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Faraday DiscussionsCite this: DOI: 10.1039/c3fd00029j

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Modeling the influence of alkane molecularstructure on secondary organic aerosolformation

Bernard Aumont,*a Marie Camredon,a Camille Mouchel-Vallon,a

Stephanie La,a Farida Ouzebidour,a Richard Valorso,a Julia Lee-Taylorb

and Sasha Madronichb

Received 6th March 2013, Accepted 21st May 2013

DOI: 10.1039/c3fd00029j

Secondary Organic Aerosols (SOA) production and ageing is a multigenerational

oxidation process involving the formation of successive organic compounds with

higher oxidation degree and lower vapor pressure. Intermediate Volatility Organic

Compounds (IVOC) emitted to the atmosphere are expected to be a substantial source

of SOA. These emitted IVOC constitute a complex mixture including linear, branched

and cyclic alkanes. The explicit gas-phase oxidation mechanisms are here generated for

various linear and branched C10–C22 alkanes using the GECKO-A (Generator for Explicit

Chemistry and Kinetics of Organics in the Atmosphere) and SOA formation is

investigated for various homologous series. Simulation results show that both the size

and the branching of the carbon skeleton are dominant factors driving the SOA yield.

However, branching appears to be of secondary importance for the particle oxidation

state and composition. The effect of alkane molecular structure on SOA yields appears

to be consistent with recent laboratory observations. The simulated SOA composition

shows, however, an unexpected major contribution from multifunctional organic

nitrates. Most SOA contributors simulated for the oxidation of the various homologous

series are far too reduced to be categorized as highly oxygenated organic aerosols

(OOA). On a carbon basis, the OOA yields never exceeded 10% regardless of carbon

chain length, molecular structure or ageing time. This version of the model appears

clearly unable to explain a large production of OOA from alkane precursors.

1 Introduction

Long carbon chain hydrocarbons (C>10) are emitted in the atmosphere frombiomass and as unburnt byproducts of biomass and fossil fuel combustion. Theseemissions form a complex mixture, including a large fraction of linear, cyclic and

aLISA, UMR CNRS 7583, Universite Paris Est Creteil et Universite Paris Diderot, France. E-mail: bernard.

[email protected] Center for Atmospheric Research, Boulder, CO, USA

This journal is ª The Royal Society of Chemistry 2013 Faraday Discuss.

http://dx.doi.org/10.1039/c3fd00029j

http://pubs.rsc.org/en/journals/journal/FD

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branched alkanes.1–4 These organic compounds of intermediate volatility (IVOC)are emitted largely in the condensed phase and then volatized by dilution5 in theatmosphere. Gas phase oxidation of IVOC leads to the production of organicspecies of low enough volatility to condense and contribute to secondary organicaerosols (SOA) production.6 Laboratory studies have shown that IVOC alkanes arepotential SOA contributors.7–12 Modeling studies have also identied IVOC assubstantial SOA precursors at the continental scale and in the plumes ofmegacities.13–17

SOA production in the Mexico City plume was explored using a detailedchemical scheme: the Generator of Explicit Chemistry and Kinetics of Organics inthe Atmosphere – GECKO-A.17 In these simulations, n-alkanes were used assurrogate species of IVOC emissions. The model was found to be able to explaintypical SOA levels but the prediction of the atomic O/C ratio fell short.17,18 SOAformation from the gas phase oxidation of intermediate volatility n-alkanes wasrecently examined using the GECKO-A tool.19 For the n-alkane series, most SOAcontributors were found to be reduced enough to be categorized as hydrocarbon-like organic aerosols (HOA), although of secondary origin.19 Simulated resultswith GECKO-A suggest that gas phase n-alkane oxidation cannot explain the largefraction of highly oxygenated organic aerosols (OOA) observed in the atmosphere.Branched alkanes are a major fraction of the emitted hydrocarbons in engineexhausts.4 These branched alkanes are more prone to fragment in the early stageof the oxidation than their corresponding linear analogues.7,9,11 This enhancedfragmentation is expected to alter both the SOA yields and the mean SOA oxida-tion state.

The aim of this study is to examine the inuence of the parent alkanemolecular structure on SOA yield and oxidation states. The study focuses on theeffect of the degree of branching of the carbon backbone for various C10–C22

alkane series. The mechanism self-generator GECKO-A20 is used to generatehighly detailed oxidation schemes, and box model simulations are performed toexplore the formation and ageing of the particle phase subjected to continuousgas phase oxidation of the organic matter. Section 2 briey presents the GECKO-Amodeling and results are discussed in Section 3.

2 Model description2.1 The GECKO-A modeling tool

The number of species involved in the multigenerational gas phase oxidation ofhydrocarbons grows exponentially with the size of the carbon skeleton.20,21 For aC8 hydrocarbon, the explicit description of the gas phase oxidation processes upto the nal production of CO2 involves more than one million species, farexceeding the size of chemical mechanisms that can be managed manually. Thechemical mechanism generator GECKO-A is a computer program designed toovercome this difficulty. This tool is used here to develop an oxidation scheme foralkane series with various degrees of branching.

Reaction pathways and rate constants are assigned during the chemicalmechanism generation on the basis of experimental data and Structure ActivityRelationships (SAR). The protocol implemented in GECKO-A to self generate themechanisms is described by Aumont et al.,20 with chemistry updates described byAumont et al.19 The relative contribution of fragmentation vs. functionalization

Faraday Discuss. This journal is ª The Royal Society of Chemistry 2013


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oxidation routes is a key ratio in the context of SOA formation.22 Evolution alongthe fragmentation or functionalization routes is mostly dictated by fate of thealkoxy radicals produced as intermediates during the gas phase oxidation.7 In theprevious version of GECKO-A,19 the alkoxy radical chemistry was based on esti-mations provided by the SAR developed by Atkinson.23 Vereecken and Peeters24

have recently reported a SAR based on quantum chemical calculations to estimatethe barrier heights for alkoxy radical decomposition for a large set of organicmoieties. In particular, the SAR includes rates for leaving groups, such as thenitrate moieties, which are prevalent in the oxidation mechanisms of long-chainalkanes. The recent SAR provided by Vereecken and Peeters24 for alkoxy radicaldecomposition has therefore been implemented in GECKO-A.

Phase partitioning is described in the model assuming that the condensedphase behaves as an ideal well mixed homogeneous liquid phase. Gas/particlephase equilibrium is described by Raoult's law:25

Pi ¼ xiPivap (1)

where Pivap is the vapor pressure of the species i, xi its mole fraction in the

condensed phase and Pi its equilibrium partial pressure. At phase equilibrium,the ratio xi of the species in the condensed phase is given by:26

xaeri

¼ Ni;aer

Ni;aer þNi;gas

¼ 1þMOAPvapi

COART� 106

� ��1

¼�1þ C*

i

COA

��1

(2)

where Ni,j is the number concentration in phase j (molecule of species i per cm3 ofair), T is the T/K, R the ideal gas constant (atm m3 K�1 mol�1), COA the aerosolmass concentration (mg m�3 of air), MOA the mean organic molar mass in theaerosol (g mol�1) and C*i is an effective saturation mass concentration (mg m�3 ofair). The Nannoolal et al.27 group contribution method was used to estimate thevapor pressure of each non radical species included in the mechanism, asdescribed by Valorso et al.28 This method was used in conjunction with theNannoolal et al.29 method to estimate boiling points. Contributions for functionalgroups not provided by Nannoolal et al.27,29 were taken from Compernolle et al.30

The explicit description of n-decane oxidation is expected to lead to a mech-anism including about 108 species,20 one order of magnitude above the size ofchemical schemes that can be both generated and solved. Simplications aretherefore required to reduce the schemes down to a manageable size. Usually,simplications in GECKO-A are performed based on the lumping of structuralisomers.17,28,31 The description of oxidative trajectories along fragmentation orfunctionalization routes is a key aspect of the modeling study performed here andrequires a detailed representation of alkoxy radical chemistry. The chemical fateof the alkoxy radical (O2 reaction, H-shi isomerisation, or C–C bond breaking)depends on the chemical structure in the vicinity of the alkoxy moiety.23,24 Nolumping is therefore performed to keep the molecular structures of the successivegenerations of secondary organic species. Mechanism reduction has been per-formed as follows. Species produced with a maximum yield below 5 � 10�6 arenot treated further in themechanism and the chemical removal of these species isconsidered as a nal sink. This approximation does not signicantly alter themass budget, the cumulative loss of carbon atoms at the end of the simulationbeing less than 1% of the initial organic carbon. Furthermore, gas phase




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oxidation becomes negligible for non-volatile species, where most of the speciesmass occurs in the condensed phase. According to eqn (2) and for COA repre-sentative of polluted conditions (10 mg m�3), a species is almost exclusively in thecondensed phase at thermodynamic equilibrium when Pvap is below 10�13 atm.The gas phase chemistry is therefore omitted for such low volatility species.Finally, to reduce the mechanisms to a manageable size, we assume high NOx

conditions, i.e. the peroxy + peroxy reactions are ignored. This approximation isappropriate under most polluted conditions. Nevertheless, this high NOx condi-tion remains hardly representative of the atmospheric oxidation on timescalesexceeding one day. Restricting the study to high NOx conditions is therefore asevere simplication.19 Simulations performed here are clearly exploratory andintended to examine some, though not all, aspects of the alkane molecularstructure on SOA formation. The numbers of species ultimately considered in themechanisms are listed in Table 1 for various alkane series.

Time integration of the set of ordinary differential equations associated withgas phase oxidation is solved using the “2-step” solver.32,33 Phase equilibrium isenforced at each time step, as described by Camredon et al.34 Reactions in thecondensed phase are not considered in this model conguration. Ageing of theparticles is driven by gas phase chemistry that progressively shis the various gas/particles equilibria as oxidation progresses.

Table 1 Number of species generated by GECKO-A for the gas phase oxidation mechanisms andnumber of non radical species for which gas/particle equilibrium is considered

Parent alkane # species # phase equilibria

C10 alkanesn-decane 1.7 � 105 5.1 � 104

2-methyl nonane 1.6 � 105 5.0 � 104

3,3 diethyl hexane 1.0 � 105 3.3 � 104

4,5-dimethyl octane 2.4 � 105 7.3 � 104

3,4,5-trimethyl heptane 2.9 � 105 9.1 � 104

C14 alkanesn-tetradecane 6.2 � 105 2.1 � 105

2-methyl tridecane 7.9 � 105 2.6 � 105

3,3 diethyl decane 5.0 � 105 1.7 � 105

6,7-dimethyl dodecane 8.6 � 105 2.9 � 105

5,6,7-trimethyl undecane 8.0 � 105 2.7 � 105

C18 alkanesn-octadecane 8.4 � 105 3.2 � 105

2-methyl heptadecane 1.4 � 106 5.3 � 105

3,3 diethyl tetradecane 1.0 � 106 3.6 � 105

8,9-dimethyl hexadecane 1.2 � 106 4.5 � 105

7,8,9-trimethyl pentadecane 1.1 � 106 4.1 � 105

C22 alkanesn-docosane 6.5 � 105 2.5 � 105

2-methyl uncosane 1.3 � 106 5.1 � 105

3,3 diethyl octadecane 1.5 � 106 5.5 � 105

10,11-dimethyl eicosane 1.2 � 106 4.5 � 105

9,10,11-trimethylnonadecane

1.3 � 106 4.6 � 105




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2.2 Simulation conditions

Simulations were conducted under conditions similar to those described byAumont et al.19 The simulations are run for constant environmental conditions.The temperature is set to 298 K. Photolysis frequencies are calculated using theTUV model35 for mid latitude conditions and for a zenith angle of 45�. A constantOH source of 2 � 107 radicals cm�3 s�1 is included to initiate oxidation in themodel. The NOx mixing ratio is set to 1 ppb. Note that the simulated SOAformation is almost insensitive to the prescribed NOx value, the fate of the organicperoxy radical being exclusively the reaction with NO as described above (i.e. highNOx conditions). In this study, COA is set to a constant value of 10 mg m�3,representative of polluted tropospheric conditions. The composition is assumedto be non-volatile organic matter with a mean molar mass MOA of 250 g mol�1,within the expected values for atmospheric organic aerosol.25,36,37 Oxidation of theparent hydrocarbon may obviously lead to the formation of SOA and thereforealters the prescribed aerosol composition and mass concentration. The hydro-carbon initial mixing ratio is set to an arbitrary value of 10 ppt carbon (ca. C0¼ 6.5� 10�3 mg m�3), a value low enough to not modify the prescribed aerosol prop-erties. For these conditions (C0 � COA), SOA yields are independent of C0.

3 Results3.1 SOA yields

The time proles of the simulated SOA yields are given in Fig. 1 for various C10,C14, C18 and C22 alkane series. Five homologous series are considered: n-alkane(Fig. 1a), 2 methyl series (Fig. 1b), 3,3-diethyl series (Fig. 1c), 2 vicinal methylgroups in the middle of the backbone (Fig. 1d) and 3 vicinal methyl groups in themiddle of the backbone (Fig. 1e). Yields (denoted YC hereaer) are here dened asthe ratio between the carbon atoms in the condensed phase to the initial carbonload included in the parent backbone. The time scales are expressed as thenumber of lifetimes Ns of the parent hydrocarbon dened as:31

Ns ¼ lnC0

Ct

¼ t

s(3)

where C0 is the initial parent compound concentration, Ct its concentration at thesimulated time t and s its e-folding lifetime. Rate constants for OH radical reac-tions for the alkanes studied here range from ca. 1� 10�11 cm3 s�1 (C10 alkane) toca. 3 � 10�11 (C22 alkane).38,39 Using a typical atmospheric OH concentration of106 radical cm�3, the corresponding atmospheric lifetimes range from a fewhours (C22 alkanes) to one day (C10 alkanes).

SOA yields typically grow with decay of the parent alkane and reach a maximumforNs� 5. This time scale is however somewhat longer for the C10 alkanes due to anenhanced contribution of the later generation oxidation products to SOA forma-tion.19 As expected, Fig. 1 shows that (i) YC increases with the size of the carbonbackbone in each homologous series and (ii) for a given carbon chain length, YCdecreases with the branching degree of the parent hydrocarbon. These trends areconsistent with experimental observations.7,9,11 Furthermore, Fig. 1 shows thatbackbone branching does not substantially change SOA time proles. This behaviorsuggests that, for a given carbon chain length, major SOA contributors arise fromspecies produced in the same generations (see Section 3.3).



Fig. 1 SOA yields YC (top row), OOA yields YC,OOA (middle row) and particle oxidation states OSC(bottom row) for the n-alkanes (1st column), 2-methyl alkanes (2nd column), 3,3 dimethyl alkanes (3rdcolumn), a,b dimethyl alkanes (4th column), a,b,g trimethyl alkanes (5th column). Red, green, blue andgrey lines are for C10, C14, C18, C22 parent compounds, respectively. Dotted lines in panels a–e denoteresults obtained using the Atkinson (2007) SAR for alkoxy radical decomposition. The time scale isdefined as multiples of lifetimes of the initial hydrocarbon (see text).


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Fig. 1a–e shows that simulations performed with the two SAR implemented inGECKO-A for alkoxy radical decomposition23,24 provide similar SOA yields forstraight and lightly branched alkanes. However, discrepancies between theresults obtained with the different SAR increase with the number of branches onthe carbon backbone. SOA production is found to be substantially lower in thesimulations including the SAR by Vereecken and Peeters. For example, themaximum SOA yield obtained for 6,7,8-trimethyl pentadecane (Fig. 1e) decreasesfrom 0.43 using the Atkinson SAR23 to 0.28 using the Vereecken and Peeters SAR.24

As stated above, the SAR developed by Vereecken and Peeters24 includes groupcontributions for a comprehensive set of moieties generated during the oxidationof alkanes. In the subsequent sections, we therefore use as a benchmark theresults obtained with the mechanisms generated with the Vereecken and PeetersSAR for alkoxy radical decomposition.

Fig. 2 shows the SOA yields simulated at Ns ¼ 5 (i.e. around the maximumyield) for various carbon chain lengths as a function of the number of branchingmethyl groups. Results given in Fig. 2 are for structures with methyl branchescentered in the middle of the backbone (e.g. 7-methyl tridecane for the C14 alkane



Fig. 2 Maximum SOA yields YC as a function of the number of branching methyl groups. Red, green,blue, and grey symbols are for C10, C14, C18 and C22 alkanes, respectively. Methyl branches are located inthe middle of the carbon backbone. For species with more than 2 branches, squares denote for structurehaving no methylene carbon (–CH2–) between each branch, circles denote structures having onemethylene groups between each branch and triangles denote structures with 2 methylene carbonsbetween each branch.


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with one branch). Various structures are considered with either 0, 1 or 2 methy-lene (i.e. –CH2–) groups between each branch (e.g. 5,6,7-, 4,6,8- and 3,6,9- tri-methyl undecane for the C14 alkanes with 3 branches). The SOA yields decreasemonotonically with the number of branching methyl groups. For example, YC forC10 alkanes decreases from 0.10 for n-decane to 0.03 and 0.01 for the dimethyloctane and tetramethyl hexane, respectively. Similarly, 6 branchingmethyl groupsdecrease YC by a factor of 4 (C22 alkanes) to 10 (C14 alkanes). The position ofbranching methyl groups is also found to have a substantial effect, with vicinalmethyl branching leading to the largest effect. For example, YC changes from 0.58to 0.38 for 7,10- and 8,9- dimethyl hexadecane, respectively.

Comparison of our modeling results with experimental observations is notstraightforward owing to different simulation conditions. The branching effectssimulated here agree nevertheless reasonably well with some experimental values.For example, the SOA yields for dodecane and 2-methyl undecane have been recentlydetermined under high NOx conditions in several experimental studies.7,9,11 For ourmodeling conditions, the 2-methyl branching reduces the SOA yield by 22% incomparison to the linear structure for C12 alkanes. This value is consistent with thereduction in the 15–50% range observed experimentally. Similarly, Lim and Zie-mann7 report a 68% reduction of the SOA yield for 2,3 dimethyl decane incomparison to dodecane. A similar reduction (55%) was observed in the simulationresults performed for the same couple of species (not shown here). Furthermore,Tkacik et al.9 have reported that the SOA yields for C14 alkanes drop by almost a factorof 5 (from 0.38 to 0.08) aer a single methyl branching in the middle of the carbonchain. The model was unable to explain this large effect: the simulated decrease forYC for the linear and same branched C14 structure is 18% only (see Fig. 2).

3.2 OOA yields

Highly oxygenated organic aerosols (OOA) are a major component identied insitu by aerosol mass spectrometer (AMS) measurements of submicron




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aerosols.40–42 OOA is usually considered as SOA and is characterized by an atomicO/C ratio larger than 0.2522 or equivalently by a mean carbon oxidation state (OSC)greater than about �1.25.43,19 We dene here OOA yields YC,OOA as the carbon-weighted concentration ratio of the species in the condensed phase with OSCgreater than �1.25 to the initial concentration of the parent compound. YC,OOAcan equivalently be dened as:

YC,OOA ¼ fOOA � YC (4)

where fOOA is the fraction of the condensed carbon included in species with OSCgreater than�1.25. Fig. 1 shows the time prole of YC,OOA and OSC for the particlephase for the various homologous series. The dominant factor controlling OSCappears to be the size of the parent backbone, OSC decreasing clearly with thenumber of carbon atoms. Branching of the backbone is of secondary importance.This result is consistent with the experimental observations by Tkacik et al.9 intheir study of SOA formation from various linear, cyclic and branched alkanesunder high NOx conditions. In these experiments, the dominant factor driving O/C ratio in the particle was found to be the carbon number of the parent alkane aswell. Furthermore, the particle O/C ratios reported by Tkacik et al.9 for various C10

to C19 alkanes are typically in the 0.1–0.3 range, corresponding roughly to an OSCin the �1.7 to �1.1 range.43,19 Simulated values reported in Fig. 1 are within asimilar range.

Fig. 3 shows fOOA and YC as a function of the carbon chain length for anoxidation progress corresponding to Ns ¼ 5. Within each homologous series, fOOAincreases when the size of the carbon backbone decreases. This outcome isconsistent with the expectations that the oxygen content of the SOA contributorsmust increase when the size of the parent backbone decreases to bring their vaporpressures to a low enough value to condense.

For the conditions tested here, YC,OOA remains below 0.1 whatever the chainlength or molecular structure of the parent alkane (see Fig. 1). For short carbonchains (C<16), branching of the carbon skeleton decreases both YC,OOA and OSC.This effect grows when the carbon backbone decreases. For a C10 molecule, YC,OOA

Fig. 3 SOA yields Yc (left panel) and fraction fOOA of the condensed carbon in highly oxygenatedorganic aerosols (right panel) as a function of the carbon chain length of the parent alkane. Symbolshapes denote distinct homologous series: linear structure (circles), 2-methyl branching (triangles), 2vicinal methyl branching groups (diamonds) and 3 vicinal methyl branching groups (diamonds). YC andfOOA are given for Ns ¼ 5.




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decreases by almost a factor of 2 for one branching methyl group and reaches oneorder of magnitude for 2 vicinal methyl groups (see Fig. 1). For short carbonchains, fragmentation almost exclusively leads to volatile products which ulti-mately end up as CO2. Branching therefore decreases both YC and fOOA (see Fig. 2and 3). For long carbon chains (C>16), the opposite trend is found with both YC,OOAand OSC increasing with the branching degree of the backbone. For a C22 back-bone, YC,OOA is increased by one order of magnitude from a linear structure to astructure with 3 vicinal methyl groups (see Fig. 1). Fragmentation leads to specieswith a long enough carbon skeleton to ultimately produce highly oxygenated lowvolatility species. Although branching decreases the overall SOA yields, itincreases fOOA signicantly, ultimately leading to a substantial increase of theOOA yield.

3.3 Number of generations

Reaction sequences involved during gas phase oxidation lead to species withsimilar chemical structures but of increasing volatility when the number ofcarbon atoms in the parent alkane decreases. Within a given homologous series,the number of generations required for the production of low volatility speciesmust therefore grow when the carbon skeleton of the parent alkane decreases inlength. Fig. 4 show the composition of the particles at Ns ¼ 5 discriminated on agenerational basis for various homologous series. For the condition simulatedhere (COA¼ 10 mgm�3), most SOA contributors are 1st and 2nd generation speciesfor C22 parent alkanes but 3rd and 4th generation for a C10 parent alkane. Asexpected, Fig. 4 shows that the condensed species are on average produced aer agrowing number of oxidation steps when the size of carbon backbone decreases.Note that branching does not substantially change the relative contribution of thespecies produced by each generation to the SOA composition, similar distribu-tions being observed for the various molecular structures with the same numberof carbon atoms in the parent alkane (see Fig. 4).

Fig. 4 also shows the fraction of SOA contributors having the same carbonskeleton as the parent compounds, i.e. the fraction produced by oxidation alongthe functionalization routes only. The remaining fraction (with fewer carbonatoms) includes all the species produced along routes involving at least onefragmentation step. Fig. 4 shows that at least 80% of the SOA composition is madeof species produced by oxidative trajectories involving functionalization stepsonly. As expected, the contribution of the fragmentation products to SOAincreases with the degree of branching of the backbone. However, that contri-bution remains weak, typically of the order of 10% for parent alkanes with 3vicinal methyl groups. Most fragmentation products follow trajectories evolvingtoward CO2 without leading to the production of low volatility (i.e. highlyoxygenated) products in signicant yields.

3.4 SOA composition

As discussed in the previous section, SOA composition is dominated by speciesretaining the carbon skeleton of the parent hydrocarbons. Furthermore, thenumber of generations required to produce the low volatility species is found tobe rather insensitive to the number of branching groups of the parent alkane.These outcomes suggest that most SOA contributors are produced by analogous



Fig. 4 Contribution of the species from a given generation to the particle composition (color histo-grams) and fraction of the species in the particle phase having the same carbon skeleton as the parentcompound (grey histograms). Four homologous series are considered: linear (1st column), 2-methylseries (2nd column), 2 and 3 methyl vicinal methyl groups in the middle of the backbone (3rd and 4thcolumn) and for C10 (first row) to C22 alkanes (4th row). Results are given for Ns¼ 5. Budget is performedon a carbon atom basis.


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oxidation sequences along the functionalization routes. For the conditionssimulated here, oxidation is driven by OH reaction. Fig. 5 shows the main reac-tions involved in the functionalization trajectories between 2 successive genera-tions. Briey, OH reaction on a paraffinic carbon atom leads to the abstraction ofa hydrogen atom, leading to an alkyl radical and then to a peroxy radical (RO2)aer O2 additions. For the high NOx conditions simulated here, peroxy radicalsreact with NO to produce an organic nitrate moiety or an alkoxy radical (RO).Three reactions pathways are considered for alkoxy radicals: (1) C–C bond



Fig. 5 The main reactions involved in the functionalization trajectories between 2 successivegenerations.


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breaking (2) O2 reaction (a minor pathway for long carbon chains) leading to aketone moiety and (3) hydrogen shi isomerization, leading to an 1–4 hydroxyalkyl radical, which then adds O2 to produce a new peroxy radical that reacts withNO as described above. Note that channel (1) is along the fragmentation route andis omitted in Fig. 5 for clarity. Finally, it is assumed the alkyl radical produced ona carbon already bearing an oxygenated functional group does not add O2 butdecomposes to form a ketone moiety. This concerns especially –C(OH)(_)– struc-tures produced either by OH reaction or alkoxy H-shi isomerisation on analcohol group and –C(ONO2)(_)– structures produced by OH reaction on a carbonbearing the nitrate group. Oxidation along the functionalization trajectories leadsto multifunctional species bearing various combination of nitrate moiety (deno-ted N hereaer), ketonemoiety (denote K hereaer) and hydroxyl moiety (denotedO hereaer). For example, the rst generation products of a long carbon chainalkane will be organic nitrate (N), ketone (K, although with a negligible yield forthe parent alkane examined here), hydroxynitrate (NO), hydroxyketone (KO),dihydroxynitrate (NOO) and dihydroxyketone (KOO) (see Fig. 5). These rstgeneration products are next oxidized to produce the set of second generationspecies. For the functional routes, this second generation products arise eitherfrom the oxidation of the already existing moieties, i.e. the oxidation of nitrate (N)or hydroxy (O) moiety to ketone (K) moiety or from the addition of a new set offunctional groups on the carbon backbone.




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Fig. 6 presents the composition of the SOA at Ns ¼ 5 for various homologousseries of alkanes. Position isomers (i.e. species with the same functional groups butwith distinct position on the backbone) are lumped into same functional family forthe purpose of presentation. Fieen families of compounds are considered,accounting for 60% to 95% of the SOA mass budget, depending on the parentcompounds considered. As already discussed above (see Section 3.3), SOA producedfrom the oxidation of C22 alkanes are dominated by rst generation products, inparticular N, KO, NO species with a C22 backbone. The number of generations

Fig. 6 Major contributors to the SOA composition. Position isomers are lumped into the same histo-gram. Colors discriminate contributors by number of functional groups borne by the species: dark blue¼monofunctional species, light blue ¼ difunctional species, orange ¼ trifunctional species and red ¼species with more than 3 functional groups. N, K, O denotes species bearing the nitrate, ketone andhydroxy group, respectively. Four homologous series are considered: linear (1st column), 2-methyl series(2nd column), 2 and 3 methyl vicinal methyl groups in the middle of the backbone (3rd and 4th column)and for C10 (first row) to C22 alkanes (4th row). Results are given for Ns ¼ 5. Budget is performed on acarbon atom basis.




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required to produce low volatility species increases when the carbon skeleton of theparent alkane decreases. These successive oxidation steps increase the diversity ofthe species produced and ultimately the complexity of the SOA composition. For theoxidation of C#14 alkanes, the simulated SOA contributors are typically speciesbearing at least 3 functional groups, with diverse combinations of N, K, O moietiesadded on the carbon skeleton along the various functionalization trajectories. Fig. 6shows a progressive evolution of the SOA composition with the carbon chain lengthof the parent compound. However, the chemical identities of the SOA contributorsshow similar patterns for parent alkanes having the same total number of carbonsbut various branching conguration. This outcome is consistent with the statementabove that oxidative sequences leading to the formation of SOA contributors arenearly identical whether themolecular structure of the parent alkane is branched ornot. Note that this nding also agrees with experimental observations. In particular,Lim and Ziemann7 have shown that the SOA reaction products of linear andbranched C12 alkane are similar.

3.5 Organic nitrates in the condensed phase

Fig. 6 shows that most simulated SOA contributors bear at least one nitratemoiety. For the C10 alkanes, this functional group is almost ubiquitous in theconstituents of the particle phase. For example, 99% of the condensed moleculesbear a least one nitrate moiety at Ns ¼ 5 in the n-decane oxidation case. Theoccurrence of the nitrate moiety decreases with the parent alkane chain length. AtNs ¼ 5, the fraction of condensed phase species bearing at least one nitrate groupis 84%, 75% and 46% for the C14, C18 and C22 n-alkane oxidation, respectively.

For our modeling conditions, organic nitrates (RONO2) result exclusively fromthe following competing reactions:

RO2 + NO / RONO2 (R1)

RO2 + NO / RO + NO2 (R2)

In the current version of GECKO-A, the branching ratio for these 2 channels isestimated using the SAR developed for the SAPRC99 mechanism44 and depends onthe carbon chain length, the presence of functional groups on the carbon backboneand the nature of the RO2 radical (i.e. primary, secondary or tertiary). The nitrateyield typically increases from a few percent for C3 peroxy radicals to �25% for C10

peroxy radicals and reach a maximum value of �30% for C>15 peroxy radicals. Thepresence of a hydroxy group on the carbon skeleton of the peroxy radical was foundto signicantly decrease the nitrate branching ratio,45 behavior not included in theoriginal SAR. In GECKO-A, we therefore reduced the nitrate yield by a factor 2 forhydroxy peroxy radicals. With this parameterization, the major rst generationproducts generated by GECKO-A for the C22 n-alkane are produced with thefollowing yields: KO ¼ 0.58, N ¼ 0.28, NO ¼ 0.10, and KOO ¼ 0.2. These valuesagree well with the yields recommended by Ziemann46 based on experimentalobservations for long carbon chain alkanes: KO ¼ 0.55, N ¼ 0.30, NO ¼ 0.15.

The SOA contributors given in Fig. 6 for the C22 species broadly result from theabove yield distribution when rst and second generation species are combinedand explain the occurrence of the nitrate moiety in the 40–50% range. Additionalfactors explain the increase of the nitrate moiety prevalence when the size the




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parent alkane decreases. As stated in Section 3.3, more generations are requiredto produce SOA contributors when the carbon chain length decreases. The nitratemoiety tends to be preserved during these successive oxidation steps. Indeed, thenitrate group reduces signicantly the reactivity of the alpha carbon atom towardH abstraction by OH radical (or RO radical via 1–5 H-shi isomerisation). Forexample, the SAR developed by Kwok and Atkinson38 shows that OH reaction on a–CH(ONO2)– group is decreased by an order of magnitude compared to theequivalent –CH2– group. Oxidation of the nitrate moiety to a carbonyl moiety istherefore a slow process. Furthermore, channel (R1) follows a functionalizationroute while channel (R2) either leads to a functionalization or a fragmentationroute, depending on the fate of the alkoxy radical (RO) produced. Although thefunctionalization route dominates the evolution of rst generation alkoxy radi-cals, the probability of fragmentation increases greatly for subsequent genera-tions as the number of functional groups borne by the carbon backboneincreases. For the highly oxygenated organic compounds produced aer a fewgenerations, channel (R1) (i.e. nitrate formation) mostly favors SOA production byretaining the carbon skeleton (in addition to lowering the vapor pressure of thespecies) while channel (R2) mostly favors fragmentation of the carbon skeletonand ultimately full oxidation into CO2.

Organic nitrates have been identied in the condensed phase during labora-tory experiments devoted to SOA formation from oxidation of alkanes andalkenes46–48 and during eld observations.49 However, the simulated fractions ofnitrogen containing species in the particle phase appears surprisingly high andseems to be overestimated. The high NOx conditions used here to generate theoxidation scheme explain only partially the large simulated organic nitrate yields.As stated in Section 2.1, oxidation of anthropogenic hydrocarbons under the highNOx assumption remains hardly representative of atmospheric conditions beyond1 day (corresponding to Ns � 1 for C10 alkanes). Nevertheless, the fraction ofnitrogen containing species in the condensed phase at Ns ¼ 0.5 is 0.94 and 0.68for n-decane and n-tetradecane, respectively. Multifunctional organic nitratestherefore appear as major SOA constituents even in the early stage of alkaneoxidation. Even under more realistic NOx conditions, a large fraction of nitrogencontaining species is simulated in the condensed phase. This has been forexample recently shown in a GECKO-A modeling study examining the SOA budgetin the Mexico City plume, where multifunctional organic nitrates were identiedas major SOA contributors.17 The causes of an organic nitrate overestimation inthe simulated SOA are unclear. Hydrolysis in aerosol of organic nitrate has beenproposed49–51 and recently observed to occur in aerosol particles in a laboratorystudy.52 Our understanding of the gas-phase chemistry of multifunctional nitratesis questionable as well. It was for example recently observed that carbonyl nitratesundergo fast photolysis and that the Kwok and Atkinson SAR38 fell short todescribe the OH reactivity of these multifunctional nitrates.53 Additional experi-mental studies are clearly required to improve our understanding of the behaviorof multifunctional organic nitrates in the gas and condensed phases.

4 Conclusion

The GECKO-A modeling tool was used to generate explicit gas phase oxidationmechanisms for various homologous series of C10–C22 alkanes. Box model




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simulations were performed under high NOx conditions to assess the sensitivity ofSOA yield and composition to the molecular structure of the parent alkane. Asexpected, simulation results show that within each homologous series, the SOAyield increases with the carbon chain length of the parent alkane while the meanoxidation state and the SOA fraction that can be categorized as OOA both decrease.For a given chain length, the SOA yield decreases with the degree of branching ofthe parent alkane. For short backbones (C<16), the branching inhibits the produc-tion of OOA due to an enhancement of the chemical transformation leading up tothe full oxidation of the carbon to CO2. This trend is reversed for long backbones(C>16) for which the branching favors the formation of long enough fragmentedspecies to produce low volatility and highly oxygenated species aer subsequentoxidation along the functionalization routes. The particle composition is domi-nated by species having the same carbon skeleton as the parent compound, i.e. byspecies formed by the functionalization mechanisms. The SOA contributors arethus position and skeleton isomers for particles produced from alkanes with thesame number of carbon atoms but various branching degree. In other words, oursimulation results suggest that the branching substantially decreases SOA yieldsbut does not greatly changes SOA composition.

In the version of themodel used in this study, the condensed phase is assumedto be chemically inert, and gas/particles equilibrium enforced at each time step.SOA ageing is thus only induced by gas phase oxidation. Simulated results for thevarious alkane homologous series have shown that most SOA contributors are fartoo reduced to be categorized in the OOA component. On a carbon basis, the OOAyields never exceeded 10% regardless of carbon chain length, molecular structure,or ageing time. This version of the model appears clearly unable to explain a largeproduction of OOA from alkane precursors. Including a detailed representation ofthe complex mixture of alkane isomers emitted in the atmosphere will thus notlikely decrease substantially the mismatch between the simulated and observedparticle oxidation states. Processes and/or other organic compound types notincluded in the model are therefore likely responsible for the large oxygen contentusually observed for particles collected in situ. Processing of organic matter in thecondensed phase is a likely process. For example, carboxylic acid is a majormoiety observed in the condensed phase in both laboratory studies and eldmeasurements.54,55,51 However, this group does not contribute substantially to thecarbon budget simulated with detailed schemes.17,56 The gas phase oxidation ofaliphatic compounds leads to a substantial production of 1–4 hydroxy ketones(see Section 3.5). These molecular structures are known to be converted to dihy-drofurans in the condensed phase and may then volatilize.7,46,57 These interme-diates have been suggested as a source of carboxylic acid aer the subsequent gasphase ozonolysis.55,51 Oxidation in the condensed aqueous phase may alsocontribute to increase the oxygen content, especially through the formation ofcarboxylic moieties in the aqueous phase or the processing (oligomerization) oflow molecular weight oxygenated intermediate like glyoxal.58,59 For example, theinclusion of parameterizations to represent the aqueous phase processing ofglyoxal in a box model was recently found to substantially increase the O/C ratiosimulated with GECKO-A for the Mexico City plume and help to explain themismatch between eld observations and modeling outputs.18 The integration ofan explicit representation of multiphase mechanisms in the GECKO-A tool is thesubject of ongoing studies.




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Acknowledgements

BA acknowledges support from the Primequal program of the French Ministry ofEcology, Sustainable Development and Energy and the Sustainable DevelopmentResearch Network (DIM-R2DS) of the Ile-de-France region and the French ANRwithin the project ONCEM. JLT and SM were supported in part by grant DE-FG02-ER65323 from the US Department of Energy Office of Science.

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