Relative Humidity Effect on the Formation of Highly Oxidized Molecules and New Particles during Monoterpene Oxidation

. It has been widely observed around the world that the frequency and intensity of new particle formation (NPF) 10 events are reduced during periods of high relative humidity (RH). The current study focuses on how RH affects the formation 11 of highly oxidized molecules (HOMs), which are key components of NPF and initial growth caused by oxidized organics. The 12 ozonolysis of α-pinene, limonene, and △ 3 -carene, with and without OH-scavenger, were carried out under low NOx conditions 13 under a range of RH (from ~3% to ~92%) in a temperature-controlled flow tube to generate secondary organic aerosol (SOA). 14 A Scanning Mobility Particle Sizer (SMPS) was used to measure the size distribution of generated particles and a novel 15 transverse-ionization chemical ionization inlet with a high-resolution time-of-fight mass spectrometer detected HOMs. A 16 major finding from this work is that neither the detected HOMs nor their abundance changed significantly with RH, which 17 indicates that the detected HOMs must be formed from water-independent pathways. In fact, the distinguished OH- and 18 O 3 -derived peroxy radicals (RO 2 ), HOM monomers, and HOM dimers could mostly be explained by the autoxidation of RO 2 19 followed by bimolecular reactions with other RO 2 or hydroperoxy radicals (HO 2 ), rather than from a water-influenced pathway 20 like through the formation of a stabilized Criegee intermediate (sCI). However, as RH increased from ~3% to ~92%, the total number concentrations decreased by a factor of 2-3 while SOA mass concentrations remains relatively constant. These observations show that, while high RH appears to inhibit NPF as evident by the decreasing number concentration, this reduction is not caused by a decrease in RO 2 -derived HOM formation. Possible explanations to these phenomena were discussed.


5
Spectrometer (AmPMS) (Hanson et al., 2011) and the cluster-CIMS (Zhao et al., 2010). In the TI inlet, a 4 -10 LPM flow of 120 sample air is passed across the inlet orifice of the mass spectrometer, where it encounters an orthogonal, 1 LPM reagent ion 121 gas flow consisting N 2 containing ionized nitrate ions (NO 3 -) as well as potential cluster ions (HNO 3 ) n NO 3 with n = 1-3. For 122 the current study, the sample flow to the inlet was set to 4.5 LPM. Chemical ionization occurs at atmospheric pressure and 123 temperature. The reagent gas is generated by passing 3 ccm of N 2 over a small vial containing nitric acid, which is then 124 ionized by a 370 MBq Po 210 radioactive source (model P-2021, NRD, LLC). An additional flow of N 2 can be added to the 125 reagent gas to change the reagent ion concentration, and the assembly can be adjusted to vary ion-molecule reaction time. 126 The latter can be controlled by adjusting the sample and reagent gas flow rates or by applying different voltages to the 127 ionization source and the main inlet block. To minimize the diffusion loss in sample lines, the inlet of the TI source was 128 connected to the flow tube outlet by a short (~10 cm) piece of electro-polished stainless steel tubing. Compared to the widely 129 used commercial nitrate inlet patterned after the design by Eisele and Tanner (1993) and marketed by Aerodyne, Inc., no 130 additional sheath flow is required thus any impurities potentially introduced by the sheath flow are eliminated. Some flow 131 disturbance may occur where the sample flow encounters the transverse reagent flow, which may lead to non-ideal behavior. 132 However, even at the maximum total flow of 11 LPM, the reynolds number in this region is ~500 thus turbulence is not 133 expected to be significant. 134 135 Another unique aspect of the TI design is the use of an N 2 curtain gas in front of the inlet orifice to the mass spectrometer to 136 reduce water clustering on reagent and sample ions. Water clusters are expected to form at high RH mainly during the 137 free-jet expansion of the sampled gas on the vacuum side of the orifice plate (Thomson and Iribarne, 1979). The presence of 138 these clusters makes the identification and quantification of both sample and reagent ions challenging (Kulmala et Figure 2 shows the details of the TI source that address this issue. 140 Small holes drilled in a radial channel blow N 2 uniformly in front of the orifice plate so that only sampled ions and this clean 141 N 2 gas pass into the vacuum chamber. Since the sampling flow rate of the mass spectrometer is ~0.7 LPM when using 0.3 142 mm orifice, the N 2 curtain flow is set to be 1 LPM to overflow the region surrounding the orifice. By applying voltages to 143 the ion source and the block, the ions can be efficiently guided into the mass spectrometer while neutral molecules such as 144 water vapor are prevented from entering by the N 2 curtain gas. 145 146 This TI inlet is suitable to all types of reagent ion chemistry, e.g. NO 3 -, I -, and H 3 O + . Nitrate ion chemistry was used as the 147 reagent ion in these experiments, which is selective to highly oxidized molecules that have at least two hydroperoxy (-OOH)  consisting of a Po210 bipolar neutralizer, a nano-Differential Mobility Analyzer (nano-DMA; model 3081, TSI, Inc.), and a 156 condensation particle counter (MCPC; model 1720, Brechtel Manufacturing) were used to measure the number-size 157 distribution of particles, which is later used to deduce the total particle number and mass concentrations (the latter assumes a 158 uniform density for organic particles of 1.2 g cm -3 ). The sampling flow rate of the MCPC was 0.3 LPM and the sheath and 159 excess flows of the nano DMA were set to 3 LPM. The flow tube particle number-size distribution was measured without 160 further drying to get a more accurate measure of the actual particle surface area and volume, which are important for HOM 161 partitioning, and also to prevent particle evaporation during the measurements. 162 163

Experimental conditions 164
Three monoterpenes were used in our experiments (see Table 1), α-pinene, limonene and Δ 3 -carene. Oxidation by ozone is 165 believed to dominate over other oxidation radicals (i.e., OH or NO 3 ) in forming SOA under atmospheric conditions 166 (Atkinson and Arey, 2003). Ozonolysis of alkenes generates a substantial amount of OH, leading to products that are 167 produced by a combination of O 3 and OH oxidation. For some experiments, in order to isolate oxidation by O 3 , cyclohexane 168 (see Table 1 for mixing ratios) was premixed with the monoterpene and added to the flow tube as an OH scavenger. For other 169 experiments, the combination of OH and O 3 chemistry were investigated to study atmospheric oxidation chemistry more 170 representative of ambient air. The "high concentration" experiments were conducted with similar mixing ratios of 171 monoterpene (~1100ppb) and O 3 (~900ppb). The "low concentration" experiments were conducted to study the particle-free 172 chemical processes with initial concentrations of monoterpenes and O 3 shown in Table 1. Since wall losses should be 173 comparable for different precursors as a function of RH, it was not taken into consideration in our analysis of HOM 174 production. 175 176

HOM volatility predictions 177
The SIMPOL.1 method (Pankow and Asher, 2008) and the molecular corridor method (Li et al., 2016) were used to predict 178 the saturation mass concentrations (C*) of some of the detected OH-and O 3 -related HOMs. SIMPOL.1 is a group 7 contribution method and requires information on molecular structure, while the molecular corridor method only requires the 180 molecular formulae. Both methods are semi-empirical and based on volatility data from hundreds or thousands of 181 compounds. The calculated volatilities were then applied to the two-dimensional volatility basis set (2D-VBS) (Donahue et 182 al., 2012) to explore the likelihood that the products participate in the initial stages of nanoparticle growth. 183 184 3 Results and discussion 185

TI-CIMS performance 186
When comparing the TI inlet with the commercial nitrate inlet in measuring α-pinene ozonolysis products, both inlets 187 produced identical mass spectra. The sensitivities of both inlets to H 2 SO 4 were determined using a home-built H 2 SO 4 188 calibration system ( Figure S2 (Eisele and Tanner, 1993), for the TI in this position and the 193 commercial inlet were 3.25×10 10 molecules cm -3 and 1.41 × 10 10 molecules cm -3 , respectively. The lower calibration factor 194 for the TI inlet is attributed to the shorter reaction time (~80 ms) compared to the commercial inlet (~200 ms). We note that 195 the reaction time of the TI inlet can be further increased by positioning the ion source assembly further upstream relative to 196 the inlet orifice, which would require a slight modification of the current design. The total ion counts (TIC) of the TI inlet are 197 more than 5 times higher than the commercial inlet, which we attribute to the more direct path of ions through the ion source 198 as well as the use of a Po 210 radioactive source as compared to the soft X-ray in the commercial nitrate inlet. The limit of 199 detection (LOD) for sulfuric acid, which is defined as three times the standard deviation of the background (Jokinen et al., 200 2012), is 9.3 × 10 4 molecules cm -3 and 1.26 × 10 5 molecules cm -3 for the TI and commercial inlets, respectively. 201 202 After applying the N 2 curtain gas flow, the TIC recorded by the TI-CIMS decreased significantly. This was compensated for 203 by increasing the ion source and reaction chamber voltages that direct ions to the orifice ( Figure S3). When RH ≈ 90%, the 204 reagent ion mass spectrum was dominated by water clusters (H 2 O) m (HNO 3 ) n NO 3 -(m = 0-30, n = 0-2) if no N 2 curtain flow 205 was applied (Figure 4a) . In contrast, after 1 LPM N 2 curtain flow was applied to 207 the inlet, most of the water clusters were removed ( Figure 4b). The reagent ions, sample ions and TIC remained stable as RH 208 8 increased, which resulted in a reliable measurement of HOM concentrations as a function of RH. The result that the N 2 209 curtain flow eliminated water clustering to a large extent confirms that most of the water clusters in the spectrum were 210 produced during the free-jet expansion into vacuum instead of formed in the ion-molecular reagent region. 211 212 3.2 Identification of HOM spectrum 213 Figure 5 shows the average mass spectra of the HOM dimers and Figure S4 shows the average mass spectra of the HOM 214 monomers and RO 2 radicals for each of the six particle generation experiments. More than 400 peaks were identified in each 215 spectrum, the majority of which were clusters with NO 3 or HNO 3 NO 3 -. [H 2 SO 4 ], which arises from the oxidation of trace 216 amounts of SO 2 in the aero air, was ~10 5 molecules cm -3 and was always less than 3% of the most abundant C 10 products, 217 suggesting that sulfuric acid plays a negligible role in nucleation and cluster growth in our experiments. After subtracting the 218 reagent ions (NO 3 or HNO 3 NO 3 -), molecular formulae for organics with an odd number of H atoms were assigned to 219 radicals, which are generally difficult to detect experimentally (Rissanen et al., 2015), and formulae with an even number of 220 H atoms were assigned to closed-shell molecules. Most of the HOM products from the three endocyclic monoterpenes were 221 very similar, while the relative abundance of different HOMs was quite different, indicating similar reaction pathways but 222 different branching ratios in the reaction mechanisms. The main products were C 5-10 H 6-16 O 3-10 for closed shell monomers and 223 RO 2 and C 15-20 H 22-34 O 6-18 for closed shell dimers. Among these, C 10 and C 20 compounds were the most abundant. C 5-9 224 products could be formed from O 3 attack on the less reactive exocyclic carbon double bond or the decomposition of 225 intermediate radicals. Some fragments were found to be unique for specific monoterpene precursors. For instance, C 5 H 6 O 7 226 (m/z 240) was much more abundant in α-pinene oxidation than from other two precursors, which might be a tri-carboxylic 227 acid (Ehn et al., 2012). 228 229 Comparing total HOM abundance for the three monoterpene oxidation reactions, limonene created the most, followed by 230 α-pinene and then △ 3 -carene. This is in qualitative agreement with prior studies ( . When comparing the average 237 spectra with and without OH scavenger, no obvious differences were seen for OH-related RO 2 or monomers ( Figure S4). In 238 9 contrast, for dimers we found that C 20 H 32 O 6-13 were more abundant in experiments without OH scavenger ( Figure 5). The 239 formation of these dimers can be explained by the reaction of one OH-related RO 2 with one O 3 -derived RO 2 (see Section 240 3.5), and can therefore be considered as markers for combined OH and O 3 chemistry. As HOM dimers are generally less 241 volatile than monomers with identical O/C ratio, rapid production of dimers is believed to play a more important role in 242 initial particle formation and growth (Zhang et al., 2015). 243 244 3.3 RH influence on HOM generation 245 Figure 6 shows a time series of experimental parameters, particle size distribution, and key ions from the limonene 246 ozonolysis experiment with OH scavenger (EXP. 2 in Table 1). The O 3 inlet and outlet concentrations were approximately 247 constant with increasing RH (Figure 6a), indicating that RH did not significantly change O 3 levels in the flow tube. This also 248 shows that the reactivity of the limonene with ozone does not change with RH. The number concentration of the generated 249 particles decreased from 4.9×10 6 cm -3 to 2.7×10 6 cm -3 with increasing RH, while the peak of the number-size distribution 250 increased slightly, due in part to water absorption. When RH was above 80%, both the integrated number and mass 251 concentrations, which were calculated from the number-size distributions, decreased (Figure 6b). 252 253 Despite the change in particle number and mass concentrations with RH, the concentration of all the main HOMs, including 254 RO 2 , monomers and dimers, did not change for both OH-and O 3 -derived products (Figure 6c). In fact, the only signals in the 255 mass spectra that changed with RH corresponded to an increases associated with water clusters. The variations in HOM 256 concentrations can be explained by the competition between production and condensational losses. As almost all of the 257 detected HOMs are ELVOCs or LVOCs (see Section 3.6), they are not likely to partition back to the gas phase after they 258 encounter a surface. The condensation sink (CS) and wall loss rate for a compound with diffusion coefficient of 8.5×10 -6 m 2 259 s -1 (e.g., sulfuric acid) were estimated using established methods ( nm. The generated SOA particle number and mass concentrations for limonene (2.2-6.0 × 10 6 cm -3 for number 269 concentrations and 470-1025 μg m -3 for mass concentrations) were ~3-12 times greater than for △ 3 -carene (0.3-2.0 × 10 6 270 cm -3 for number concentrations and 56-86 μg m -3 for mass concentrations) and α-pinene (0.4-2.2 × 10 6 cm -3 for number 271 concentrations and 61-130 μg m -3 for mass concentrations). This is because the theoretical ozone reactivity of limonene is 272 3~5 times higher than the latter two and molar yield from limonene ozonolysis is also the highest. Peaks in the particle 273 number-size distributions were between 40 and 70 nm ( Figure S5). In most of the experiments, generated SOA mass 274 concentrations increased or decreased slightly when RH increased from ~0% to ~60% and decreased as RH further increased 275 to ~90%. The variability in particle mass concentration as a function of RH for different experiments can be attributed to 276 combined effects of gas phase reactions, condensed phase reactions, physical uptake of water, as well as the re-evaporation 277 of semi-volatile compounds from the wall. We cannot accurately quantify these effects. As a result, although the measured 278 SOA mass concentration remained relatively constant, we cannot draw conclusions from this observation. In contrast, while 279 particle number concentrations may also be affected by the factors mentioned above, they decreased by a factor of 2~3 with 280 increasing RH. In that study, SOA number concentrations decreased by a factor of 1.1~2.5 as RH increased, while the variation in 289 volume concentrations was negligible (within ±10%). They concluded that water's influence on non-volatile products, which 290 are responsible for the initial steps of nucleation, was much larger than its influence for semi-volatile compounds which 291 mainly determined the final volume concentrations of particles. Thus, it was highly suspected that water influenced new 292 particle formation through influencing the generation of NPF precursors. However, our measurements indicate that at least 293 the formation of the detected HOMs is independent of water vapor concentrations. There may be other species that are 294 crucial to the initial steps of NPF and are affected by water vapor but are not detected by nitrate CIMS (see section 3.5). 295 Another possible explanation is that a fraction of HOMs cluster with water at high RH in such a way that they may no longer 296 be able to participate in further cluster formation, thereby suppressing NPF. If the CIMS measurement only detected the 297 declustered molecule, then such a mechanism may still be consistent with our observations. 298 11 299 3.5 Possible formation pathways of water-relevant C 10 and C 20 HOMs 300 Although the oxidation of BVOCs has been widely studied, it has mostly been constrained to the early stages (first and 301 second generation intermediates) and many uncertainties still exist (Johnson and Marston, 2008; Isaacman-VanWertz et al., 302 2018; Atkinson and Arey, 2003). The first step of ozonolysis for the three BVOCs (α-pinene, △ 3 -carene and limonene) is 303 ozone attack on the endocyclic carbon double bond to form a primary ozonide. Figure 9 shows the O 3 -initiated oxidation 304 pathways of α-pinene that may be related to the detected C 10 and C 20 HOMs for representative isomers. The primary ozonide 305 rapidly transforms to two excited Criegee intermediates (eCIs), one of which (branching ratio= 0.4) (Kamens et al., 1999) is 306 shown in Figure 9. The reaction pathways of the eCI are complex, the most important two under ambient and most chamber 307 conditions are the sCI channel (reaction Ⅰ) and the hydroperoxide channel (reaction Ⅱ) (Bonn et al., 2002). The sCI either 308 reacts with aldehydes to form a secondary ozonide (when the aldehyde is C 10 , then the formed SOZ is C 20 and is marked as 309 sCI-C 10 ) or with water or other acidic compounds such as alcohols and carboxylic acids to form hydroxy-hydroperoxide, 310 which then decomposes to carboxylic acids or aldehydes. For α-pinene, the main decomposition product is pinonic acid. In 311 the hydroperoxide channel (reaction Ⅱ), the formed hydroperoxide quickly decomposes to a first generation alkyl radical (R) 312 and OH (Johnson and Marston, 2008). R reacts with O 2 immediately to form the first generation RO 2 , which can undergo 313 numerous reactions, including reaction with HO 2 , R'O 2 and autoxidation. The reaction with HO 2 mainly forms 314 hydroperoxides, with a small fraction forming hydroxyl or carbonyl-containing compounds. When reacted with another R'O 2 , 315 either ROOR' or an alkoxy radical (RO) or a carbonyl and a hydroxyl are formed. The RO can undergo isomerization, or 316 form a carbonyl and HO 2 , for which the branching ratios are extremely difficult to evaluate. RO can also undergo 317 decomposition, which is one of the pathways to form C 5~C9 . The autoxidation process is key to HOM formation. Each anhydride is more likely in condensed phase, whereas there is also a possibility it can also happen in gas phase (Kamens et 343 al., 1999). However, it is unknown whether these sCI-C 10 can be detected using nitrate-CIMS as they may lack hydrogen 344 bond donor moieties. The semi-volatile pinonic acid can also form HOMs after further oxidation by OH (Ehn et al., 2014), 345 provided that excess α-pinene is not present to compete with pinonic acid for the generated OH. 346 347 Water vapor's influence on HOM formation can be direct or indirect. For monoterpene oxidation, the direct participation of 348 water vapor is to react with sCI, favoring the formation of the hydroperoxide and its decomposition products (reaction Ⅲ) 349 over the secondary ozonides (sCI-C 10 , reaction Ⅳ) or possible anhydrides (sCI-C 10 , reaction Ⅴ). Since the formation of 350 sCI-C 10 is more likely to contribute to NPF than the products from sCI and water vapor (Kamens et al. The indirect water effect on HOM formation includes the water influence on HO 2 fate. As water promotes HO 2 self-reaction 361 (Equation 2), reaction of HO 2 with RO 2 should decrease and the related HOM monomers should likewise decrease with 362 increasing RH. However, as the formation of both HOM monomers and dimers was not affected by H 2 O, it was likely that 363 water does not significantly increase HO 2 self-reaction or that HO 2 chemistry was not important in our experiments. 364 groups (e.g., aromatic rings, aldehydes, ketones, hydroxyls, peroxides, hydro-peroxides) was estimated and used to derive 372 saturation vapor pressure using SIMPOL.1 (Table S1). To simplify the calculation, the functional groups used here were 373 directly predicted from the proposed formation pathways in Figure 10  difference of C* predicted from the two methods was ~1-4 orders of magnitudes. Despite these differences, most of the C 20 386 HOMs can be classified as ELVOCs, while C 10 products were mostly LVOCs. Typically, t for those compounds with 387 14 identical ̅̅̅̅ , such as C 20 H 30 O 10 (O 3 -derived dimer, log 10 (C*)=-2.74), C 20 H 32 O 11 (OH and O 3 combined dimer, 388 log 10 (C*)=-5.11), C 20 H 34 O 12 (OH-derived dimer, log 10 (C*)=-7.41), OH-derived HOMs have lower volatilities than 389 O 3 -derived HOMs due to a greater number of (hydro) peroxide groups. 390 391

Conclusions 392
The RH influence on HOM formation and NPF during monoterpene oxidation was explored in this study. HOMs were 393 detected with a TI-CIMS, using nitrate as reagent ions; C 10 and C 20 dominated the spectra. There are mainly three potential 394 paths for water vapor influence on the formation of C 10 and C 20 HOMs. One is water reacting with sCI (Equation 1), thereby 395 influencing the branching ratio between formation of more volatile compounds decomposed from hydroxyl hydroperoxide, 396 such as pinonic acid, and accretion products with sCI such as secondary ozonide (sCI-C 10 ) and anhydride (sCI-C 10 ). The 397 second hypothesized water influence is on the HOMs formed from hydration reactions (Equation 2). The third is that water 398 increases the rate of self-reaction of HO 2 (Equation 3), thus indirectly impacts the loss pathways of RO 2 . Our experimental 399 results, both with high particle loading and particle-free conditions, demonstrated that neither the detected HOM species nor 400 their signal abundance changed significantly with RH. This indicates that the detected HOMs, which can mostly be 401 explained by RO 2 autoxidation, must be formed from water-independent pathways rather than by those reactions mentioned 402 above. One implication of this result is that HO 2 self-reaction was not significantly promoted by water or that the RO 2 403 reaction with HO 2 was not be significant in our system, but instead that RO 2 reacts with another peroxy radical, R'O 2 , to 404 generate both closed shell monomers and dimers. Another implication is that the sCI pathway is not responsible for the 405 generation of the detected HOMs while the role of sCI-related HOMs (SOZ or anhydride) formation by accretion with long 406 chain products, which may not be detected with nitrate CIMS, may be important in causing the decrease in SOA number 407 concentrations with increased RH. Another possible explanation for the decreasing SOA number concentration is that water 408 may cluster with HOMs and suppress NPF. 409

410
The detected HOMs, which could mostly be explained by autoxidation of RO 2 followed by reactions with R'O 2 or HO 2 , were 411 distinguished as OH-related, O 3 -related RO 2 , closed shell HOM monomers, and HOM dimers. The volatility of the identified 412 products were estimated with the SIMPOL.1 group contribution method and with the molecular corridor technique. That 413 analysis confirmed that C 20 closed shell products have significantly lower volatility compared to C 10 products and are thus 414 more likely to contribute to NPF. For the HOM products with identical ̅̅̅̅ , OH-derived HOMs have lower volatilities than 415 O 3 -derived HOMs due to a greater number of (hydro) peroxide groups. As a result, OH chemistry is suspected to be more 416 15 likely to lead to NPF than O 3 chemistry, given the same level of oxidants and VOCs precursors. 417 418