Parameterising secondary organic aerosol from α-pinene using a detailed oxidation and aerosol formation model

Abstract. A new parameter model for α-pinene secondary organic aerosol (SOA) is presented, based on simulations with the detailed model BOREAM (Biogenic hydrocarbon Oxidation and Related Aerosol formation Model). The parameterisation takes into account the influence of temperature, type of oxidant, NOx-regime, photochemical ageing and water uptake, and is suitable for use in global chemistry transport models. BOREAM is validated against recent photooxidation smog chamber experiments, for which it reproduces SOA yields to within a factor of 2 in most cases. In the simple chemical mechanism of the parameter model, oxidation of α-pinene generates peroxy radicals, which, upon reaction with NO or HO2, yield products corresponding to high or low-NOx conditions, respectively. The model parameters – i.e. the temperature-dependent stoichiometric coefficients and partitioning coefficients of 10 semi-volatile products – are obtained from simulations with BOREAM, including a prescribed diurnal cycle for the radiation, oxidant and emission levels, as well as a deposition sink for the particulate and gaseous products. The effects of photooxidative ageing are implicitly included in the parameterisation, since it is based on near-equilibrium SOA concentrations, obtained through simulations of a two-week period. In order to mimic the full BOREAM model results both during SOA build-up and when SOA has reached an equilibrium concentration, the revolatilisation of condensable products due to photochemical processes is taken into account through a fitted pseudo-photolysis reaction of the lumped semi-volatile products. Modelled SOA mass yields are about ten times higher in low-NOx than in high-NOx conditions, with yields of more than 50% in the low-NOx OH-initiated oxidation of α-pinene, considerably more than in previous parameterisations based on smog chamber experiments. Sensitivity calculations indicate that discrepancies between the full model and the parameterisation due to variations in assumed oxidant levels are limited, but that changes in the radiation levels can lead to larger deviations. Photolysis of species in the particulate phase is found to strongly reduce SOA yields in the full model. Simulations of ambient conditions at 17 different sites (using oxidant, radiation and meteorological data from a global chemistry-transport model) show that overall, the parameterisation displays only little bias (2%) compared with the full model, whereas averaged relative deviations amount to about 11%. Water uptake is parameterised using fitted activity coefficients, resulting in a good agreement with the full model.

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Parameterising secondary organic aerosol from α-pinene using a detailed oxidation and aerosol formation model

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-23421-2011 ◽

2011 ◽

Vol 11 (8) ◽

pp. 23421-23468

Author(s):

K. Ceulemans ◽

S. Compernolle ◽

J.-F. Müller

Keyword(s):

Water Uptake ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Chemical Mechanism ◽

Model Parameters ◽

Peroxy Radicals ◽

Smog Chamber ◽

Aerosol Formation ◽

Formation Model ◽

Detailed Model

Abstract. A new 10-product parameter model for α-pinene secondary organic aerosol (SOA) is presented, based on simulations with the detailed model BOREAM (Biogenic hydrocarbon Oxidation and Related Aerosol formation Model). The parameterisation takes into account the influence of temperature, type of oxidant, NOx-regime, photochemical ageing and water uptake, and is suitable for use in global chemistry transport models. BOREAM is validated against recent photooxidation smog chamber experiments, for which it reproduces SOA yields to within a factor of 2 in most cases. In the simple chemical mechanism of the parameter model, oxidation of α-pinene generates peroxy radicals, which, upon reaction with NO or HO2, yield products corresponding to high or low-NOx conditions, respectively. The model parameters – i.e. the temperature-dependent stoichiometric coefficients and partitioning coefficients of the 10 semi-volatile products – are obtained from simulations with BOREAM, including a prescribed diurnal cycle for the radiation, oxidant and emission levels, as well as a deposition sink for the particulate and gaseous products. The effects of photooxidative ageing are implicitly included in the parameterisation, since it is based on near-equilibrium SOA concentrations, obtained through simulations of a two-week period. Modelled SOA mass yields are about ten times higher in low-NOx than in high-NOx conditions, with yields of about 50 % in the low-NOx OH-initiated oxidation of α-pinene, considerably more than in previous parameterisations based on smog chamber experiments. The parameterisation is only moderately sensitive to the assumed oxidant levels. However, photolysis of species in the particulate phase is found to strongly reduce SOA yields. Water uptake is parameterised using fitted activity coefficients, resulting in a good agreement with the full model.

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Secondary organic aerosol formation from smoldering and flaming combustion of biomass: a box model parametrization based on volatility basis set

10.5194/acp-2018-1308 ◽

2019 ◽

Cited By ~ 2

Author(s):

Giulia Stefenelli ◽

Jianhui Jiang ◽

Amelie Bertrand ◽

Emily A. Bruns ◽

Simone M. Pieber ◽

...

Keyword(s):

Mass Spectrometer ◽

Secondary Organic Aerosol ◽

Time Of Flight ◽

Organic Aerosol ◽

Box Model ◽

Basis Set ◽

Model Parameters ◽

Oh Radicals ◽

Smog Chamber ◽

Aerosol Formation

Abstract. Box model simulations based on the volatility basis set (VBS) approach were used to assess secondary organic aerosol (SOA) precursors and volatility distributions from residential wood combustion. Emissions were sampled from three different residential stoves at different combustion conditions (flaming vs. smoldering-dominated), aging temperatures (−10 °C, 2 °C and 15 °C), and emission loads, then exposed to hydroxyl (OH) radicals in a smog chamber. Primary emissions of SOA precursor compounds, organic aerosol and their evolution during aging in the smog chamber were monitored by a comprehensive suite of gas and particle instrumentation, including a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) and a high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). SOA precursors were classified according to their chemical composition and the identification of the nature of the precursors revealed useful to better constrain model parameters, in particular SOA production rates and molecular characteristics of the condensable gases formed. The general aim of the model was the determination of the parameters describing the volatility distributions of the oxidation products from the different chemical classes considered and their temperature dependence. Novel parameterization methods based on a genetic algorithm (GA) approach allowed estimation of precursor class contributions to SOA and evaluation of the effect of emission variability on SOA yield predictions. Significant differences were observed in the gas-phase composition between smoldering and flaming emissions. Smoldering phase emissions were dominated by oxidized VOCs with less than six carbon atoms family (OVOCc

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Impact of aftertreatment devices on primary emissions and secondary organic aerosol formation potential from in-use diesel vehicles: results from smog chamber experiments

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-16055-2010 ◽

2010 ◽

Vol 10 (6) ◽

pp. 16055-16109 ◽

Cited By ~ 5

Author(s):

R. Chirico ◽

P. F. DeCarlo ◽

M. F. Heringa ◽

T. Tritscher ◽

R. Richter ◽

...

Keyword(s):

Urban Areas ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Oxidation Catalyst ◽

Particulate Filter ◽

Exhaust Aftertreatment ◽

Smog Chamber ◽

Aerosol Formation ◽

Diesel Particulate ◽

Diesel Vehicles

Abstract. Diesel particulate matter (DPM) is a significant source of aerosol in urban areas and has been linked to adverse health effects. Although newer European directives have introduced increasingly stringent standards for primary PM emissions, gaseous organics emitted from diesel cars can still lead to large amounts of secondary organic aerosol (SOA) in the atmosphere. Here we present results from smog chamber investigations characterizing the primary organic aerosol (POA) and the corresponding SOA formation at atmospherically relevant concentrations for three in-use diesel vehicles with different exhaust aftertreatment systems. One vehicle lacked exhaust aftertreatment devices, one vehicle was equipped with a diesel oxidation catalyst (DOC) and the final vehicle used both a DOC and diesel particulate filter (DPF). The experiments presented here were obtained from the vehicles at conditions representative of idle mode, and for one car in addition at a speed of 60 km/h. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was used to measure the organic aerosol (OA) concentration and to obtain information on the chemical composition. For the conditions explored in this paper, primary aerosols from vehicles without a particulate filter consisted mainly of black carbon (BC) with a low fraction of organic matter (OM, OM/BC<0.5), while the subsequent aging by photooxidation resulted in a consistent production of SOA only for the vehicles without a DOC and with a deactivated DOC. After 5 h of aging ~80% of the total organic aerosol was on average secondary and the estimated "emission factor" for SOA was 0.23–0.56 g/kg fuel burned. In presence of both a DOC and a DPF, primary particles with a mobility diameter above 5 nm were 300±19 cm−3, and only 0.01 g SOA per kg fuel burned was produced within 5 h after lights on. The mass spectra indicate that POA was mostly a non-oxidized OA with an oxygen to carbon atomic ratio (O/C) ranging from 0.097 to 0.190. Five hours of oxidation led to a more oxidized OA with an O/C range of 0.208 to 0.369.

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Application of the Statistical Oxidation Model (SOM) to Secondary Organic Aerosol formation from photooxidation of C<sub>12</sub> alkanes

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-1591-2013 ◽

2013 ◽

Vol 13 (3) ◽

pp. 1591-1606 ◽

Cited By ~ 33

Author(s):

C. D. Cappa ◽

X. Zhang ◽

C. L. Loza ◽

J. S. Craven ◽

L. D. Yee ◽

...

Keyword(s):

Formation Process ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Atomic Ratio ◽

Model Parameters ◽

Reaction Products ◽

Aerosol Formation ◽

Volatile Organic ◽

Oxidation Model ◽

Mechanism Of Oxidation

Abstract. Laboratory chamber experiments are the main source of data on the mechanism of oxidation and the secondary organic aerosol (SOA) forming potential of volatile organic compounds. Traditional methods of representing the SOA formation potential of an organic do not fully capture the dynamic, multi-generational nature of the SOA formation process. We apply the Statistical Oxidation Model (SOM) of Cappa and Wilson (2012) to model the formation of SOA from the formation of the four C12 alkanes, dodecane, 2-methyl undecane, cyclododecane and hexylcyclohexane, under both high- and low-NOx conditions, based upon data from the Caltech chambers. In the SOM, the evolution of reaction products is defined by the number of carbon (NC) and oxygen (NO) atoms, and the model parameters are (1) the number of oxygen atoms added per reaction, (2) the decrease in volatility upon addition of an oxygen atom and (3) the probability that a given reaction leads to fragmentation of the molecules. Optimal fitting of the model to chamber data is carried out using the measured SOA mass concentration and the aerosol O:C atomic ratio. The use of the kinetic, multi-generational SOM is shown to provide insights into the SOA formation process and to offer promise for application to the extensive library of existing SOA chamber experiments that is available.

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Simulating regional scale secondary organic aerosol formation during the TORCH 2003 campaign in the southern UK

Atmospheric Chemistry and Physics ◽

10.5194/acp-6-403-2006 ◽

2006 ◽

Vol 6 (2) ◽

pp. 403-418 ◽

Cited By ~ 131

Author(s):

D. Johnson ◽

S. R. Utembe ◽

M. E. Jenkin ◽

R. G. Derwent ◽

G. D. Hayman ◽

...

Keyword(s):

Secondary Organic Aerosol ◽

Regional Scale ◽

Organic Aerosol ◽

Chemical Mechanism ◽

Background Concentration ◽

Aerosol Formation ◽

Organic Species ◽

Voc Oxidation ◽

Partitioning Coefficients

Abstract. A photochemical trajectory model has been used to simulate the chemical evolution of air masses arriving at the TORCH field campaign site in the southern UK during late July and August 2003, a period which included a widespread and prolonged photochemical pollution episode. The model incorporates speciated emissions of 124 non-methane anthropogenic VOC and three representative biogenic VOC, coupled with a comprehensive description of the chemistry of their degradation. A representation of the gas/aerosol absorptive partitioning of ca. 2000 oxygenated organic species generated in the Master Chemical Mechanism (MCM v3.1) has been implemented, allowing simulation of the contribution to organic aerosol (OA) made by semi- and non-volatile products of VOC oxidation; emissions of primary organic aerosol (POA) and elemental carbon (EC) are also represented. Simulations of total OA mass concentrations in nine case study events (optimised by comparison with observed hourly-mean mass loadings derived from aerosol mass spectrometry measurements) imply that the OA can be ascribed to three general sources: (i) POA emissions; (ii) a "ubiquitous" background concentration of 0.7 µg m-3; and (iii) gas-to-aerosol transfer of lower volatility products of VOC oxidation generated by the regional scale processing of emitted VOC, but with all partitioning coefficients increased by a species-independent factor of 500. The requirement to scale the partitioning coefficients, and the implied background concentration, are both indicative of the occurrence of chemical processes within the aerosol which allow the oxidised organic species to react by association and/or accretion reactions which generate even lower volatility products, leading to a persistent, non-volatile secondary organic aerosol (SOA). The contribution of secondary organic material to the simulated OA results in significant elevations in the simulated ratio of organic carbon (OC) to EC, compared with the ratio of 1.1 assigned to the emitted components. For the selected case study events, [OC]/[EC] is calculated to lie in the range 2.7-9.8, values which are comparable with the high end of the range reported in the literature.

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Mapping gas-phase organic reactivity and concomitant secondary organic aerosol formation: chemometric dimension reduction techniques for the deconvolution of complex atmospheric data sets

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-8077-2015 ◽

2015 ◽

Vol 15 (14) ◽

pp. 8077-8100 ◽

Cited By ~ 7

Author(s):

K. P. Wyche ◽

P. S. Monks ◽

K. L. Smallbone ◽

J. F. Hamilton ◽

M. R. Alfarra ◽

...

Keyword(s):

Phase Composition ◽

Dimension Reduction ◽

Gas Phase ◽

Secondary Organic Aerosol ◽

Ambient Air ◽

Organic Aerosol ◽

Chemical Mechanism ◽

Particle Phase ◽

Aerosol Formation ◽

Holistic View

Abstract. Highly non-linear dynamical systems, such as those found in atmospheric chemistry, necessitate hierarchical approaches to both experiment and modelling in order to ultimately identify and achieve fundamental process-understanding in the full open system. Atmospheric simulation chambers comprise an intermediate in complexity, between a classical laboratory experiment and the full, ambient system. As such, they can generate large volumes of difficult-to-interpret data. Here we describe and implement a chemometric dimension reduction methodology for the deconvolution and interpretation of complex gas- and particle-phase composition spectra. The methodology comprises principal component analysis (PCA), hierarchical cluster analysis (HCA) and positive least-squares discriminant analysis (PLS-DA). These methods are, for the first time, applied to simultaneous gas- and particle-phase composition data obtained from a comprehensive series of environmental simulation chamber experiments focused on biogenic volatile organic compound (BVOC) photooxidation and associated secondary organic aerosol (SOA) formation. We primarily investigated the biogenic SOA precursors isoprene, α-pinene, limonene, myrcene, linalool and β-caryophyllene. The chemometric analysis is used to classify the oxidation systems and resultant SOA according to the controlling chemistry and the products formed. Results show that "model" biogenic oxidative systems can be successfully separated and classified according to their oxidation products. Furthermore, a holistic view of results obtained across both the gas- and particle-phases shows the different SOA formation chemistry, initiating in the gas-phase, proceeding to govern the differences between the various BVOC SOA compositions. The results obtained are used to describe the particle composition in the context of the oxidised gas-phase matrix. An extension of the technique, which incorporates into the statistical models data from anthropogenic (i.e. toluene) oxidation and "more realistic" plant mesocosm systems, demonstrates that such an ensemble of chemometric mapping has the potential to be used for the classification of more complex spectra of unknown origin. More specifically, the addition of mesocosm data from fig and birch tree experiments shows that isoprene and monoterpene emitting sources, respectively, can be mapped onto the statistical model structure and their positional vectors can provide insight into their biological sources and controlling oxidative chemistry. The potential to extend the methodology to the analysis of ambient air is discussed using results obtained from a zero-dimensional box model incorporating mechanistic data obtained from the Master Chemical Mechanism (MCMv3.2). Such an extension to analysing ambient air would prove a powerful asset in assisting with the identification of SOA sources and the elucidation of the underlying chemical mechanisms involved.

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Impact of chamber wall loss of gaseous organic compounds on secondary organic aerosol formation: explicit modeling of SOA formation from alkane and alkene oxidation

Atmospheric Chemistry and Physics ◽

10.5194/acp-16-1417-2016 ◽

2016 ◽

Vol 16 (3) ◽

pp. 1417-1431 ◽

Cited By ~ 47

Author(s):

Y. S. La ◽

M. Camredon ◽

P. J. Ziemann ◽

R. Valorso ◽

A. Matsunaga ◽

...

Keyword(s):

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Chamber Wall ◽

Smog Chamber ◽

Aerosol Formation ◽

Organic Vapors ◽

Alkene Oxidation ◽

Branched Alkanes ◽

Wall Loss ◽

Kinetics Of

Abstract. Recent studies have shown that low volatility gas-phase species can be lost onto the smog chamber wall surfaces. Although this loss of organic vapors to walls could be substantial during experiments, its effect on secondary organic aerosol (SOA) formation has not been well characterized and quantified yet. Here the potential impact of chamber walls on the loss of gaseous organic species and SOA formation has been explored using the Generator for Explicit Chemistry and Kinetics of the Organics in the Atmosphere (GECKO-A) modeling tool, which explicitly represents SOA formation and gas–wall partitioning. The model was compared with 41 smog chamber experiments of SOA formation under OH oxidation of alkane and alkene series (linear, cyclic and C12-branched alkanes and terminal, internal and 2-methyl alkenes with 7 to 17 carbon atoms) under high NOx conditions. Simulated trends match observed trends within and between homologous series. The loss of organic vapors to the chamber walls is found to affect SOA yields as well as the composition of the gas and the particle phases. Simulated distributions of the species in various phases suggest that nitrates, hydroxynitrates and carbonylesters could substantially be lost onto walls. The extent of this process depends on the rate of gas–wall mass transfer, the vapor pressure of the species and the duration of the experiments. This work suggests that SOA yields inferred from chamber experiments could be underestimated up a factor of 2 due to the loss of organic vapors to chamber walls.

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Secondary organic aerosol formation exceeds primary particulate matter emissions for light-duty gasoline vehicles

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-23173-2013 ◽

2013 ◽

Vol 13 (9) ◽

pp. 23173-23216 ◽

Cited By ~ 10

Author(s):

T. D. Gordon ◽

A. A. Presto ◽

A. A. May ◽

N. T. Nguyen ◽

E. M. Lipsky ◽

...

Keyword(s):

Particulate Matter ◽

Secondary Organic Aerosol ◽

Cold Start ◽

Organic Aerosol ◽

Smog Chamber ◽

Aerosol Formation ◽

Gas Emissions ◽

Light Duty ◽

Gasoline Vehicles ◽

Diesel Trucks

Abstract. The effects of photochemical aging on emissions from 15 light-duty gasoline vehicles were investigated using a smog chamber to probe the critical link between the tailpipe and ambient atmosphere. The vehicles were recruited from the California in-use fleet; they represent a wide range of model years (1987 to 2011), vehicle types and emission control technologies. Each vehicle was tested on a chassis dynamometer using the unified cycle. Dilute emissions were sampled into a portable smog chamber and then photochemically aged under urban-like conditions. For every vehicle, substantial secondary organic aerosol (SOA) formation occurred during cold-start tests, with the emissions from some vehicles generating as much as 6 times the amount of SOA as primary particulate matter after three hours of oxidation inside the chamber at typical atmospheric oxidant levels. Therefore, the contribution of light duty gasoline vehicle exhaust to ambient PM levels is likely dominated by secondary PM production (SOA and nitrate). Emissions from hot-start tests formed about a factor of 3–7 less SOA than cold-start tests. Therefore, catalyst warm-up appears to be an important factor in controlling SOA precursor emissions. The mass of SOA generated by photo-oxidizing exhaust from newer (LEV1 and LEV2) vehicles was only modestly lower (38%) than that formed from exhaust emitted by older (pre-LEV) vehicles, despite much larger reductions in non-methane organic gas emissions. These data suggest that a complex and non-linear relationship exists between organic gas emissions and SOA formation, which is not surprising since SOA precursors are only one component of the exhaust. Except for the oldest (pre-LEV) vehicles, the SOA production could not be fully explained by the measured oxidation of speciated (traditional) SOA precursors. Over the time scale of these experiments, the mixture of organic vapors emitted by newer vehicles appear to be more efficient (higher yielding) in producing SOA than the emissions from older vehicles. About 30% of the non-methane organic gas emissions from the newer (LEV1 and LEV2) vehicles could not be speciated, and the majority of the SOA formed from these vehicles appears to be associated with these unspeciated organics. These results for light-duty gasoline vehicles contrast with the results from a companion study of on-road heavy-duty diesel trucks; in that study late model (2007 and later) diesel trucks equipped with catalyzed diesel particulate filters emitted very little primary PM, and the photo-oxidized emissions produced negligible amounts of SOA.

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Simulating regional scale secondary organic aerosol formation during the TORCH 2003 campaign in the southern UK

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-5-7829-2005 ◽

2005 ◽

Vol 5 (4) ◽

pp. 7829-7874 ◽

Cited By ~ 3

Author(s):

D. Johnson ◽

S. R. Utembe ◽

M. E. Jenkin ◽

R. G. Derwent ◽

G. D. Hayman ◽

...

Keyword(s):

Secondary Organic Aerosol ◽

Regional Scale ◽

Organic Aerosol ◽

Chemical Mechanism ◽

Background Concentration ◽

Aerosol Formation ◽

Organic Species ◽

Voc Oxidation ◽

Partitioning Coefficients

Abstract. A photochemical trajectory model has been used to simulate the chemical evolution of air masses arriving at the TORCH field campaign site in the southern UK during late July and August 2003, a period which included a widespread and prolonged photochemical pollution episode. The model incorporates speciated emissions of 124 non-methane anthropogenic VOC and three representative biogenic VOC, coupled with a comprehensive description of the chemistry of their degradation. A representation of the gas/aerosol absorptive partitioning of ca. 2000 oxygenated organic species generated in the Master Chemical Mechanism (MCM v3.1) has been developed and implemented, allowing simulation of the contribution to organic aerosol (OA) made by semi- and non-volatile products of VOC oxidation; emissions of primary organic aerosol (POA) and elemental carbon (EC) are also represented. Simulations of total OA mass concentrations in nine case study events (optimised by comparison with observed mass loadings derived from aerosol mass spectrometry measurements) imply that the OA can be ascribed to three general sources: (i) POA emissions; (ii) a ubiquitous background concentration of 0.7 µg m−3; and (iii) gas-to-aerosol transfer of lower volatility products of VOC oxidation generated by the regional scale processing of emitted VOC, but with all partitioning coefficients increased by a species-independent factor of 500. The requirement to scale the partitioning coefficients, and the implied background concentration, are both indicative of the occurrence of chemical processes within the aerosol which allow the oxidised organic species to react by association and/or accretion reactions which generate even lower volatility products, leading to a persistent, non-volatile secondary organic aerosol (SOA). The contribution of secondary organic material to the simulated OA results in significant elevations in the simulated ratio of organic carbon (OC) to EC, compared with the ratio of 1.1 assigned to the emitted components. For the selected case study events, [OC]/[EC] is calculated to lie in the range 2.7–9.8, values which are comparable with the high end of the range reported in the literature.

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Secondary organic aerosol formation from OH-initiated oxidation of <i>m</i>-xylene: effects of relative humidity on yield and chemical composition

10.5194/acp-2019-20 ◽

2019 ◽

Author(s):

Qun Zhang ◽

Yongfu Xu ◽

Long Jia

Keyword(s):

Relative Humidity ◽

Mass Spectrometer ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Chemical Mechanism ◽

Oh Radicals ◽

Aerosol Formation ◽

Scanning Mobility Particle Sizer ◽

Significant Discrepancy ◽

Liquid Chromatograph

Abstract. The effect of relative humidity (RH) on the secondary organic aerosol (SOA) formation from the photooxidation of m-xylene initiated by OH radicals in the absence of seed particles was investigated in a smog chamber. The SOA yields were determined based on the particle mass concentrations measured with a scanning mobility particle sizer (SMPS) and reacted m-xylene concentrations measured with a gas chromatograph-mass spectrometer (GC-MS). The SOA components were analyzed using Fourier transform infrared spectrometer (FTIR) and ultrahigh performance liquid chromatograph-electrospray ionization-high-resolution mass spectrometer (UPLC-ESI-HRMS). A significant discrepancy was observed in SOA mass concentration and yield variation with the RH conditions. The SOA yield is 13.8 % and 0.8 % at low RH (13.7 %) and high RH (79.1 %), respectively, with the difference being over an order of magnitude. The relative increase of C-O-C at high RH from the FTIR analysis of functional groups indicates that the oligomers from carbonyl compounds cannot well explain the suppression of SOA yield. Highly oxygenated molecules (HOMs) were observed to be suppressed in the HRMS spectra. The chemical mechanism for explaining the RH effects on SOA formation from m-xylene-OH system is proposed based on the analysis of both FTIR and HRMS measurements as well as Master Chemical Mechanism (MCM) simulations. The reduced SOA at high RH is mainly ascribed to the less formation of oligomers and the suppression of RO2 autoxidation. As a result, high RH can obstruct the oligomerization and autoxidation that contribute to the SOA formation.

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