scholarly journals Evolution of NO<sub>3</sub> reactivity during the oxidation of isoprene

2020 ◽  
Author(s):  
Patrick Dewald ◽  
Jonathan M. Liebmann ◽  
Nils Friedrich ◽  
Justin Shenolikar ◽  
Jan Schuladen ◽  
...  
Keyword(s):  
Rate Coefficient ◽  
Chemical Mechanism ◽  
Organic Nitrates ◽  
Peroxy Radicals ◽  
Reaction Products ◽  
Flow Tube ◽  
Mixing Ratios ◽  
Tube Reactor ◽  
Master Chemical Mechanism ◽  
Series Of Experiments

Abstract. In a series of experiments in an atmospheric simulation chamber (SAPHIR, Forschungszentrum Jülich, Germany) NO3 reactivity (kNO3) resulting from the reaction of NO3 with isoprene and stable trace gases formed as products was measured directly using a flow-tube reactor coupled to a cavity-ring-down spectrometer (FT-CRDS). The experiments were carried out in both dry and humid air with variation of the initial mixing ratios of ozone (50–100 ppbv), isoprene (3–22 ppbv) and NO2 (5–30 ppbv). kNO3 was in excellent agreement with values calculated from the isoprene mixing ratio and the rate coefficient for the reaction of NO3 with isoprene. This result serves both to confirm that the FT-CRDS returns accurate values of kNO3 even at elevated NO2 concentrations and to show that reactions of NO3 with stable reaction products like non-radical organic nitrates do not contribute significantly to NO3 reactivity during the oxidation of isoprene. A comparison of kNO3 with NO3 reactivities calculated from NO3 mixing ratios and NO3 production rates suggests that organic peroxy radicals and HO2 account for ~ 50 % of NO3 losses. This contradicts predictions based on numerical simulations using the Master Chemical Mechanism (MCM version 3.3.1) unless the rate coefficient for reaction between NO3 and isoprene-derived RO2 is roughly doubled to ≈ 5 × 10−12 cm3 molecule−1 s−1.

scholarly journals Evolution of NO<sub>3</sub> reactivity during the oxidation of isoprene

2020 ◽  
Vol 20 (17) ◽  
pp. 10459-10475
Author(s):  
Patrick Dewald ◽  
Jonathan M. Liebmann ◽  
Nils Friedrich ◽  
Justin Shenolikar ◽  
Jan Schuladen ◽  
...  
Keyword(s):  
Rate Coefficient ◽  
Chemical Mechanism ◽  
Organic Nitrates ◽  
Peroxy Radicals ◽  
Reaction Products ◽  
Flow Tube ◽  
Mixing Ratios ◽  
Tube Reactor ◽  
Master Chemical Mechanism ◽  
Series Of Experiments

Abstract. In a series of experiments in an atmospheric simulation chamber (SAPHIR,1 Forschungszentrum Jülich, Germany), NO3 reactivity (kNO3) resulting from the reaction of NO3 with isoprene and stable trace gases formed as products was measured directly using a flow tube reactor coupled to a cavity ring-down spectrometer (FT-CRDS). The experiments were carried out in both dry and humid air with variation of the initial mixing ratios of ozone (50–100 ppbv), isoprene (3–22 ppbv) and NO2 (5–30 ppbv). kNO3 was in excellent agreement with values calculated from the isoprene mixing ratio and the rate coefficient for the reaction of NO3 with isoprene. This result serves to confirm that the FT-CRDS returns accurate values of kNO3 even at elevated NO2 concentrations and to show that reactions of NO3 with stable reaction products like non-radical organic nitrates do not contribute significantly to NO3 reactivity during the oxidation of isoprene. A comparison of kNO3 with NO3 reactivities calculated from NO3 mixing ratios and NO3 production rates suggests that organic peroxy radicals and HO2 account for ∼50 % of NO3 losses. This contradicts predictions based on numerical simulations using the Master Chemical Mechanism (MCM version 3.3.1) unless the rate coefficient for reaction between NO3 and isoprene-derived RO2 is roughly doubled to ∼5×10-12 cm3 molecule−1 s−1.

scholarly journals Observations of speciated isoprene nitrates in Beijing: implications for isoprene chemistry

2021 ◽  
Vol 21 (8) ◽  
pp. 6315-6330
Author(s):  
Claire E. Reeves ◽  
Graham P. Mills ◽  
Lisa K. Whalley ◽  
W. Joe F. Acton ◽  
William J. Bloss ◽  
...  
Keyword(s):  
Chemical Mechanism ◽  
Organic Nitrates ◽  
Isomer Ratio ◽  
Peroxy Radicals ◽  
Model Calculations ◽  
Mixing Ratios ◽  
Field Evidence ◽  
Isoprene Nitrates ◽  
Field Deployment ◽  
The Impact

Abstract. Isoprene is the most important biogenic volatile organic compound in the atmosphere. Its calculated impact on ozone (O3) is critically dependent on the model isoprene oxidation chemical scheme, in particular the way the isoprene-derived organic nitrates (IN) are treated. By combining gas chromatography with mass spectrometry, we have developed a system capable of separating and unambiguously measuring individual IN isomers. In this paper we use measurements from its first field deployment, which took place in Beijing as part of the Atmospheric Pollution and Human Health in a Chinese Megacity programme, to test understanding of the isoprene chemistry as simulated in the Master Chemical Mechanism (MCM) (v.3.3.1). Seven individual isoprene nitrates were identified and quantified during the campaign: two β-hydroxy nitrates (IHN), four δ-carbonyl nitrates (ICN), and propanone nitrate. Our measurements show that in the summertime conditions experienced in Beijing the ratio of (1-OH, 2-ONO2)-IHN to (4-OH, 3-ONO2)-IHN (the numbers indicate the carbon atom in the isoprene chain to which the radical is added) increases at NO mixing ratios below 2 ppb. This provides observational field evidence of the redistribution of the peroxy radicals derived from OH oxidation of isoprene away from the kinetic ratio towards a new thermodynamic equilibrium consistent with box model calculations. The observed amounts of δ-ICN demonstrate the importance of daytime addition of NO3 to isoprene in Beijing but suggest that the predominant source of the δ-ICN in the model (reaction of NO with δ-nitrooxy peroxy radicals) may be too large. Our speciated measurements of the four δ-ICN exhibit a mean C1 : C4 isomer ratio of 1.4 and a mean trans : cis isomer ratio of 7 and provide insight into the isomeric distribution of the δ-nitrooxy peroxy radicals. Together our measurements and model results indicate that propanone nitrate was formed from the OH oxidation of δ-ICN both during the day and night, as well as from NO3 addition to propene at night. This study demonstrates the value of speciated IN measurements in testing understanding of the isoprene degradation chemistry and shows how more extensive measurements would provide greater constraints. It highlights areas of the isoprene chemistry that warrant further study, in particular the impact of NO on the formation of the IHN and the NO3-initiated isoprene degradation chemistry, as well as the need for further laboratory studies on the formation and the losses of IN, in particular via photolysis of δ-ICN and hydrolysis.

scholarly journals Comparisons of observed and modeled OH and HO<sub>2</sub> concentrations during the ambient measurement period of the HO<sub>x</sub>Comp field campaign

2011 ◽  
Vol 11 (10) ◽  
pp. 28851-28894 ◽  
Author(s):  
Y. Kanaya ◽  
A. Hofzumahaus ◽  
H.-P. Dorn ◽  
T. Brauers ◽  
H. Fuchs ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
Peroxy Radicals ◽  
Field Campaign ◽  
Mixing Ratios ◽  
Measurement Mode ◽  
Master Chemical Mechanism ◽  
Regeneration Processes ◽  
Measurement Period

Abstract. A photochemical box model constrained by ancillary observations was used to simulate OH and HO2 concentrations for three days of ambient observations during the HOxComp field campaign held in Jülich, Germany in July 2005. OH and HO2 levels, observed by four and three instruments, respectively, were fairly well reproduced to within 33% by a base model run (Regional Atmospheric Chemistry Mechanism with updated isoprene chemistry adapted from Master Chemical Mechanism ver. 3.1) with high R2 values (0.72–0.97) over a range of isoprene (0.3–2 ppb) and NO (0.1–10 ppb) mixing ratios. Adding isomerization of isoprene peroxy radicals to the model increased OH and HO2 by 43% and 48% on average. Although these are still only 15% and 21% higher than the observations made by one of the instruments, larger overestimations (>60%) occurred with respect to the observations made by the other three instruments, suggesting that the rates of the isomerization were not readily supported by the ensemble of radical observations. These model runs tend to underestimate observed OH reactivity which may be explained by unmeasured hydrocarbon species. By selecting hydrocarbon types to be added to the model in amounts that accounted for the missing fractions of observed OH reactivity, the gaps between HOx observations and model results with and without isomerization could be individually diminished to within uncertainty levels. In this case, however, the HO2/OH ratio rose on addition of hydrocarbons and diverged from observations. In the case where we used modeled HO2(*), taking into account the sensitivity toward speciated RO2 (organic peroxy) radicals, as recently reported from one of the participating instruments in the HO2 measurement mode, the model's overestimation for HO2 became evident (by factors of more than 1.8). These results strongly indicated that more loss processes for peroxy radicals were necessary to explain the observations. One of the measurement days was characterized by low isoprene concentrations (~0.5 ppb) and OH reactivity that was well explained by the observed species, especially before noon. For this selected period, as opposed to the general behavior, the model tended to underestimate HO2 (and HO2(*)) with respect to observations made by the three instruments. We found that this tendency is associated with high NOx concentrations, suggesting that some HO2 production or HO2 regeneration processes under high NOx conditions were being overlooked; this might require revision of ozone production regimes.

scholarly journals Comparisons of observed and modeled OH and HO2concentrations during the ambient measurement period of the HOxComp field campaign

2012 ◽  
Vol 12 (5) ◽  
pp. 2567-2585 ◽  
Author(s):  
Y. Kanaya ◽  
A. Hofzumahaus ◽  
H.-P. Dorn ◽  
T. Brauers ◽  
H. Fuchs ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
High Yield ◽  
Base Case ◽  
Peroxy Radicals ◽  
Field Campaign ◽  
Mixing Ratios ◽  
Measurement Mode ◽  
Single Instrument

Abstract. A photochemical box model constrained by ancillary observations was used to simulate OH and HO2 concentrations for three days of ambient observations during the HOxComp field campaign held in Jülich, Germany in July 2005. Daytime OH levels observed by four instruments were fairly well reproduced to within 33% by a base model run (Regional Atmospheric Chemistry Mechanism with updated isoprene chemistry adapted from Master Chemical Mechanism ver. 3.1) with high R2 values (0.72–0.97) over a range of isoprene (0.3–2 ppb) and NO (0.1–10 ppb) mixing ratios. Daytime HO2(*) levels, reconstructed from the base model results taking into account the sensitivity toward speciated RO2 (organic peroxy) radicals, as recently reported from one of the participating instruments in the HO2 measurement mode, were 93% higher than the observations made by the single instrument. This also indicates an overprediction of the HO2 to OH recycling. Together with the good model-measurement agreement for OH, it implies a missing OH source in the model. Modeled OH and HO2(*) could only be matched to the observations by addition of a strong unknown loss process for HO2(*) that recycles OH at a high yield. Adding to the base model, instead, the recently proposed isomerization mechanism of isoprene peroxy radicals (Peeters and Müller, 2010) increased OH and HO2(*) by 28% and 13% on average. Although these were still only 4% higher than the OH observations made by one of the instruments, larger overestimations (42–70%) occurred with respect to the OH observations made by the other three instruments. The overestimation in OH could be diminished only when reactive alkanes (HC8) were solely introduced to the model to explain the missing fraction of observed OH reactivity. Moreover, the overprediction of HO2(*) became even larger than in the base case. These analyses imply that the rates of the isomerization are not readily supported by the ensemble of radical observations. One of the measurement days was characterized by low isoprene concentrations (∼0.5 ppb) and OH reactivity that was well explained by the observed species, especially before noon. For this selected period, as opposed to the general behavior, the model tended to underestimate HO2(*). We found that this tendency is associated with high NOx concentrations, suggesting that some HO2 production or regeneration processes under high NOx conditions were being overlooked; this might require revision of ozone production regimes.

scholarly journals OH and HO<sub>2</sub> radical chemistry in a midlatitude forest: Measurements and model comparisons

2019 ◽  
Author(s):  
Michelle L. Lew ◽  
Pamela S. Rickly ◽  
Brandon P. Bottorff ◽  
Sofia Sklaveniti ◽  
Thierry Léonardis ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Global Climate ◽  
Oxidation Products ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
Peroxy Radicals ◽  
Mixing Ratios ◽  
Photolysis Rates ◽  
Model Predictions ◽  
Ho2 Radical

Abstract. Reactions of the hydroxyl (OH) and peroxy radicals (HO2 and RO2) play a central role in the chemistry of the atmosphere. In addition to controlling the lifetimes of many trace gases important to issues of global climate change, OH radical reactions initiate the oxidation of volatile organic compounds (VOCs) which can lead to the production of ozone and secondary organic aerosols in the atmosphere. Previous measurements of these radicals in forest environments characterized by high mixing ratios of isoprene and low mixing ratios of nitrogen oxides (NOx) have shown serious discrepancies with modeled concentrations. These results bring into question our understanding of the atmospheric chemistry of isoprene and other biogenic VOCs under low NOx conditions. During the summer of 2015, OH and HO2 radical concentrations as well as total OH reactivity were measured using Laser-Induced Fluorescence - Fluorescence Assay by Gas Expansion (LIF-FAGE) techniques as part of the Indiana Radical, Reactivity and Ozone Production Intercomparison (IRRONIC). This campaign took place in a forested area near the Indiana University, Bloomington campus characterized by high mixing ratios of isoprene and low mixing ratios of NOx. Supporting measurements of photolysis rates, VOCs, NOx, and other species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism (RACM2) and the Master Chemical Mechanism (MCM). Using an OH chemical scavenger technique, the study revealed the presence of an interference with the LIF-FAGE measurements of OH that increased with both ambient concentrations of ozone and temperature. Subtraction of the interference resulted in measured OH concentrations that were in better agreement with model predictions, although the model still underestimated the measured concentrations, likely due to an underestimation of the concentration of NO at this site. Measurements of HO2 radical concentrations during the campaign included a fraction of isoprene-based peroxy radicals (HO2* = HO2 + αRO2) and were found to agree with model predictions. On average, the measured reactivity was consistent with that calculated from measured OH sinks to within 20 %, with modeled oxidation products accounting for the missing reactivity, although significant missing reactivity (approximately 40 % of the total measured reactivity) was observed on some days.

scholarly journals Evaluating the sensitivity of radical chemistry and ozone formation to ambient VOCs and NO<sub><i>x</i></sub> in Beijing

2021 ◽  
Vol 21 (3) ◽  
pp. 2125-2147
Author(s):  
Lisa K. Whalley ◽  
Eloise J. Slater ◽  
Robert Woodward-Massey ◽  
Chunxiang Ye ◽  
James D. Lee ◽  
...  
Keyword(s):  
Propagation Rate ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
Peroxy Radicals ◽  
Atom Concentration ◽  
Mixing Ratios ◽  
Budget Analysis ◽  
Order Of Magnitude ◽  
Atom Chemistry ◽  
Cl Atom

Abstract. Measurements of OH, HO2, complex RO2 (alkene- and aromatic-related RO2) and total RO2 radicals taken during the integrated Study of AIR Pollution PROcesses in Beijing (AIRPRO) campaign in central Beijing in the summer of 2017, alongside observations of OH reactivity, are presented. The concentrations of radicals were elevated, with OH reaching up to 2.8×107moleculecm-3, HO2 peaking at 1×109moleculecm-3 and the total RO2 concentration reaching 5.5×109moleculecm-3. OH reactivity (k(OH)) peaked at 89 s−1 during the night, with a minimum during the afternoon of ≈22s-1 on average. An experimental budget analysis, in which the rates of production and destruction of the radicals are compared, highlighted that although the sources and sinks of OH were balanced under high NO concentrations, the OH sinks exceeded the known sources (by 15 ppbv h−1) under the very low NO conditions (<0.5 ppbv) experienced in the afternoons, demonstrating a missing OH source consistent with previous studies under high volatile organic compound (VOC) emissions and low NO loadings. Under the highest NO mixing ratios (104 ppbv), the HO2 production rate exceeded the rate of destruction by ≈50ppbvh-1, whilst the rate of destruction of total RO2 exceeded the production by the same rate, indicating that the net propagation rate of RO2 to HO2 may be substantially slower than assumed. If just 10 % of the RO2 radicals propagate to HO2 upon reaction with NO, the HO2 and RO2 budgets could be closed at high NO, but at low NO this lower RO2 to HO2 propagation rate revealed a missing RO2 sink that was similar in magnitude to the missing OH source. A detailed box model that incorporated the latest Master Chemical Mechanism (MCM3.3.1) reproduced the observed OH concentrations well but over-predicted the observed HO2 under low concentrations of NO (<1 ppbv) and under-predicted RO2 (both the complex RO2 fraction and other RO2 types which we classify as simple RO2) most significantly at the highest NO concentrations. The model also under-predicted the observed k(OH) consistently by ≈10s-1 across all NOx levels, highlighting that the good agreement for OH was fortuitous due to a cancellation of missing OH source and sink terms in its budget. Including heterogeneous loss of HO2 to aerosol surfaces did reduce the modelled HO2 concentrations in line with the observations but only at NO mixing ratios <0.3 ppbv. The inclusion of Cl atoms, formed from the photolysis of nitryl chloride, enhanced the modelled RO2 concentration on several mornings when the Cl atom concentration was calculated to exceed 1×104atomscm-3 and could reconcile the modelled and measured RO2 concentrations at these times. However, on other mornings, when the Cl atom concentration was lower, large under-predictions in total RO2 remained. Furthermore, the inclusion of Cl atom chemistry did not enhance the modelled RO2 beyond the first few hours after sunrise and so was unable to resolve the modelled under-prediction in RO2 observed at other times of the day. Model scenarios, in which missing VOC reactivity was included as an additional reaction that converted OH to RO2, highlighted that the modelled OH, HO2 and RO2 concentrations were sensitive to the choice of RO2 product. The level of modelled to measured agreement for HO2 and RO2 (both complex and simple) could be improved if the missing OH reactivity formed a larger RO2 species that was able to undergo reaction with NO, followed by isomerisation reactions reforming other RO2 species, before eventually generating HO2. In this work an α-pinene-derived RO2 species was used as an example. In this simulation, consistent with the experimental budget analysis, the model underestimated the observed OH, indicating a missing OH source. The model uncertainty, with regards to the types of RO2 species present and the radicals they form upon reaction with NO (HO2 directly or another RO2 species), leads to over an order of magnitude less O3 production calculated from the predicted peroxy radicals than calculated from the observed peroxy radicals at the highest NO concentrations. This demonstrates the rate at which the larger RO2 species propagate to HO2, to another RO2 or indeed to OH needs to be understood to accurately simulate the rate of ozone production in environments such as Beijing, where large multifunctional VOCs are likely present.

scholarly journals Modelling of the photooxidation of toluene: conceptual ideas for validating detailed mechanisms

2003 ◽  
Vol 3 (1) ◽  
pp. 89-106 ◽  
Author(s):  
V. Wagner ◽  
M. E. Jenkin ◽  
S. M. Saunders ◽  
J. Stanton ◽  
K Wirtz ◽  
...  
Keyword(s):  
First Generation ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
Peroxy Radicals ◽  
Reaction Systems ◽  
Master Chemical Mechanism ◽  
Reactive Nitrogen Oxides ◽  
Toluene Photooxidation ◽  
Radical Transformation ◽  
Validation Experiments

Abstract. Toluene photooxidation is chosen as an example to examine how simulations of smog-chamber experiments can be used to unravel shortcomings in detailed mechanisms and to provide information on complex reaction systems that will be crucial for the design of future validation experiments. The mechanism used in this study is extracted from the Master Chemical Mechanism Version 3 (MCM v3) and has been updated with new modules for cresol and g-dicarbonyl chemistry. Model simulations are carried out for a toluene-NOx experiment undertaken at the European Photoreactor (EUPHORE). The comparison of the simulation with the experimental data reveals two fundamental shortcomings in the mechanism: OH production is too low by about 80%, and the ozone concentration at the end of the experiment is over-predicted by 55%. The radical budget was analysed to identify the key intermediates governing the radical transformation in the toluene system. Ring-opening products, particularly conjugated g-dicarbonyls, were identified as dominant radical sources in the early stages of the experiment. The analysis of the time evolution of radical production points to a missing OH source that peaks when the system reaches highest reactivity. First generation products are also of major importance for the ozone production in the system. The analysis of the radical budget suggests two options to explain the concurrent under-prediction of OH and over-prediction of ozone in the model: 1) missing oxidation processes that produce or regenerate OH without or with little NO to NO2 conversion or 2) NO3 chemistry that sequesters reactive nitrogen oxides into stable nitrogen compounds and at the same time produces peroxy radicals. Sensitivity analysis was employed to identify significant contributors to ozone production and it is shown how this technique, in combination with ozone isopleth plots, can be used for the design of validation experiments.

scholarly journals OH regeneration from methacrolein oxidation investigated in the atmosphere simulation chamber SAPHIR

2014 ◽  
Vol 14 (15) ◽  
pp. 7895-7908 ◽  
Author(s):  
H. Fuchs ◽  
I.-H. Acir ◽  
B. Bohn ◽  
T. Brauers ◽  
H.-P. Dorn ◽  
...  
Keyword(s):  
Reaction Pathway ◽  
Chemical Mechanism ◽  
Isomerization Reaction ◽  
Initial Reaction ◽  
Oh Radicals ◽  
Peroxy Radicals ◽  
Atmospheric Conditions ◽  
Model Calculations ◽  
Mixing Ratios ◽  
Simulation Chamber

Abstract. Hydroxyl radicals (OH) are the most important reagent for the oxidation of trace gases in the atmosphere. OH concentrations measured during recent field campaigns in isoprene-rich environments were unexpectedly large. A number of studies showed that unimolecular reactions of organic peroxy radicals (RO2) formed in the initial reaction step of isoprene with OH play an important role for the OH budget in the atmosphere at low mixing ratios of nitrogen monoxide (NO) of less than 100 pptv. It has also been suggested that similar reactions potentially play an important role for RO2 from other compounds. Here, we investigate the oxidation of methacrolein (MACR), one major oxidation product of isoprene, by OH in experiments in the simulation chamber SAPHIR under controlled atmospheric conditions. The experiments show that measured OH concentrations are approximately 50% larger than calculated by the Master Chemical Mechanism (MCM) for conditions of the experiments (NO mixing ratio of 90 pptv). The analysis of the OH budget reveals an OH source that is not accounted for in MCM, which is correlated with the production rate of RO2 radicals from MACR. In order to balance the measured OH destruction rate, 0.77 OH radicals (1σ error: ± 0.31) need to be additionally reformed from each reaction of OH with MACR. The strong correlation of the missing OH source with the production of RO2 radicals is consistent with the concept of OH formation from unimolecular isomerization and decomposition reactions of RO2. The comparison of observations with model calculations gives a lower limit of 0.03 s−1 for the reaction rate constant if the OH source is attributed to an isomerization reaction of MACR-1-OH-2-OO and MACR-2-OH-2-OO formed in the MACR + OH reaction as suggested in the literature (Crounse et al., 2012). This fast isomerization reaction would be a competitor to the reaction of this RO2 species with a minimum of 150 pptv NO. The isomerization reaction would be the dominant reaction pathway for this specific RO2 radical in forested regions, where NO mixing ratios are typically much smaller.

scholarly journals Oxidation capacity of the city air of Santiago, Chile

2009 ◽  
Vol 9 (6) ◽  
pp. 2257-2273 ◽  
Author(s):  
Y. F. Elshorbany ◽  
R. Kurtenbach ◽  
P. Wiesen ◽  
E. Lissi ◽  
M. Rubio ◽  
...  
Keyword(s):  
Chemical Mechanism ◽  
Oh Radical ◽  
Production Rates ◽  
Oxidation Capacity ◽  
Sources And Sinks ◽  
Mixing Ratios ◽  
Photostationary State ◽  
Master Chemical Mechanism ◽  
The City ◽  
Calculated Average

Abstract. The oxidation capacity of the highly polluted urban area of Santiago, Chile has been evaluated during a summer measurement campaign carried out from 8–20 March 2005. The hydroxyl (OH) radical budget was evaluated employing a simple quasi-photostationary-state model (PSS) constrained with simultaneous measurements of HONO, HCHO, O3, NO, NO2, j(O1D), j(NO2), 13 alkenes and meteorological parameters. In addition, a zero dimensional photochemical box model based on the Master Chemical Mechanism (MCMv3.1) has been used to estimate production rates and total free radical budgets, including OH, HO2 and RO2. Besides the above parameters, the MCM model has been constrained by the measured CO and volatile organic compounds (VOCs) including alkanes and aromatics. Both models simulate the same OH concentration during daytime indicating that the primary OH sources and sinks included in the simple PSS model predominate. Mixing ratios of the main OH radical precursors were found to be in the range 0.8–7 ppbv (HONO), 0.9–11 ppbv (HCHO) and 0–125 ppbv (O3). The alkenes average mixing ratio was ~58 ppbC accounting for ~12% of the total identified non-methane hydrocarbons (NMHCs). During the daytime (08:00 h–19:00 h), HONO photolysis was shown to be the most important primary OH radical source comprising alone ~55% of the total initial production rate, followed by alkene ozonolysis (~24%) and photolysis of HCHO (~16%) and O3 (~5%). The calculated average and maximum daytime OH production rates from HONO photolysis was 1.7 ppbv h−1 and 3.1 ppbv h−1, respectively. Based on the experimental results a strong photochemical daytime source of HONO is proposed. A detailed analysis of the sources of OH radical precursors has also been carried out.

scholarly journals Missing peroxy radical sources within a summertime ponderosa pine forest

2014 ◽  
Vol 14 (9) ◽  
pp. 4715-4732 ◽  
Author(s):  
G. M. Wolfe ◽  
C. Cantrell ◽  
S. Kim ◽  
R. L. Mauldin III ◽  
T. Karl ◽  
...  
Keyword(s):  
Ponderosa Pine ◽  
Forest Canopy ◽  
Strong Dependence ◽  
Rocky Mountain ◽  
Peroxy Radical ◽  
Pine Forest ◽  
Chemical Mechanism ◽  
Peroxy Radicals ◽  
Mixing Ratios ◽  
Ponderosa Pine Forest

Abstract. Organic peroxy (RO2) and hydroperoxy (HO2) radicals are key intermediates in the photochemical processes that generate ozone, secondary organic aerosol and reactive nitrogen reservoirs throughout the troposphere. In regions with ample biogenic hydrocarbons, the richness and complexity of peroxy radical chemistry presents a significant challenge to current-generation models, especially given the scarcity of measurements in such environments. We present peroxy radical observations acquired within a ponderosa pine forest during the summer 2010 Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen – Rocky Mountain Organic Carbon Study (BEACHON-ROCS). Total peroxy radical mixing ratios reach as high as 180 pptv (parts per trillion by volume) and are among the highest yet recorded. Using the comprehensive measurement suite to constrain a near-explicit 0-D box model, we investigate the sources, sinks and distribution of peroxy radicals below the forest canopy. The base chemical mechanism underestimates total peroxy radicals by as much as a factor of 3. Since primary reaction partners for peroxy radicals are either measured (NO) or underpredicted (HO2 and RO2, i.e., self-reaction), missing sources are the most likely explanation for this result. A close comparison of model output with observations reveals at least two distinct source signatures. The first missing source, characterized by a sharp midday maximum and a strong dependence on solar radiation, is consistent with photolytic production of HO2. The diel profile of the second missing source peaks in the afternoon and suggests a process that generates RO2 independently of sun-driven photochemistry, such as ozonolysis of reactive hydrocarbons. The maximum magnitudes of these missing sources (~120 and 50 pptv min−1, respectively) are consistent with previous observations alluding to unexpectedly intense oxidation within forests. We conclude that a similar mechanism may underlie many such observations.

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