Kinetics and impacting factors of HO 2 uptake onto submicron atmospheric aerosols during a 1 2019 air quality study (AQUAS) in Yokohama, Japan

28 HO 2 uptake kinetics onto ambient aerosols play pivotal roles in tropospheric chemistry but are not fully 29 understood. Field measurements of aerosol chemical and physical properties should be linked to 30 molecular level kinetics; however, given that the HO 2 reactivity of ambient aerosols is low, traditional 31 analytical techniques are unable to achieve this goal. We developed an online approach to precisely 32 investigate the lower limit values of (i) the HO 2 reactivities of ambient gases and aerosols and (ii) HO 2 33 uptake coefficients onto ambient aerosols (  ) during 2019 air quality study (AQUAS) in Yokohama, 34 Japan. We identified the effects of individual chemical components of ambient aerosols on  . The results 35 verified in laboratory studies on individual chemical components: transition metals play a key role in 36 HO 2 uptake processes and chemical components indirectly influence such processes (i.e., through 37 altering aerosol surface properties or providing active sites), with smaller particles tending to yield 38 higher  values than larger particles owing to the limitation of gas phase diffusion is smaller with 39 micrometer particles and the distribution of depleting species such as transition metal ions is mostly 40 distributed in accumulation mode of aerosol. The modeling of  utilized transition metal chemistry 41 derived by previous studies, further confirming our conclusion. However, owing to the high NO 42 concentrations in Yokohama, peroxy radical loss onto submicron aerosols has a negligible impact on O 3 43 production

The enrichment factor of VACES for the surface area was estimated as 12.5 ±2.5 from the ratio 162 between S2 and S1, where S2 and S1 are the averaged surface areas measured by SMPS2 and SMPS1 of 163 each day, respectively. According to the test from previous study of the VACES system, there is no 164 distortion of the size distribution of the original ultrafine aerosols as the particle concentration 165 enrichment occurs without any coagulation (Sioutas et al., 1999), here we listed the mean radius and 166 geometric standard deviation (Geo. Std. Dev.) of the ambient aerosols before and after VACES during 167 the enrichment factor measurement periods, as shown in Table 1. We could see that the mean radius 168 before and after VACES are not statistically different within the standard deviation. 169

172
The enriched surface area of ambient aerosols with aerodynamic diameter < 0.74 μm (PM0.74) was 173 calculated from the surface area of ambient aerosol measured by SMPS1 and the enrichment factor. The 174 enriched surface area of PM2.5 was then calculated by multiplying the enriched surface area of PM0.74 175 by the mass ratio between PM2.5 and PM0.75 (~1.1), where we assume the surface area are increased in 176 proportional to the mass concentration. However, as the larger particles (here referred to particles ranged 177 from 0.74 to 2.5 μm) tend to have lower surface area than the smaller particles, we consider the obtained 178 enriched surface area of PM2.5 as the upper limit value. More details can be found in SI. 179 HO2 uptake kinetics After passing through the VACES system, the ambient air was sampled using a 180 three-port valve (Bolt, Flon Industry Co., LTD) and injected into the LFP-LIF system. The valve was 181 switched automatically between two sampling lines, one with the aerosol filter on, and the other one 182 with the aerosol filter off, HO2 reactivities in ambient air caused by two modes were measured: (a) the 183 gas phase mode with aerosol filter on, the HO2 reactivities are represented as kg, and (b) the gas + 184 enriched aerosol phase mode with aerosol filter off, the HO2 reactivities are represented as kg+Eka, where 185 represents the enrichment factor of ka, Eka represents the total HO2 reactivities caused by enriched 186 ambient aerosols, the usage of Eka is based on the assumption that HO2 uptake with aerosol particles 187 follows the pseudo-first-order rate law. We modeled kg in both modes using a theory identified 188 previously (see SI: HO2 reactivity of ambient gas phase) (Zhou et al., 2019b) and compared it with the 189 measured values. The differences between measured and modeled kg in mode (a) enabled us to establish 190 their interrelationship and to check instrument stability. The differences between (kg+Eka) and the 191 modeled kg in mode (b) are considered as the enriched aerosol phase HO2 reactivity (Eka). The total HO2 192 reactivity decay profile follows single-exponential decay: 193 where bg denotes the zero air background obtained by injecting zero air with the same RH as the real-195 time ambient value into the reaction cell every 24 h for 30 min. The RH was controlled by passing some 196 of the zero air through a water bubbler. The value of kbg was subtracted separately on each day. The 197 variability of kbg (i.e., the reproducibility of the laser system) was calculated as the standard deviation 198 of the response of repeated measurements on different days. It was found to be ~4%, which is slightly 199 higher than the instrument precision (3%). A 30-min average calculation was applied to the data to 200 reduce data fluctuation. The observed HO2 uptake coefficients onto ambient aerosols ( obs ) can be 201 calculated from the dependence of a on obs : 202 a = obs ω HO 2 S 4 (2) 203 where ES and HO 2 represent the enriched surface area of ambient aerosol after VACES and the mean 204 thermal velocity of HO2 (~437.4 m s −1 ), respectively. The uncertainty of the enriched surface area was 205 estimated from the instrument systematic error of SMPS (~ 8%) and the uncertainty of the enrichment 206 factor (±2.5), which are shown in Fig.1b (see SI). The HO2 reactivity of ambient aerosol (ka) can be 207 obtained from by dividing by the enrichment factor E. 208

High resolution-time of flight-aerosol mass spectrometry (HR-ToF-AMS) A field-deployable HR-209
ToF-AMS (Aerodyne Research Inc.) (DeCarlo et al., 2006) was used for the characterization of the 210 non-refractory aerosol mass with a time resolution of ~3 min. The HR-ToF-AMS measured the total 211 organic aerosol (OA), sulfate (SO4 2-), nitrate (NO3 -), ammonium (NH4 + ), chloride (Cl -), and the two 212 most dominant oxygen-containing ions in the OA spectra, i.e., mass-to-charge ratios of m/z = 44 (Org44, 213 mostly CO2 + ) and m/z = 43 (Org43, mainly C2H3O + for the oxygenated OA and C3H7 + for the 214 hydrocarbon-like OA) (Ng et al., 2011). The fractions of Org44 and Org43 in OA are represented as f44 215 and f43, respectively. Ambient air was sampled through a critical orifice into an aerodynamic lens, which 216 efficiently transmitted particles between 80 nm and up to at least 1 µm. Particles were flash-vaporized 217 by impaction on a resistively heated surface (~600 o C) and ionized by electron ionization (70 eV). The 218 m/z values of the resulting fragments were determined using a ToF mass spectrometer. Data were 219 analyzed using the ToF-AMS software SQUIRREL and PIKA. Data were not corrected for lens 220 transmission efficiency. Standard relative ionization efficiencies (RIE) were used for organics (RIE = 221 1.4), nitrate (RIE = 1.1), chloride (RIE = 1.3), sulfate (RIE = 1.12), and ammonium (RIE = 4). 222 Concentration data were obtained from background-subtracted stick-mass data (low-mass-resolution-223 base mass concentration data, which are calibrated using ammonium sulfate particles) and determined 224 assuming a collection efficiency (CE) of 0.5. 225 Filter-based photometer Real-time measurement of the equivalent black carbon (eBC) was performed 226 using a 5-wavelength dual-spot absorption photometer (MA300, AethLabs, San Francisco, CA, USA), 227 which performed an online correction for possible artefacts resulting from filter loading and multiple 228 scattering (Drinovec et al., 2015). In this study, eBC data obtained from light attenuation at a wavelength 229 of 880 nm were used to avoid possible contributions from brown carbon; the time resolution was ~1 230

min. 231
Trace elements Fourteen trace elements (Al, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Sr, Cd, Ba, and Pb) 232 were measured using an offline method at two-day intervals from 21 July to 5 August 2019. The 233 suspended particulate matter (SPM) was collected onto 623.7 cm 2 size quartz fiber filters (Pallflex 234 Tissuquartz 2500QAT-UP), which had an available collecting area of 405.84 cm 2 , using a high-volume 235 sampler (1000 L min -1 ). Approximately 2 cm 2 of each filter was cut into pieces and placed into a 236 polytetrafluoroethylene (PTFE) pressure digestion tank with 1 mL 49% hydrofluoric acid (HF) and 5 237 mL 69% nitric acid (HNO3). A Thermo Fisher X2 Series ICP-MS was then used to determine metal 238 concentrations. By assuming that the metal fractions were the same in SPM and PM1 (aerosol 239 particles with aero-dynamic diameters less than 1 µm), the concentrations in PM1 were estimated 240 according to the tested metal concentrations in SPM and the ratio between SPM and PM1 measured in-241

NO3
-, Cl -, Ca 2+ , K + , Mg 2+ ) used for the ISORROPIA-II model were also measured using offline method,   ISORROPIA-II model NR-PM1 water-soluble inorganic species (including Na + , SO4 2-, NH4 + , NO3 -, Cl 258 -, Ca 2+ , K + , Mg 2+ ) and meteorological parameters including temperature and RH were used to calculate 259 the aerosol pH and liquid water content based on the ISORROPIA-II model (Fountoukis and Nenes, 260 2007). We ran ISORROPIA-II in "reverse" mode and the particles were assumed to be deliquescent, i.e., 261 in metastable mode (Hennigan et al., 2015). The thermodynamic equilibrium of the NH4 + -SO4 2--NO3 262 system case was used for modeling. 263 3 Results and discussion  the median value of 0.25 μm), the gas-phase diffusion effects on  were estimated to be ~ 6.6 % (further 282 details are given in the SI). The absolute increase of  due to the gas-phase diffusion is 0.03 on average, 283 which is negligible compared to  uncertainty (~0.21 on average). Therefore, we ignored the gas-phase 284 diffusion effects to .

323
Statistical significance analysis showed that the average  value of group i (0.35 ± 0.28) is 324 significantly higher than that of group ii (0.21 ± 0.16) (calculated p = 4.9E-5; Mann-Whitney), 325 indicating that the air masses from the ocean yield higher  values than the air masses from mainland 326 Japan. The difference in  values between group i and group ii may due to the different chemical 327 components contained in the ambient aerosols arrived from the ocean or mainland, which we will 328 discuss in the following sections. The average value of ka at Yokohama (0.005 ± 0.005 s -1 ) was much 329 higher than that at Kyoto city (0.0017 ± 0.0015 s -1 ) (with calculated p < 0.05; Mann-Whitney), this may 330 due to the different aerosol properties in Kyoto and Yokohama city. We list some of them as follows: 331 1) mass composition, the aerosols at the coast city (Yokohama) tend to contain more sea salts thus 332 increased ka, 2) particle size distribution, smaller particles tend to yield higher  values than larger 333 particles owing to the depleting species (e.g., transition metal ions) are mostly distributed in 334 accumulation mode of aerosol, 3) the water content and the metal concentrations, which will highly 335 influence the HO2 uptake capacity of the ambient aerosols. However, the average value of HO2 uptake 336 coefficient onto ambient aerosols () at Yokohama is ~0.23, which is comparable with previous 337   During this period, PM1 ranged from ~1 to 35 µ g m -3 (average  13 µg m -3 ) and was dominated by OA, 350 SO4 2-, and NH4 + , with contributions of 39 ± 11%, 30 ± 12%, and 12 ± 4%, respectively; these were 351 followed by eBC and metals, with contributions of 10 ± 7% and 8 ± 8%, respectively. Clcontributed 352 < 1% in both groups, which is similar to that reported for an urban area in winter in Bern (Switzerland) 353 (Zhou et al., 2019a). However, NO3contributed much less (~2 ± 0.7%) compared with that reported 354 for Bern (~19 ± 4%), which may be due to the reverse reaction of NH4NO3 converting to HNO3. Since 355 Yokohama is a coastal city, and HNO3 is easy vaporized in summer, gaseous HNO3 may sink with sea 356 salt particles by forming NaNO3 through heterogeneous reactions (Finlayson-Pitts and Pitts, 2000). 357 Cu and Fe contained in ambient aerosols can be chelated by organics (Lakey et al., 2016b). Therefore, 381 we produced a Pearson correlation matrix of all the testing factors at Yokohama city, including different 382 chemical components, ka and  . Here we note that the different chemical components were measured 383 using HR-ToF-AMS for ambient aerosols with aerodynamic diameters < 1 μm, while ka and  were 384 measured using VACES-LFP-LIF system for ambient aerosols with aerodynamic diameters < 2.5 μm, 385 but due to most "fine-mode" aerosols have the mean diameter ranged from 0.09 µm to 0.47 µm (with 386 the median value of 0.25 µm, measured by SMPS), we assume the chemical components of ambient 387 aerosols with the aerodynamic diameter ranged between 1 μm and 2.5 μm have negligible impact on 388 Pearson correlation matrix result. However, present results do not include the effects of coarse particles 389 (with aerodynamic diameters > 2.5 μm) to the HO2 uptake kinetics, and we may partially miss 390 measuring sea spray (with diameters ranged from ~ 0.05 to 10 µm) effects. When Cl measured by 391 AMS increased, coarse particles may exist and our results may not represent the real ambient 392 conditions. Consequently, we consider our results as the lower limit of the HO2 uptake kinetics onto 393 real ambient aerosols. 394 To exclude the effects of the different fractions of chemical components in groups i and ii, the 395 bootstrap method, which is based on the creation of replicate the inputs by perturbing the original data 396 through resampling, was employed. The resampling was performed by randomly reorganizing the rows 397 of the original time series such that some rows of the original data were present several times while 398 other rows were removed. The final results were obtained by running the data for 1000 bootstrap 399 replicates. The average values of these 1000 bootstrap replicates are listed in Fig. 3. 400

406
Most of the chemical components had strong or moderate Pearson correlation coefficients with each 407 other (Fig. 3), although ka and  showed only a moderate correlation with each other (0.56). As  can be coated by other chemical components, thereby increase  . This is further confirmed by the classification 426 of the air masses, i.e., the air mass from the ocean (group i), which contained less OA and more metals 427 than that from mainland Japan (group ii), had a higher HO2 uptake capacity. We further compared the 428 measured  values with the modeled  values using previously proposed mechanisms, as shown below. (ii) chemistry with transition metals playing a role. In this study, the liquid content of the total ambient 433 aerosol mass ranged from 70% to 88%, as obtained from the ISORROPIA-II model. As the solubility of 434 Fe is rather small in ambient aerosol, the reaction rates of Fe/Mn for liquid phase HO2 in aerosol is ~ 435 100 times slower than it is for Cu, thus the influence of Fe and Mn on HO2 uptake can be neglected 436 the destruction of peroxy radicals, the activity coefficient for total Cu was assumed to be 0.1 (upper 442 limit) based on a study of (NH4)2SO4 particles at 68% RH (Ross and Noone, 1991;Robinson and Stokes, 443 1970). Using copper ions as a surrogate metal for transition metal ions (TMIs), the potential HO2 loss 444 onto aqueous ambient aerosols via mechanisms involving TMIs was estimated as (Hanson et al., 1994): where HO 2 is the mass accommodation coefficient of HO2, is the mean HO2 molecular speed (cm 447 s -1 ), Heff is the effective Henry's Law coefficient, R is the gas constant (J K -1 mol -1 ), and T the 448  Table S1 shows more details of the parameters used for modeling. Previous laboratory studies suggest  shows that  TMI has a weak correlation with measured  values when  ≥ 0.4 (Fig. S7), which may due 469 to the higher fraction of metals in the total mass at measured  ≥ 0.4 (~12%) than at < 0.4 (~7%); 470 therefore, the impact of the other chemical components is much lower. The  values obtained here are 471 comparable with those in previous ambient aerosol studies (Taketani et al., 2008;Zhou et al., 2019b) (Fig.  472  5b). When compare with single-compound aerosols obtained from laboratory studies,  values were 473 generally higher than the HO2 uptake coefficients onto organic species (Lakey et al., 2015), soot particles 474 (Bedjanian et al., 2005), and the dry state of inorganic aerosols (i.e., (NH4)2SO4, NaCl, and H2SO4), but 475 comparable or lower than aqueous and copper-doped aqueous phases of inorganic species (Fig. 4b)   study, ka and  showed no linear dependence on the mean ambient particle diameters (see Fig. S10).

508
Identifying the fractional contributions of aerosols in different particle size ranges to ka and  is highly 509 desirable in terms of understanding their influence. However, it seems that high  values (> 0.8) occur 510 when the surface area is < 2× 10 −6 cm 2 cm -3 and the mean particle diameter is < 110 nm. This is in 511 accordance with a previous study showing that aerosols yield the highest fractional contribution to the 512 total heterogeneous loss rate of HO2 radicals of size < 0.1 µm (Morita et al., 2004) and that the mass 513 accommodation process plays the determining role for small and medium sized aerosols in controlling 514 HO2 uptake. Guo et al. (2019) states the HO2 radicals experience less loss upon its diffusion into larger 515 droplets than its diffusion into small droplets due to dilution effects make the larger aerosols having 516 lower depleting species concentrations (Cu 2+ ). However, this was based on the assumption that the total 517 mass of Cu 2+ is constant during the hygroscopic growth of particles which is not always true in the 518 ambient conditions. Further studies about Cu 2+ content in particles with different sizes are needed to 519 fully understand the result here. 520

Significance of ka to O3 formation potential 521
In urban atmosphere, XO2 (=HO2+RO2) fate is important to the photochemical production of ozone 522 (P(O3)). Here, the loss rates of XO2 due to three factors were compared: (i) uptake onto the ambient 523 aerosols ( P−XO 2 in Eq. 5), since no experiment or reference available for RO2 uptake onto ambient 524 particles, we assume the RO2 reactivities caused by its interaction with ambient aerosols were the same 525 as ka, (ii) XO2 self-reactions ( R−XO 2 in Eq. 6), and (iii) reaction with NO ( N−XO 2 in Eq. 7), which can 526 produce NO2, a precursor of O3; therefore Eq. 7 can also be regarded as P(O3). 527 where HO 2 −HO 2 and HO 2 −RO 2 are the second-order rate constants of HO2 self-reaction and its reaction 531 with RO2, respectively. NO−HO 2 is the second-order rate constant of the reaction of HO2 with NO. The 532 HO2 concentration was estimated from O3 concentration using the method described by Kanaya et al., 533 (2007a). The RO2 concentration is then estimated by assuming a steady state of HO2 in the HOX cycle; 534 the reaction rates of HO2 radicals are approximated as 0: 535 where CO−OH and H 2 CO−OH are the second-order rate constants of the reactions of CO and H2CO with 538 OH, respectively. The different XO2 loss rates described in Eqs. 5-7, along with the measurement times, 539 are shown in Fig. 5a. Generally, P−XO 2 is much greater than R−XO 2 , indicating that the XO2 taken up 540 by ambient aerosols will compete with the XO2 self-reaction, thus influencing XO2 concentration. 541 However, such an influence may have a negligible impact on P(O3) because P−XO 2 is tens of thousands 542 of times lower than NO−XO 2 owing to the relatively high NOx concentration at Yokohama. We further 543 where = P−XO 2 /( P−XO 2 + R−XO 2 ). The results indicate that both LN/Q and LN/Q_without_aerosol 553 (calculated with and without including ′ in Eq. 9, respectively) were higher than LN/Q_transition, 554 indicating that ozone formation was VOC-sensitive throughout the campaign and that the aerosol uptake 555 of XO2 (ka') showed no impact on the O3 formation regime (see in ambient aerosols may act as a catalyst, thus accelerating the depletion of HO2, however, they can be 607 chelated by OA. OA can also cover the aerosol surface and alter the viscosity of ambient aerosols, 608 thereby decreasing , and that more oxidized organic aerosols tend to be highly viscous thus decrease may be owing to BC can provide active sites or be coated by other chemical components thus facilitating 617 the physical uptake of HO2. Here, we observed higher  values (> 0.8) when the mean particle diameter 618 is < 110 nm, identifying the fractional contributions of aerosols in different particle size ranges to ka and 619  is highly desirable in terms of understanding their influence.

620
In summary, the chemical components and physical properties of ambient aerosols may dominate 621  variation during field campaign; to yield more accurate  value, total suspended particles in ambient 622 air should be measured, and the metal-catalyzed reactions, chemical components, and aerosol states 623 should be considered. Also, improvements to the time-resolution of metal measurements are needed for 624 more precise analysis. For more detailed investigation of HO2 uptake mechanisms, an offline 625 methodology that can maintain constant chemical compositions and experimental conditions (such as 626 RH and T) will be useful. The HO2 loss onto ambient aerosols was identified to have a negligible impact 627 on the O3 production rate and formation regime owing to the high NOX concentrations at Yokohama.   (Table S1), summary of equations and values used for XO2 644 (=HO2+RO2) loss and O3 formation sensitivity regime (Table S1).