scholarly journals Oxidative capacity of the Mexico City atmosphere – Part 2: A RO<sub>x</sub> radical cycling perspective

2010 ◽  
Vol 10 (14) ◽  
pp. 6993-7008 ◽  
Author(s):  
P. M. Sheehy ◽  
R. Volkamer ◽  
L. T. Molina ◽  
M. J. Molina
Keyword(s):  
Chain Length ◽  
Mexico City ◽  
Oxidative Capacity ◽  
Early Morning ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
Vapor Concentration ◽  
Oh Radicals ◽  
Radical Initiation ◽  
Very High

Abstract. A box model using measurements from the Mexico City Metropolitan Area study in the spring of 2003 (MCMA-2003) is presented to study oxidative capacity (our ability to predict OH radicals) and ROx (ROx=OH+HO2+RO2+RO) radical cycling in a polluted (i.e., very high NOx=NO+NO2) atmosphere. Model simulations were performed using the Master Chemical Mechanism (MCMv3.1) constrained with 10 min averaged measurements of major radical sources (i.e., HCHO, HONO, O3, CHOCHO, etc.), radical sink precursors (i.e., NO, NO2, SO2, CO, and 102 volatile organic compounds (VOC)), meteorological parameters (temperature, pressure, water vapor concentration, dilution), and photolysis frequencies. Modeled HOx (=OH+HO2) concentrations compare favorably with measured concentrations for most of the day; however, the model under-predicts the concentrations of radicals in the early morning. This "missing reactivity" is highest during peak photochemical activity, and is least visible in a direct comparison of HOx radical concentrations. We conclude that the most likely scenario to reconcile model predictions with observations is the existence of a currently unidentified additional source for RO2 radicals, in combination with an additional sink for HO2 radicals that does not form OH. The true uncertainty due to "missing reactivity" is apparent in parameters like chain length. We present a first attempt to calculate chain length rigorously i.e., we define two parameters that account for atmospheric complexity, and are based on (1) radical initiation, n(OH), and (2) radical termination, ω. We find very high values of n(OH) in the early morning are incompatible with our current understanding of ROx termination routes. We also observe missing reactivity in the rate of ozone production (P(O3)). For example, the integral amount of ozone produced could be under-predicted by a factor of two. We argue that this uncertainty is partly accounted for in lumped chemical codes that are optimized to predict ozone concentrations; however, these codes do not reflect the true uncertainty in oxidative capacity that is relevant to other aspects of air quality management, such as the formation of secondary organic aerosol (SOA). Our analysis highlights that apart from uncertainties in emissions, and meteorology, there is an additional major uncertainty in chemical mechanisms that affects our ability to predict ozone and SOA formation with confidence.

scholarly journals Oxidative capacity of the Mexico City atmosphere – Part 2: A RO<sub>x</sub> radical cycling perspective

2008 ◽  
Vol 8 (2) ◽  
pp. 5359-5412 ◽  
Author(s):  
P. M. Sheehy ◽  
R. Volkamer ◽  
L. T. Molina ◽  
M. J. Molina
Keyword(s):  
Mexico City ◽  
Oxidative Capacity ◽  
Early Morning ◽  
Photochemical Activity ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
Vapor Concentration ◽  
Photolysis Frequencies ◽  
Pressure Water ◽  
Major Chemical

Abstract. A box model using measurements from the Mexico City Metropolitan Area study in the spring of 2003 (MCMA-2003) is presented to study ROx (ROx=OH+HO2+RO2+RO) radical cycling in the troposphere. Model simulations were performed with the Master Chemical Mechanism (MCMv3.1) constrained with 10 min averaged measurements of major radical sources (i.e., HCHO, HONO, O3, CHOCHO, etc.), radical sink precursors (i.e., NO, NO2, SO2, CO, and 102 volatile organic compounds VOC), meteorological parameters (temperature, pressure, water vapor concentration, dilution), and photolysis frequencies. Modeled HOx concentrations compare favorably with measured concentrations for most of the day; however, the model under-predicts the concentrations of radicals in the early morning. This "missing reactivity" is highest during peak photochemical activity, and is least visible in a direct comparison of HOx radical concentrations. The true uncertainty due to "missing reactivity" is apparent in parameters like chain length, and ozone production (P(O3)). For example, the integral amount of ozone produced could be under-predicted by a factor of two. Our analysis highlights that apart from uncertainties in emissions, and meteorology, there is an additional major chemical uncertainty in current models.

scholarly journals Oxidative capacity of the Mexico City atmosphere – Part 1: A radical source perspective

2010 ◽  
Vol 10 (14) ◽  
pp. 6969-6991 ◽  
Author(s):  
R. Volkamer ◽  
P. Sheehy ◽  
L. T. Molina ◽  
M. J. Molina
Keyword(s):  
Mexico City ◽  
Oxidative Capacity ◽  
Chemical Mechanism ◽  
Oh Radicals ◽  
Small Contribution ◽  
Radical Chain ◽  
Chain Reactions ◽  
Time Profiles ◽  
Concentration Time ◽  
Radical Production

Abstract. A detailed analysis of OH, HO2 and RO2 radical sources is presented for the near field photochemical regime inside the Mexico City Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign) an extensive set of measurements was collected to quantify time-resolved ROx (sum of OH, HO2, RO2) radical production rates from day- and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was constrained by measurements of (1) concentration time-profiles of photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde (HCHO), ozone (O3), glyoxal (CHOCHO), and other oxygenated volatile organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values); (3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103 compound are treated) and oxidants, i.e., OH- and NO3 radicals, O3; and (4) NO, NO2, meteorological and other parameters. The ROx production rate was calculated directly from these observations; the MCM was used to estimate further ROx production from unconstrained sources, and express overall ROx production as OH-equivalents (i.e., taking into account the propagation efficiencies of RO2 and HO2 radicals into OH radicals). Daytime radical production is found to be about 10–25 times higher than at night; it does not track the abundance of sunlight. 12-h average daytime contributions of individual sources are: Oxygenated VOC other than HCHO about 33%; HCHO and O3 photolysis each about 20%; O3/alkene reactions and HONO photolysis each about 12%, other sources <3%. Nitryl chloride photolysis could potentially contribute ~15% additional radicals, while NO2* + water makes – if any – a very small contribution (~2%). The peak radical production of ~7.5 107 molec cm−3 s−1 is found already at 10:00 a.m., i.e., more than 2.5 h before solar noon. O3/alkene reactions are indirectly responsible for ~33% of these radicals. Our measurements and analysis comprise a database that enables testing of the representation of radical sources and radical chain reactions in photochemical models. Since the photochemical processing of pollutants in the MCMA is radical limited, our analysis identifies the drivers for ozone and SOA formation. We conclude that reductions in VOC emissions provide an efficient opportunity to reduce peak concentrations of these secondary pollutants, because (1) about 70% of radical production is linked to VOC precursors; (2) lowering the VOC/NOx ratio has the further benefit of reducing the radical re-cycling efficiency from radical chain reactions (chemical amplification of radical sources); (3) a positive feedback is identified: lowering the rate of radical production from organic precursors also reduces that from inorganic precursors, like ozone, as pollution export from the MCMA caps the amount of ozone that accumulates at a lower rate inside the MCMA. Continued VOC reductions will in the future result in decreasing peak concentrations of ozone and SOA in the MCMA.

scholarly journals Oxidative capacity of the Mexico City atmosphere – Part 1: A radical source perspective

2007 ◽  
Vol 7 (2) ◽  
pp. 5365-5412 ◽  
Author(s):  
R. Volkamer ◽  
P. M. Sheehy ◽  
L. T. Molina ◽  
M. J. Molina
Keyword(s):  
Mexico City ◽  
Near Field ◽  
Oxidative Capacity ◽  
Chemical Mechanism ◽  
Oh Radicals ◽  
Time Resolved ◽  
Time Profiles ◽  
Concentration Time ◽  
Radical Production ◽  
Photolysis Frequencies

Abstract. A detailed analysis of OH, HO2 and RO2 radical sources is presented for the near field photochemical regime inside the Mexico City Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign) an extensive set of measurements was collected to quantify time resolved ROx (sum of OH, HO2, RO2) radical production rates from day- and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was constrained by measurements of (1) concentration time-profiles of photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde (HCHO), ozone (O3), glyoxal (CHOCHO), and other oxygenated volatile organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values); (3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103 compound are treated) and oxidants, i.e., OH- and NO3 radicals, O3; and (4) NO, NO2, meteorological and other parameters. The ROx production rate was calculated directly from these observations; MCM was used to estimate further ROx production from unconstrained sources, and express overall ROx production as OH-equivalents (i.e., taking into account the propagation efficiencies of RO2 and HO2 radicals into OH radicals). Daytime radical production is found to be about 10-25 times higher than at night; it does not track the abundance of sunlight. 12-h average daytime contributions of individual sources are: HCHO and O3 photolysis, each about 20%; O3/alkene reactions and HONO photolysis, each about 15%; unmeasured sources about 30%. While the direct contribution of O3/alkene reactions appears to be moderately small, source-apportionment of ambient HCHO and HONO identifies O3/alkene reactions as being largely responsible for jump-starting photochemistry about one hour after sunrise. The peak radical production is found to be higher than in any other urban influenced environment studied to date; further, differences exist in the timing of radical production. Our measurements and analysis comprise a database that enables testing of the representation of radical sources in photochemical models. Since the photochemical processing of pollutants is radical-limited in the MCMA, our analysis identifies the drivers for such processing. Three pathways are identified by which reductions in VOC emissions induce reductions in peak concentrations of secondary pollutants, such as O3 and secondary organic aerosol (SOA).

scholarly journals Development of a chlorine chemistry module for the Master Chemical Mechanism

2015 ◽  
Vol 8 (6) ◽  
pp. 4823-4849
Author(s):  
L. K. Xue ◽  
S. M. Saunders ◽  
T. Wang ◽  
R. Gao ◽  
X. F. Wang ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Southern China ◽  
Oxidative Capacity ◽  
Early Morning ◽  
Chemical Mechanism ◽  
Atmospheric Photochemistry ◽  
Nitryl Chloride ◽  
Chemistry Research ◽  
Master Chemical Mechanism ◽  
Chemistry Module

Abstract. The chlorine atom (Cl·) has a high potential to perturb atmospheric photochemistry by oxidizing volatile organic compounds (VOCs), but the exact role it plays in the polluted troposphere remains unclear. The Master Chemical Mechanism (MCM) is a near explicit mechanism that has been widely applied in the atmospheric chemistry research. While it addresses comprehensively the chemistry initiated by the OH, O3 and NO3 radicals, its representation of the Cl· chemistry is incomplete as it only considers the reactions for alkanes. In this paper, we develop a more comprehensive Cl· chemistry module that can be directly incorporated within the MCM framework. A suite of 199 chemical reactions describes the Cl·-initiated degradation of alkenes, aromatics, aldehydes, ketones, alcohols, and some organic acids and nitrates, along with the inorganic chemistry involving Cl· and its precursors. To demonstrate the potential influence of the new chemistry module, it was incorporated into a MCM box model to evaluate the impacts of nitryl chloride (ClNO2), a product of nocturnal halogen activation by nitrogen oxides (NOx), on the following-day's atmospheric photochemistry. With constraints of recent observations collected at a coastal site in Hong Kong, southern China, the modeling analyses suggest that the Cl· produced from ClNO2 photolysis may substantially enhance the atmospheric oxidative capacity, VOC oxidation, and O3 formation, particularly in the early morning period. The results demonstrate the critical need for photochemical models to include more fully chlorine chemistry in order to better understand the atmospheric photochemistry in polluted environments subject to intense emissions of NOx, VOCs and chlorine-containing constituents.

scholarly journals How does the OH reactivity affect the ozone production efficiency: case studies in Beijing and Heshan

2016 ◽  
Author(s):  
Yudong Yang ◽  
Min Shao ◽  
Stephan Keβel ◽  
Yue Li ◽  
Keding Lu ◽  
...  
Keyword(s):  
Production Efficiency ◽  
Ambient Air ◽  
Early Morning ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
University Campus ◽  
Oxygenated Volatile Organic Compounds ◽  
Volatile Organic ◽  
Peking University ◽  
Significant Diurnal Variation

Abstract. Total OH reactivity measurements have been conducted in August 2013 on the Peking University campus, Beijing and from October to November 2014 in Heshan, Guangdong Province. The daily median result for OH reactivity was 19.98 ± 11.03 s−1 in Beijing and 30.62 ± 19.76 s−1 in Heshan. Beijing presented a significant diurnal variation with maxima over 27 s−1 in the early morning and minima below 16 s−1 in the afternoon. Measurements in Heshan gave a much flatter diurnal pattern. Missing reactivity was observed at both sites, with 21 % missing in Beijing and 32 % missing in Heshan. Unmeasured primary species, such as branched-alkenes could contribute to missing reactivity in Beijing, especially in morning rush hour. An observation-based model with the Regional Atmospheric Chemical Mechanism 2 was used to understand the daytime missing reactivity in Beijing by adding unmeasured oxygenated volatile organic compounds and simulated intermediates of primary VOCs degradation. However, the model failed to explain the missing reactivity in Heshan, where the ambient air was found to be more aged, and the missing reactivity was presumably to attribute to oxidized species, such as aldehydes, acids and di-carbonyls. The ozone production efficiency was 27 % higher in Beijing and 35 % higher in Heshan when constrained by the measured reactivity, compared to the calculation with measured and modeled species included, indicating the importance of quantifying the OH reactivity for better understanding ozone chemistry.

scholarly journals Observationally constrained modeling of atmospheric oxidation capacity and photochemical reactivity in Shanghai, China

2020 ◽  
Vol 20 (3) ◽  
pp. 1217-1232 ◽  
Author(s):  
Jian Zhu ◽  
Shanshan Wang ◽  
Hongli Wang ◽  
Shengao Jing ◽  
Shengrong Lou ◽  
...  
Keyword(s):  
Air Quality ◽  
Chain Length ◽  
Ambient Air ◽  
Chemical Mechanism ◽  
Atmospheric Oxidation ◽  
Oh Radicals ◽  
Ozone Pollution ◽  
Oxidation Capacity ◽  
High Level ◽  
Ozone Levels

Abstract. An observation-based model coupled to the Master Chemical Mechanism (V3.3.1) and constrained by a full suite of observations was developed to study atmospheric oxidation capacity (AOC), OH reactivity, OH chain length and HOx (=OH+HO2) budget for three different ozone (O3) concentration levels in Shanghai, China. Five months of observations from 1 May to 30 September 2018 showed that the air quality level is lightly polluted or worse (Ambient Air Quality Index, AQI, of > 100) for 12 d, of which ozone is the primary pollutant for 10 d, indicating ozone pollution was the main air quality challenge in Shanghai during summer of 2018. The levels of ozone and its precursors, as well as meteorological parameters, revealed the significant differences among different ozone levels, indicating that the high level of precursors is the precondition of ozone pollution, and strong radiation is an essential driving force. By increasing the input JNO2 value by 40 %, the simulated O3 level increased by 30 %–40 % correspondingly under the same level of precursors. The simulation results show that AOC, dominated by reactions involving OH radicals during the daytime, has a positive correlation with ozone levels. The reactions with non-methane volatile organic compounds (NMVOCs; 30 %–36 %), carbon monoxide (CO; 26 %–31 %) and nitrogen dioxide (NO2; 21 %–29 %) dominated the OH reactivity under different ozone levels in Shanghai. Among the NMVOCs, alkenes and oxygenated VOCs (OVOCs) played a key role in OH reactivity, defined as the inverse of the OH lifetime. A longer OH chain length was found in clean conditions primarily due to low NO2 in the atmosphere. The high level of radical precursors (e.g., O3, HONO and OVOCs) promotes the production and cycling of HOx, and the daytime HOx primary source shifted from HONO photolysis in the morning to O3 photolysis in the afternoon. For the sinks of radicals, the reaction with NO2 dominated radical termination during the morning rush hour, while the reactions of radical–radical also contributed to the sinks of HOx in the afternoon. Furthermore, the top four species contributing to ozone formation potential (OFP) were HCHO, toluene, ethylene and m/p-xylene. The concentration ratio (∼23 %) of these four species to total NMVOCs is not proportional to their contribution (∼55 %) to OFP, implying that controlling key VOC species emission is more effective than limiting the total concentration of VOC in preventing and controlling ozone pollution.

scholarly journals Development of a chlorine chemistry module for the Master Chemical Mechanism

2015 ◽  
Vol 8 (10) ◽  
pp. 3151-3162 ◽  
Author(s):  
L. K. Xue ◽  
S. M. Saunders ◽  
T. Wang ◽  
R. Gao ◽  
X. F. Wang ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Southern China ◽  
Oxidative Capacity ◽  
Early Morning ◽  
Chemical Mechanism ◽  
Atmospheric Photochemistry ◽  
Nitryl Chloride ◽  
Chemistry Research ◽  
Master Chemical Mechanism ◽  
Chemistry Module

Abstract. The chlorine atom (Cl·) has a high potential to perturb atmospheric photochemistry by oxidizing volatile organic compounds (VOCs), but the exact role it plays in the polluted troposphere remains unclear. The Master Chemical Mechanism (MCM) is a near-explicit mechanism that has been widely applied in the atmospheric chemistry research. While it addresses comprehensively the chemistry initiated by the OH, O3 and NO3 radicals, its representation of the Cl· chemistry is incomplete as it only considers the reactions for alkanes. In this paper, we develop a more comprehensive Cl· chemistry module that can be directly incorporated within the MCM framework. A suite of 205 chemical reactions describes the Cl·-initiated degradation of alkenes, aromatics, alkynes, aldehydes, ketones, alcohols, and some organic acids and nitrates, along with the inorganic chemistry involving Cl· and its precursors. To demonstrate the potential influence of the new chemistry module, it was incorporated into a MCM box model to evaluate the impacts of nitryl chloride (ClNO2), a product of nocturnal halogen activation by nitrogen oxides (NOX), on the following day's atmospheric photochemistry. With constraints of recent observations collected at a coastal site in Hong Kong, southern China, the modeling analyses suggest that the Cl· produced from ClNO2 photolysis may substantially enhance the atmospheric oxidative capacity, VOC oxidation and O3 formation, particularly in the early morning period. The results demonstrate the critical need for photochemical models to include more detailed chlorine chemistry in order to better understand the atmospheric photochemistry in polluted environments subject to intense emissions of NOX, VOCs and chlorine-containing constituents.

scholarly journals Higher measured than modeled ozone production at increased NO<sub><i>x</i></sub> levels in the Colorado Front Range

2017 ◽  
Vol 17 (18) ◽  
pp. 11273-11292 ◽  
Author(s):  
Bianca C. Baier ◽  
William H. Brune ◽  
David O. Miller ◽  
Donald Blake ◽  
Russell Long ◽  
...  
Keyword(s):  
Formation Rate ◽  
Chemical Species ◽  
Peroxy Radical ◽  
Early Morning ◽  
Mitigation Strategies ◽  
Chemical Mechanism ◽  
Colorado Front Range ◽  
Ozone Production ◽  
Front Range ◽  
Reduction Strategies

Abstract. Chemical models must correctly calculate the ozone formation rate, P(O3), to accurately predict ozone levels and to test mitigation strategies. However, air quality models can have large uncertainties in P(O3) calculations, which can create uncertainties in ozone forecasts, especially during the summertime when P(O3) is high. One way to test mechanisms is to compare modeled P(O3) to direct measurements. During summer 2014, the Measurement of Ozone Production Sensor (MOPS) directly measured net P(O3) in Golden, CO, approximately 25 km west of Denver along the Colorado Front Range. Net P(O3) was compared to rates calculated by a photochemical box model that was constrained by measurements of other chemical species and that used a lumped chemical mechanism and a more explicit one. Median observed P(O3) was up to a factor of 2 higher than that modeled during early morning hours when nitric oxide (NO) levels were high and was similar to modeled P(O3) for the rest of the day. While all interferences and offsets in this new method are not fully understood, simulations of these possible uncertainties cannot explain the observed P(O3) behavior. Modeled and measured P(O3) and peroxy radical (HO2 and RO2) discrepancies observed here are similar to those presented in prior studies. While a missing atmospheric organic peroxy radical source from volatile organic compounds co-emitted with NO could be one plausible solution to the P(O3) discrepancy, such a source has not been identified and does not fully explain the peroxy radical model–data mismatch. If the MOPS accurately depicts atmospheric P(O3), then these results would imply that P(O3) in Golden, CO, would be NOx-sensitive for more of the day than what is calculated by models, extending the NOx-sensitive P(O3) regime from the afternoon further into the morning. These results could affect ozone reduction strategies for the region surrounding Golden and possibly other areas that do not comply with national ozone regulations. Thus, it is important to continue the development of this direct ozone measurement technique to understand P(O3), especially under high-NOx regimes.

scholarly journals How the OH reactivity affects the ozone production efficiency: case studies in Beijing and Heshan, China

2017 ◽  
Vol 17 (11) ◽  
pp. 7127-7142 ◽  
Author(s):  
Yudong Yang ◽  
Min Shao ◽  
Stephan Keßel ◽  
Yue Li ◽  
Keding Lu ◽  
...  
Keyword(s):  
Volatile Organic Compounds ◽  
Organic Compounds ◽  
Production Efficiency ◽  
Ambient Air ◽  
Early Morning ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
Diurnal Pattern ◽  
Volatile Organic ◽  
The Difference

Abstract. Total OH reactivity measurements were conducted on the Peking University campus (Beijing) in August 2013 and in Heshan (Guangdong province) from October to November 2014. The daily median OH reactivity was 20 ± 11 s−1 in Beijing and 31 ± 20 s−1 in Heshan, respectively. The data in Beijing showed a distinct diurnal pattern with the maxima over 27 s−1 in the early morning and minima below 16 s−1 in the afternoon. The diurnal pattern in Heshan was not as evident as in Beijing. Missing reactivity, defined as the difference between measured and calculated OH reactivity, was observed at both sites, with 21 % missing reactivity in Beijing and 32 % missing reactivity in Heshan. Unmeasured primary species, such as branched alkenes, could contribute to missing reactivity in Beijing, especially during morning rush hours. An observation-based model with the RACM2 (Regional Atmospheric Chemical Mechanism version 2) was used to understand the daytime missing reactivity in Beijing by adding unmeasured oxygenated volatile organic compounds and simulated intermediates of the degradation from primary volatile organic compounds (VOCs). However, the model could not find a convincing explanation for the missing reactivity in Heshan, where the ambient air was found to be more aged, and the missing reactivity was presumably attributed to oxidized species, such as unmeasured aldehydes, acids and dicarbonyls. The ozone production efficiency was 21 % higher in Beijing and 30 % higher in Heshan when the model was constrained by the measured reactivity, compared to the calculations with measured and modeled species included, indicating the importance of quantifying the OH reactivity for better understanding ozone chemistry.

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.

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