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.
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
(O3) and hydroxyl radical (OH), HOM formation in the oxidation by
nitrate radical (NO3), an important oxidant at nighttime and dawn, has received less attention. In this study, HOM formation in the reaction of
isoprene with NO3 was investigated in the SAPHIR chamber (Simulation of
Atmospheric PHotochemistry In a large Reaction chamber). A large number of
HOM, including monomers (C5), dimers (C10), and trimers (C15), both closed-shell compounds and open-shell peroxy radicals (RO2), 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 NO3 addition to carbon double bond, forming peroxy radicals, followed by autoxidation. 1N-monomers were formed by both the direct reaction of
NO3 with isoprene and of NO3 with first-generation products.
2N-monomers (e.g., C5H8N2On(n=7–13), C5H10N2On(n=8–14)) were likely the termination products of C5H9N2On⚫, which was formed by
the addition of NO3 to C5-hydroxynitrate (C5H9NO4), a
first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for ∼34 % of all HOM, indicating the important role of
second-generation oxidation in HOM formation in the isoprene + NO3
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 RO2 and
via the termination reactions of dimer RO2 formed by further reaction
of closed-shell dimers with NO3 and possibly by the reaction of
C5–RO2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO2 with monomer RO2. 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
∼3.6 % assuming the molecular weight of
C5H9NO6 as the lower limit. This yield suggests that HOM may
contribute a significant fraction to SOA yield in the reaction of isoprene
with NO3.
The formation of organic nitrates (ONs) in the gas phase and their impact on mass formation of secondary organic aerosol (SOA) was investigated in a laboratory study for α-pinene and β-pinene ...photooxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by NOx, and therein the role of highly oxygenated multifunctional molecules (HOMs). We observed that with increasing NOx concentration (a) the portion of HOM
organic nitrates (HOM-ONs) increased, (b) the fraction of accretion products (HOM-ACCs) decreased, and (c) HOM-ACCs contained on average smaller carbon numbers. Specifically, we investigated HOM organic nitrates (HOM-ONs), arising from
the termination reactions of HOM peroxy radicals with NOx, and HOM permutation products (HOM-PPs), such as ketones, alcohols, or hydroperoxides, formed by other termination reactions. Effective uptake coefficients γeff of HOMs on particles were determined. HOMs with more than six O atoms efficiently condensed on particles (γeff>0.5 on average), and for HOMs containing more than eight O atoms, every collision led to loss. There was no systematic difference in γeff for HOM-ONs and HOM-PPs arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOMs: as functional groups in HOMs arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character of the final termination group. As a consequence, the suppressing effect of NOx on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups. According to their γeff all HOM-ONs with more than six O atoms will contribute to organic bound nitrate (OrgNO3) in the particulate phase. However, the fraction of OrgNO3 stored in condensable HOMs with molecular masses > 230 Da appeared to be substantially higher than the fraction of particulate OrgNO3 observed by aerosol mass spectrometry. This result suggests losses of OrgNO3 for organic nitrates in particles, probably due to hydrolysis of OrgNO3 that releases HNO3 into the gas phase but leaves behind the organic rest in the particulate phase. However, the loss of HNO3 alone could not explain the observed suppressing effect of NOx on particle mass formation from α-pinene and β-pinene. Instead we can attribute most of the reduction in SOA mass yields with
increasing NOx to the significant suppression of gas phase HOM-ACCs, which have high molecular mass and are potentially important for SOA mass formation at low-NOx conditions.
Aromatic hydrocarbons are a class of volatile organic compounds associated with anthropogenic activity and make up a significant fraction of urban volatile organic compound (VOC) emissions that ...contribute to the formation of secondary organic aerosol (SOA). Benzene is one of the most abundant species emitted from vehicles, biomass burning and industry. An iodide time-of-flight chemical ionisation mass spectrometer (ToF-CIMS) and nitrate ToF-CIMS were deployed at the Jülich Plant Atmosphere Chamber as part of a series of experiments examining benzene oxidation by OH under high- and low-NO.sub.x conditions, where a range of organic oxidation products were detected. The nitrate scheme detects many oxidation products with high masses, ranging from intermediate volatile organic compounds (IVOCs) to extremely low volatile organic compounds (ELVOCs), including C.sub.12 dimers. In comparison, very few species with C.sub.â¥6 and O.sub.â¥8 were detected with the iodide scheme, which detected many more IVOCs and semi-volatile organic compounds (SVOCs) but very few ELVOCs and low volatile organic compounds (LVOCs). A total of 132 and 195 CHO and CHON oxidation products are detected by the iodide ToF-CIMS in the low- and high-NO.sub.x experiments respectively. Ring-breaking products make up the dominant fraction of detected signal and 21 and 26 of the products listed in the Master Chemical Mechanism (MCM) were detected. The time series of highly oxidised (O.sub.â¥6) and ring-retaining oxidation products (C.sub.6 and double-bond equivalent = 4) equilibrate quickly, characterised by a square form profile, compared to MCM and ring-breaking products which increase throughout oxidation, exhibiting sawtooth profiles. Under low-NO.sub.x conditions, all CHO formulae attributed to radical termination reactions of first-generation benzene products, and first-generation auto-oxidation products are observed. Several N-containing species that are either first-generation benzene products or first-generation auto-oxidation products are also observed under high-NO.sub.x conditions. Hierarchical cluster analysis finds four clusters, of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO2/NOx ratio, indicating it is sensitive to NO concentration and thus likely to contain NO addition products and alkoxy-derived termination products. This cluster has the highest average carbon oxidation state (OSC-) and the lowest average carbon number. Where nitrogen is present in a cluster member of cluster 2, the oxygen number is even, as expected for alkoxy-derived products. In contrast, cluster 1 shows no dependency on the NO2/NOx ratio and so is likely to contain more NO.sub.2 addition and peroxy-derived termination products. This cluster contains fewer fragmented species, as the average carbon number is higher and OSC- lower than cluster 2, and more species with an odd number of oxygen atoms. This suggests that clustering of time series which have features pertaining to distinct chemical regimes, for example, NO2/NOx perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.
Aromatic hydrocarbons are a class of volatile organic compounds associated
with anthropogenic activity and make up a significant fraction of urban volatile organic compound (VOC)
emissions that ...contribute to the formation of secondary organic aerosol
(SOA). Benzene is one of the most abundant species emitted from vehicles,
biomass burning and industry. An iodide time-of-flight chemical ionisation
mass spectrometer (ToF-CIMS) and nitrate ToF-CIMS were deployed at the
Jülich Plant Atmosphere Chamber as part of a series of experiments
examining benzene oxidation by OH under high- and low-NOx conditions,
where a range of organic oxidation products were detected. The nitrate
scheme detects many oxidation products with high masses, ranging from
intermediate volatile organic compounds (IVOCs) to extremely low volatile
organic compounds (ELVOCs), including C12 dimers. In comparison, very
few species with C≥6 and O≥8 were detected with
the iodide scheme, which detected many more IVOCs and semi-volatile organic
compounds (SVOCs) but very few ELVOCs and low volatile organic compounds
(LVOCs). A total of 132 and 195 CHO and CHON oxidation products are detected by the
iodide ToF-CIMS in the low- and high-NOx experiments respectively. Ring-breaking products make up the dominant fraction of detected signal and 21
and 26 of the products listed in the Master Chemical Mechanism (MCM) were
detected. The time series of highly oxidised (O≥6) and ring-retaining oxidation products (C6 and double-bond equivalent = 4)
equilibrate quickly, characterised by a square form profile, compared to MCM
and ring-breaking products which increase throughout oxidation, exhibiting
sawtooth profiles. Under low-NOx conditions, all CHO formulae
attributed to radical termination reactions of first-generation benzene
products, and first-generation auto-oxidation products are observed. Several
N-containing species that are either first-generation benzene products or
first-generation auto-oxidation products are also observed under high-NOx conditions. Hierarchical cluster analysis finds four clusters, of
which two describe photo-oxidation. Cluster 2 shows a negative dependency on
the NO2/NOx ratio, indicating it is sensitive to NO concentration
and thus likely to contain NO addition products and alkoxy-derived
termination products. This cluster has the highest average carbon oxidation
state (OSC‾) and the lowest average carbon number.
Where nitrogen is present in a cluster member of cluster 2, the oxygen
number is even, as expected for alkoxy-derived products. In contrast,
cluster 1 shows no dependency on the NO2/NOx ratio and so is
likely to contain more NO2 addition and peroxy-derived termination
products. This cluster contains fewer fragmented species, as the average
carbon number is higher and OSC‾ lower than
cluster 2, and more species with an odd number of oxygen atoms. This
suggests that clustering of time series which have features pertaining to
distinct chemical regimes, for example, NO2/NOx perturbations, coupled
with a priori knowledge, can provide insight into identification of potential
functionality.
PDF