OH and HO2 radical chemistry in a midlatitude forest: Measurements and model comparisons (original) (raw)
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Atmospheric Chemistry and Physics, 2012
A photochemical box model constrained by ancillary observations was used to simulate OH and HO 2 concentrations for three days of ambient observations during the HO x Comp 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 R 2 values (0.72-0.97) over a range of isoprene (0.3-2 ppb) and NO (0.1-10 ppb) mixing ratios. Daytime HO 2 (*) levels, reconstructed from the base model results taking into account the sensitivity toward speciated RO 2 (organic peroxy) radicals, as recently reported from one of the participating instruments in the HO 2 measurement mode, were 93 % higher than the observations made by the single instrument. This also indicates an overprediction of the HO 2 to OH recycling. Together with the good modelmeasurement agreement for OH, it implies a missing OH source in the model. Modeled OH and HO 2 (*) could only be matched to the observations by addition of a strong unknown loss process for HO 2 (*) 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 HO 2 (*) 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 HO 2 (*) 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 Published by Copernicus Publications on behalf of the European Geosciences Union. 2568 Y. Kanaya et al.: HO x Comp: observed and modeled ambient OH and HO comparisons period, as opposed to the general behavior, the model tended to underestimate HO 2 (*). We found that this tendency is associated with high NO x concentrations, suggesting that some HO 2 production or regeneration processes under high NO x conditions were being overlooked; this might require revision of ozone production regimes.
2009
Measurements of hydroxyl (OH) and hydroperoxy (HO 2) radicals were made during the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These measurements provide a unique opportunity to test current models of atmospheric RO x (OH + HO 2 + RO 2) photochemistry under polluted conditions. A zero-dimensional box model based on the Regional Atmospheric Chemical Mechanism (RACM) was constrained by 10-min averages of 24 J-values and the concentrations of 97 chemical species. Several issues related to the RO x chemistry under polluted conditions
Journal of Geophysical Research, 2002
1] An observationally constrained box model has been used to investigate the chemistry of the marine boundary layer at the Mace Head Atmospheric Research Station, a remote site on the west coast of Ireland. The model aims to simulate concentrations of the hydroxyl (OH) and hydroperoxy (HO 2 ) radicals measured by an in situ fluorescence assay by gas expansion instrument, and the sum of peroxy radicals ([HO 2 ] + AE[RO 2 ]) as determined by a peroxy radical chemical amplification instrument. The model has been constructed based on observed concentrations of nonmethane hydrocarbons, measured in situ during the campaign by gas chromatography. The chemical mechanism for the model is a subset of a comprehensive master chemical mechanism. This paper details comparisons of the concentrations of modeled and measured radical species from a field campaign held at Mace Head during spring 1997. The air masses arriving at the site have been split into three categories depending on their origin and chemical characteristics and model-measurement comparisons carried out for each air mass. The average model-measurement ratios are 2.4 for [OH], 3.6 for [HO 2 ], and 0.9 for ([HO 2 ] + AE[RO 2 ]), respectively, between 1100 and 1500 hours: the level of agreement is better for all three sets of radicals in the cleanest air mass. Possible reasons for the observed discrepancies are discussed. In addition, a rate of production analysis is used to identify key OH and HO 2 reactions in the three air masses. The rate of OH production from HO 2 with NO exceeds that from ozone photolysis by factors of 2-6 in the polluted air masses studied. In cleaner air from the northern polar region, primary production from ozone photolysis exceeds that from HO 2 + NO by a factor of 2.5. HO 2 and CH 3 O 2 dominate the peroxy radical composition in all air masses, but peroxy radicals derived from the oxidation of nonmethane hydrocarbons are more important in polluted air masses.
Journal of Geophysical Research, 2002
An observationally constrained box model has been used to investigate the chemistry of the marine boundary layer at the Mace Head Atmospheric Research Station, a remote site on the west coast of Ireland. The model aims to simulate concentrations of the hydroxyl (OH) and hydroperoxy (HO 2) radicals measured by an in situ fluorescence assay by gas expansion instrument, and the sum of peroxy radicals ([HO 2 ] + AE[RO 2 ]) as determined by a peroxy radical chemical amplification instrument. The model has been constructed based on observed concentrations of nonmethane hydrocarbons, measured in situ during the campaign by gas chromatography. The chemical mechanism for the model is a subset of a comprehensive master chemical mechanism. This paper details comparisons of the concentrations of modeled and measured radical species from a field campaign held at Mace Head during spring 1997. The air masses arriving at the site have been split into three categories depending on their origin and chemical characteristics and model-measurement comparisons carried out for each air mass. The average model-measurement ratios are 2.4 for [OH], 3.6 for [HO 2 ], and 0.9 for ([HO 2 ] + AE[RO 2 ]), respectively, between 1100 and 1500 hours: the level of agreement is better for all three sets of radicals in the cleanest air mass. Possible reasons for the observed discrepancies are discussed. In addition, a rate of production analysis is used to identify key OH and HO 2 reactions in the three air masses. The rate of OH production from HO 2 with NO exceeds that from ozone photolysis by factors of 2-6 in the polluted air masses studied. In cleaner air from the northern polar region, primary production from ozone photolysis exceeds that from HO 2 + NO by a factor of 2.5. HO 2 and CH 3 O 2 dominate the peroxy radical composition in all air masses, but peroxy radicals derived from the oxidation of nonmethane hydrocarbons are more important in polluted air masses.
ROOOH: the Missing Piece of the Puzzle for OH measurements in low NO Environments
Field campaigns have been carried out with the FAGE technique in remote biogenic environments in the last decade to quantify the in situ concentrations of OH, the main oxidant in the atmosphere. These data have revealed concentrations of OH radicals up to a factor of 10 higher than predicted by models, whereby the disagreement increases with decreasing NO concentration. This was interpreted as a major lack in our understanding of the chemistry of biogenic VOCs, particularly isoprene, which are dominant in remote pristine conditions. But interferences in these measurements of unknown origin have also been discovered for some FAGE instruments. We present in this paper convincing experimental and modeling evidence that the disagreement between model and measurement is due to interference by the unexpected decomposition of a new class of molecule, ROOOH, in the FAGE instruments. Including ROOOH reflects the missing piece of the puzzle in our understanding of OH in the atmosphere. 1 Introduction OH radicals are the most important oxidant in the atmosphere, and the detailed understanding of their formation and reactivity is key for the understanding of the overall chemistry. Upon reaction with Volatile Organic Compounds (VOCs, such as methane and isoprene), OH oxidation leads to the production of organic peroxy radicals (RO 2) who play a crucial role in the chemistry of tropospheric ozone and secondary organic aerosol (Monks et al., 2015). The concentration of OH radicals has been measured for several decades now (Holland et al., 2003;Creasey et al., 1997;Brune et al., 1995), and comparison of OH concentration profiles with model outputs is taken as a good indicator on the degree of understanding of the chemistry going on. Good agreement is often obtained between measurements and models for polluted environments (where levels of nitrogen oxides (NO x =NO+NO 2) are in excess of 500 pmol/mol, or ppt), however remote and clean