scholarly journals Importance of isomerization reactions for OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR

2020 ◽  
Vol 20 (6) ◽  
pp. 3333-3355 ◽  
Author(s):  
Anna Novelli ◽  
Luc Vereecken ◽  
Birger Bohn ◽  
Hans-Peter Dorn ◽  
Georgios I. Gkatzelis ◽  
...  
Keyword(s):  
Relative Yield ◽  
Oh Radicals ◽  
Oh Radical ◽  
Peroxy Radicals ◽  
Unimolecular Reactions ◽  
Model Calculations ◽  
Hydrogen Shift ◽  
Wide Range ◽  
Regeneration Efficiency ◽  
Simulation Chamber

Abstract. Theoretical, laboratory, and chamber studies have shown fast regeneration of the hydroxyl radical (OH) in the photochemistry of isoprene, largely due to unimolecular reactions which were previously thought not to be important under atmospheric conditions. Based on early field measurements, nearly complete regeneration was hypothesized for a wide range of tropospheric conditions, including areas such as the rainforest where slow regeneration of OH radicals is expected due to low concentrations of nitric oxide (NO). In this work the OH regeneration in isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospherically relevant NO levels (between 0.15 and 2 ppbv – parts per billion by volume) in the atmospheric simulation chamber SAPHIR. These conditions cover remote areas partially influenced by anthropogenic NO emissions, giving a regeneration efficiency of OH close to 1, and areas like the Amazonian rainforest with very low NO, resulting in a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3 higher than explainable in the absence of unimolecular reactions. The measured radical concentrations were compared to model calculations, and the best agreement was observed when at least 50 % of the total loss of isoprene peroxy radicals conformers (weighted by their abundance) occurs via isomerization reactions for NO lower than 0.2 ppbv. For these levels of NO, up to 50 % of the OH radicals are regenerated from the products of the 1,6 α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO2 radicals through the photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehydic hydrogen shift (rate constant ∼10 s−1 at 300 K) in di-hydroperoxy carbonyl peroxy radicals (di-HPCARP-RO2), depending on their relative yield. The agreement between all measured and modelled trace gases (hydroxyl, hydroperoxy, and organic peroxy radicals, carbon monoxide, and the sum of methyl vinyl ketone, methacrolein, and hydroxyl hydroperoxides) is nearly independent of the adopted yield of HPALD and di-HPCARP-RO2 as both degrade relatively fast (<1 h), forming the OH radical and CO among other products. Taking into consideration this and earlier isoprene studies, considerable uncertainties remain on the distribution of oxygenated products, which affect radical levels and organic aerosol downwind of unpolluted isoprene-dominated regions.

Importance of isomerization reactions for the OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR

2020 ◽  
Author(s):  
Anna Novelli ◽  
Luc Vereecken ◽  
Birger Bohn ◽  
Hans-Peter Dorn ◽  
Georgios Gkatzelis ◽  
...  
Keyword(s):  
Oh Radicals ◽  
Oh Radical ◽  
Peroxy Radicals ◽  
Atmospheric Conditions ◽  
Unimolecular Reactions ◽  
Model Calculations ◽  
Hydrogen Shift ◽  
Wide Range ◽  
Regeneration Efficiency ◽  
Simulation Chamber

&lt;p&gt;Theoretical, laboratory and chamber studies have shown fast regeneration of hydroxyl radical (OH) in the photochemistry of isoprene largely due to previously disregarded unimolecular reactions which were previously thought not to be important under atmospheric conditions. Based on early field measurements, nearly complete regeneration was hypothesized for a wide range of tropospheric conditions, including areas such as the rainforest where slow regeneration of OH radicals is expected due to low concentrations of nitric oxide (NO). In this work the OH regeneration in the isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospheric conditions (i.e. NO between 0.15 and 2 ppbv and temperature between 25 and 41&amp;#176;C) in the atmospheric simulation chamber SAPHIR. These conditions cover remote areas partially influenced by anthropogenic NO emissions, giving a regeneration efficiency of OH close to one, and areas like the Amazonian rainforest with very low NO, resulting in a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3 higher than explainable in the absence of unimolecular reactions. The measured radical concentrations were compared to model calculations and the best agreement was observed when at least 50% of the total loss of isoprene peroxy radicals conformers (weighted by their abundance) occurs via isomerization reactions for NO lower than 0.2 parts per billion (ppbv). For these levels of NO, up to 50% of the OH radicals are regenerated from the products of the 1,6 &amp;#945;-hydroxy-hydrogen shift (1,6-H shift) of Z-&amp;#948;-RO&lt;sub&gt;2 &lt;/sub&gt;radicals through photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehyde hydrogen shift (rate constant ~10 s&lt;sup&gt;-1&lt;/sup&gt; at 300K) in di-hydroperoxy carbonyl peroxy radicals (di-HPCARP-RO&lt;sub&gt;2&lt;/sub&gt;), depending on their relative yield. The agreement between all measured and modelled trace gases (hydroxyl, hydroperoxy and organic peroxy radicals, carbon monoxide and the sum of methyl vinyl ketone, methacrolein and hydroxyl hydroperoxides) is nearly independent on the adopted yield of HPALD and di-HPCARP-RO&lt;sub&gt;2&lt;/sub&gt; as both degrade relatively fast (&lt; 1 h), forming OH radical and CO among other products. Taking into consideration this and earlier isoprene studies, considerable uncertainties remain on the oxygenated products distribution, which affect radical levels and organic aerosol downwind of unpolluted isoprene dominated regions.&lt;/p&gt;

scholarly journals Importance of isomerization reactions for the OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR

2019 ◽  
Author(s):  
Anna Novelli ◽  
Luc Vereecken ◽  
Hans-Peter Dorn ◽  
Andreas Hofzumahaus ◽  
Frank Holland ◽  
...  
Keyword(s):  
Relative Yield ◽  
Oh Radicals ◽  
Peroxy Radicals ◽  
Atmospheric Conditions ◽  
Unimolecular Reactions ◽  
Model Calculations ◽  
Hydrogen Shift ◽  
Wide Range ◽  
Regeneration Efficiency ◽  
Simulation Chamber

Abstract. Theoretical, laboratory and chamber studies have shown fast regeneration of hydroxyl radical (OH) in the photochemistry of isoprene largely due to previously disregarded unimolecular reactions. Based on early field measurements, nearly complete regeneration was hypothesized for a wide range of tropospheric conditions, including areas such as the rainforest where slow regeneration of OH radicals is expected due to low concentrations of nitric oxide (NO). In this work the OH regeneration in the isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospheric conditions (i.e. NO between 0.15 and 2 ppbv and temperature between 25 and 41 °C) in the atmospheric simulation chamber SAPHIR. These conditions cover remote areas partially influenced by anthropogenic NO emissions, giving a regeneration efficiency of OH close to one, and areas like the Amazonian rainforest with very low NO, resulting in a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3 higher than explainable in the absence of unimolecular reactions. The measured radical concentrations were compared to model calculations and the best agreement was observed when at least 50 % of the total loss of isoprene peroxy radicals conformers (weighted by their abundance) occurs via isomerization reactions for NO lower than 0.2 parts per billion (ppbv). For these levels of NO, up to 50 % of the OH radicals are regenerated from the products of the 1,6 α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO2 radicals through photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehyde hydrogen shift (rate constant ∼ 10 s−1 at 300 K) in di-hydroperoxy carbonyl peroxy radicals (di-HPCARP-RO2), depending on their relative yield. The agreement between all measured and modelled trace gases (hydroxyl, hydroperoxy and organic peroxy radicals, carbon monoxide and the sum of methyl vinyl ketone, methacrolein and hydroxyl hydroperoxides) is nearly independent on the adopted yield of HPALD and di-HPCARP-RO2 as both degrade relatively fast (

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

2014 ◽  
Vol 14 (4) ◽  
pp. 5197-5231 ◽  
Author(s):  
H. Fuchs ◽  
I.-H. Acir ◽  
B. Bohn ◽  
T. Brauers ◽  
H.-P. Dorn ◽  
...  
Keyword(s):  
Nitrogen Monoxide ◽  
Reaction Rate Constant ◽  
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 current chemical models for conditions of the experiments (NO mixing ratio of 90 pptv). The analysis of the OH budget reveals a so far unaccounted OH source, which is correlated with the production rate of RO2 radicals from MACR. In order to balance the measured OH destruction rate, (0.77±0.3) OH radicals need to be additionally reformed from each OH that has reacted 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 one RO2 species formed in the MACR+OH reaction as suggested in literature. This fast isomerization reaction would be competitive to the reaction of this RO2 species with minimum 150 pptv NO.

scholarly journals Comparison of OH reactivity measurements in the atmospheric simulation chamber SAPHIR

2017 ◽  
Author(s):  
Hendrik Fuchs ◽  
Anna Novelli ◽  
Michael Rolletter ◽  
Andreas Hofzumahaus ◽  
Eva Y. Pfannerstill ◽  
...  
Keyword(s):  
Water Vapor ◽  
Organic Compounds ◽  
Limit Of Detection ◽  
Measurement Techniques ◽  
Oh Radical ◽  
Atmospheric Measurements ◽  
Radical Reactivity ◽  
Wide Range ◽  
Chemical Conditions ◽  
Simulation Chamber

Abstract. Hydroxyl (OH) radical reactivity (kOH) has been measured for 18 years with different measurement techniques. In order to compare the performances of instruments deployed in the field, two campaigns were conducted performing experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich in October 2015 and April 2016. Chemical conditions were chosen either to be representative of the atmosphere or to test potential limitations of instruments. All types of instruments that are currently used for atmospheric measurements took part in one of the two campaigns. The results of these campaigns demonstrate that OH reactivity can be accurately measured for a wide range of atmospherically relevant chemical conditions (e.g. water vapor, nitrogen oxides, various organic compounds) by all instruments. The precision of the measurements (limit of detection

scholarly journals Secondary Organic Aerosol Reduced by Mixture of Atmospheric Vapours

2020 ◽  
Author(s):  
Thomas Mentel ◽  
Gordon McFiggans ◽  
Jürgen Wildt ◽  
Astrid Kiendler-Scharr ◽  
Keyword(s):  
Secondary Organic Aerosol ◽  
Organic Aerosol ◽  
Oh Radicals ◽  
Peroxy Radicals ◽  
Model Calculations ◽  
Volatile Organic ◽  
Single Precursor ◽  
Real Atmosphere ◽  
Isoprene Oxidation ◽  
Plant Emissions

&lt;p&gt;Biogenic volatile organic compounds (VOC) are important secondary organic aerosol (SOA) precursors. Whilst isoprene dominates VOC plant emissions globally, its yield of SOA mass is found to be modest in comparison to that of monoterpenes (MT). Tracers from isoprene oxidation have been observed in particles showing that they condense from the gas phase and yet new particle formation is suppressed by the presence of isoprene in mixtures of plant emissions containing MT.&lt;/p&gt;&lt;p&gt;Experiments were performed in the JPAC chamber in J&amp;#252;lich. We showed that isoprene can suppress both the instantaneous mass formation and overall yield of monoterpenes in mixtures by two effects: oxidant and product scavenging. Isoprene scavenged OH radicals from reacting with MT (oxidant scavenging). Subsequently, the resulting isoprene peroxy radicals reacted with highly oxygenated peroxy radicals from MT oxidation (product scavenging). These effects from isoprene, also demonstrated using CO or CH&lt;sub&gt;4&lt;/sub&gt;, reduced the yield of low-volatility, highly oxygenated molecules (HOM) from MT that would otherwise form SOA.&lt;/p&gt;&lt;p&gt;Our results show that in mixtures changes in particle mass and number are not additive, and yields from single precursor experiments cannot simply be linearly combined. Reactive, modest SOA yield compounds are not necessarily net SOA producers and isoprene oxidation can suppress both SOA number and mass. Global model calculations support that OH scavenging and product scavenging can also operate in the real atmosphere. Our results highlight a need for more realistic consideration of SOA formation in the atmosphere analogous to the treatment of ozone formation, where interactions between the mechanistic pathways involving peroxy radicals are recognised to be essential.&lt;/p&gt;

New insights into the gas-phase oxidation of isoprene by the nitrate radical from experiments in the atmospheric simulation chamber SAPHIR

2020 ◽  
Author(s):  
Philip Carlsson ◽  
Patrick Dewald ◽  
Justin Shenolikar ◽  
Nils Friedrich ◽  
John Crowley ◽  
...  
Keyword(s):  
Chemical Mechanism ◽  
Organic Nitrate ◽  
Nitrate Radical ◽  
Peroxy Radicals ◽  
Night Time ◽  
Alkyl Nitrates ◽  
Model Calculations ◽  
Alkyl Nitrate ◽  
Chemical Conditions ◽  
Simulation Chamber

&lt;p&gt;Experiments at a set of atmospherically relevant conditions were performed in the simulation chamber SAPHIR, investigating the oxidation of isoprene by the nitrate radical (NO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;.&lt;/sub&gt; An extremely comprehensive set of instruments detected trace gases, radicals, aerosol properties and hydroxyl (OH) and NO&lt;sub&gt;3&lt;/sub&gt;&amp;#160;radical reactivity. The chemical conditions in the chamber were varied to change the fate of the peroxy radicals (RO&lt;sub&gt;2&lt;/sub&gt;) formed after the reaction between NO&lt;sub&gt;3&lt;/sub&gt;&amp;#160;and isoprene from either mainly recombining with other RO&lt;sub&gt;2&lt;/sub&gt;&amp;#160;or mainly reacting with hydroperoxyl radicals (HO&lt;sub&gt;2&lt;/sub&gt;). These major atmospheric pathways for RO&lt;sub&gt;2&lt;/sub&gt;&amp;#160;radicals lead to the formation of organic nitrate compounds which then have different atmospheric fates. The experimental concentration profiles are compared to box model calculations using both the current Master Chemical Mechanism (MCM) as well as recently available literature data alongside new quantum chemical calculations. The discussion here focusses on the resulting RO&lt;sub&gt;2&lt;/sub&gt;&amp;#160;distribution and deviations in the predictions of early products and total alkyl nitrate yields for the different chemical conditions. Preliminary results for instance show too high night time losses of alkyl nitrates due to ozonolysis in the current MCM.&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;

Structure-activity relationships for unimolecular reactions of peroxy radicals, RO2, at atmospheric temperatures

2020 ◽  
Author(s):  
Luc Vereecken ◽  
Giang H. T. Vu ◽  
Hue M. T. Nguyen
Keyword(s):  
Ring Closure ◽  
Specific Rate ◽  
Extensive Literature ◽  
Peroxy Radicals ◽  
Unimolecular Reactions ◽  
Radical Site ◽  
Critical Step ◽  
Structure Activity Relationships ◽  
Structure Activity ◽  
Wide Range

&lt;p&gt;The oxidation of most organic matter emitted to the atmosphere proceeds by radical reaction steps, where peroxy radicals, ROO&lt;sup&gt;&amp;#8226;&lt;/sup&gt;, are critical intermediates formed by addition of O&lt;sub&gt;2&lt;/sub&gt; molecules to carbon-based radicals. The chemistry of these RO&lt;sub&gt;2&lt;/sub&gt; radicals in high-NOx conditions is well-known, forming alkoxy radicals and NO&lt;sub&gt;2&lt;/sub&gt;. In low-NOx and pristine conditions, the RO&lt;sub&gt;2&lt;/sub&gt; radicals react with HO&lt;sub&gt;2&lt;/sub&gt; and other R'O&lt;sub&gt;2&lt;/sub&gt; radicals, but can have a sufficiently long lifetime to also undergo unimolecular reactions. Hydrogen atom migration, forming a hydroperoxide (-OOH) and a new peroxy radical site after addition of an additional O&lt;sub&gt;2&lt;/sub&gt; on the newly formed radical site, has been studied extensively in some compounds, such as isoprene where it was shown to be the a critical step in OH radical regeneration. RO&lt;sub&gt;2&lt;/sub&gt; ring closure reactions have likewise been studied, where for &amp;#946;-pinene it has been shown to be a critical step governing the yield of the decomposition products such as acetone and nopinone.&lt;/p&gt;&lt;p&gt;Despite the interest in RO&lt;sub&gt;2&lt;/sub&gt; unimolecular reactions, and the potential impact on atmospheric chemistry, no widely applicable structure-activity relationships (SARs) have been proposed to allow systematic incorporation of such unimolecular reactions in gas phase atmospheric kinetic models. In this work, we present a series of systematic theoretical predictions on the site-specific rate coefficients for such reactions for a wide range of molecular substitutions. Combined with extensive literature data this allows for the formulation of a SAR for RO&lt;sub&gt;2&lt;/sub&gt; unimolecular reactions, covering aliphatic, branched, and unsaturated RO&lt;sub&gt;2&lt;/sub&gt; with oxo, hydroxy, hydroperoxy, nitrate, carboxylic acid, and ether substitutions.&lt;/p&gt;&lt;p&gt;The predictions are compared to experimental and theoretical data, including multi-functionalized species. Though some molecular classes are well represented in the training set (e.g. aliphatic RO&lt;sub&gt;2&lt;/sub&gt;), other classes have little data available and additional work is needed to enhance and validate the reliability of the SAR. Direct experimental data is scarce for all RO&lt;sub&gt;2&lt;/sub&gt; classes. The fastest H-migrations are found to be for unsaturated RO&lt;sub&gt;2&lt;/sub&gt;, with the double bond outside the H-migration TS ring. Ring closure of unsaturated RO&lt;sub&gt;2&lt;/sub&gt; are likewise fast if the product radical carbon is exocyclic to the newly formed peroxide ring.&lt;/p&gt;

scholarly journals Enhancement of atmospheric H<sub>2</sub>SO<sub>4</sub>/H<sub>2</sub>O nucleation: organic oxidation products versus amines

2013 ◽  
Vol 13 (6) ◽  
pp. 16301-16335 ◽  
Author(s):  
T. Berndt ◽  
M. Sipilä ◽  
F. Stratmann ◽  
T. Petäjä ◽  
J. Vanhanen ◽  
...  
Keyword(s):  
Oxidation Products ◽  
Radical Scavenger ◽  
Oh Radicals ◽  
Oh Radical ◽  
Flow Tube ◽  
Experimental Conditions ◽  
Wide Range ◽  
Nucleation Mechanisms ◽  
Comparison Of The Results ◽  
Organic Oxidation

Abstract. Atmospheric H2SO4/H2O nucleation influencing effects have been studied in the flow tube IfT-LFT (Institute for Tropospheric Research – Laminar Flow Tube) at 293 ± 0.5 K and a pressure of 1 bar using synthetic air as the carrier gas. The presence of a possible background amine concentration in the order of 107–108 molecule cm−3 throughout the experiments has to be taken into account. In a first set of investigations, ozonolysis of olefins (tetramethylethylene, 1-methyl-cyclohexene, α-pinene and limonene) for close to atmospheric concentrations, served as the source of OH radicals and possibly other oxidants initiating H2SO4 formation starting from SO2. The oxidant generation is inevitably associated with the formation of a series of organic oxidation products arising from the parent olefins. These products (first generation mainly) showed no clear effect on the number of nucleated particles within a wide range of experimental conditions for H2SO4 concentrations higher than ~107 molecule cm−3. A comparison of the results of two different particle counters (50% cut-off size: about 1.5 nm or 2.5–3 nm) suggested that the early growth process of the nucleated particles was not significantly influenced by the organic oxidation products. An additional, H2SO4-independent process of particle (nano-CN) formation was observed in the case of α-pinene and limonene ozonolysis for H2SO4 concentrations smaller than ~10 7 molecule cm−3. Furthermore, the findings confirm the existence of an additional oxidant for SO2 beside OH radicals, very likely stabilized Criegee Intermediate (sCI). In the case of the ozonolysis of tetramethylethylene, the H2SO4 measurements in the absence and presence of an OH radical scavenger were well described by modelling using recently obtained kinetic data for the sCI reactivity in this system. A second set of experiments has been performed in the presence of added amines (trimethylamine, dimethylamine, aniline and pyridine) in the concentration range of a few 107–1010 molecule cm−3. Here, photolytic OH radical generation was applied for H2SO4 production with no addition of other organics. All amines showed a significant nucleation enhancement with increasing efficiency in the order pyridine < aniline < dimethylamine < trimethylamine. This result supports the idea of H2SO4 cluster stabilization by amines due to strong H2SO4 &amp;leftrightarrow; amine interactions. On the other hand, this study reveals that for organic oxidation products (in presence of the possible amine background as stated) a distinct H2SO4/H2O nucleation enhancement can be due to increased H2SO4 formation caused by additional organic oxidant production (sCI) rather than by stabilization of H2SO4 clusters due to H2SO4 &amp;leftrightarrow; organics interactions. However, because the molecular composition of nucleating clusters was not measured, the role of any background substances, unavoidably present in any system, to experimental data remains unclear. Also the experimental conditions do not cover fully the range of atmospheric observations, e.g., the concentration of precursor vapours represents rather the upper end of the atmospheric range. More experimental work is needed before definite conclusions about the nucleation mechanisms in the atmosphere can be drawn.

scholarly journals The role of radical chemistry in the product formation from nitrate radical initiated gas-phase oxidation of isoprene

2021 ◽  
Author(s):  
Philip T. M. Carlsson ◽  
Luc Vereecken ◽  
Anna Novelli ◽  
François Bernard ◽  
Birger Bohn ◽  
...  
Keyword(s):  
Gas Phase ◽  
First Generation ◽  
Product Distribution ◽  
Chem Phys ◽  
Oxidation Products ◽  
Rate Coefficients ◽  
Peroxy Radicals ◽  
Model Calculations ◽  
Alkoxy Radicals ◽  
Wide Range

&lt;p&gt;Experiments at atmospherically relevant conditions were performed in the simulation chamber SAPHIR, investigating the reaction of isoprene with NO&lt;sub&gt;3&lt;/sub&gt; and its subsequent oxidation. Due to the production of NO&lt;sub&gt;3&lt;/sub&gt; from the reaction of NO&lt;sub&gt;2&lt;/sub&gt; with O&lt;sub&gt;3&lt;/sub&gt; as well as the formation of OH in subsequent reactions, the reactions of isoprene with O&lt;sub&gt;3&lt;/sub&gt; and OH were estimated to contribute up to 15% of the total isoprene consumption each in these experiments. The ratio of RO&lt;sub&gt;2&lt;/sub&gt; to HO&lt;sub&gt;2&lt;/sub&gt; concentrations was varied by changing the reactant concentrations, which modifies the product distribution from bimolecular reactions of the nitrated RO&lt;sub&gt;2&lt;/sub&gt;. The reaction with HO&lt;sub&gt;2&lt;/sub&gt; or NO&lt;sub&gt;3&lt;/sub&gt; was found to be the main bimolecular loss process for the RO&lt;sub&gt;2&lt;/sub&gt; radicals under all conditions examined.&lt;/p&gt;&lt;p&gt;Yields of the first-generation isoprene oxygenated nitrates as well as the sum of methyl vinyl ketone (MVK) and methacrolein (MACR) were determined by high resolution proton mass spectrometry using the Vocus PTR-TOF. The experimental time series of these products are compared to model calculations based on the MCM v3.3.1,&lt;sup&gt;1&lt;/sup&gt; the isoprene mechanism as published by Wennberg &lt;em&gt;et al.&lt;/em&gt;&lt;sup&gt;2&lt;/sup&gt; and the newly developed FZJ-NO&lt;sub&gt;3&lt;/sub&gt;-isoprene mechanism,&lt;sup&gt;3&lt;/sup&gt; which incorporates theory-based rate coefficients for a wide range of reactions.&lt;/p&gt;&lt;p&gt;Among other changes, the FZJ-NO&lt;sub&gt;3&lt;/sub&gt;-isoprene mechanism contains a novel fast oxidation route through the epoxidation of alkoxy radicals, originating from the formation of nitrated peroxy radicals. This inhibits the formation of MVK and MACR from the NO&lt;sub&gt;3&lt;/sub&gt;-initiated oxidation of isoprene to practically zero, which agrees with the observations from chamber experiments. In addition, the FZJ-NO&lt;sub&gt;3&lt;/sub&gt;-isoprene mechanism increases the level of agreement for the main first-generation oxygenated nitrates.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; M. E. Jenkin, J. C. Young and A. R. Rickard, The MCM v3.3.1 degradation scheme for isoprene, &lt;em&gt;Atmospheric Chem. Phys.&lt;/em&gt;, 2015, &lt;strong&gt;15&lt;/strong&gt;, 11433&amp;#8211;11459.&lt;/p&gt;&lt;p&gt;&lt;sup&gt;2&lt;/sup&gt; P. O. Wennberg &lt;em&gt;at al.&lt;/em&gt;, Gas-Phase Reactions of Isoprene and Its Major Oxidation Products, &lt;em&gt;Chem. Rev.&lt;/em&gt;, 2018, &lt;strong&gt;118&lt;/strong&gt;, 3337&amp;#8211;3390.&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;3&lt;/sup&gt; L. Vereecken &lt;em&gt;et al.&lt;/em&gt;, Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene, &lt;em&gt;Phys. Chem. Chem. Phys.&lt;/em&gt;, submitted.&lt;/p&gt;

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