Ultraviolet and Visible Complex Refractive Indices of Secondary Organic Material Produced by Photooxidation of the Aromatic Compounds Toluene and m -Xylene by

Secondary organic material (SOM) produced by the oxidation of anthropogenic volatile 2 organic compounds can be light-absorbing (i.e., brown carbon). Spectral data of the optical 3 properties, however, are scarce. The present study obtained the continuous spectra of the real and 4 imaginary refractive indices ( m = n – i k ) in the ultraviolet (UV)-to-visible region using 5 spectroscopic ellipsometry for n and UV-visible spectrometry for k . Several different types of 6 SOM were produced in an oxidation flow reactor by photooxidation of toluene and m -xylene for 7 variable concentrations of nitrogen oxides (NO x ). The results show that the k values of the 8 anthropogenically derived material were at least ten times greater than those of the biogenically 9 derived material. The presence of NO x was associated with the production of organonitrogen 10 compounds, such as nitro-aromatics and organonitrates, which enhanced light absorption. 11 Compared with the SOM derived from m -xylene, the toluene-derived SOM had larger k values, 12 as well as a greater NO x -induced enhancement, suggesting different brown-carbon-forming 13 potentials of different aromatic precursor compounds. The results imply that anthropogenic SOM 14 produced around urban environments can have an important influence on ultraviolet irradiance, 15 which might consequently influence photochemical cycles of urban pollution.


Introduction 17
Aerosol particles affect many atmospheric processes. Globally, aerosol particles 18 influence Earth's radiative balance both directly by scattering and absorbing solar radiation and 19 indirectly by acting as cloud nuclei. At the urban and regional scale, aerosol particles contribute 20 to the degradation of visibility  and adversely influence human health 21 (Dockery et al., 1993). A major fraction of the ambient particle population is secondary organic 22 material (SOM) produced by oxidation of anthropogenic and biogenic gaseous precursors 23 (Kanakidou et al., 2005;Hallquist et al., 2009). The magnitude and properties of SOM, however, 24 are still poorly represented in models (Heald et al., 2005;Volkamer et al., 2006). 25 Ultraviolet-absorbing components in atmospheric particles can significantly reduce 26 ultraviolet (UV) irradiation, thus affecting photochemistry in the atmospheric boundary layer 27 (Jacobson, 1999;Barnard et al., 2008). Modeling the optical properties and radiative effects, 28 however, requires spectral data of complex refractive indices (m = n -i k), including both the 29 refractory index n and the absorptive index k. Lack of this information, especially in the UV 30 region (300-400 nm), hampers an understanding of the photochemical effect of anthropogenic 31 SOM. 32 In highly polluted urban regions, anthropogenic aromatic compounds constitute up to 33 70% of the non-methane hydrocarbons . Among the anthropogenic aromatics, 34 toluene and xylenes are the most abundant compounds, and the yields for the production of SOM 35 from these compounds are high (Odum et al., 1996;Odum et al., 1997;Ng et al., 36 2007;Hilderbrandt et al., 2009;Zhang et al., 2014). Quantitative estimates of the climate effects 37 of anthropogenic SOM, however, remain limited for several different reasons, including the 38 incomplete knowledge of chemical composition and optical properties. 39 The gas-and particle-phase chemistry of aromatic oxidation is complex, and a broad 40 spectrum of products is produced from the oxidation of a single precursor (Forstner et al., 41 1997;Jang and Kamens, 2001a). The concentration of nitrogen oxides (NO x ) further influences 42 the product distribution (Sato et al., 2007). Some of the molecular products resulting from the 43 oxidation of aromatic precursors have been identified and quantified (Forstner et al., 44 1997;Cocker III et al., 2001;Jang and Kamens, 2001a;Hamilton et al., 2005;Sato et al., 2007). 45 These products, however, typically constitute less than 50% of the reacted carbon (Forstner et al., 46 1997;Hamilton et al., 2005;Sato et al., 2007). Particle-phase reactions have been proposed as an 47 The imaginary refractive indices k were calculated from the data sets, as follows (Sun et 130 al., 2007): 131 (1) 132 for an absorbance A, an optical path length L (m), a material density ρ (kg m -3 ), and a 133 concentration c (kg m -3 ). A material density of (1.4 ± 0.1) × 10 3 kg m -3 was used in the analysis 134 for toluene-and m-xylene-derived SOMs (Ng et al., 2007). For Suwannee river fulvic acid, a 135 material density of (1.47 ± 0.02) × 10 3 kg m -3 was used (Dinar et al., 2006). For the k values 136 derived from UV-visible spectroscopy, propagation of uncertainties in ρ, c, and A(λ) leads to an 137 overall uncertainty of ±15% (10 and 90% confidence interval) for λ < 420 nm. 138 In an alternative to the UV-visible spectroscopy, the k values are also retrieved by 139 ellipsometry (Liu et al., 2013). A comparison of k values derived from UV-visible spectroscopy 140 and to those from ellipsometry is provided in the supporting information (cf. Fig. S1). Overall 141 agreement is good. For the smallest k values (<0.005), the ellipsometry retrievals have 142 uncertainties approaching >50%. The uncertainties for UV-visible spectroscopy are considerably 143 smaller (< 15%). Even so, this method requires sample extraction, and artifacts associated with 144 extraction efficiency, material density, and solvent effect can be introduced. All factors 145 considered, the k values derived from UV-visible spectroscopy were adopted in this study for 146 further analysis. 147

Infrared Spectroscopy 148
Aerosol particles were collected on Teflon filters (Sartorius Stem, 0.2 μm) at a flow rate 149 of 2 L min -1 for up to 24 h. The collected mass on the filters ranged from 0.8 to 2.0 mg. The 150 Teflon filter was cut to the shape of the germanium element of an Attenuated Total Reflectance 151 (ATR) accessory (Pike Technologies). The assembly was then screw-pressed to the crystal 152 surface, the holder was opened, and the filter was peeled off. A thin layer of secondary organic 153 material remained on the surface of the crystal (Hung et al., 2012). An experiment using a blank 154 filter showed no residual signal from the Teflon filter after peeling. This preparation method 155 avoided interference from Teflon filters across 900-1250 cm -1 (Russell et al., 2009b;Russell et al., 156 2011;Takahama et al., 2012), which otherwise obscured the absorption bands of C-O stretching. 157 After preparation, the filter samples were taken for spectroscopy analysis. Infrared 158 spectra were recorded using the ATR accessory in a Fourier Transform Infrared Spectrometer 159 (FTIR, Nicolet 670). The spectral resolution was 0.5 cm -1 . The number of scans was 16. 160 Additional information about the ATR-FTIR protocols is provided in Hung et al. (2012). A band 161 fitting algorithm, implemented in MATLAB, was used to analyze the infrared spectra. The 162 algorithm was adopted from Russell et al. (2009a) and Takahama et al. (2012). SOMs produced by toluene photooxidation in an environmental chamber. 175 The increase of k for high NO x can in part be explained by the production of light-176 absorptive organonitrogen compounds, mostly nitro-aromatic compounds, such as nitrophenols, 177 nitrocatechols, and dinitrophenols (cf. Section 3.2). These compounds have been identified as 178 products from toluene photooxidation under high-NO x conditions (Forstner et al., 1997;Jang and 179 Kamens, 2001b;Sato et al., 2007;Zhong et al., 2012;Nakayama et al., 2013). Nitro-aromatic 180 compounds have also been identified in brown carbon sampled in urban plumes dominated by 181 anthropogenic SOM Zhang et al., 2013). The spectra for several methyl-182 nitrophenol isomers were measured (cf. Section S1), and the results confirm that these 183 compounds are strong UV absorbers. In particular, the aromatic compounds having hydroxyl and 184 nitro groups in para substitution, such as 2-methyl-4-nitrophenol (a major product of aromatic 185 photooxidation), have a strong absorption band at 320 nm. Compounds having this configuration 186 are good candidates for contribution to the main peak in the difference spectra Δk (i.e., Δk = k -187 k NO 0 =0 ) (Fig. 2b). 188 For similar reaction conditions, the m-xylene-derived SOMs are less absorptive than the 189 toluene-derived SOMs ( Fig. 3a; Table 1 The k values of the aromatic-derived SOMs can be compared to those of other light-194 absorbing material relevant to atmospheric aerosol particles (Fig. 4). The k values decrease for 195 increasing wavelength for the aromatic-derived SOMs. Similar wavelength-dependent behavior 196 is observed for light-absorbing carbonaceous materials referred to as "brown carbon" in literature 197 (Kirchstetter et al., 2004;Andreae and Gelencser, 2006;Hoffer et al., 2006;Alexander et al., 198 2008;Dinar et al., 2008;Chakrabarty et al., 2010;Cappa et al., 2012;Lack et al., 2013). In 199 contradistinction, the value of k for black carbon is independent of wavelength (Kirchstetter et al., 200 2004). Compared to the k values of SOMs derived from examples of biogenic precursors (B-201 SOM), such as α-pinene and limonene SOM (Liu et al., 2013), the k values of the studied 202 anthropogenic SOMs (A-SOM) are one order of magnitude more absorptive in the UV-visible 203 region, even for those produced at low NO x . These higher values suggest that conjugated double 204 bonds are retained in some oxidation products, which have absorption transitions in the 205 ultraviolet to near visible (Lambe et al., 2013). Even so, the k values in the low-NO x experiments 206 are smaller than those of a reference compound like Suwannee river fulvic acid, which is often 207 cited as a surrogate of atmospheric humic-like substances (HULIS) (Gelencsér et al., 2003). In 208 the high-NO x experiments, however, the k values are within the range of atmospheric brown 209 carbon (cf. shaded region in Fig. 4). 210 The real refractive indices n of toluene-and m-xylene-derived SOMs are shown in Fig The curves can be parameterized by the three-term form of Cauchy's equation (cf . Table S2). 213 The Cauchy-form of the curves for the studied anthropogenic SOMs also holds for the biogenic 214 SOMs reported previously (Liu et al., 2013). 215 The refractive indices n shift +0.02 for both toluene-and m-xylene-derived SOMs for an 216 increase of the initial NO concentration from 0 to 10 ppm (Fig. 5). This upward shift of n for 217 increasing initial NO concentration is possibly attributed to an increasing abundance of nitrogen 218 in the SOM produced at higher initial NO concentrations. Nitrogen has a higher atomic 219 polarizability (1.03 Å 3 ) than both oxygen (0.57 Å 3 ) and hydrogen (0.17 Å 3 ) (Bosque and Sales, 220 2002). The n value of a material is related to its polarizability by the Lorentz-Lorenz equation 221 (Bosque and Sales, 2002). Within the tolerance of the measurement uncertainty, the n values do 222 not differ between SOMs derived from the two different aromatic precursors at the same initial 223 NO x concentration. The implication could be that the n values of SOMs are mainly determined 224 by bulk chemical properties, such as the elemental ratios or functional groups (cf. Section 3.2 225 and 3.3). Detailed chemical properties, such as the molecular structure, might play a minor role 226 in determining the value of n. In this case, upscaling of the laboratory parameterizations to large-227 scale models of the effects of different types of SOMs on radiative forcing and climate is 228 simplified (Lambe et al., 2013;Flores et al., 2014;Kim et al., 2014). As a caveat, the SOMs of 229 this study were produced at mass concentrations much higher than typical atmospheric 230 concentrations. Both elemental composition and refractive index can depend on mass 231 concentration (Shilling et al., 2009;Kim et al., 2012;Kim and Paulson, 2013), and further 232 investigations are needed to quantify this possible effect. 233

Production of Organonitrogen Compounds and Light Absorption 234
The infrared spectra in the presence and absence of NO x are similar, except for 235 organonitrogen groups, such as -NO 2 and -ONO 2 (Fig. 6). This similarity suggests that the 236 oxygen-containing functional groups, excluding nitrogen-containing groups, are substantially 237 similar for SOMs produced at the different NO x concentrations. SOMs derived from toluene and 238 m-xylene also have similar overall compositions. 239 Organonitrogen compounds are detected in SOM collected in high-NO x experiments. The 240 bands at 846, 1281, and 1647 cm -1 , corresponding to organonitrate groups (-ONO 2 ), are present 241 in both toluene-and m-xylene-derived SOMs (Roberts, 1990;Liu et al., 2012). The area of the -242 ONO 2 band from 1610 to 1690 cm -1 , when normalized by the alkane C-H bands from 2790 to 243 2980 cm -1 to account for different masses on the filters, increases for greater initial NO 244 concentrations (Fig. 3c). Comparison of the spectrum of the toluene-derived SOM to that of m-245 xylene-derived SOM for fixed initial NO x concentration shows that the -ONO 2 fractions are 246 approximately equal for both types of SOMs produced. The dominant mechanism for -ONO 2 247 production, which is the reaction of peroxy radicals (RO 2 ) with NO (Roberts, 1990), can explain 248 why the fraction of -ONO 2 increases for greater NO concentrations. 249 In the high-NO x experiments, -NO 2 groups are produced. For the toluene-derived SOM, 250 the production of -NO 2 groups is indicated by a strong band at 1558 cm -1 and a weak band at 251 1342 cm -1 (Fig. 6a). For m-xylene-derived SOM, only the strong band at 1558 cm -1 is observed 252 (Fig. 6b). Based on an analysis of area ratios, the mole fraction of -NO 2 groups in the m-xylene-253 derived SOM is 35 to 50% lower than that in the toluene-derived SOM for fixed initial NO x 254 concentration (Fig. 3c). The production mechanism of -NO 2 group has been proposed as the 255 adduction of -NO 2 to phenoxy radicals to produce nitrophenols (Forstner et al., 1997;Jang and 256 Kamens, 2001b;Nakayama et al., 2013). Compared to toluene, the alkyl substitution at a meta 257 site of m-xylene can inhibit the production of stable -NO 2 adducts of phenoxy radicals 258 (Nakayama et al., 2013), which can explain the lower -NO 2 fraction observed for m-xylene-259 derived SOM. 260 The difference in -NO 2 fraction explains in part but not entirely the differences in k 261 values for toluene-compared to m-xylene-derived SOMs (cf. Section 3.1). When normalized by 262 -NO 2 fractions and for similar reaction conditions, Δk at 320 nm for toluene-derived SOM is 50% 263 higher than that of m-xylene-derived SOM. The implication is that the compounds in toluene-264 derived SOM are more UV-absorptive than those in m-xylene-derived SOM. Differences in the 265 extent of conjugation of the oxygenated products can be important. The k values in the UV 266 region at low NO x (k NO 0 =0 ) provide a baseline to quantify this influence. When normalized by 267 both k NO 0 =0 and -NO 2 fraction, the two types of SOM are similarly absorptive (Fig. 3d). 268 Organonitrogen groups attached to a conjugated chain can have increased light absorption as 269 well as shifts in absorption to longer wavelengths. 270

Oxygenated Groups and the Importance of Particle-Phase Reactions 271
The infrared spectra show that several different types of oxygenated functional groups are 272 present in the SOMs (Fig. 6) to those reported in the literature for related SOMs (Jang and Kamens, 2001a;Liu et al., 2012). 276 The new finding of the present study is the presence of a strong C-O stretch at 1000-1260 cm -1 277 (cf . Table S3). This band was obscured in previous studies by ammonium sulfate or Teflon filter.  (Forstner et al., 1997). The cluster representing the toluene-and m-287 xylene-derived SOMs is uniquely situated and differentiated from the reference compounds 288 because of the C-O stretch at 1000-1260 cm -1 . 289 This absorption band at 1000-1260 cm -1 is plausibly contributed by an ether group (C-O-290 C) of acetals and hemiacetals produced via particle-phase reactions (Jang et al., 2002;Kroll and 291 Seinfeld, 2008;Lim et al., 2010). These reactions tend to drive product distribution toward the C-292 O vertex of the composition diagram (cf. arrow in Fig. 7). The gas-phase oxidation of aromatic 293 precursors produces dialdehydes in high yields, including glyoxal and methylglyoxal. These 294 dialdehydes readily oligomerize along hemiacetal and acetal pathways, with associated changes 295 in the C-O/C=O stretch band ratio (Loeffler et al., 2006). Hemiacetal/acetal production reactions 296 leading to oligomerization can occur in SOM produced by photooxidation of trimethylbenzene 297 (TMB), even in the absence of catalysis by sulfuric acid (Kalberer et al., 2004). 298 The mole fraction of each functional group is estimated using the absorptivity of Russell 299 et al. (2009b) and Takahama et al. (2012), along with the area ratios of ether (C-O-C) to alkane 300 (C-H) bands for 19 ether and acetal compounds appearing in the NIST database. The analysis 301 concludes that ether groups constitute up to 50% of the SOM mass. This result agrees with a 302 modeling study suggesting that 20-80% of the SOM derived from toluene is produced by 303 particle-phase reactions (Cao and Jang, 2009). These particle-phase reactions can produce 304 oligomers having conjugated structures that contribute to the light absorption even in the absence 305 of nitrogen moieties (Zhong et al., 2012). 306

Atmospheric Implications 307
For the obtained spectral data sets of n and k (Section 3.1), the optical effects of brown 308 carbon (BrC) from anthropogenic SOM can be assessed. A model case study is formulated to 309 represent light-absorbing particles in a fresh urban plume close to the anthropogenic sources. 310 Parameters defining the case study are listed in Table 2. The case study considers a population of 311 brown carbon particles produced by photooxidation of anthropogenic aromatic precursors in the 312 presence of NO x . This population is compared with populations of black carbon (BC) particles 313 (representing emissions from fossil fuel combustion) and externally mixed at variable ratios with 314 ammonium sulfate particles (representing the regional background atmospheric aerosol). The 315 number-diameter distributions of the BrC and sulfate particle populations are representative of 316 polluted urban regions (Wu et al., 2008). The BC particle population having a relatively smaller 317 mode diameter is typical for fresh soot particles emitted from motor vehicles (Kleeman et al., 318 2000). The investigated ratios of BC and ammonium sulfate are representative of Asian outflows 319 (Ramana et al., 2010). The external mixing assumption is consistent with the small absorption 320 enhancement of BC in urban regions (Cappa et al., 2012). The single-scattering albedo ω, 321 defined as the ratio of scattering to total extinction, is calculated for each population and their 322 mixtures using a Mie-theory-based optical model (Bohren and Huffman, 1983;Liu et al., 2013) 323 (Fig. 8a). The relative contribution of BrC absorption to total light absorption (i.e., BrC/(BrC + 324 BC)) is calculated as a function of the mass ratio of organic matter to BC. The calculated results 325 for λ = 320, 405, and 550 nm are plotted in Fig. 8b. These three wavelengths are selected 326 because solar radiation in these bands respectively regulates O 3 photolysis, NO 2 photolysis, and 327 energy balance (cf. Fig. 8a). 328 Results of the case study have several implications for climate and atmospheric chemistry 329 modeling. The ω values of the BrC particle populations are close to unity for λ > 500 nm (Fig.  330 8a). When externally mixed with BC, the studied BrC has a negligible contribution to light 331 absorption at 550 nm (Fig. 8b). These results indicate that these BrC populations have a net 332 cooling effect. The ω values, however, decrease below unity for λ < 400 nm, meaning that the 333 particle population becomes absorptive in the UV region. Although the solar irradiance in the 334 UV region contributes only 10% of the total solar irradiation, meaning a small heating effect by 335 brown carbon for the conditions of the case study, the effect can still be important because UV 336 irradiance determines the photolysis rates of many chemical species (Fig. 8a). For example, 337 reduced UV irradiance for λ <320 nm slows ozone photolysis, thus suppressing the production of 338 OH radicals (Martin et al., 2003;Tie et al., 2003). The case study suggests that BrC populations 339 can have a substantial contribution for light absorption in this band (Fig. 8b). For a mass ratio of 340 organic matter to BC in a range of 2 to 20, which is typical for urban atmosphere (Turpin et al., 341 1991), BrC accounts for 15-80% of the UV absorption at 320 nm. The photolysis of NO 2 is 342 similarly suppressed by reduced UV irradiance for λ < 405 nm, thus inhibiting the production of 343 ozone (Dickerson et al., 1997;Martin et al., 2003). The implication is that, as an effective UV 344 absorber, BrC influences the production of O 3 and OH by reducing UV irradiance and 345 consequently affects the oxidation capacity of the regional atmosphere. 346 In conclusion, photooxidation of toluene and m-xylene in the presence of NO x can 347 produce SOMs having k values similar to those reported for brown carbon in biomass burning 348 and urban plumes (Kirchstetter et al., 2004;Hoffer et al., 2006;Alexander et al., 2008;Dinar et al., 349 2008;Chakrabarty et al., 2010;Cappa et al., 2012;Lack et al., 2013). The implication is that the 350 photooxidation of anthropogenic precursors can be a significant source of atmospheric brown 351 carbon. These findings are consistent with atmospheric observations in urban regions, such as the 352 Los Angeles basin Cappa et al., 2012;Zhang et al., 2013), Mexico city 353 (Barnard et al., 2008), and Beijing (Cheng et al., 2011). The case studies considered in the 354 present study suggest that anthropogenic brown carbon, along with brown carbon from biomass 355 burning, can have a major influence on light absorption at wavelengths that drive photochemical         Fig. 4. The cases for "BC + sulfate" represent a BC population externally mixed with an ammonium sulfate population at variable mixing ratios representative of typical ambient values in a pollution plume (Ramana et al., 2010). Table 2       Fulvic acid (this work) BrC spheres (Alexander et al., 2008) BB (Kirchstetter et al., 2004) BB (Chakrabarty et al., 2010) BB (Lack et al., 2013) HULIS (Hoffer et al., 2006) HULIS (Dinar et al., 2008) Urban BrC (Cappa et al., 2012) Brown carbon Anthropogenic SOM (this work) Biogenic SOM (Liu et al., 2013) (Kirchstetter et al., 2004) Black carbon     Toluene + OH + NO x m-Xylene + OH + NO x λ = 320 nm λ = 405 nm λ = 550 nm Cumulative fraction (%) of solar irradiance/photolysis rate (b) (a) Figure 8