Figure  summarizes the ROS concentrations and the chemical and phase transfer rates for the cloud case at pH = 4.5; the corresponding figures for simulations at pH = 3 and 6 are shown in Fig. S1 (Supplement). The net transfer rates (moles per gram of air per second) into the aqueous phase are shown in the yellow boxes. For FeN<100, the relative contributions (%) of this rate into the iron-free and iron-containing droplets are shown next to the arrows. The phase transfer rates are additionally shown in units of moles per liter per second (using the air density of 1.13 × 10-3 g cm-3 and LWCs in Table ), together with the chemical source and loss rates in the aqueous phase (M s-1) and the corresponding aqueous-phase ROS concentrations (M) (white boxes). If the effect of iron is negligible on aqueous-phase concentrations and aqueous-phase rates, there should be no concentration difference between any droplets, neither between the results for FeBulk and FeN<100 nor between the iron-free and iron-containing droplets. In addition, the ratio of the phase transfer rates should also be equal to the ratio of the LWCs of the two drop classes (98 % : 2 %).

All ROS are predicted to be taken up into cloud water, independently of the iron distribution at pH = 4.5 (Fig. ), and also their ratio of the phase transfer rates nearly corresponds to the ratio of the LWCs of the two droplet classes (98 % : 2 %). Significant deviations from this theoretical value occur at pH = 3 for HO2 (Fig. S1b) and at pH = 6 for H2O2 that is predicted to evaporate from iron-free droplets (Fig. S1d).

The H2O2 concentrations in all droplets under all conditions and pH values (Fig. S1) correspond to the equilibrium concentrations according to Henry's law, in agreement with previous studies that have shown that H2O2 is at thermodynamic equilibrium in cloud water . At pH = 3 and 4.5, the efficient H2O2 consumption is compensated by a relatively higher uptake rate, resulting in the efficient replenishment of H2O2 and equilibrium concentration. The uptake rates (in M s-1) between FeBulk and the iron-free droplets in the FeN<100 approach are similar (e.g., for H2O2 : 0.56 and 0.53 M s-1), whereas they are higher for uptake into the iron-containing droplets (2.2 M s-1). Since these droplets only comprise 2 % of the total LWC, this difference is not reflected in the net uptake rate. At pH = 6 (Fig. S1c and d), the majority of cloud droplets are predicted to be a source of H2O2; the only exception is the iron-containing droplets in the FeN<100 approach, from which H2O2 evaporates due to less efficient chemical H2O2 consumption in the aqueous phase.

The OH concentrations differ by nearly 1 order of magnitude between the iron-free and iron-containing cloud droplets (5.8 × 10-14 M vs. 3.3 × 10-13 M; Fig. b); the concentration in the FeBulk model is even slightly lower (4.5 × 10-14 M; Fig. a). The reasons for these differences are explored in Sect. , where the individual chemical processes affecting the concentrations are analyzed.

The bulk aqueous-phase OH concentration in the FeBulk approach is approximately 50 % higher (4.5 × 10-14 M) as compared to 6.4 × 10-14 M for FeN<100 (Fig. ). A much greater discrepancy of nearly 1 order of magnitude between the two approaches is predicted at pH = 6, whereas the difference in the bulk concentrations at pH = 3 is smaller (Fig. S1). These concentrations corresponded to those in bulk cloud water samples from cloud droplet populations with FN,Fe = 2 %, provided that chemical processes in the sample upon sampling do not change the ROS levels (Sect. ). For all cases, the chemical source and loss rates of the OH radical compensate for each other, leading to a relatively small uptake rate of the moderately soluble OH radical into the aqueous phase (∼ 10-16 mol g-1 s-1), which is not significantly affected by FN,Fe.

The concentration difference of HO2 / O2- between iron-free and iron-containing droplets is nearly 2 orders of magnitude (2.4 × 10-8 M vs. 8.9 × 10-10 M), whereas the concentration for FeBulk is predicted to be between these two values (8.9 × 10-9 M) (Fig. ). The difference in the concentrations is even greater (factor > 200) at pH = 6 (Fig. S1d), whereas it is less than a factor of 3 at pH = 3. These trends suggest that the concentrations are dependent on pH (Sect. ). The comparison of the bulk aqueous-phase HO2 / O2- concentrations resulting from the FeN<100 approach (bottom of Figs. b, S2b and d) to those in FeBulk shows that at pH = 4.5 and 6, the total HO2 / O2- is underestimated by about 1 order of magnitude. At pH = 3, the chemical loss rate of HO2 / O2- in iron-containing droplets is relatively larger than its chemical source rate (74 × 10-8 M s-1 vs. 66 × 10-8 M s-1; Fig. S1b). This imbalance in the chemical rates leads to a very efficient HO2 uptake into iron-containing droplets. Consequently, the phase transfer into these droplets comprises 13 % of the total uptake rate, whereas at higher pH values the contributions correspond to the ratio of LWCs between the two droplet classes (98 % : 2 %). The higher Henry's law constant of HO2 and rate constants of O2- reactions as compared to those of HO2 at higher pH lead to increasingly lower HO2 / O2- concentrations with increasing pH.

Since the LWC of the aqueous aerosol particles is smaller by a factor of 2 × 105 than in clouds, the aqueous-phase iron concentration is accordingly higher, and so are all chemical rates of iron reactions. Similar to cloud water, the H2O2 concentration is neither significantly affected by the iron distribution nor by iron reactions; i.e., at a given pH value its aqueous-phase concentration is nearly the same in all particles. At pH = 4.5 and 6, evaporation of H2O2 is predicted for FeBulk and from the iron-free particles in FeN<100 (Figs. b and S2b and d). However, since the uptake rate of H2O2 into the iron-containing droplets is much higher, exceeding the evaporation rate by a factor of 25 (-4 % vs. 104 %, pH = 4.5) and ∼ 2 (-143 % vs. 243 % at pH = 6), aqueous particles represent a net sink of H2O2.

Similar to the cloud case, the OH concentrations in the aqueous phase of FeBulk are between those in the iron-free and iron-containing particles for FeN<100. The OH concentrations do not show a strong pH dependence, but their bulk values are about a factor of 3 lower for the FeN<100 approach (FN,Fe = 2 %) than for the FeBulk approach (1.6 × 10-15 M vs. ∼ 4.6 × 10-15 M). Even at the high iron concentrations as present in aerosol water (≤ 0.41 M), the chemical source and loss rates of OH nearly cancel out, leading to a relatively small uptake rate.

The HO2 / O2- concentrations in the FeBulk approach are comparable to those as predicted for cloud water at the same pH value. However, the concentrations in the iron-free and iron-containing particles differ by 4 orders of magnitude at pH = 6 (1.5 × 10-6 and 1.3 × 10-10 M, Fig. S2d). The resulting bulk aqueous-phase concentrations are at least 1 order of magnitude underestimated by FeBulk (pH = 3); this difference is even a factor of 2000 at pH = 6 (Figs. d and S2d). While the chemical loss rate of HO2 / O2- is nearly equal to or even larger than its source rate at pH = 4.5 and 6, at pH = 3 the source rate exceeds the loss rate resulting in evaporation of HO2 from the 2 % iron-containing particles (Fig. S2b). However, since the uptake rate into the iron-containing particles is larger than this evaporation rate, the net flux of HO2 is an uptake into the particle phase.

The gas-phase concentrations of OH and HO2 and mixing ratios of H2O2 are shown in Fig. S3 as a function of time (t≤7200 s) in the presence of cloud droplets and aerosol particles, respectively, at the three pH values considered here and for the two assumed iron distributions in the aqueous phase. The H2O2 mixing ratio in the presence of cloud water is very quickly reduced to about 50 % of its initial concentration (1 ppb) due to partitioning to the aqueous phase. Both the pH value and FN,Fe have a small effect on the H2O2 mixing ratio. The small increase of H2O2 over time is in agreement with findings in a recent multiphase box model intercomparison .

The OH concentrations are on the order of 106 cm-3, i.e., typical concentrations as found for continental daytime conditions. These concentrations build up within a few seconds and do not largely change over the time period considered here. The difference of up to a factor of 5 in the OH concentrations in the presence of cloud droplets with a pH value of 3 and 6, respectively, cannot be directly explained by any OH reactions in the aqueous phase but is rather a consequence of the OH–HO2 cycle in the multiphase system (Fig. ). At pH = 6, the effective Henry's law constant for HO2 is highest, and therefore, most HO2 is partitioned to the aqueous phase, resulting in the lowest fractions of HO2 in the gas phase. The gas-phase HO2 concentrations are on the order of 108 cm-3, in agreement with predictions from other atmospheric multiphase chemistry models (e.g., ). Generally, the pH value and iron distributions cause smaller differences in the presence of aerosol particles as compared to the cloud case, since the liquid water content of particles is smaller by several orders of magnitude, and therefore a smaller fraction of the species is dissolved.

The partitioning coefficient ϵ is often used to describe the distribution of species between the aqueous and gas phases in the atmospheric multiphase system, e.g., . It is defined as 4ϵ=Caqpg⋅KH(eff), with Caq and pg being the aqueous-phase concentration (M) and gas-phase partial pressure (atm) and KH(eff) the (effective) Henry's law constant (M atm-1). Accordingly, ϵ<1 indicates situations when the aqueous phase is subsaturated.

The ϵ values for the three ROS are summarized in Fig.  for the cloud and aerosol simulations at three pH values. The fact that ϵH2O2 is ∼ 1 shows that H2O2 is in thermodynamic equilibrium and explains why its concentrations do not differ among iron-containing and iron-free droplets (particles) (Figs. , and S2).

The ϵOH values in the presence of cloud droplets (red symbols, Fig. b) are on the order of ∼ 0.01–0.1. The values for iron-containing aerosol particles show a distinct trend with highest ϵOH values in iron-containing particles, whereas ϵOH in iron-free particles is lower by 2 and 3 orders of magnitude, respectively. The lack of a clear trend in ϵOH with pH suggests that the main formation and loss processes of OH are pH-independent. The differences in ϵOH scale with the OH concentrations in aerosol particles that differ by 3 orders of magnitude between iron-containing and iron-free particles in the FeN<100 approach and the concentration in the FeBulk case (Fig. ). The ϵOH values in cloud water do not exhibit such strong differences, which is also reflected in the more similar concentrations in all droplets (Fig. ).

The difference between the concentration in iron-free and iron-containing particles is up to 3 orders of magnitude (pH = 3, Fig. S2b), which is also reflected in the same difference in ϵ (Fig. b).

For the HO2 radical, iron reactions cause a significant deviation from thermodynamic equilibrium at high pH, resulting in decreasing ϵHO2 with increasing pH. At high pH, HO2 is more efficiently consumed due to the higher rate constants of the O2- radical anion as compared to those with the undissociated HO2 radical. Unlike for OH, for which the smallest ϵ is seen in iron-free particles, the lowest ϵHO2 is found for iron-containing particles – which shows that the concentration differences due to iron distribution for OH and HO2 have different reasons: whereas iron leads to efficient OH formation, it causes significant HO2 / O2- loss.

The values of FN,Fe = 2 % and 100 % likely represent extreme values for the iron distribution in cloud droplet or particle populations. Depending on abundance and proximity of iron emissions sources, fewer particles (and thus droplets) may also contain iron; this would translate into an even higher iron aqueous-phase concentration. Under such conditions, iron would not be completely dissolved and available for aqueous-phase reactions. Therefore, in the following, we limit our discussion to values of FN,Fe ≥ 1 %. In order to illustrate the total ROS budget in the aqueous phase, Fig.  shows the bulk aqueous-phase concentrations [ROS]aq,bulk (M) calculated based on the LWC-weighted average ROS concentration in the iron-containing and iron-free droplets or particles: 5ROSaq,bulk=[ROS]ironfree⋅1-FN,Fe/100%+[ROS]iron⋅FN,Fe/100%, where [ROS]ironfree and [ROS]iron are the ROS concentrations in the iron-free and iron-containing droplets, respectively. (Note that the same calculation was performed for the bulk aqueous-phase concentrations in Figs. , , S1 and S2). Results are shown for the full range of 1 % ≤ FN,Fe ≤ 100 %.

The H2O2 concentrations do not show any dependence on the iron distribution FN,Fe (Fig. a and d). This trend can be explained because (i) in cloud water the main chemical source and loss processes are independent of iron, whereas (ii) in aerosol water, both the main chemical source and loss reactions are dependent on the Fe(II) concentration. The same set of processes can also explain why the resulting aqueous-phase H2O2 concentrations are independent of the total iron mass. In a sensitivity study, we increased mFe to 50 ng m-3, which results in very similar H2O2 concentrations (Fig. S5). Thus, a higher iron mass concentration increases both rates by the same factor, resulting in nearly equal H2O2 concentrations for both mFe.

The OH concentration in cloud water is underestimated by FeBulk by up to a factor of ∼ 5 (pH = 6, Fig. b). The large difference at the highest pH is the consequence of the more efficient OH formation at pH = 6 by the reaction of O2- with O3 (Sect. ). Since in the FeN<100 approach the HO2 / O2- concentration is about 1 order of magnitude higher at this pH value than for FeBulk, the OH concentration is generally underestimated by assuming FN,Fe = 100 %. It can be concluded that in clean conditions, i.e., when the pH value of cloud water is ≥∼5, even for relatively low iron mass concentrations, iron distribution should be taken into account to obtain the correct OH concentrations in cloud water. The deviations are only slightly higher for mFe = 50 ng m-3 than for 10 ng m-3 (Fig. S5). In aerosol water, the OH concentration is generally overestimated by FeBulk by factors of ≤∼ 2 and ∼ 5 for mFe = 10 and mFe = 50 ng m-3 (Figs. e and S5e), respectively. The lower OH concentration in the FeN<100 approach is a consequence of the lower Fe(II) concentration (Sect. ), which leads to less OH formation by the Fenton reaction.

The HO2 / O2- concentrations are underestimated in cloud water by the FeBulk approach, approximately by the same factors (∼ 2–5) as the OH radical, with the highest bias at pH = 6 (Fig. c). At pH = 3, the difference between the HO2 / O2- concentrations for the two iron mass concentrations is much larger than for the other pH values. Unlike for the H2O2 and OH concentrations, that are very similar for both mFe, the HO2 / O2- concentrations more clearly deviate for the two iron masses considered in Fig. S5. This can be explained by the less efficient Fe(III) reduction in the cloud droplets at mFe at all pH values (Sect. ).

In aerosol water, the aqueous-phase HO2 / O2- concentrations are nearly identical for wide ranges of FN,Fe at pH = 3 and 4.5, whereas the difference at FN,Fe = 1 % and 100 % is about 1 order of magnitude at pH = 6 (Fig. f). Thus, the aqueous-phase budget of HO2 / O2- may be significantly underestimated – independently of mFe – with increasing pH if the iron distribution across the particle distribution is not properly accounted for.

Strictly, Henry's law constants are not applicable to highly concentrated aqueous solutions such as aerosol water. Simultaneous measurements of particle-bound and gas-phase H2O2 concentrations suggest that it partitions much more efficiently to the particle phase than predicted based on its physical Henry's law constant (KH,H2O2 = 1.02 × 105 M atm-1). Despite large uncertainties in such measurements and of related parameters such as the aerosol water content, various studies revealed that the partitioning of H2O2 between the gas and the aerosol aqueous phases may be more appropriately described with an effective Henry's law constant of KH,H2O2 ≤ 2.7 × 108 M atm-1 . The observation by of higher H2O2 concentrations in coarse-mode particles than in fine-mode particles might point to a size-dependent chemical particle composition with higher iron content in coarse particles that often contain dust.

To explore the effects of such enhanced partitioning, we performed a sensitivity study, using KH,H2O2 ≤ 2.7 × 108 M atm-1. In Fig. S6, the aqueous-phase concentrations and net phase transfer rates of the three ROS are compared to those using the physical Henry's law constant (KH,H2O2 = 1.02 × 105 M atm-1). No other parameter was changed as to the best of our knowledge corresponding KH(eff) values for OH and/or HO2 are not available. We perform this comparison only for the aerosol case, as numerous measurements have shown that the partitioning of H2O2 into cloud water can be satisfactorily described by its KH (e.g., ).

As expected, a higher KH,H2O2 results in higher H2O2 aqueous-phase concentrations by 3 orders of magnitude. Unlike for the low KH,H2O2, the higher partitioning leads to smaller H2O2 concentrations for FeBulk than for FeN<100 by up to 1 order of magnitude and pH = 6. The OH concentration in the aqueous phase is predicted to decrease with increasing FN,Fe, unlike in the base case simulation where we showed the opposite trend (Fig. S6b). The higher H2O2 concentration generally increases the importance of the Fenton reaction and photolysis for OH production in the aqueous phase. While the HO2 / O2- concentrations for the two KH,H2O2 values do not differ at low FN,Fe, they are significantly smaller by several orders of magnitude for the higher KH,H2O2 (Fig. S6c).

The uptake of OH and HO2 into aerosol particles is often parameterized by the dimensionless reactive uptake coefficient γ that is derived as a function of the first-order radical loss (kloss) onto aerosol particles. For the derivation of γ based on lab studies, it is assumed that γ solely depends on the molecular speed and the droplet surface. However, it has been discussed that the reactive uptake coefficient in atmospheric applications should also include a term to account for gas-phase diffusion : 6kloss=rdDg+4ωγ-1S, with S being the total droplet surface area per air volume and ω the molecular speed, 7ω=8RTπM, where M is the molar mass (g mol-1), R is the gas constant and T is the absolute temperature (K) (e.g., ). Accordingly, the phase transfer rate of radicals into the particle phase can be described as 8d[Radical]gdt=kloss[Radical]g, and the reactive uptake coefficient γ can be derived as 9γ=4ωSkloss-rdDg.

The γ values for OH and HO2 as a function of FN,Fe are displayed in Fig. . If gas-phase diffusion is neglected in the derivation of γ, the last term in the denominator of Eq. () is set to zero (Eq. S.3). To demonstrate the impact of gas-phase diffusion on the reactive uptake parameter, we compare γ values calculated with Eq. (10) to values without consideration of gas-phase diffusion (Sect. S3 in the Supplement).

The derived values for γOH over the full range of FN,Fe (∼ 2.2 × 10-3 ≤γOH(cloud) ≤ 6.4 × 10-3; γOH(aerosol) ∼ 3.8 × 10-2) are in good agreement with the average value suggested for pure water surfaces (γOH = 0.035) . It should be noted that this latter experimental value was derived assuming a Henry's law constant of KH,OH ≥ 40 M atm-1 and [OH] ≥ 108 cm-3. If it were adjusted to atmospherically more relevant conditions ([OH] ∼ 106 cm-3) and KH,OH = 25 M atm-1 as used in our study (Table S4), the resulting γOH might be smaller by more than an order of magnitude than the reported one. Molecular dynamics simulations for the mass accommodation of OH on water surfaces showed αOH = 0.1, which can be considered the upper limit of reactive OH uptake onto water surfaces . The difference between such a value for pure water and the values from our model study demonstrates the high reactivity of the OH radical in the aqueous phase. As its solubility is limited, this high reactivity results in subsaturated conditions (ϵOH<1; Fig. b). The γOH values show little difference at different pH values and FN,Fe. Thus, the reactive uptake of OH into cloud droplets could be parameterized with a value of γOH∼ 2.3 × 10-3 for all conditions explored here. Similar to the findings for cloud water, the uptake of the OH radical could be parameterized by a single γ value for the conditions applied here (γOH∼ 0.0038). The comparison of the values in Fig. a and c to those in Fig. S8a and c reveals that the gas-phase diffusion leads to a significant decrease of γOH by a factor of ∼ 3 in clouds and ∼10% in aerosol particles.

Several previous studies have determined γOH on surfaces other than pure water that are more comparable to atmospheric aerosol particles. showed that γOH increases on acidic solutions from 0.07 (28 % H2SO4) to unity on 96 % H2SO4. On organic surfaces, γOH values are generally higher than on inorganic surfaces which demonstrates the high reactivity of OH with organics in the condensed phase and/or on organic surfaces. In several lab studies, squalane was used as a proxy for long-chain alkanes; the reactive uptake coefficients have been determined in a range of ∼ 0.2 <γOH<0.5 (e.g., ). A similar value range was also found for a wide variety of other organics . In other experimental studies on organic particles, γOH>1 was found (γOH = 1.64 on wet succinic acid, ; 1.3 ± 0.4 on bis(2-ethylhexyl) sebacate, ). γOH on organic aerosols may decrease with increasing OH gas-phase concentration because the viscosity of the organic condensed phase prevents efficient uptake and diffusion of OH towards the particle center . Such effects were included in recently developed frameworks to parameterize γOH as a function of particle viscosity, size and gas-phase OH concentration .

We did not explore any of these parameters in detail in our current model study. However, the comparison of the tight range of our γOH values in Fig. , independently of FN,Fe and cloud or aerosol aqueous conditions, to the wide ranges of literature values leads us to the following conclusions: (i) the similarity of our γOH value based on an explicit chemical mechanism to that measured on pure water suggests that ∼ 10-3<γOH ∼ 10-2 may be a good approximation under cloud conditions. (ii) The fact that we cannot reproduce the high γOH values as observed on organic aerosols suggests that organic reactions control the reactive uptake of OH, which are not included in our explicit chemistry scheme. (iii) Since organics are likely present in all aerosol particles, the effect of the iron distribution (FN,Fe) for γOH may be overestimated by our chemical mechanism on ambient aerosol particles.

The γHO2 values into cloud water are in the range of 0.001–0.004, with higher values at the highest pH (Fig. b). In aerosol water, γHO2 shows a strong dependence on FN,Fe, leading to an overestimate of the reactive uptake by FeBulk by up to 2 orders of magnitude at pH = 3 (Fig. d). Note that at FN,Fe = 1 %. no γHO2 value is shown as under such conditions, HO2 is predicted to evaporate. This evaporation is due to the inefficient chemical HO2 consumption in iron-free particles, whereas efficient uptake occurs into the iron-containing particles. This leads to a great imbalance of the phase transfer rates from and into the two droplet classes as shown in Fig. b. We use identical α values for droplet and aqueous particle surfaces. The mass accommodation coefficient α is the upper limit of the maximum value for the uptake coefficient γ. This limit can be seen in Fig. (d), where γHO2 approaches the value of αHO2 = 0.05, i.e., the regime in which the uptake is fully controlled by the mass accommodation and not by the chemical loss in the aqueous phase. For the HO2 radical, gas-phase diffusion may decrease γHO2 by a factor of ∼ 2 at pH = 6, whereas the two γ values are nearly identical at pH = 3 (Fig. S8b). In aerosol water, gas-phase diffusion has an insignificant impact on its reactive uptake coefficient onto aerosol water (Fig. S8d).

It has been shown in global model studies that the reactive loss of HO2 on aerosol surfaces can significantly impact the atmospheric oxidant budget (e.g., ). A value of γHO2 = 0.2 based on the study by has been commonly used in global models, which has also been supported by molecular dynamics simulations . However, recent model studies suggest that this value may be an overestimate and that values on the order of γHO2 = 0.05 or 0.08 may be more appropriate .

On dry aerosol surfaces, γHO2 is usually low: for example, measured γHO2 = 0.075 ± 0.015 on dry soot surfaces, which is comparable to the value of γHO2 ≤ 0.031 on dust and γHO2 < 0.01 on dry NaCl . γHO2 increases by more than 1 order of magnitude when RH increases from ∼ 20 % to >90 % . showed much higher γHO2 values on two humic acids (0.0007≤γHO2≤0.06 and 0.043≤γHO2)≤0.09, respectively) than on pure long-chain carboxylic acids (γHO2<0.004). They ascribed the higher HO2 uptake to the small concentrations of copper and iron ions in the humic acids, in agreement with previous studies that showed the large impact of TMIs on HO2 reactive uptake (e.g., ). It was suggested by that the HO2 uptake cannot be solely controlled by the TMI concentration as they did not see any correlation with γHO2. This lack of trend is similar to findings by , who also did not find any clear relationship in γHO2 on Cu-doped NH4HSO4 and LiNO3 particles with Cu concentration.

Based on our model results, the lack of such correlation can be qualitatively understood by simultaneous increase of the main chemical HO2 source and sink rates (Reactions S1 and L4–L6 in Fig. f). The absence of a dependence of HO2 uptake on wet particle diameter in the study by is also in agreement with our sensitivity studies in which we varied the particle diameter by a factor of ± 2 (Fig. S9). This implies that the interfacial mass transfer is not a limiting step in the HO2 uptake. The HO2 uptake on Cu-doped ammonium sulfate particles was observed to be higher under neutral (γHO2 = 0.2) as opposed to acidic conditions (γHO2 = 0.05) . Similar results were observed by who generally found somewhat higher values than in our study (γHO2>0.4) and highest values for the lowest Cu/Fe ratios (i.e., highest iron concentration at a constant copper concentration) and highest pH. However, overall their differences in γHO2 due to the Fe/Cu ratio and/or pH are smaller than those that we predict for different FN,Fe (∼ 10-4 ≤γHO2 ≤∼ 10-2, at pH = 3). This suggests that γHO2 parameterizations that take into account the dependence on Cu concentration, total aerosol mass and particle size (e.g., and ) should be further refined to also include the iron distribution.

We note that our results are not quantitatively comparable to those on Cu-doped particles, as our chemical scheme includes only reactions of iron as the only TMI. The corresponding reactions with Cu+/2+ generally have higher rate constants than those of iron reactions ; however, as copper concentrations in ambient aerosols are usually lower than iron concentrations, the effect of both TMIs might be similar. We did not perform the equivalent simulations for copper in the present study as to our knowledge, there are no data available that give single-particle information on the distribution of iron and copper within droplet or particle populations. However, if iron and copper originated from different emission sources, the number concentration of TMI-containing particles might be relatively high. However, in such situations the synergistic effects of Fe and Cu affecting HOx cycling and uptake as suggested by might be reduced.

Measurements of γHO2 on ambient aerosol populations showed ranges of 0.13 ≤γHO2 ≤ 0.34 (Mt. Tai) and 0.09 ≤γHO2≤ 0.4 (Mt. Mang) . No systematic trend with total iron concentration that exceeded 1 µg m-3 at Mt. Mang and 10 µg m-3 at Mt. Tai was observed. The acidity of the aerosols was not reported in that study; however, the derived γHO2 values did not show any significant trend with the molar NH4+ / (SO42-+ NO3-) ratios that ranged from ∼ 1 to ∼ 2 and ∼ 2 to ∼ 3, pointing to moderately acidic to fully neutralized aerosol. The fact that the ambient data do not show any trend with acidity might either point to a high number fraction of TMI-containing particles and/or to a relatively high pH, i.e., to conditions under which our model results do not show a large effect of FN,Fe (Fig. d). Two studies of ambient aerosol at urban locations revealed similar average values as compared to the mountain sites, with γHO2 = 0.23 ± 0.22 in Yokohama, Japan , and 0.24 (0.08 ≤γHO2 ≤ 0.36) in Kyoto, Japan . No significant trend in the γ values was observed as a function of aerosol or air mass characteristics (coastal or mainland).

Given the large ranges of γHO2 values on lab-generated and ambient aerosol, it is difficult to reconcile differences between lab data and observations with our model results. However, our results might provide an explanation for the discrepancies of lab-derived or theoretical γHO2 values in the range of 0.2–1 to those determined on ambient aerosol (<0.4). We cannot say for sure that these differences indeed stem from different assumptions of iron distribution or other factors affecting γHO2. However, our strong predicted decrease of γHO2 values at low pH and FN,Fe suggests that applying lab-derived γHO2 on aerosol particles with identical composition might lead to an overestimate of HO2 loss on atmospheric aerosol surfaces, even if all other aerosol properties (size, total iron mass, pH, etc.) are identical.