An updated and expanded representation of organics in the chemistry general circulation model EMAC
(ECHAM5/MESSy for Atmospheric Chemistry) has been evaluated. First, the
comprehensive Mainz Organic ...Mechanism (MOM)
in the submodel MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere) was activated with
explicit degradation of organic species up to five carbon atoms and a
simplified mechanism for larger molecules. Second, the ORACLE submodel (version 1.0)
now considers condensation on aerosols for all
organics in the mechanism. Parameterizations for aerosol yields are used only for the lumped species
that are not included in the explicit mechanism.
The simultaneous usage of MOM and ORACLE
allows an efficient estimation of not only the chemical degradation of
the simulated volatile organic compounds but also the contribution
of organics to the growth and fate of (organic) aerosol,
with the complexity of the mechanism largely increased
compared to EMAC simulations with more simplified chemistry.
The model evaluation presented here reveals that the OH concentration is reproduced well globally,
whereas significant biases for observed oxygenated organics are present.
We also investigate the general properties
of the aerosols and their composition, showing that the more
sophisticated and process-oriented secondary aerosol formation does not
degrade the good agreement of previous model configurations with observations at the surface,
allowing further research in the field of gas–aerosol interactions.
Condensation of carboxylic acids on mineral particles leads to coatings and impacts the particles' potential to act as cloud condensation nuclei (CCN). To determine how the CCN activity of mineral ...particles is impacted by carboxylic acid coatings, the CCN activities of CaCO3 particles and CaCO3 particles with oleic acid and malonic acid coatings were compared in this study. The results revealed that small amounts of oleic acid coating (volume fraction (vf) ≤4.3 %) decreased the CCN activity of CaCO3 particles, while more oleic acid coating (vf ≥16 %) increased the CCN activity of CaCO3 particles. This phenomenon has not been reported before. In contrast, the CCN activity of CaCO3 particles coated with malonic acid increased with the thickness of the malonic acid coating (vf =0.4–40 %). Even the smallest amounts of malonic acid coating (vf =0.4 %) significantly enhanced the CCN activity of CaCO3 particles from κ=0.0028±0.0001 to κ=0.0123±0.0005. This indicates that a small amount of water-soluble organic acid coating may significantly enhance the CCN activity of mineral particles. The presence of water vapor during the coating process with malonic acid additionally increased the CCN activity of the coated CaCO3 particles, probably because more CaCO3 reacts with malonic acid when sufficient water is available.
Secondary organic aerosols (SOAs) play a key role in climate change and air
quality. Determining the fundamental parameters that distribute organic
compounds between the phases is essential, as ...atmospheric lifetime and
impacts change drastically between the gas and particle phase. In this work,
gas-to-particle partitioning of major biogenic oxidation products was
investigated using three different aerosol chemical characterization
techniques. The aerosol collection module, the collection thermal desorption unit, and the
chemical analysis of aerosols online are different aerosol sampling inlets connected to a proton-transfer reaction time-of-flight
mass spectrometer (ACM-PTR-ToF-MS, TD-PTR-ToF-MS, and CHARON-PTR-ToF-MS, respectively, referred to hereafter as
ACM, TD, and CHARON). These techniques
were deployed at the atmosphere simulation chamber SAPHIR to perform
experiments on the SOA formation and aging from different monoterpenes
(β-pinene, limonene) and real plant emissions (Pinus sylvestris L.). The saturation mass
concentration C* and thus the volatility of the individual ions was
determined based on the simultaneous measurement of their signal in the gas and particle phase. A method to identify and exclude ions affected by thermal dissociation
during desorption and ionic dissociation in the ionization chamber of the
proton-transfer reaction mass spectrometer (PTR-MS) was developed and tested for each technique. Narrow volatility
distributions with organic compounds in the semi-volatile (SVOCs – semi-volatile
organic compounds) to
intermediate-volatility (IVOCs – intermediate-volatility organic compounds) regime were found for all systems studied.
Despite significant differences in the aerosol collection and desorption
methods of the proton-transfer-reaction (PTR)-based techniques, a comparison of the C* values obtained
with different techniques was found to be in good agreement (within 1 order
of magnitude) with deviations explained by the different operating
conditions of the PTR-MS. The C* of the identified organic compounds were mapped onto the
two-dimensional volatility basis set (2D-VBS), and results showed a decrease in C* with increasing oxidation state. For all experiments conducted in
this study, identified partitioning organic compounds accounted for
20–30 % of the total organic mass measured from an aerosol mass spectrometer (AMS). Further
comparison between observations and theoretical calculations was performed
for species found in our experiments that were also identified in previous
publications. Theoretical calculations based on the molecular structure of
the compounds showed, within the uncertainties ranges, good agreement with
the experimental C* for most SVOCs, while IVOCs deviated by up to a factor of
300. These latter differences are discussed in relation to two main
processes affecting these systems: (i) possible interferences by thermal and
ionic fragmentation of higher molecular-weight compounds, produced by
accretion and oligomerization reactions, that fragment in the m∕z range
detected by the PTR-MS and (ii) kinetic influences in the distribution
between the gas and particle phase with gas-phase condensation, diffusion in
the particle phase, and irreversible uptake.
The gas-phase reaction of isoprene with the nitrate radical (NO.sub.3) was investigated in experiments in the outdoor SAPHIR chamber under atmospherically relevant conditions specifically with ...respect to the chemical lifetime and fate of nitrato-organic peroxy radicals (RO.sub.2). Observations of organic products were compared to concentrations expected from different chemical mechanisms: (1) the Master Chemical Mechanism, which simplifies the NO.sub.3 isoprene chemistry by only considering one RO.sub.2 isomer; (2) the chemical mechanism derived from experiments in the Caltech chamber, which considers different RO.sub.2 isomers; and (3) the FZJ-NO3 isoprene mechanism derived from quantum chemical calculations, which in addition to the Caltech mechanism includes equilibrium reactions of RO.sub.2 isomers, unimolecular reactions of nitrate RO.sub.2 radicals and epoxidation reactions of nitrate alkoxy radicals. Measurements using mass spectrometer instruments give evidence that the new reactions pathways predicted by quantum chemical calculations play a role in the NO.sub.3 oxidation of isoprene. Hydroperoxy aldehyde (HPALD) species, which are specific to unimolecular reactions of nitrate RO.sub.2, were detected even in the presence of an OH scavenger, excluding the possibility that concurrent oxidation by hydroxyl radicals (OH) is responsible for their formation. In addition, ion signals at masses that can be attributed to epoxy compounds, which are specific to the epoxidation reaction of nitrate alkoxy radicals, were detected. Measurements of methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations confirm that the decomposition of nitrate alkoxy radicals implemented in the Caltech mechanism cannot compete with the ring-closure reactions predicted by quantum chemical calculations. The validity of the FZJ-NO3 isoprene mechanism is further supported by a good agreement between measured and simulated hydroxyl radical (OH) reactivity. Nevertheless, the FZJ-NO3 isoprene mechanism needs further investigations with respect to the absolute importance of unimolecular reactions of nitrate RO.sub.2 and epoxidation reactions of nitrate alkoxy radicals. Absolute concentrations of specific organic nitrates such as nitrate hydroperoxides would be required to experimentally determine product yields and branching ratios of reactions but could not be measured in the chamber experiments due to the lack of calibration standards for these compounds. The temporal evolution of mass traces attributed to product species such as nitrate hydroperoxides, nitrate carbonyl and nitrate alcohols as well as hydroperoxy aldehydes observed by the mass spectrometer instruments demonstrates that further oxidation by the nitrate radical and ozone at atmospheric concentrations is small on the timescale of one night (12 h) for typical oxidant concentrations. However, oxidation by hydroxyl radicals present at night and potentially also produced from the decomposition of nitrate alkoxy radicals can contribute to their nocturnal chemical loss.
The gas-phase reaction of isoprene with the nitrate radical (NO3) was investigated in experiments in the outdoor SAPHIR chamber under atmospherically relevant conditions specifically with respect to ...the chemical lifetime and fate of nitrato-organic peroxy radicals (RO2). Observations of organic products were compared to concentrations expected from different chemical mechanisms: (1) the Master Chemical Mechanism, which simplifies the NO3 isoprene chemistry by only considering one RO2 isomer; (2) the chemical mechanism derived from experiments in the Caltech chamber, which considers different RO2 isomers; and (3) the FZJ-NO3 isoprene mechanism derived from quantum chemical calculations, which in addition to the Caltech mechanism includes equilibrium reactions of RO2 isomers, unimolecular reactions of nitrate RO2 radicals and epoxidation reactions of nitrate alkoxy radicals. Measurements using mass spectrometer instruments give evidence that the new reactions pathways predicted by quantum chemical calculations play a role in the NO3 oxidation of isoprene. Hydroperoxy aldehyde (HPALD) species, which are specific to unimolecular reactions of nitrate RO2, were detected even in the presence of an OH scavenger, excluding the possibility that concurrent oxidation by hydroxyl radicals (OH) is responsible for their formation. In addition, ion signals at masses that can be attributed to epoxy compounds, which are specific to the epoxidation reaction of nitrate alkoxy radicals, were detected. Measurements of methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations confirm that the decomposition of nitrate alkoxy radicals implemented in the Caltech mechanism cannot compete with the ring-closure reactions predicted by quantum chemical calculations. The validity of the FZJ-NO3 isoprene mechanism is further supported by a good agreement between measured and simulated hydroxyl radical (OH) reactivity. Nevertheless, the FZJ-NO3 isoprene mechanism needs further investigations with respect to the absolute importance of unimolecular reactions of nitrate RO2 and epoxidation reactions of nitrate alkoxy radicals. Absolute concentrations of specific organic nitrates such as nitrate hydroperoxides would be required to experimentally determine product yields and branching ratios of reactions but could not be measured in the chamber experiments due to the lack of calibration standards for these compounds. The temporal evolution of mass traces attributed to product species such as nitrate hydroperoxides, nitrate carbonyl and nitrate alcohols as well as hydroperoxy aldehydes observed by the mass spectrometer instruments demonstrates that further oxidation by the nitrate radical and ozone at atmospheric concentrations is small on the timescale of one night (12 h) for typical oxidant concentrations. However, oxidation by hydroxyl radicals present at night and potentially also produced from the decomposition of nitrate alkoxy radicals can contribute to their nocturnal chemical loss.
The composition of secondary organic aerosols (SOAs) formed by β-pinene ozonolysis was experimentally investigated in the Juelich aerosol chamber. Partitioning of oxidation products between gas and ...particles was measured through concurrent concentration measurements in both phases. Partitioning coefficients (
K
p
) of 2.23 × 10
−5
± 3.20 × 10
−6
m
3
μg
−1
for nopinone, 4.86 × 10
−4
± 1.80 × 10
−4
m
3
μg
−1
for apoverbenone, 6.84 × 10
−4
± 1.52 × 10
−4
m
3
μg
−1
for oxonopinone and 2.00 × 10
−3
± 1.13 × 10
−3
m
3
μg
−1
for hydroxynopinone were derived, showing higher values for more oxygenated species. The observed
K
p
values were compared with values predicted using two different semi-empirical approaches. Both methods led to an underestimation of the partitioning coefficients with systematic differences between the methods. Assuming that the deviation between the experiment and the model is due to non-ideality of the mixed solution in particles, activity coefficients of 4.82 × 10
−2
for nopinone, 2.17 × 10
−3
for apoverbenone, 3.09 × 10
−1
for oxonopinone and 7.74 × 10
−1
for hydroxynopinone would result using the vapour pressure estimation technique that leads to higher
K
p
. We discuss that such large non-ideality for nopinone could arise due to particle phase processes lowering the effective nopinone vapour pressure such as diol- or dimer formation. The observed high partitioning coefficients compared to modelled results imply an underestimation of SOA mass by applying equilibrium conditions.
Measured particle phase concentrations of semi-volatile organic compounds exceed those predicted by absorption equilibrium gas-particle partitioning by orders of magnitude.
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 (HO.sub.2 ), and organic peroxy (RO.sub.2) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (k.sub.OH, 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 x 10.sup.6 cm.sup.-3 at noon, and median concentrations of both HO.sub.2 and RO.sub.2 radicals were 3 x 10.sup.8 cm.sup.-3 . The measured OH reactivity was between 4 and 18 s.sup.-1 in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (O.sub.x = NO.sub.2 + O.sub.3) determined from NO.sub.2 and O.sub.3 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 HO.sub.2 and RO.sub.2 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.sup.-1 for OH, HO.sub.2, and RO.sub.2 . Total RO.sub.X (OH, HO.sub.2, and RO.sub.2) initiation and termination rates were below 3 ppbv h.sup.-1 . The highest OH radical turnover rate of 13 ppbv h.sup.-1 was observed during a high-temperature (max. 40 .sup." C) period in August. In this period, the highest HO.sub.2, RO.sub.2, and RO.sub.X turnover rates were around 11, 10, and 4 ppbv h.sup.-1, respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO.sub.2 production and destruction rates were balanced, but unexplained RO.sub.2 and RO.sub.X production reactions with median rates of 2 and 0.4 ppbv h.sup.-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 HO.sub.2 and RO.sub.2 measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO.sub.2 were found with maximum rates of 3.0 and 2.0 ppbv h.sup.-1, respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO.sub.2 to OH conversion reaction (up to 2 ppbv h.sup.-1) and a missing primary radical source (0.5-1.4 ppbv h.sup.-1). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO.sub.2 isomerization reactions from isoprene, HO.sub.2 uptake on aerosol, and RO.sub.2 production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study.
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.
Highly oxygenated organic molecules (HOM) are found to play an important role in the formation and growth of secondary organic aerosol (SOA). SOA is an important type of aerosol with significant ...impact on air quality and climate. Compared with the oxidation of volatile organic compounds by ozone (O.sub.3) and hydroxyl radical (OH), HOM formation in the oxidation by nitrate radical (NO.sub.3 ), an important oxidant at nighttime and dawn, has received less attention. In this study, HOM formation in the reaction of isoprene with NO.sub.3 was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). A large number of HOM, including monomers (C.sub.5 ), dimers (C.sub.10 ), and trimers (C.sub.15 ), both closed-shell compounds and open-shell peroxy radicals (RO.sub.2 ), were identified and were classified into various series according to their formula. Their formation pathways were proposed based on the peroxy radicals observed and known mechanisms in the literature, which were further constrained by the time profiles of HOM after sequential isoprene addition to differentiate first- and second-generation products. HOM monomers containing one to three N atoms (1-3N-monomers) were formed, starting with NO.sub.3 addition to carbon double bond, forming peroxy radicals, followed by autoxidation. 1N-monomers were formed by both the direct reaction of NO.sub.3 with isoprene and of NO.sub.3 with first-generation products. 2N-monomers (e.g., C.sub.5 H.sub.8 N.sub.2 O.sub.n(n=7-13), C.sub.5 H.sub.10 N.sub.2 O.sub.n(n=8-14)) were likely the termination products of C.sub.5 H.sub.9 N.sub.2 O.sub.n â«, which was formed by the addition of NO.sub.3 to C5-hydroxynitrate (C.sub.5 H.sub.9 NO.sub.4 ), a first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for â¼34 % of all HOM, indicating the important role of second-generation oxidation in HOM formation in the isoprene + NO.sub.3 reaction under our experimental conditions. H shift of alkoxy radicals to form peroxy radicals and subsequent autoxidation ("alkoxy-peroxy" pathway) was found to be an important pathway of HOM formation. HOM dimers were mostly formed by the accretion reaction of various HOM monomer RO.sub.2 and via the termination reactions of dimer RO.sub.2 formed by further reaction of closed-shell dimers with NO.sub.3 and possibly by the reaction of C5-RO.sub.2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO.sub.2 with monomer RO.sub.2 . The concentrations of different HOM showed distinct time profiles during the reaction, which was linked to their formation pathway. HOM concentrations either showed a typical time profile of first-generation products, second-generation products, or a combination of both, indicating multiple formation pathways and/or multiple isomers. Total HOM molar yield was estimated to be 1.2 %-0.7%+1.3%, which corresponded to a SOA yield of â¼3.6 % assuming the molecular weight of C.sub.5 H.sub.9 NO.sub.6 as the lower limit. This yield suggests that HOM may contribute a significant fraction to SOA yield in the reaction of isoprene with NO.sub.3.
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