Chlorine-initiated oxidation of n -alkanes under high-NO x conditions: insights into secondary organic aerosol composition and volatility using a FIGAERO–CIMS

. Chlorine-initiated oxidation of n -alkanes (C 8 − 12 ) under high-nitrogen oxide conditions was investigated. Observed secondary organic aerosol yields (0.16 to 1.65) are higher than those for OH-initiated oxidation of C 8 − 12 alkanes (0.04 to 0.35). A high-resolution time-of-ﬂight chemical ionization mass spectrometer coupled to a Filter Inlet for Gases and AEROsols (FIGAERO–CIMS) was used to characterize the gas- and particle-phase molecular composition. Chlorinated organics were observed, which likely originated from chlorine addition to the double bond present on the heterogeneously produced dihydrofurans. A two-dimensional thermogram representation was developed to visualize the composition and relative volatility of organic aerosol components using unit-mass resolution data. Evidence of oligomer formation and thermal decomposition was observed. Aerosol yield and oligomer formation were suppressed under humid conditions (35 % to 67 % RH) relative to dry conditions (un-der 5 % RH). The temperature at peak desorption signal, T max , a proxy for aerosol volatility, was shown to change with aerosol ﬁlter loading, which should

Recent field studies have identified reactive chlorine compounds in diverse locales from natural and anthropogenic sources (Faxon and Allen, 2013;Finlayson-Pitts, 2010;Saiz-Lopez and von Glasow, 2012;Simpson et al., 2015). Tropospheric chlorine chemistry can enhance ozone production (Tanaka et al., 2003). Night-time production of nitryl chloride mediated by heterogeneous uptake of NO x onto chloride-containing particles represents an ubiquitous source of reactive chlorine (Thornton et al., 2010). In addition, biomass burning could also act as a source of ClNO 2 (Ahern et al., 2018). ClNO 2 photolysis in the early morning produces Cl and NO x , which has been shown to enhance RO 2 production from alkane oxidation near coastal regions (Riedel et al., 2012) as well as OH radical propagation in urban environments (Young et al., 2014). In addition to reactive chlorine emissions from water treatment (Chang et al., 2001) and fuel combustion (Osthoff et al., 2008;Parrish et al., 2009), the rising usage of volatile chemical products such as pesticides, cleaning products, and personal care products may be a significant source of reactive chlorine compounds and VOCs in urban environments (Khare and Gentner, 2018;McDonald et al., 2018). VOC-Cl oxidation products such as isomers of 1-chloro-3-methyl-3-butene-2-one, a tracer for isoprene-Cl chemistry (Nordmeyer et al., 1997), have been observed in highly polluted environments (Le Tanaka et al., 2003).
Akin to OH radicals, Cl radicals initiate reactions with alkanes via hydrogen abstraction, forming hydrogen chloride (HCl) and alkylperoxy radicals (RO 2 ). According to structure-activity relationships, terminal hydrogen abstraction occurs more frequently with alkane-Cl than with alkane-OH reactions (Kwok and Atkinson, 1995), resulting in different product distributions, for example increased formation of primary alkyl nitrates. The biomolecular gasphase reaction rate constants with linear alkanes for OH radicals and Cl radicals increase with the alkane chain length. Consequently, the reaction rate constants for Cl and linear C 8−12 alkanes range from 4.05 × 10 −10 (octane) to 5.36 × 10 −10 cm −3 molecules −1 s −1 (dodecane) at 298 K and 1 atm (Aschmann and Atkinson, 1995), which is over an order of magnitude higher than the rate constants for OH radicals, 8.11×10 −12 (octane) to 1.32×10 −11 cm −3 molecules −1 s −1 (dodecane; Atkinson and Arey, 2003). Studies show that Clinitiated oxidation of volatile organic compounds such as isoprene, monoterpenes, toluene, and polycyclic aromatic hydrocarbons can lead to rapid SOA formation with high yields (Cai et al., 2008;Cai and Griffin, 2006;Huang et al., 2014;Karlsson et al., 2001;Ofner et al., 2013;Riva et al., 2015;. Compounds consistent with isoprene-derived organochloride were recently observed in filter samples collected in Beijing (Le . Although all initial C 8−12 alkane-Cl oxidation products are expected be nonchlorinated, formation of alkanederived organochlorides is possible from Cl addition to multigenerational products. Studies show that the oxidation of alkanes with five or more linear carbons produces 1,4-hydroxycarbonyl that can undergo a rate-limiting, acid-catalyzed heterogeneous reaction to produce dihydrofuran (DHF) compounds Holt et al., 2005;Jordan et al., 2008;Ziemann, 2009b, 2009c). DHF is highly reactive: for 2,5-dihydrofuran, the bimolecular reaction rate constants with O 3 , OH and Cl are 1.65 ± 0.31 × 10 −17 , 6.45 ± 1.69 × 10 −11 , and 4.48 ± 0.59 × 10 −10 cm 3 molecule −1 s −1 , respectively (Alwe et al., 2013(Alwe et al., , 2014. Chlorine radicals can react with dihydrofuran via both H abstraction and Cl addition, producing chlorinated (e.g., dichlorotetrahydrofurans) and nonchlorinated compounds (e.g., furanones) under low NO x conditions (Alwe et al., 2013). Similarly, formation of both chloronitrates (via Cl addition) and organonitrates (via H abstraction) from alkane-Cl oxidation is possible in the presence of NO x . In this study, environmental chamber studies were conducted using long-chain (C 8−12 ) n-alkanes and Cl radicals under high-NO x conditions to evaluate the Cl-initiated SOA formation from alkanes and to characterize the molecular composition of gas and aerosol compounds.

Environmental chamber experiments and instrumentation
Experiments were conducted inside a 10 m 3 Teflon ® chamber at 298 K, using UV lights to generate radicals. The NO 2 photolysis rate was measured to characterize the UV intensity (Carter et al., 2005) and was found to be similar to ambient levels (0.53 min −1 at 0 • zenith angle) at approximately 0.5 min −1 . The chamber relative humidity (RH) ranged from 0 to 67 %. For dry experiments, the chamber was filled with dried clean air supplied by a clean-air generator (model 737R, Aadco). Under typical atmospheric conditions, elevated RH can be expected, especially within the marine boundary layer near coastal regions, where Cl-alkane chemistry may be important (Riedel et al., 2012). Therefore, SOA formation under humid conditions was also investigated. For humid experiments (i.e., RH > 20 %), the chamber was flushed overnight with humidified clean air. To reduce wall loss, dried ammonium sulfate seed particles were injected into the chamber. The seed particles were generated from a 0.01 M aqueous ammonium sulfate solution using an aerosol generation system (AGS 2002, Brechtel). Chlorine gas (101 ppm in N 2 , Airgas) was injected as the Cl radical precursor. Each n-alkane (n-octane at 99 %, n-decane to the sum of all organic ion fragments observed at peak SOA concentration. Used as a proxy for the SOA extent of oxidation. d Mass ratio of organic ion fragments at m/z 57 (presumably C 4 H + 9 ) to the sum of all organic ion fragments observed at peak SOA concentration. Used as a proxy for hydrocarbon-like organic aerosol. e Ratio of particulate chlorine mass (estimated using HCl + ion fragments in the ACSM) to organic mass. Lower value is the ratio observed at peak organic aerosol concentration and the higher value is the ratio observed at the peak particulate chlorine concentration. Particulate chlorine and organics peak at different times due to the rate-limiting heterogeneous production of dihydrofurans. Chloride concentrations were near-detection limits for octane experiments and may be more sensitive towards vaporizer interference effects . f The amount of ozone observed when peak SOA concentration was observed.
at 99 %, dodecane > 99%; Sigma-Aldrich) was first injected into a glass gas sampling tube (Kimble-Chase, 250 mL), which was flushed with gently heated clean air into the chamber at 2 L min −1 for at least 30 min. Precursor NO (9.98 ppm in N 2 , Airgas) and NO 2 (9.86 ppm in N 2 , Airgas) were injected into the chamber using a mass flow controller (GFC17, Aalborg). Initial concentrations of VOC and oxidant precursors are summarized in Table 1. Gas-phase NO and NO 2 concentrations were monitored by a chemiluminescence monitor (200E, Teledyne). The O 3 concentration was monitored using a photometric ozone analyzer (400E, Teledyne). The start of photooxidation, initiated by turning on all the UV lights after the precursors were well mixed in the chamber, was designated as the reference point (i.e., time 0 min) for each experiment. UV lights were turned off after 60 min. The day before and after each SOA formation experiment, ammonium sulfate seed particles and 50 ppb of chlorine gas were injected into the chamber. UV lights were turned on to generate Cl radicals and remove residual reactive organic compounds in the chamber. Minimal SOA formation was observed during these cleaning experiments.
Particle size distributions were characterized using a scanning electrical mobility system (SEMS, Brechtel model 2002). The particle-phase bulk chemical composition was measured using an aerosol chemical speciation monitor (ACSM, Aerodyne). The ACSM was calibrated with 300 nm size-selected ammonium nitrate and ammonium sulfate aerosols generated from nebulized 0.005 M solutions using the AGS to determine the nitrate response factor (RF) and relative ionization efficiencies (RIE) for ammonium and sul-fate, which are required for ion-to-mass signal conversions. Using electron impact ionization, the ACSM can measure the submicron, nonrefractory aerosol bulk composition at 1 min intervals (Budisulistiorini et al., 2013;Ng et al., 2011a). Using a standard fragmentation table (Allan et al., 2004), the ACSM can speciate the aerosol content into organics, nitrate, sulfate, ammonium, and chloride (Ng et al., 2011a). The ability of the ACSM to detect organic chloride using HCl + (m/z 36) has been demonstrated previously for isoprene-Cl SOA . The default RIE Chl value of 1.3 was used for chloride mass conversion. The Cl + (m/z 35) ion was excluded from chlorine quantification as it showed inconsistent response to nonrefractory chlorides (e.g., ammonium chloride). ACSM data were analyzed in Igor Pro V6.37 (Wavemetrics, Inc.) using ACSM local v1603 (Aerodyne) and other custom routines. Organic aerosol concentrations were calculated using ACSM measurements assuming a collection efficiency of 0.5 and RIE of 1.4, corrected for depositional particle wall loss (Pathak et al., 2007) but not for organic vapor loss (Huang et al., 2018b;Krechmer et al., 2017;Nah et al., 2017).

FIGAERO-CIMS
A high-resolution time-of-flight chemical ionization mass spectrometer (CIMS, Aerodyne) was used to measure the gas-phase chemical composition using I − as the chemical ionization reagent. Humidified UHP N 2 was flushed over a methyl iodide permeation tube and then through 210 Po ionizer into the ion-molecule reaction chamber of the CIMS. Theory and operation of the CIMS are described in detail elsewhere (Aljawhary et al., 2013;Bertram et al., 2011;Lee et al., 2014;. The CIMS inlet was coupled to a Filter Inlet for Gases and AEROsols (FIGAERO), which has been used in a number of studies to investigate the chemical composition and volatility of particle-phase compounds (D'Ambro et al., 2017;Gaston et al., 2016;Huang et al., 2018a;Lee et al., 2016;Lopez-Hilfiker et al., 2014). The FIGAERO system alternated between two operational modes. In the aerosol collection mode, a gasaerosol mixture was drawn through a PTFE filter (Zefluor ® 2.0 µm 24 mm, Pall Corp.) at 3 SLPM for 15 to 45 min while gas species were sampled and analyzed. In the desorption mode, clean air or UHP N 2 was passed through the filter and heated to 200 • C (measured just above the filter) at a rate of approximately 5 to 10 • C min −1 for 40 to 20 min, respectively. The filter was then soaked at 200 • C for 20 min. The volatilized vapor was sampled by the CIMS, and the desorption signal as a function of temperature can be used to construct a one-dimensional (1-D) thermogram (e.g., Fig. 6a). For a monomeric compound, the desorption signal as a function of temperature is expected to be monomodal . The temperature at the peak desorption signal, T max correlates with the enthalpy of sublimation (dH sub ) and the saturation vapor pressure (C * ) of the compound . However, some compounds may exhibit bimodal or multimodal behavior, with a second desorption mode occurring at much higher temperatures than expected, which is often interpreted as the result of thermal decomposition of larger oligomeric compounds (Huang et al., 2018a;Lopez-Hilfiker et al., 2014;Wang et al., 2016).
A FIGAERO-CIMS data analysis was conducted in Igor Pro with Tofware v2.5.10 (Tofwerk) and custom routines. Hundreds of ions belonging to diverse chemical families can be retrieved from the mass spectra. To simultaneously represent the chemical composition, relative aerosol volatility (i.e., T max distribution), and multimodal thermal desorption behaviors, a two-dimensional (2-D) thermogram framework was developed. The 2-D thermogram is comprised of normalized unit-mass resolution 1-D thermograms, each expressed as a percentage color scale of the maximum desorption signal. Two-dimensional thermogram applications are discussed in Sect. 3.2. The advantage of using UMR over HR data is the ability to investigate the SOA thermal desorption behavior over the entire m/z and volatility (i.e., T max ) range without having to assign chemical formulae to all ion. This HR analysis is time consuming, especially for highmolecular-weight compounds, the exact molecular composition of which can be difficult to ascertain. The disadvantage of using UMR over HR data is the overlapping of ions and potential interference by isotopic signals or nonadduct ions. Therefore, the 2-D thermogram should be used as a complement rather than a replacement of the HR analysis.
Based on the SOA molecular composition as observed by the FIGAERO-CIMS, the average oxidation state of carbon (OS C ) may be estimated: where NO 3 : C, Cl : C, O : C, and H : C are the molecular ratios of the number of -NO 3 functional groups, -Cl functional groups, non-NO 3 oxygen atoms, and H atoms to the number of carbon atoms for any given compound. The average SOA OS C is calculated based on iodide adducts only. For simplicity, all organic ions were assumed to have equal sensitivity, which is known to vary with ion cluster binding energy and sample RH (Hyttinen et al., 2018;Iyer et al., 2016;Lopez-Hilfiker et al., 2016).
3 Results and discussion

SOA and organic chloride formation
Experimental conditions and results are summarized in Table  1. Chlorine-alkane SOA yields increased with VOC precursor length, consistent with the trend observed for OH-alkane SOA (Jordan et al., 2008;Lim and Ziemann, 2009b;Presto et al., 2010;Yee et al., 2012Yee et al., , 2013. For similarly functionalized alkane oxidation products, the vapor pressure decreases roughly loglinearly as the precursor carbon number increases from 8 to 15 (Jordan et al., 2008;Presto et al., 2010). Comparison of the unit-mass ACSM spectra for octane (Exp. 3), decane (Exp. 7), and dodecane (Exp. 11) SOA in Fig. S1 in the Supplement shows a consistent increase in the fractional contributions to the bulk OA mass (i.e., f m/z ) by organic ions at m/z 27 (C 2 H + 3 ), 41 (C 3 H + 5 ), 55 (C 4 H + 7 ) and 57 (C 4 H + 9 ) with increasing alkane length. Select odd m/z ions, noticeably m/z 55 and 57, have been used as tracers for hydrocarbon-like organic aerosol (HOA), and sometimes for primary aerosol emissions as well (Ng et al., 2011b;Ulbrich et al., 2009). Given the same oxidation conditions, SOA products derived from longer alkane precursors appeared less oxidized, as seen by the decrease in f 44 (presumably mostly CO + 2 ), but are more hydrocarbonlike, as seen by the increase in f 57 , consistent with previous observations Loza et al., 2014). As the alkane chain length increases, the increased SOA production also favors the partitioning of semivolatile compounds into the particle phase (Donahue et al., 2006;Pankow, 1994), further lowering f 44 .
Time series of gas-phase NO, NO 2 , O 3 , Cl 2 , and suspended particle-phase organics and chlorine concentrations during photooxidation are shown in Fig. 1 using dodecane-Cl oxidation under low RH (Exp. 11) as the representative example. The Cl 2 concentration was estimated using I − CIMS by tracking the Cl 2 I − ion. In addition, ions consistent with HO 2 NO 2 , HONO, and ClNO 2 were observed in the gas phase, as shown in Fig to correspond to both ClNO 2 (< 20 %) and chlorine nitrite (ClONO, > 80 %) from the reaction between Cl q and NO 2 (Golden, 2007;Niki et al., 1978). Formation of HO 2 NO 2 from the reaction of HO 2 and NO 2 has been observed using I − CIMS previously (Veres et al., 2015). Formation of HONO was likely due to the interactions between HO x and NO (see Reactions S2 and S3 in the Supplement). Under UV, HONO decays to background levels. Production of HO 2 NO 2 and HONO is indicative of secondary HO x chemistry enabled by primary Cl-alkane oxidation chemistry, consistent with previous studies Young et al., 2014). The NO concentration decreased sharply at the beginning of the photooxidation accompanied by increases in the concentrations of NO 2 and ozone. SOA production was most rapid during this initial period (0 to 10 min). Afterwards, at around 15 min in Fig. 1, the concentrations of NO 2 and ozone stabilized. Oxidation continued under UV driven by Cl radicals, and the SOA concentration began to decay due to oxidative fragmentation (Kroll et al., 2011;Lambe et al., 2012; and wall loss. Ozone production continued slowly under UV at ∼ 0.24 ppb min −1 . Slight NO 2 production (< 0.1 ppb min −1 ) was also observed, which may be due to production of nitrous acid (HONO) on a Teflon ® surface, a common background contaminant in environmental chambers (Carter et al., 2005;Rohrer et al., 2005).
SOA production was dependent on the initial NO and NO 2 concentrations, as shown in Fig. S2. Higher initial NO concentrations led to higher-SOA yields and lower ozone production for all precursors, as shown in Table 1 and Fig. S2. This is similar to alkane-OH SOA formation, where higher NO concentrations lead to more abundant organic nitrate formation, which increases the SOA volume (Schilling Fahne-stock et al., 2015) and density . This is consistent with FIGAERO-CIMS results where the particulate organonitrate molar ratio (calculated as the average number of -NO 3 per molecule) and mass fraction were the highest in SOA produced under NO-only conditions (see Table S1 in Supplement). The organonitrate molar ratio also increased with the alkane precursor length, from 0.57-0.64 for octane to 0.75-0.93 for dodecane. A similar trend was observed for the organonitrate mass fraction, which increased from 0.53-0.58 for octane to 0.66-0.72 for dodecane. The alkane-Cl SOA yields, ranging from 0.16 (octane) to 1.65 (dodecane), are 4 times higher than the alkane-OH SOA yields (0.04 for octane and 0.35 for dodecane) that were obtained using 1 ppm alkane, 10 ppm methyl nitrite, and 10 ppm NO (Lim and Ziemann, 2009a), which are much higher than the precursor concentrations used here (13 to 15 ppb alkanes, < 40 ppb NO x , 40 ppb Cl 2 ). Formation of primary alkyl nitrates due to terminal H abstraction by Cl could contribute to the higher SOA yields observed for alkane-Cl than for alkane-OH oxidation, as primary alkyl nitrates have lower saturation vapor pressures than secondary alkyl nitrates (Lim and Ziemann, 2009b;Ziemann, 2014, 2015). The apparent SOA density, calculated using the ACSM SOA mass measurement and the SEMS SOA volume measurement, was 2.1 ± 0.3 g cm −3 , which is substantially higher than that reported for OH-alkane SOA formed under high-NO x conditions, 1.06 to 1.28 g cm −3 (Lim and Ziemann, 2009a;Loza et al., 2014). These differences could be due to uncertainties associated with the ACSM and similar instruments, specifically the RIE (Li et al., 2018;Xu et al., 2018) and CE (Docherty et al., 2013;Middlebrook et al., 2012;Robinson et al., 2017), which may vary with SOA composition, oxidation state, or phase state. Assuming an 15540 D. S. Wang and L. Hildebrandt Ruiz: Chlorine-initiated oxidation of n-alkanes  Figure 2. Formation pathway of chlorinated organics via chlorine addition to the DHF. H abstraction from DHF is also possible. Ozone may also react with the double-bond on the DHF, competing with chlorine radicals. Reaction pathways leading to DHF are adapted from OH-initiated oxidation of alkanes (Lim and Ziemann, 2009b).
SOA density of 1.06 g cm −3 , the SOA yield (0.10 to 0.99) is still higher for Cl-alkane oxidation compared to OH-alkane oxidation. Molar SOA yields, calculated using the average molecular weight of species identified with the FIGAERO-CIMS, are summarized in Table S1.
Although the initial Cl-alkane reactions proceed via a Habstraction pathway, particulate chlorine was observed using the ACSM as shown in Fig. 1. Direct halogenation of the C 8−12 alkyl radicals is expected to be minimal, given the low amounts of Cl 2 present (maximum of 40 ppb). A carbon-carbon double bond is required to enable Cl-addition reactions and organic chlorine formation. The heterogeneous production of dihydrofuran (DHF) via 1,4-hydroxycarbonyl uptake, acid-catalyzed isomerization, and dehydration reactions Jordan et al., 2008;Lim and Ziemann, 2009c, b), followed by Cl addition to the DHF double bond, could be the source of observed organic chlorine. A condensed reaction pathway is shown in Fig. 2. The delay in particulate chlorine formation relative to that of bulk organics, as shown in Fig. 1, is consistent with the rate-limiting heterogeneous DHF production (Holt et al., 2005;Zhang et al., 2014). Under high-NO x conditions, the peroxy radical product from the chlorine-addition pathway could react with NO to form a cyclic hemiacetal chloronitrate or an alkoxy radical that would undergo ring-opening reactions. Compounds resembling chloronitrates (e.g., ONO 2 −C 12 H 18 ClO 3 · I − for dodecane) were tentatively identified in the particle phase using the FIGAERO, but they were not well separated from the shoulder of nearby organonitrate peaks (e.g., ONO 2 −C 12 H 21 O 5 · I − ), as shown in Fig. S9. Select C 2−6 organochlorides were well separated from nearby peaks and confirmed by the distinct Cl isotopic signal, the thermograms of which are shown in Fig. S10. The most abundant gas-phase organic chlorine compounds observed were products of ring-opening reaction pathways such as C 2 H 3 ClO 2 and C 4−6 compounds. Small amounts of C 2 H 2 Cl 2 were also observed.
Organochloride formation is expected to be lower under humid conditions, where DHF formation is inhibited (Holt et al., 2005;Zhang et al., 2014;Ziemann, 2011). Evidence consistent with organochloride suppression under humid conditions was observed for dodecane SOA only, where the organochloride (including chloronitrates) mass fraction de-creased from 0.15 (Exp. 11, < 5 % RH) to 0.13 (Exp. 12, 67 % RH) as measured by the FIGAERO-CIMS. The mass fraction of the -Cl functional group decreased from 1.8 E −2 to 1.6 E −2 as measured by the FIGAERO-CIMS (Table S1) or from 1.4 E −2 to 1.1 E −2 as measured by the ACSM (Table 1) as the RH increased. No clear differences were observed for octane or decane SOA, which may be due to the less extreme RH conditions investigated, uncertainties with organochloride ion identification in the CIMS, or the lower organochloride concentrations observed in Exps. 1-8, which is especially challenging for chloride quantification using the ACSM.
DHF ozonolysis can compete with chlorination. For instance, under typical marine boundary layer (MBL) conditions, the chlorine-initiated oxidation was estimated to be a significant sink of 2,5-DHF, accounting for 29 % of the reaction in the presence of OH and O 3 (Alwe et al., 2014). In contrast, only 1.8 % of 2,3-DHF consumption was attributed to Cl radical chemistry in the MBL, owing to the increased reactivity of 2,3-DHF (relative to 2,5-DHF) towards ozone and OH radicals (Alwe et al., 2014). The reported alkane-OH reaction mechanisms expect the formation of substituted 2,3-DHF (Ziemann, 2011). The alkane-derived organochloride yield under ambient conditions is therefore expected to be smaller in the presence of elevated RH and O 3 levels. Although ozone can impose an upper limit on alkane SOA yields and oxidation state, suppressing the multigenerational OH-alkane oxidation chemistry , the continued gas-phase processing of OH-alkane and DHFozonolysis products via H abstraction by chlorine radicals could counteract these limitations.
The gas-phase alkane-Cl oxidation products formed under high-NO x conditions were dominated by organonitrates, which was also reflected in the dominance of organonitrates in the particles phase (see Table 1 and Sect. 3.2). Figure 3 shows the formation of early-generation (e.g., hydroxynitrates, ONO 2 −C n H 2n+1 O) and multigenerational (e.g., oxidized organonitrates, ONO 2 −C n H 2n−5 O 4 ) oxidation products from different alkane precursors. Like the bulk particle-phase composition, gas-phase compounds derived from smaller alkane precursor were more oxidized given similar oxidation conditions (e.g., NO, NO 2 , and Cl 2 ): the signal of oxidation products with similar oxygen numbers   Figure 3. Trends of early-generation gas-phase oxidation products (hydroxynitrates) and multigeneration-oxidation products (hydroxycarbonyl nitrates) observed during chlorine-initiated oxidation of octane (Exp. 3), decane (Exp. 7), and dodecane (Exp. 11). Species shown were first normalized against the I − reagent ion signal and then normalized against their respective maxima.
(and higher oxidation state) peaked earlier in the photooxidation period for smaller precursors (e.g., ONO 2 −C 8 H 11 O 4 vs. ONO 2 −C 12 H 19 O 4 in Fig. 3). As oxidation continued, driven primarily by gas-phase chemistry , the importance of fragmentation reactions increased relative to that of functionalization reactions . The heterogeneous oxidation of SOA (Bertram et al., 2001;George and Abbatt, 2010), which is expected to drive the oxidation of very large (n > 30) alkanes (Lim and Ziemann, 2009b), may also contribute to oxidation and fragmentation observed here, but its impacts are beyond the scope of this work. Assuming a uniform CIMS sensitivity, Figure 4a shows that the gas phase was dominated by C 3 to C 5 organic nitrates. Figure 4b shows that the OS C increased, while n C decreased as photooxidation continued, indicative of fragmentation reactions. In addition to hydroxynitrates and hydroxycarbonyl nitrates, dinitrates and trinitrates were also observed. The CIMS was not sensitive towards simple, alkane-derived alkyl nitrates (ONO 2 −C n H 2n+1 ). The lack of sensitivity of the I − reagent ion towards simple alkyl and keto nitrates has been reported previously for isoprene-and monoterpene-derived organic nitrates .

Two-dimensional thermogram
The FIGAERO filter spectra are shown in Fig. 4c for octane (Exp. 3), decane (Exp. 7), and dodecane (Exp. 11). A logscale version of Fig. 4c is shown in Fig. S3. The spectra are calculated from the average desorption ion signals observed when the filter temperature was between 40 and 140 • C. As shown below and in Fig. 5, SOA components desorbed most effectively in this temperature range, with most organic ions having T max within this temperature range. At desorption temperature above 140 • C, inorganic ions began to dominate the spectra. The C 8−12 alkane-Cl SOA share many similarities towards the lower m/z (< 320) range, which consisted of C ≤7 oxidized organic compounds. For larger alkane precursors, the particle-phase composition was dominated by C n organic nitrates, which are grouped by their degree of oxygenation (i.e., number of nonnitrate O atoms), O x , as shown in Fig. 4c Fig. S4. Assuming the same sensitivity towards the different organic nitrates observed, the organic nitrate abundance follows a bell-shaped distribution, similarly to field observations for C 5 and C 10 organic nitrates derived from isoprene and monoterpenes, respectively  As the precursor length increased, the SOA appeared less oxidized, as shown in Fig. 4d, consistent with ACSM observations for f 44 (see Table 1 and Fig. S1), which is correlated with OS C (Canagaratna et al., 2015). Simple hydroxynitrates (e.g., ONO 2 −C n H 2n+1 O) were not observed in the particle phase as they were completely oxidized in the gas phase, as shown in Fig. 3. In addition to the identification of aerosol chemical composition, the FIGAERO-CIMS can be used to estimate the   Color-coded ions in (a) and (c) indicate molecules that were much more abundant in reaction products derived from a particular alkane precursor. In (c), O x , dO x , and tO x were used as shorthand notations for oxidized organic nitrate, dinitrate, and trinitrates with x number of oxygens (excluding those from ONO 2 ). Sticks in (a) and (c) were not stacked, and no x-axis offset was applied during plotting. In (b), OS C increased and n C decreased as photooxidation continued, consistent with photooxidative fragmentation. In (d), SOA derived from larger alkane precursors appeared less oxidized, consistent with results shown in Table 1 and Fig. S1. Methods used to calculate the average OS C and n C are detailed in the Supplement.
aerosol volatility using empirical correlations between C * and T max . The particlephase chemical composition and volatility distribution can be represented in several ways. For instance, desorption signals of select compounds can be normalized against the maximum and plotted on the same 1-D thermogram (ion signal vs. filter temperature) to compare their relative volatilities . Scatterplots of oxygen number or O : C ratio vs. carbon number colored by T max or particle-phase mass fraction have been used to show changes in aerosol composition (Huang et al., 2018a;Lopez-Hilfiker et al., 2014. To show the differences between various types of thermal desorption products (i.e., monomers, thermal decomposition products, and oligomers), averages of the 1-D thermograms can be shown (Huang et al., 2018a;Lopez-Hilfiker et al., 2015). Scatterplots of T max or the OS c vs. molecular weight have been used to distinguish monomers and oligomers for select compounds (Wang et al., 2016).
Here we propose a new framework to represent aerosol composition, relative volatility, and oligomer decomposition simultaneously using 2-D thermograms shown in Fig. 5a-e.
As illustrated in Fig. 5a for octane-chlorine SOA, the thermal desorption products can be separated into five different groups. Region 1 (m/z < 350, 40 < T max < 90 • C) was composed of a group of semivolatile compounds with similar T max values. Iodide adducts in this region correspond to species that are too volatile to be present as molecular compounds in the particle phase and are likely lowtemperature decomposition products. A prominence of ions smaller than m/z 127 including Cl − and a range of organic ions could be due to acid exchange or charge transfer products (as opposed to I − adducts) of low-temperature decomposition products. Although I − is a relatively soft ionization method compared to electron impact ionization, collision-induced ion fragmentation and cluster dissociation cannot be ruled out as a potential source for nonadducts. Region 2 (350 < m/z < ∼ 700, 40 • C < T max < 120 • C) consisted of monomers, dimers, and some oligomers. Oligomerization may proceed via condensed-phase reactions between cyclic hemiacetal compounds, forming acetal dimers, or reactions between cyclic hemiacetal and 1,4-hydroxycarbonyl compounds, forming hemiacetal dimers (Aimanant and Ziemann,   D. S. Wang and L. Hildebrandt Ruiz: Chlorine-initiated oxidation of n-alkanes 2013). Condensed-phase, acid-catalyzed isomerization of hemiacetal and acetal dimers is also possible . For OH-initiated oxidation of dodecane under high-NO x conditions, hemiacetals and nitrate hemiacetals are reported to dominate the oligomer composition , where nitrate hemiacetals could form via the reactive uptake of hydroxy dihydrofuran and hydroxycarbonyl nitrate compounds . The three oligomerization pathways are illustrated in Fig. S11. Ions consistent with acetal, hemiacetal, and nitrate hemiacetal compounds were observed in the particle phase using the FIGAERO-CIMS in regions 2 and 3. Oligomer formation via aldol condensation by highly oxidized compounds is also possible . Oligomerization may also proceed through a NO 3 elimination and a reverse esterification process involving two alkyl nitrate compounds to form a single nitrate oligomer, which was recently proposed for α-Pinene SOA, though the exact mechanism remains elusive (Faxon et al., 2018). An overall positive correlation between T max and molecular weight was observed in region 2. A similar correlation was also reported for SOA derived from oleic acid (Wang et al., 2016) and from α-Pinene (Faxon et al., 2018). In other words, overall, volatility decreased with molecular weight for compounds in region 2. However, within small m/z segments in region 2, the correlation was not strictly linear and the T max varied cyclically. When the carbon and oxygen numbers were fixed (i.e., fixed O : C), the T max decreased (i.e., volatility increased) with increasing H : C (i.e., decreasing OS C ), as shown in Fig. 5e for C 12 organic nitrates (e.g., ONO 2 −C 12 H 19−25 O 3 · I − from m/z 400 to 406) observed in dodecane-Cl SOA desorption. Although decreases in OS C are often associated with increases in the aerosol volatility Kroll et al., 2011), fragmentation notwithstanding, the increase in H : C in each fixed O : C ion group was expected to decrease in aerosol volatility. The increase in H : C was likely due to increased hydroxy functionalization over ketone functionalization, which should result in lowering of the saturation vapor pressure according to group contribution theory (Pankow and Asher, 2008). In addition, the T max decreased from O 3 : C 12 organonitrates (e.g., ONO 2 −C 12 H 21 O 3 , T max 95 • C) to O 4 : C 12 organonitrates (e.g., ONO 2 −C 12 H 21 O 4 , T max 89 • C). The inconsistencies observed within small m/z segments may be due to loadingdependent T max shift (see Sect. 3.3), where T max increases with mass loading. As shown in Fig. 4c, the organonitrate ion intensities decreased from O 3 onwards. Overlapping of monomer and oligomer desorption peaks could also increase the apparent T max .
Region 3 (T max > 90 • C) consisted of thermal decomposition products across a wide m/z range. Select ions were present in both region 2 and region 3, resulting in bimodal thermal desorption, which can complicate T max estimation using the 2-D thermogram, especially when ions from the two desorption modes overlap. The thermal decomposition products may be the result of reversible dissociation of oligomers (Faxon et al., 2018;Schobesberger et al., 2018) or the cleavage of C-O or C-C bonds, which has been observed in the thermal decomposition of monomeric organic acids . Additionally, SOA may exist in a "glassy" state (Koop et al., 2011), trapping volatile components (Perraud et al., 2012) that are released as the SOA mass is depleted. Region 4 (150 • C < T max < 170 • C) consisted of mostly thermal decomposition products of the (NH 4 ) 2 SO 4 seed particles. There was a clear gap between region 3 and 4. It is possible that the edge of regions 2 and 3, towards a high T max range, is the FIGAERO-CIMS's volatility detection limit for alkane-Cl SOA, where the energy required for vaporization exceeds that for thermal decomposition . Plateauing of T max towards the high m/z range of region 2 appeared to agree with this hypothesis, for which further increases in molecular weight, carbon number, and the degree of oxidation translated to negligible T max increase. Region 5 (m/z > 700) likely consisted of high molecular weight, low volatility oligomers. Region 5 therefore likely consisted of C 24−36+ (dimers and above) compounds. Thermal desorption of some high-molecularweight compounds at temperatures much lower than expected (T max ∼ 80 • C) was observed for decane SOA and dodecane SOA. No definitive molecular composition was determined for these ions due to low signal intensity and wide range of potential chemical formulae.
The region boundaries can vary depending on the SOA composition as well as mass loading, as will be discussed in Sect. 3.3. The distribution and general features of the regions are consistent for alkane-Cl SOA formed in different experiments (Figs. 5 and S8). Comparison of 2-D thermograms for octane-Cl SOA (Fig. 5a) and dodecane-Cl SOA (Fig. 5b) shows that the dodecane SOA contains unique, high-molecular-weight oligomers (m/z > 750), which are accompanied by significantly stronger thermal decomposition features in region 3. Bimodal thermal desorption behavior was observed, as shown in Fig. 5d, which masked the volatility m/z dependence expected for region 2. Compared to SOA formed under dry conditions (Fig. 5b, Exp. 11), SOA formed under humid conditions (Exp. 12, 67 % RH) contained less high-molecular-weight oligomers in region 5 and exhibited much less thermal decomposition behavior in region 3, as shown in Fig. 5e, where a more distinct T max -m/z correlation could be established compared to Fig. 5d. RH-induced oligomer suppression has been reported for toluene and α-Pinene SOA formation (Hinks et al., 2018;Huang et al., 2018a). The oligomer decrease under high RH could also be due to RH-volatility dependent organic vapor wall loss (Huang et al., 2018b). A clear m/z dependence of signal reduction under humid conditions compared to the dry conditions is shown in Fig. S5. Additionally, SOA yield decreased (37 % for decane and 22 % for dodecane) under humid conditions while ozone production increased, which could be related to humidity-induced inhibition of 1,4-hydroxycarbonyl  Figure 6. (a) The 1-D thermograms for levoglucosan (C 6 H 10 O 5 ) at different loading conditions and (b) the correlation between filter loading and T max . Levoglucosan aerosol was generated by nebulizing a 1.2 E −2 M aqueous solution. The aerosol was injected into the clean Teflon chamber, collected onto the FIGAERO filter, and analyzed. The T max -loading correlation for pure levoglucosan could be described by a sigmoid function, leveling off at 41 and 64 • C under very low and very high loading conditions, respectively. uptake and heterogeneous DHF formation (Holt et al., 2005). DHF is a key intermediate product for multigenerational oxidation chemistry and is reactive towards ozone Ziemann, 2011). Reduction in SOA yields under humid conditions has also been reported for dodecane-OH SOA formation . The combined effects of DHF reduction and ozone enhancement would suppress organic chloride formation, which was observed for dodecane SOA, as discussed in Sect. 3.1.

T max shift
As the desorption temperature increased above 140 • C, all dominant ions observed shared the same T max , except for some background ions (e.g., IHNO − 3 ), as shown in Fig. 5ac, which were identified to be related to the thermal decomposition of (NH 4 ) 2 SO 4 seed particles. From this process, H 2 SO 4 vapor molecules produced were either detected as H 2 SO 4 · I − or were deprotonated to produce HSO − 4 , acting as a secondary chemical ionization reagent. Examples of first-order (e.g., HSO − 4 ), second-order (e.g., H 2 SO 4 · HSO − 4 ) and third-order sulfate (e.g., (H 2 SO 4 ) 2 · HSO − 4 ) clusters are shown in Fig. S6. It is worth noting that, while the T max of sulfate ions was uniform within each filter desorption run, the T max varied between filter runs, which can be seen in Fig. 5a-c to be between 146 and 153 • C. Characterization experiments suggested that T max may increase with filter loading, as shown in Fig. 6a for pure levoglucosan aerosols and in Fig. S7a for pure (NH 4 ) 2 SO 4 aerosols. Whereas the T max -mass loading correlation appeared to be linear for pure (NH 4 ) 2 SO 4 , as shown in Fig. S7b. for T max between 136 and 149 • C, a roughly sigmoidal correlation was observed for pure levoglucosan, as shown in Fig. 6b, where T max increased quickly from 43 to 62 • C as filter loading increased from 0.2 to 0.8 µg.
T max shift phenomena have been observed in several FIGAERO-CIMS studies (D'Ambro et al., 2017;Gaston et al., 2016;Huang et al., 2018a;Lopez-Hilfiker et al., 2015;Thompson et al., 2017). Explanations vary for the observed T max shifts. For example, pinonic acid (C 10 H 16 O 3 ) identified in α-Pinene ozonolysis SOA had a much higher T max (40 • C) than expected from calibration (T max < 32 • C), which was attributed to interactions between pinonic acid and other SOA components, causing a decrease in the measured apparent vapor pressure (Lopez-Hilfiker et al., 2015). However, no matrix effects were reported for the pinonic acid T max value (now around 65 • C instead of < 32 • C) obtained from a synthetic mixture of organic acid calibrants , where some interactions between different organic molecules might have been expected. Whereas the pure compounds are shown to have sharp monomodal desorption peaks , the field campaign-average thermograms for individual compounds have broader desorption peaks , which could be due to the presence of isomers , variations in filter loading, differences in aerosol viscosity (Huang et al., 2018a), or interferences from oligomer decomposition products . Another T max shift example was reported for ambient biomass burning measurements, where the levoglucosan T max varied between 58-70 • C, in the range of the 61.5 • C literature value  and the values observed here (41-64 • C), between 17:00 and 10:00 over the course of the campaign (Gaston et al., 2016). Between 10:00 and 19:00, the levoglucosan T max was significantly higher at 100 • C, which was attributed to thermal decomposition of oligomers produced from acid-catalyzed heterogenous reactions, as indicated by the increase in sulfate ion intensities during the same period (Gaston et al., D. S. Wang and L. Hildebrandt Ruiz: Chlorine-initiated oxidation of n-alkanes 2016). It was not clear whether the sulfate ions measured were derived from sulfuric acid or from the decomposition of (NH 4 ) 2 SO 4 , which could change the assumption made for aerosol acidity. A T max shift was also observed for isoprene SOA, where the mass and T max of nonnitrated OA decreased with increasing NO x concentration (D'Ambro et al., 2017). A recent investigation reported increases in T max for α-Pinene ozonolysis SOA produced under dry conditions compared to SOA produced under humid conditions (Huang et al., 2018a). A positive correlation between aerosol loading and T max was reported for the first time, and the uniform thermal desorption peak shape assumption was questioned (Huang et al., 2018a). The loading dependence was reported to plateau at around 2 to 4 µg (converted from CIMS ion intensity assuming maximum sensitivity, i.e., that of formic acid), similarly to observations for levoglucosan standards as shown in Fig. 6b, possibly due to saturation effects. Because increased oligomer content was observed under dry conditions, the authors suggested that viscosity effects were responsible for the observed T max shift (Huang et al., 2018a). It should be noted that plateauing of the T max -loading dependence was demonstrated using averaged values, where ions were lumped based on carbon number. Those that had less than or equal to 10 carbons were designated as monomers, which likely included desorption ions across multiple desorption regions (see Fig. 5). This categorization was perhaps too broad, and the loading dependence of individual ions could be lost after averaging. In contrast, the T max of oligomers, which were defined as compounds that contained more than 10 carbons -thus were a more precisely defined group -only began to plateau at higher concentrations.
The T max mass dependence was also observed for alkane-Cl SOA. For Exp. 6, three filters were collected and analyzed. The first filter was collected at 3 SLPM for 45 min, followed by the second filter at 3 SLPM for 30 min, and afterwards the third filter at 3 SLPM for 15 min. The ratios of the unit-mass integrated ion signals during the desorption period for the first two filters relative to that of the third filter are shown in Fig. S8a. The associated 2-D thermograms are shown in Fig. S8b-d. Compounds in the T max -m/z dependence region (Region 2) showed a 10 to 20 • C T max increase from the lowest filter-loading to the highest filter-loading conditions. T max of sulfate-related ions increased from 149 to 172 • C with filter loading. The increase of enhancement ratio with m/z, as shown in Fig. S8a, reflects the changes in the composition of suspended aerosol in the chamber over time. Between the first (high-loading) filter collection and the third (low-loading) filter collection, volatility-dependent vapor wall loss may lead to a disproportionate decrease in high-molecular-weight, low-volatility compounds (Huang et al., 2018b;Krechmer et al., 2016), resulting in increasingly greater enhancement towards the high m/z region shown in Fig. S8a. The correlation between T max shift and integrated desorption signal for organics was not linear, unlike for ammonium sulfate, which may be due to matrix or saturation effects as previously suggested (Huang et al., 2018a). T max -loading dependence may differ for individual FIGAEROs due to design variations or artifacts such as sample distribution within the filter matrix or nonuniform heating, which would result in different aerosol evaporation kinetics (Schobesberger et al., 2018). Overall, our results show that the T max for organic and inorganic compounds varies with loading, which needs to be accounted for when estimating aerosol component volatility from the empirical correlations between T max and C * using the FIGAERO-CIMS.

Conclusions
Environmental chamber experiments were carried out to investigate the chlorine-initiated oxidation of C 8−12 linear alkanes under high-NO x conditions. Rapid SOA formation and ozone production were observed. SOA yields increase with precursor length, consistent with alkane-OH SOA formation. Under similar oxidation conditions, alkane-Cl SOA exhibited more hydrocarbon-like characteristics as the alkane precursor length increased, as indicated by the ACSM measurements. This bulk SOA observation is consistent with gasand particle-phase CIMS measurements, which identified more oxidized reaction products derived from smaller alkane precursors. CIMS measurements also suggest that organonitrates dominated the gas-and particle-phase composition. Trace amounts of alkane-derived organochlorides were observed using the ACSM and the FIGAERO-CIMS, likely produced via chlorine addition to the heterogeneously produced dihydrofuran compounds. Organochloride and bulk SOA production were suppressed under humid conditions, whereas ozone production increased. Under such conditions, the SOA yields observed for chlorine-initiated oxidation of octane (0.24), decane (0.50), and dodecane (1.10) were still much higher than those observed for OH-initiated oxidation of the respective alkanes. Overall, these results show that chlorine-alkane oxidation could be an important pathway for SOA production and aging, especially in highly polluted environments replete with alkane, NO x , and reactive chlorine emission sources.
A clear mass-loading dependence for T max from FIGAERO-CIMS data was demonstrated using levoglucosan, ammonium sulfate, and alkane-Cl SOA, indicating that the quantitative assessment of C * from T max using the FIGAERO-CIMS needs to account for variations in filter loading. A unit-mass-resolution 2-D FIGAERO-CIMS thermogram framework was developed. The 2-D thermograms demonstrate a clear relationship between molecular weight and volatility of aerosol components, and RH-induced suppression of oligomer formation. When used in conjunction with high-resolution ion fitting, the 2-D thermogram can be a powerful tool for interpreting the chemical composition and volatility distribution of particle-phase compounds.
Data availability. Data published in this paper's figures and tables are available via the Texas Data Repository, https://doi.org/10. 18738/T8/4PFPSO (Wang and Hildebrandt Ruiz, 2018). Underlying research data are also available by request to Lea Hildebrandt Ruiz: lhr@che.utexas.edu.
The Supplement related to this article is available online at: https://doi.org/10.5194/acp-18-15535-2018supplement Author contributions. DSW performed the experiments, data analysis, and manuscript preparation, overseen by LHR. Both authors contributed to the experimental design and interpretation of results.
Competing interests. The authors declare that they have no conflict of interest.
(c) SOA molar yield, calculated using Eqs. S1-3, in mol mol -1 (d) Density calculated based ACSM mass measurements and SEMS volume measurements (e) SOA mass yield, recalculated assuming the lowest reported alkane-OH density, 1.06 g cm -3 (Lim and Ziemann, 2009), in µg m -3 (f) Mass fraction of -Cl functional group, estimated using the ACSM measurement of HCl + ion, relative to the bulk SOA (g) Mass fraction of -Cl functional group, estimated using the SOA molecular composition as measured by the FIGAERO-CIMS, 10 relative to the total organic ions observed (h) Mass fraction of organochlorides (including chloronitrates), estimated using the SOA molecular composition as measured by the FIGAERO-CIMS, relative to the total organic ions observed (i) Mass fraction of organonitrates (excluding chloronitrates), estimated using the SOA molecular composition as measured by the FIGAERO-CIMS, relative to the total organic ions observed (j) Average ratio of -NO3 functional group to the number of carbon, estimated using the SOA molecular composition as measured by the FIGAERO-CIMS. Individual -NO3 functional groups in dinitrates and trinitrates are counted separately (e.g. a C12 dinitrate has the same RON as a C6 mononitrate).

Figure S1
: Change in the fractional contribution by different organic ion fragments to the overall organic aerosol mass as measured by the ACSM. Dodecane-Cl SOA (Exp. 11) was used as the reference for octane-Cl SOA (Exp. 3) and decane-Cl SOA (Exp. 7) produced under similar oxidation conditions under low RH. Mass spectra shown are 50-minute averages from minute 10 to 60 during the photooxidation 5 period. Ion identities indicated (C2H3 + , C3H5 + , C3H7 + , C2H3O + , C4H7 + , C4H9 + ) were assumed, as the quadruple ACSM used lacks the necessary mass resolution for exact identification. At m/z 43, the C2H3O + is expected to dominate. Highlighted CxHy ions are often associated with hydrocarbon-like organic aerosol (Ng et al., 2011;Ulbrich et al., 2009). Clear enhancement of m/z 44 and associated ion fragments (e.g. m/z 16, 17, and 18) was also observed for octane and decane SOA.
3 Figure S2: Example of NO, NO2, O3 and SOA trends under different starting NOx conditions. Exp. 9 (high initial NO), 10 (high initial NO2), and 11 (balanced initial NO and NO2) for chlorine-initiated oxidation of dodecane under low RH conditions are shown. Ozone production slowed down significantly as the NOx concentrations plateaued and as the SOA concentrations approached their maxima. By the end of the photooxidation period (60 min), lowest final NOx concentrations (consisting of NO2 and interferences, which may include alkyl nitrates) and 5 highest SOA concentrations were observed for high initial NO concentrations (Exp. 1 for octane, Exp. 5 for decane, and Exp. 9 for dodecane).

Figure S3:
Comparison of particle-phase composition during FIGAERO desorption for octane (Exp. 3), decane (Exp. 7), and dodecane (Exp. 11). Same as Figure 4 in the main text but with the y-axis in logarithmic scale and the x-axis (m/z) extended to the maximum sampled range. Desorption signals are not stacked. Figure S4: Particle-phase distribution of C12 organic (mono-)nitrates. Assuming equal sensitivity, the particle-phase abundance roughly followed a bell-shape distribution across the different oxygenation groups, peaking at the O3 or O4 group. Within each oxygenation group, the product distribution follows a bell-shape around H19 and H21 compounds. Similar bell-shaped organic nitrate distribution has been observed for ambient isoprene-and monoterpene-derived organonitrates .

Figure S5
: Ratio of integrated unit-mass resolution FIGAERO-CIMS desorption signal during temperature ramp from a high-RH SOA formation experiment (Exp. 12, 67 % RH) to that from a low-RH SOA formation experiment (Exp. 11, < 5 % RH). Integrated ion intensity is shown on a logarithmic scale as the color. The ratio of bulk organics concentration during the filter collection period calculated using ACSM data is shown as the dotted black line. Except for the low molecular weight thermal fragmentation/decomposition products and the 5 high molecular weight, low-volatility compounds, the unit-mass ion ratios appear in agreement with bulk measurement. Under high RH conditions, the higher molecular weight (m/z > 600) compounds become increasingly suppressed. Figure S6: Example of secondary HSO4ionization products. Ion signals observed during (NH4)2SO4 thermal decomposition were first normalized by Isignal and then normalized against the maximum desorption signal observed for each ion. Linear (1 st order), quadratic (2 nd 6 Figure S7: The 1-D thermograms for two (NH4)2SO4 decomposition product ions. H2SO4•Iwas the primary adduct and H2SO4•HSO4was a secondary ionization product. The lowest time-integrated desorption ion intensity for H2SO4 Iwas used as the basis for calculating the loading ratio (LRmass). The loading ratio for H2SO4•HSO4 -(not shown) is the square of that for H2SO4 I -(b) Linear regression fitting of Tmax as a function of the LRmass. Tmax increased linearly with mass loading of (NH4)2SO4.
7 Figure S8: Effects of aerosol loading on Tmax. Three SOA filter samples were collected during Exp. 6 for 45 mins, 30 mins, and 15 mins, in that order. Each filter was subjected to two full desorption runs to minimize carry-over effects and to determine the filter background. (a) Enhancement ratios calculated using unit-mass integrated signals for the lowest filter loading run as the referenced condition. Color scale 5 shows Tmax shifts in o C, using the Tmax values observed in the low loading run as the reference point. (b) 2-D thermogram for the first, high loading filter, (c) for the second, medium loading filter, and for (d) the third, low loading filter. For (b-d), the color scale represents the normalized desorption intensity as a percentage of the maximum, same as in Fig. 5 in the main text. Between the time it took for filter collection and filter desorption, the ion intensity of some high molecular weight, low volatility compounds had decreased to below the limits of detection (i.e. 3σ of the background signal) in the particle phase, likely due to volatility-dependent wall loss. Disappearance of low m/z Figure S9: High resolution fitting at m/z (a) 434 and (b) 436 for FIGAERO-CIMS data from Exp. 11. The C12 chloronitrate (ONO2-C12H18ClO3•I -) peak is tentatively identified in (a) but it overlaps with the nearby stronger organonitrate (ONO2-C12H21O5•I -) peak. Because the chloronitrate peak is a weaker peak with significant overlap with a stronger peak, quantitative assessment would be challenging 5 due to peak fitting uncertainties (Cubison and Jimenez, 2015). The presence of chloronitrate cannot be confirmed or rejected based on its unique isotopic signature at m/z+2 positions, as shown in (b), where nonchlorinated compounds also dominate.
Figure S10: 1-D thermogram of select organochlorides observed for Exp 10. All compounds shown exhibited multimodal desorption behaviors. C2H3ClO2 is too volatile to be present as a molecular compound in the particle phase and is therefore a thermal decomposition product. C2H3ClO2 shows three local maxima at ~87 o C, ~109 o C, and ~167 o C. The Tmax for the least volatile desorption mode of C2H3ClO2 is higher than that for ammonium sulfate seed particles (~155 o C) from the same filter run, and may be produced from thermal decomposition of extremely low volatility organochlorides. However, larger organochloride ions were not observed to have any distinct Tmax modes over these very high temperature ranges (> 160 o C) Figure S11: Oligomerization via the reaction between (a) carbonyl hydroxyl nitrate and hydroxy dihydrofuran proposed by Schilling Fahnestock et al. (2015) and (b) cyclic hemiacetal with either a 1,4-hydroxycarbonyl or another cyclic hemiacetal proposed by . Condensed-phase isomerization between acetal and hemiacetal dimers is also possible, as shown in (b).

S2. Interaction between Cl, NOx, and HOx
Formation of HO2NO2 likely proceeds via 2 + 2 → 2 2 R. (S1) Formation of HONO under UV can proceed via 5 2 + → + 2 R. (S2) + → R. (S3) The trends for HO2NO2, HONO, and select gas-phase species observed by the I -CIMS are shown in Fig. S12. Figure S12: Representative trends of SOA and trace gas species during the photooxidation period. Data from dodecane oxidation (Exp. 11) 10 are shown, similar to Fig. 1 in the main text. Additional species shown include ClONO2 (from reaction between ClONO with NO2, Lesar et al., 2006), ClO (possibly a ion fragment of ClONO), HCl (from H-abstraction by Cl • or perhaps Cl-elimination reactions), and HNO3 (due to the oxidation of NO2 by OH).

S3. Calculation of Bulk SOA Properties using the FIGAERO-CIMS
If equal sensitivity is assumed for all ions (i.e. Iadducts detected by the FIGAERO-CIMS), the average molecular weight, 15 mwavg is estimated based on the (desorption) intensity Ii and the molecular weight, mwi of all ions identified, where ∆Td is the step change in desorption temperature. It should be noted that because of thermal decomposition, which has also been observed for monomers , the mwavg and mwSOA calculated using the FIGAERO-CIMS data likely underestimate the actual average SOA molecular weight. For octane SOA, mwSOA calculated in this way ranges from 206 to 11 226 g mol -1 ; for decane SOA, mwSOA ranges from 241 to 265 g mol -1 ; for dodecane SOA, mwSOA ranges from 260 to 270 g mol -1 . The molar SOA yield can then be calculated as, where nSOA and nVOC are the molar concentrations of SOA and VOC. The molecular weight of VOC, mwVOC is known for the alkane precursors. The corresponding molar yield ranges from 0.08 to 0.14 for octane SOA, 0.26 to 0.47 for decane SOA, and 5 0.72 to 1.04 for dodecane SOA. The above-unity molar SOA yield observed for dodecane (Exp. 9) indicates that SOA mass may be overestimated, which may be the result of uncertainties with the collection efficiency (CE) and relative ionization efficiency (RIE) assumed for the SOA.
Equations S1 and S2 can also be used to calculate various other bulk SOA properties, including elemental ratios such as the oxygen-to-carbon ratio (O:C), hydrogen-to-carbon ratio (H:C), nitrate-to-carbon ratio (NO3:C), chloride-to-carbon ratio 10 (Cl:C), or the oxidation state of carbon (OSC). Using the oxygen-to-carbon ratio (O:C) as the example, the bulk SOA elemental ratios as a function of the FIGAERO desorption temperature, Td can be calculated as where nO is the total number of oxygen atoms present in the desorbed ions at Td, nC is the total number of carbon atoms, nO,i is the number of oxygen atom present in compound i as determined by its assigned molecular formula (which is independent of 15 Td), nC,i is the number of carbon present in compound i, and Ii is the desorption ion intensity for compound i at Td. Equal sensitivity is assume for ions used for the analysis such that Ii can be used as the molar amount for compound i.