The oxidation of Δ3-carene and one of its main oxidation products, caronaldehyde, by the OH radical and O3 was investigated in the atmospheric simulation chamber SAPHIR under atmospheric conditions ...for NOx mixing ratios below 2 ppbv.
Within this study, the rate constants of the reaction of Δ3-carene with OH and O3 and of the reaction of caronaldehyde with OH were determined to be (8.0±0.5)×10-11 cm3 s−1 at 304 K, (4.4±0.2)×10-17 cm3 s−1 at 300 K and (4.6±1.6)×10-11 cm3 s−1 at 300 K, in agreement with previously published values.
The yields of caronaldehyde from the reaction of OH and ozone with Δ3-carene were determined to be 0.30±0.05 and 0.06±0.02, respectively. Both values are in reasonably good agreement with reported literature values. An organic nitrate (RONO2) yield from the reaction of NO with RO2 derived from Δ3-carene of 0.25±0.04 was determined from the analysis of the reactive nitrogen species (NOy) in the SAPHIR chamber. The RONO2 yield of the reaction of NO with RO2 derived from the reaction of caronaldehyde with OH was found to be 0.10±0.02. The organic nitrate yields of Δ3-carene and caronaldehyde oxidation with OH are reported here for the first time in the gas phase. An OH yield of 0.65±0.10 was determined from the ozonolysis of Δ3-carene.
Calculations of production and destruction rates of the sum of hydroxyl and peroxy radicals (ROx=OH+HO2+RO2) demonstrated that there were no unaccounted production or loss processes of radicals in the oxidation of Δ3-carene for conditions of the chamber experiments.
In an OH-free experiment with added OH scavenger, the photolysis frequency of caronaldehyde was obtained from its photolytical decay. The experimental photolysis frequency was a factor of 7 higher than the value calculated from the measured solar actinic flux density, an absorption cross section from the literature and an assumed effective quantum yield of unity for photodissociation.
The oxidation of Î.sup.3 -carene and one of its main oxidation products, caronaldehyde, by the OH radical and O.sub.3 was investigated in the atmospheric simulation chamber SAPHIR under atmospheric ...conditions for NO.sub.x mixing ratios below 2 ppbv. Within this study, the rate constants of the reaction of Î.sup.3 -carene with OH and O.sub.3 and of the reaction of caronaldehyde with OH were determined to be (8.0±0.5)x10-11 cm.sup.3 s.sup.-1 at 304 K, (4.4±0.2)x10-17 cm.sup.3 s.sup.-1 at 300 K and (4.6±1.6)x10-11 cm.sup.3 s.sup.-1 at 300 K, in agreement with previously published values. The yields of caronaldehyde from the reaction of OH and ozone with Î.sup.3 -carene were determined to be 0.30±0.05 and 0.06±0.02, respectively. Both values are in reasonably good agreement with reported literature values. An organic nitrate (RONO.sub.2) yield from the reaction of NO with RO.sub.2 derived from Î.sup.3 -carene of 0.25±0.04 was determined from the analysis of the reactive nitrogen species (NO.sub.y) in the SAPHIR chamber. The RONO.sub.2 yield of the reaction of NO with RO.sub.2 derived from the reaction of caronaldehyde with OH was found to be 0.10±0.02. The organic nitrate yields of Î.sup.3 -carene and caronaldehyde oxidation with OH are reported here for the first time in the gas phase. An OH yield of 0.65±0.10 was determined from the ozonolysis of Î.sup.3 -carene. Calculations of production and destruction rates of the sum of hydroxyl and peroxy radicals (ROx=OH+HO2+RO2) demonstrated that there were no unaccounted production or loss processes of radicals in the oxidation of Î.sup.3 -carene for conditions of the chamber experiments. In an OH-free experiment with added OH scavenger, the photolysis frequency of caronaldehyde was obtained from its photolytical decay. The experimental photolysis frequency was a factor of 7 higher than the value calculated from the measured solar actinic flux density, an absorption cross section from the literature and an assumed effective quantum yield of unity for photodissociation.
Ambient measurements of nitryl chloride (ClNO2) were
performed at a rural site in Germany, covering three periods in winter,
summer, and autumn 2019, as part of the JULIAC campaign (Jülich
...Atmospheric Chemistry Project) that aimed to understand the
photochemical processes in air masses typical of midwestern Europe.
Measurements were conducted at 50 m aboveground, which was mainly located
in the nocturnal boundary layer and thus uncoupled from local surface
emissions. ClNO2 is produced at night by the heterogeneous reaction of
dinitrogen pentoxide (N2O5) on chloride (Cl−) that contains
aerosol. Its photolysis during the day is of general interest, as it produces chlorine (Cl) atoms that react with different atmospheric trace gases to form radicals. The highest-observed ClNO2 mixing ratio
was 1.6 ppbv (parts per billion by volume; 15 min average) during the night of 20 September. Air masses reaching the measurement site either originated from long-range transport from the southwest and had an oceanic influence or circulated in the nearby region and were influenced by anthropogenic activities. Nocturnal maximum ClNO2 mixing ratios were around 0.2 ppbv if originating from long-range transport in nearly all seasons, while the values were higher, ranging from 0.4 to 0.6 ppbv for regionally influenced air. The chemical composition of long-range transported air was similar in all investigated seasons, while the regional air exhibited larger differences between the seasons. The N2O5 necessary for ClNO2 formation comes from the reaction of nitrate radicals (NO3) with nitrogen dioxide (NO2), where NO3
itself is formed by a reaction of NO2 with ozone (O3). Measured
concentrations of ClNO2, NO2, and O3 were used to quantify
ClNO2 production efficiencies, i.e., the yield of ClNO2 formation
per NO3 radical formed, and a box model was used to examine the
idealized dependence of ClNO2 on the observed nocturnal O3 and
NO2 concentrations. Results indicate that ClNO2 production
efficiency was most sensitive to the availability of NO2 rather than
that of O3 and increased with decreasing temperature. The average
ClNO2 production efficiency was highest in February and September, with values of 18 %, and was lowest in December, with values of 3 %. The average ClNO2 production efficiencies were in the range of 3 % and 6 % from August to November for air masses originating from long-range transportation. These numbers are at the high end of values reported in the literature, indicating the importance of ClNO2 chemistry in rural environments in midwestern Europe.
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.
The photooxidation of pinonaldehyde, one product of the α-pinene degradation, was investigated in the atmospheric simulation chamber SAPHIR under natural sunlight at low NO concentrations (<0.2 ppbv) ...with and without an added hydroxyl radical (OH) scavenger. With a scavenger, pinonaldehyde was exclusively removed by photolysis, whereas without a scavenger, the degradation was dominated by reaction with OH. In both cases, the observed rate of pinonaldehyde consumption was faster than predicted by an explicit chemical model, the Master Chemical Mechanism (MCM, version 3.3.1). In the case with an OH scavenger, the observed photolytic decay can be reproduced by the model if an experimentally determined photolysis frequency is used instead of the parameterization in the MCM. A good fit is obtained when the photolysis frequency is calculated from the measured solar actinic flux spectrum, absorption cross sections published by Hallquist et al. (1997), and an effective quantum yield of 0.9. The resulting photolysis frequency is 3.5 times faster than the parameterization in the MCM. When pinonaldehyde is mainly removed by reaction with OH, the observed OH and hydroperoxy radical (HO2) concentrations are underestimated in the model by a factor of 2.
Using measured HO2 as a model constraint brings modeled and measured OH concentrations into agreement. This suggests that the chemical mechanism includes all relevant OH-producing reactions but is missing a source for HO2. The missing HO2 source strength of (0.8 to 1.5) ppbv h−1 is similar to the rate of the pinonaldehyde consumption of up to 2.5 ppbv h−1. When the model is constrained by HO2 concentrations and the experimentally derived photolysis frequency, the pinonaldehyde decay is well represented. The photolysis of pinonaldehyde yields 0.18 ± 0.20 formaldehyde molecules at NO concentrations of less than 200 pptv, but no significant acetone formation is observed. When pinonaldehyde is also oxidized by OH under low NO conditions (maximum 80 pptv), yields of acetone and formaldehyde increase over the course of the experiment from 0.2 to 0.3 and from 0.15 to 0.45, respectively. Fantechi et al. (2002) proposed a degradation mechanism based on quantum-chemical calculations, which is considerably more complex than the MCM scheme and contains additional reaction pathways and products. Implementing these modifications results in a closure of the model–measurement discrepancy for the products acetone and formaldehyde, when pinonaldehyde is degraded only by photolysis. In contrast, the underprediction of formed acetone and formaldehyde is worsened compared to model results by the MCM, when pinonaldehyde is mainly degraded in the reaction with OH. This shows that the current mechanisms lack acetone and formaldehyde sources for low NO conditions like in these experiments. Implementing the modifications suggested by Fantechi et al. (2002) does not improve the model–measurement agreement of OH and HO2.
The oxidation of Δ3-carene and one of its main oxidation products, caronaldehyde, by the OH radical and O3 was investigated in the atmospheric simulation chamber SAPHIR under atmospheric conditions ...for NOx mixing ratios below 2 ppbv. Within this study, the rate constants of the reaction of Δ3-carene with OH and O3 and of the reaction of caronaldehyde with OH were determined to be (8.0±0.5)×10-11 cm3s-1 at 304 K, (4.4±0.2)×10-17 cm3s-1 at 300 K and (4.6±1.6)×10-11 cm3s-1 at 300 K, in agreement with previously published values. The yields of caronaldehyde from the reaction of OH and ozone with Δ3-carene were determined to be 0.30±0.05 and 0.06±0.02, respectively. Both values are in reasonably good agreement with reported literature values. An organic nitrate (RONO2) yield from the reaction of NO with RO2 derived from Δ3-carene of 0.25±0.04 was determined from the analysis of the reactive nitrogen species (NOy) in the SAPHIR chamber. The RONO2 yield of the reaction of NO with RO2 derived from the reaction of caronaldehyde with OH was found to be 0.10±0.02. The organic nitrate yields of Δ3-carene and caronaldehyde oxidation with OH are reported here for the first time in the gas phase. An OH yield of 0.65±0.10 was determined from the ozonolysis of Δ3-carene. Calculations of production and destruction rates of the sum of hydroxyl and peroxy radicals (ROx=OH+HO2+RO2) demonstrated that there were no unaccounted production or loss processes of radicals in the oxidation of Δ3-carene for conditions of the chamber experiments. In an OH-free experiment with added OH scavenger, the photolysis frequency of caronaldehyde was obtained from its photolytical decay. The experimental photolysis frequency was a factor of 7 higher than the value calculated from the measured solar actinic flux density, an absorption cross section from the literature and an assumed effective quantum yield of unity for photodissociation.
Photochemical processes in ambient air were studied using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient air was continuously drawn into the chamber through a ...50 m high inlet line and passed through the chamber for 1 month in each season throughout 2019. The residence time of the air inside the chamber was about 1 h. As the research center is surrounded by a mixed deciduous forest and is located close to the city Jülich, the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements of hydroxyl (OH), hydroperoxyl (HO2), and organic peroxy (RO2) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (kOH, the inverse of the OH lifetime) and a comprehensive set of trace gas concentrations and aerosol properties allowed for the investigation of the seasonal and diurnal variation of radical production and destruction pathways. In spring and summer periods, median OH concentrations reached 6 × 106 cm-3 at noon, and median concentrations of both HO2 and RO2 radicals were 3 × 108 cm-3. The measured OH reactivity was between 4 and 18 s-1 in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (Ox= NO2+ O3) determined from NO2 and O3 concentration measurements. The chemical budgets of radicals were analyzed for the spring and summer seasons, when peroxy radical concentrations were above the detection limit. For most conditions, the concentrations of radicals were mainly sustained by the regeneration of OH via reactions of HO2 and RO2 radicals with nitric oxide (NO). The median diurnal profiles of the total radical production and destruction rates showed maxima between 3 and 6 ppbv h-1 for OH, HO2, and RO2. Total ROX (OH, HO2, and RO2) initiation and termination rates were below 3 ppbv h-1. The highest OH radical turnover rate of 13 ppbv h-1 was observed during a high-temperature (max. 40 ∘C) period in August. In this period, the highest HO2, RO2, and ROX turnover rates were around 11, 10, and 4 ppbv h-1, respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO2 production and destruction rates were balanced, but unexplained RO2 and ROX production reactions with median rates of 2 and 0.4 ppbv h-1, respectively, were required to balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy radical reaction rates with NO were highly uncertain due to the low peroxy radical concentrations close to the limit of NO interferences in the HO2 and RO2 measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO2 were found with maximum rates of 3.0 and 2.0 ppbv h-1, respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO2 to OH conversion reaction (up to 2 ppbv h-1) and a missing primary radical source (0.5–1.4 ppbv h-1). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO2 isomerization reactions from isoprene, HO2 uptake on aerosol, and RO2 production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study.
Besides isoprene, monoterpenes are the non-methane volatile organic compounds (VOCs) with the highest global emission rates. Due to their high reactivity towards OH, monoterpenes can dominate the ...radical chemistry of the atmosphere in forested areas. In the present study the photochemical degradation mechanism of β-pinene was investigated in the Jülich atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). One focus of this study is on the OH budget in the degradation process. Therefore, the SAPHIR chamber was equipped with instrumentation to measure radicals (OH, HO2, RO2), the total OH reactivity, important OH precursors (O3, HONO, HCHO), the parent VOC β-pinene, its main oxidation products, acetone and nopinone and photolysis frequencies. All experiments were carried out under low-NO conditions ( ≤ 300 ppt) and at atmospheric β-pinene concentrations ( ≤ 5 ppb) with and without addition of ozone. For the investigation of the OH budget, the OH production and destruction rates were calculated from measured quantities. Within the limits of accuracy of the instruments, the OH budget was balanced in all β-pinene oxidation experiments. However, even though the OH budget was closed, simulation results from the Master Chemical Mechanism (MCM) 3.2 showed that the OH production and destruction rates were underestimated by the model. The measured OH and HO2 concentrations were underestimated by up to a factor of 2, whereas the total OH reactivity was slightly overestimated because the model predicted a nopinone mixing ratio which was 3 times higher than measured. A new, theory-derived, first-generation product distribution by Vereecken and Peeters (2012) was able to reproduce the measured nopinone time series and the total OH reactivity. Nevertheless, the measured OH and HO2 concentrations remained underestimated by the numerical simulations. These observations together with the fact that the measured OH budget was closed suggest the existence of unaccounted sources of HO2. Although the mechanism of additional HO2 formation could not be resolved, our model studies suggest that an activated alkoxy radical intermediate proposed in the model of Vereecken and Peeters (2012) generates HO2 in a new pathway, whose importance has been underestimated so far. The proposed reaction path involves unimolecular rearrangement and decomposition reactions and photolysis of dicarbonyl products, yielding additional HO2 and CO. Further experiments and quantum chemical calculations have to be made to completely unravel the pathway of HO2 formation.
The photo-oxidation of myrcene, a monoterpene species emitted by plants, was
investigated at atmospheric conditions in the outdoor simulation chamber
SAPHIR (Simulation of Atmospheric
PHotochemistry ...In a Large Reaction Chamber). The chemical structure of myrcene consists of one moiety that is a
conjugated π system (similar to isoprene) and another moiety that is a
triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen
shift reactions of organic peroxy radicals (RO2) formed in the
reaction of isoprene with atmospheric OH radicals are known to be of
importance for the regeneration of OH. Structure–activity relationships
(SARs) suggest that similar hydrogen shift reactions like in isoprene may apply
to the isoprenyl part of RO2 radicals formed during the OH oxidation
of myrcene. In addition, SAR predicts further isomerization reactions that
would be competitive with bimolecular RO2 reactions for chemical
conditions that are typical for forested environments with low concentrations
of nitric oxide. Assuming that OH peroxy radicals can rapidly
interconvert by addition and elimination of O2 like in isoprene, bulk
isomerization rate constants of 0.21 and 0.097 s−1
(T=298 K) for the three isomers resulting from the 3′-OH and
1-OH addition, respectively, can be derived from SAR. Measurements of
radicals and trace gases in the experiments allowed us to calculate radical
production and destruction rates, which are expected to be balanced. The largest
discrepancies between production and destruction rates were found for
RO2. Additional loss of organic peroxy radicals due to isomerization
reactions could explain the observed discrepancies. The uncertainty of the
total radical (ROx=OH+HO2+RO2) production rates was high
due to the uncertainty in the yield of radicals from myrcene
ozonolysis. However, results indicate that radical production can only be
balanced if the reaction rate constant of the reaction between hydroperoxy
(HO2) and RO2 radicals derived from myrcene is lower (0.9 to
1.6×10-11 cm3 s−1) than predicted by SAR. Another
explanation of the discrepancies would be that a significant fraction of
products (yield: 0.3 to 0.6) from these reactions include OH and
HO2 radicals instead of radical-terminating organic
peroxides. Experiments also allowed us to determine the yields of organic
oxidation products acetone (yield: 0.45±0.08) and formaldehyde (yield:
0.35±0.08). Acetone and formaldehyde are produced from different oxidation
pathways, so that yields of these compounds reflect the branching ratios of
the initial OH addition to myrcene. Yields determined in the
experiments are consistent with branching ratios expected from SAR. The yield
of organic nitrate was determined from the gas-phase budget analysis of
reactive oxidized nitrogen in the chamber, giving a value of 0.13±0.03. In
addition, the reaction rate constant for myrcene + OH was determined
from the measured myrcene concentration, yielding a value of (2.3±0.3)×10-10 cm3 s−1.
Hydroxyl and hydroperoxy radicals are key species for the understanding of
atmospheric oxidation processes. Their measurement is challenging due to
their high reactivity; therefore, very sensitive ...detection methods are needed.
Within this study, the measurement of hydroperoxy radicals (HO2)
using chemical ionisation combined with a high-resolution time-of-flight
mass spectrometer (Aerodyne Research Inc.) employing bromide as the primary ion
is presented. The sensitivity reached is equal to 0.005×108 HO2 cm−3 for 106 cps of bromide and 60 s of integration time, which is
below typical HO2 concentrations found in the atmosphere. The
detection sensitivity of the instrument is affected by the presence of water
vapour. Therefore, a water-vapour-dependent calibration factor that decreases
approximately by a factor of 2 if the water vapour mixing ratio increases from
0.1 % to 1.0 % needs to be applied. An instrumental background, most
likely generated by the ion source that is equivalent to a HO2
concentration of (1.5±0.2)×108 molecules cm−3, is
subtracted to derive atmospheric HO2 concentrations. This background
can be determined by overflowing the inlet with zero air. Several experiments
were performed in the atmospheric simulation chamber SAPHIR at the
Forschungszentrum Jülich to test the instrument performance in comparison
to the well-established laser-induced fluorescence (LIF) technique for
measurements of HO2. A highly linear correlation coefficient of
R2=0.87 is achieved. The slope of the linear regression of 1.07
demonstrates the good absolute agreement of both measurements. Chemical
conditions during experiments allowed for testing the instrument's behaviour in
the presence of atmospheric concentrations of H2O, NOx, and
O3. No significant interferences from these species were observed.
All of these facts demonstrate a reliable measurement of HO2 by
the chemical ionisation mass spectrometer presented.
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