40 pptv, more than observed in most U.S. cities. The OH reactivity was also measured, even during the highly polluted morning rush hour. MCMA's OH reactivity was 25 s<sup>−1</sup> during most of the day and 120 s<sup>−1</sup> at morning rush hour, which was several times greater than has been measured in any U.S. city. Median measured and modeled OH and HO<sub>2</sub> agreed to within combined uncertainties, although for OH, the model exceeded the measurement by more than 30% during midday. OH production and loss, which were calculated from measurements, were in balance to within uncertainties, although production exceeded loss during morning rush hour. This imbalance has been observed in other cities. The HO<sub>2</sub>/OH ratio from measurements and steady-state analyses have essentially the same dependence on NO, except when NO was near 100 ppbv. This agreement is unlike other urban studies, in which HO<sub>2</sub>/OH ratio decreased much less than expected as NO increased. As a result of the active photochemistry in MCMA 2003, the median calculated ozone production from measured HO<sub>2</sub> reached 50 ppb h<sup>−1</sup>, a much higher rate than observed in U.S. cities.
Atmospheric Chemistry and Physics. 01/2006;
