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Nighttime NO3-initiated oxidation of biogenic volatile organic compounds (BVOCs) such as monoterpenes is important for the atmospheric formation and growth of secondary organic aerosol (SOA), which has significant impact on climate, air quality, and human health. In such SOA formation and growth, highly oxygenated organic molecules (HOM) may be crucial, but their formation pathways and role in aerosol formation have yet to be clarified. Among monoterpenes, limonene is of particular interest for its high emission globally and high SOA yield. In this work, HOM formation in the reaction of limonene with nitrate radical (NO3) was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). About 280 HOM products were identified, grouped into 19 monomer families, 11 dimer families, and 3 trimer families. Both closed-shell products and open-shell peroxy radicals (RO2⚫) were observed, and many of them have not been reported previously. Monomers and dimers accounted for 47 % and 47 % of HOM concentrations, respectively, with trimers making up the remaining 6 %. In the most abundant monomer families, C10H15−17NO6−14, carbonyl products outnumbered hydroxyl products, indicating the importance of RO2⚫ termination by unimolecular dissociation. Both RO2⚫ autoxidation and alkoxy–peroxy pathways were found to be important processes leading to HOM. Time-dependent concentration profiles of monomer products containing nitrogen showed mainly second-generation formation patterns. Dimers were likely formed via the accretion reaction of two monomer RO2⚫, and HOM-trimers via the accretion reaction between monomer RO2⚫ and dimer RO2⚫. Trimers are suggested to play an important role in new particle formation (NPF) observed in our experiment. A HOM yield of 1.5%-0.7%+1.7% was estimated considering only first-generation products. SOA mass growth could be reasonably explained by HOM condensation on particles assuming irreversible uptake of ultra-low volatility organic compounds (ULVOCs), extremely low volatility organic compounds (ELVOCs), and low volatility organic compounds (LVOCs). This work provides evidence for the important role of HOM formed via the limonene +NO3 reaction in NPF and growth of SOA particles.

Several previous field studies have reported unexpectedly large concentrations of hydroxyl and hydroperoxyl radicals (OH and HO2, respectively) in forested environments that could not be explained by the traditional oxidation mechanisms that largely underestimated the observations. These environments were characterized by large concentrations of biogenic volatile organic compounds (BVOC) and low nitrogen oxide concentration. In isoprene-dominated environments, models developed to simulate atmospheric photochemistry generally underestimated the observed OH radical concentrations. In contrast, HO2 radical concentration showed large discrepancies with model simulations mainly in non-isoprene-dominated forested environments. An abundant BVOC emitted by lodgepole and ponderosa pines is 2-methyl-3-butene-2-ol (MBO), observed in large concentrations for studies where the HO2 concentration was poorly described by model simulations. In this work, the photooxidation of MBO by OH was investigated for NO concentrations lower than 200 pptv in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. Measurements of OH and HO2 radicals, OH reactivity (kOH), MBO, OH precursors, and organic products (acetone and formaldehyde) were used to test our current understanding of the OH-oxidation mechanisms for MBO by comparing measurements with model calculations. All the measured trace gases agreed well with the model results (within 15 %) indicating a well understood mechanism for the MBO oxidation by OH. Therefore, the oxidation of MBO cannot contribute to reconciling the unexplained high OH and HO2 radical concentrations found in previous field studies.

A global chemistry‐climate model is used to assess the impact on atmospheric composition of the regeneration and recycling of HOx in the photo‐oxidation of isoprene. The impact is explored subject to present‐day, pre‐industrial and future climate/emission scenarios. Our calculations show that, in all cases, the inclusion of uni‐molecular isomerisations of the isoprene hydroxy‐peroxy radicals leads to enhanced production of HOx radicals and ozone. The global burden of ozone increases by 25–36 Tg (8–18%), depending on the climate/emissions scenario, whilst the changes in OH lead to decreases in the methane lifetime of between 11% in the future and 35% in the pre‐industrial. Critically the size of the change in methane lifetime depends on the VOC/NOx emission ratio. The results of the present‐day calculations suggest a certain amount of parameter refinement is still needed to reconcile the updated chemistry with field observations (particularly for HO2+RO2). However, the updated chemistry could have far‐reaching implications for: future‐climate predictions; projections of future oxidising capacity; and our understanding of past changes in oxidising capacity.

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.

Recent studies have shown that measured OH under NOx‐limited, high‐isoprene conditions are many times higher than modeled OH. In this study, a detailed analysis of the HOx radical budgets under low‐NOx, rural conditions was performed employing a box model based on the Master Chemical Mechanism (MCMv3.2). The model results were compared with HOx radical measurements performed during the international HOxComp campaign carried out in Jülich, Germany, during summer 2005. Two different air masses influenced the measurement site denoted as high‐NOx (NO, 1–3 ppbv) and low‐NOx (NO, < 1 ppbv) periods. Both modeled OH and HO2 diurnal profiles lay within the measurement range of all HOx measurement techniques, with correlation slopes between measured and modeled OH and HO2 around unity. Recently discovered interference in HO2 measurements caused by RO2 cross sensitivity was found to cause a 30% increase in measured HO2 during daytime on average. After correction of the measured HO2 data, the model HO2 is still in good agreement with the observations at high NOx but overpredicts HO2 by a factor of 1.3 to 1.8 at low NOx. In addition, for two different set of measurements, a missing OH source of 3.6 ± 1.6 and 4.9 ± 2.2 ppb h−1 was estimated from the experimental OH budget during the low‐NOx period using the corrected HO2 data. The measured diurnal profile of the HO2/OH ratio, calculated using the corrected HO2, is well reproduced by the MCM at high NOx but is significantly overestimated at low NOx. Thus, the cycling between OH and HO2 is better described by the model at high NOx than at low NOx. Therefore, similar comprehensive field measurements accompanied by model studies are urgently needed to investigate HOx recycling under low‐NOx conditions. Key Points Hox budget and radical recycling at low NOx conditions Comparison of measured to modeled HOx budgets The influence of HO2 inteferences due to RO2 cross sensitivity on HOx Budgets

Accurate accounting of the partition between anthropogenic and biogenic carbon dioxide mixing ratios (CO2Anth and CO2Bio) in urban-based CO2 measurements is key to developing effective emission reduction strategies since such measurements can provide an independent catalogue of local and regional CO2 emission inventories. In an attempt to delineate the contribution of CO2Bio to the overall urban CO2 mixing ratio enhancements, carbon monoxide (CO) was utilized as a tracer, following CO2 and CO mixing ratio measurements using a wavelength-scanned cavity ring-down spectrometer (CRDS). These measurements were performed in Cookeville, TN, (36.1628° N, 85.5016° W), a medium-sized city within the Eastern Highland Rim region of the United States. Between the years 2017 and 2019, the average seasonal wintertime CO2Bio mixing ratios varied between −0.65 ± 3.44 ppm and 0.96 ± 2.66 ppm. During the springtime, the observed CO2Bio signals were largely negative while the CO2Anth values were generally lower than the wintertime values. The contribution of CO from the isoprene oxidation reaction with the hydroxyl radical (OH) (COisoprene) to the overall CO enhancement during the growing season was estimated to be ~17–27 ppb, underscoring the importance of considering the contribution of COisoprene to untangling different CO2Anth and CO2Bio sources and sinks in high isoprene-emitting urban environments.

Atmospheric oxidation of isoprene and its oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK) have an important impact on the photochemical activity in the boundary layer, in particular in forested areas. The oxidation of isoprene by OH radicals was investigated in chamber experiments conducted under tropospheric conditions in the atmosphere simulation chamber SAPHIR at the Research Center Jülich. The aim was to determine the product yield of MVK and MACR in the OH-induced isoprene oxidation and the rate constant of their reaction with OH under real atmospheric conditions. The recently published updated degradation scheme for isoprene from Geiger et al. (2003) was used to determine rate constants and product yields. The fractional yields in the isoprene peroxy radical reaction with NO were found to be 0.41±0.03 for MVK and 0.27±0.03 for MACR. The rate coefficient for MACR with OH was found to be in very good agreement with the recommended value of IUPAC Atkinson (Atkinson et al., 2005). while the rate coefficient for MVK with OH was 27% lower.[PUBLICATION ABSTRACT]

A new condensed isoprene oxidation mechanism for global atmospheric modeling (MIM) was derived from a highly detailed master chemical mechanism (MCM). In a box model intercomparison covering a wide range of boundary layer conditions the MIM was compared with the MCM and with five other condensed mechanisms, some of which have already been used in global modeling studies of nonmethane hydrocarbon chemistry. The results of MCM and MIM were generally in good agreement, but the other tested mechanisms exhibited substantial differences relative to the MCM as well as relative to each other. Different formation yields, reactivities and degradation pathways of organic nitrates formed in the course of isoprene oxidation were identified as a major reason for the deviations. The relevance of the box model results for chemistry transport models is discussed, and the need for a validated reference mechanism and for an improved representation of isoprene chemistry in global models is pointed out.

Data on a variety of organic gases are presented, obtained with a protontransfer mass spectrometer (PTR-MS) operated during the March 1998 LBA/CLAIREairborne measurement campaign, between 60 and 12500 m over the rainforest inSurinam (2° N-5° N, 54° W-57° W). The instrumentcan detect molecules with a proton affinity greater than water, includingalkenes, dialkenes, carbonyls, alcohols, and nitriles. Many such molecules areemitted from the rainforest (e.g., isoprene) or formed from the oxidation ofprimary emissions (e.g., methylvinylketone (MVK) and methacrolein (MACR)).From a comparison with modelled data; the variation with altitude; previouslyreported biogenic emissions and the time and location of the measurement,possible and probable identities for the significant masses encountered in therange 33-140 amu have been deduced.The main observed protonated masses, postulated identities and observedaverage boundary layer mixing ratios over the rainforest were: 33 methanol(1.1 nmol/mol); 42 acetonitrile (190 pmol/mol); 43 multiple possibilities (5.9nmol/mol), 45 acetaldehyde (1.7 nmol/mol), 47 formic acid (not quantified);59 acetone (2.9 nmol/mol), 61 acetic acid (not quantified), 63 dimethylsulphide (DMS) (289 pmol/mol), 69 isoprene (1.7 nmol/mol), 71 MVK + MACR (1.3nmol/mol), 73 methyl ethyl ketone (1.8 nmol/mol), 75 hydroxyacetone (606pmol/mol), 83 C^sub 5^ isoprene hydroxy carbonylsC^sub 5^H^sub 8^O^sub 2^, methyl furan, and cis 3-hexen-1-ol(732 pmol/mol), 87 C^sub 5^ carbonyls and methacrylic acid, 95 possibly2-vinyl furan (656 pmol/mol), 97 unknown (305 pmol/mol), 99 cis hexenal (512pmol/mol) and 101 isoprene C^sub 5^ hydroperoxides (575 pmol/mol). Somespecies agreed well with those derived from an isoprene only photochemicalmodel (e.g., mass 71 MVK + MACR) while others did not and were observed athigher than previously reported mixing ratios (e.g., mass 59 acetone, mass 63DMS). Monoterpenes were not detected above the detection limit of 300pmol/mol. Several species postulated are potentially important sources ofHO^sub x^ in the free troposphere, e.g., methanol, acetone, methyl ethylketone, methyl vinyl ketone and methacrolein.[PUBLICATION ABSTRACT]

Airborne measurements of volatile organic compounds (VOC) were performed overthe tropical rainforest in Surinam (0-12 km altitude,2°-7° N, 54°-58° W) using the proton transferreaction mass spectrometry (PTR-MS) technique, which allows online monitoringof compounds like isoprene, its oxidation products methyl vinyl ketone,methacrolein, tentatively identified hydroxy-isoprene-hydroperoxides, andseveral other organic compounds. Isoprene volume mixing ratios (VMR) variedfrom below the detection limit at the highest altitudes to about 7 nmol/molin the planetary boundary layer shortly before sunset. Correlations betweenisoprene and its product compounds were made for different times of day andaltitudes, with the isoprene-hydroperoxides showing the highest correlation.Model calculated mixing ratios of the isoprene oxidation products using adetailed hydrocarbon oxidation mechanism, as well as the intercomparisonmeasurement with air samples collected during the flights in canisters andlater analysed with a GC-FID, showed good agreement with the PTR-MSmeasurements, in particular at the higher mixing ratios.Low OH concentrations in the range of 1-3 × 10^sup 5^molecules cm^sup -3^ averaged over 24 hours were calculated due to lossof OH and HO^sub 2^ in the isoprene oxidation chain, thereby stronglyenhancing the lifetime of gases in the forest boundary layer.[PUBLICATION ABSTRACT]