Role of Criegee intermediates in the formation of sulfuric acid at a Mediterranean (Cape Corsica) site under influence of biogenic emissions

. Reaction of stabilized Criegee Intermediates (SCIs) with SO 2 was proposed as an additional pathway of of unsaturated VOCs and steady state SCIs concentrations estimated by adopting rate coefficients for SCIs reactions based on structure–activity relationships (SARs). The estimated concentration of the sum of SCIs was in the range of (1 – 3) × 10 3 molecule cm -3 . During the day the reaction of SCIs with SO 2 was found to account for about 10% and during the night for about 40% of the H 2 SO 4 production, closing the H 2 SO 4 budget during the day but leaving unexplained about 50% of the H 2 SO 4 formation during the night. Despite large uncertainties in used kinetic parameters, these results indicate that the SO 2 oxidation 40 by SCIs may represent an important H 2 SO 4 source in VOCs-rich environments, especially during night-time.

by SCIs may represent an important H 2 SO 4 source in VOCs-rich environments, especially during night-time.

Introduction
Sulfuric acid, H 2 SO 4 , is an important atmospheric component identified to play a key role in formation of secondary atmospheric aerosol through new particles formation processes (Dunne et al., 2016;Paasonen et al., 2010;Sipilä et al., 2010;Weber et al., 1997). H 2 SO 4 is considered as a major precursor of newly formed atmospheric nucleation-mode particles and 45 may play a significant role in their subsequent growth (Boy et al., 2005;Smith et al., 2005;Zhang et al., 2012). It is therefore important to well understand the atmospheric mechanisms determining the H 2 SO 4 concentrations in different atmospheric environments.
Until recently it was generally accepted that the dominant atmospheric source of H 2 SO 4 is the reaction of OH radicals with SO 2 (R1) presenting a rate limiting step leading in the troposphere to a fast production of H 2 SO 4 in presence of water vapour 50 and oxygen via reactions (R2 -R3) (Finlayson-Pitts and Pitts Jr, 2000). It was assumed that H 2 SO 4 atmospheric concentrations are determined predominantly by this source and the loss of sulphuric acid on the surface of existing particles with the loss rate depending on the efficiency of H 2 SO 4 uptake. Another possible atmospheric source of H 2 SO 4 via oxidation of SO 2 by stabilized Criegee intermediates (SCIs), compounds formed by ozonolysis of unsaturated organic compounds, was suggested by Cox and Penkett (1971) and discussed first in view of its atmospheric importance by Calvert and Stockwell (1983). For a long time the reactions of SO 2 with SCIs were considered as being too slow to represent an important atmospheric source of H 2 SO 4 , until in the more recent study of Welz et al. (2012) a rate constant of (3.9±0.7)×10 -11 cm -3 molecule -1 s -1 was derived for the reaction of SO 2 with the simplest SCI, formaldehyde oxide (CH 2 OO), which is significantly larger than previous estimates of around 4×10 -15 cm -3 molecule -1 s -1 for the reactions of SO 2 with CH 2 OO (Hatakeyama et al., 1986) and for the reactions of SO 2 with other SCIs (Johnson and Marston, 2008). The importance of this additional source of H 2 SO 4 which is still under discussion depends on the atmospheric SCIs concentrations and the kinetics and mechanisms of the SCIs reactions with SO 2 .
For the loss of SCIs via unimolecular decomposition and reactions with water monomers and dimers, the results of experimental and theoretical studies show that the corresponding rate coefficients may vary by orders of magnitude depending on the SCI substituents and conformers (see e.g. Vereecken et al. (2017) and references therein). In recent years experimental studies of these reactions with direct detection and generation of specific SCIs were performed for several among the simplest of them, such as formaldehyde oxide (CH 2 OO) Lewis et al., 2015;Sheps et al., 2017;Smith et al., 2015; 85 Stone et al., 2018), acetaldehyde oxide (CH 3 CHOO) (Li et al., 2020;Lin et al., 2016;Sheps et al., 2014) and acetone oxide ((CH 3 ) 2 COO) (Chhantyal-Pun et al., 2017;Fang et al., 2017;Huang et al., 2015;Lester and Klippenstein, 2018;Smith et al., 2016). Very recently the decomposition rate and an estimation of the rate coefficients for the reaction with water vapor were obtained for the first time in direct kinetic studies for syn Methyl Vinyl Ketone oxide (syn-MVK-oxide), and anti Methacrolein Oxide (anti-MACR-oxide), four-carbon unsaturated Criegee intermediates derived from the ozonolysis of isoprene (Barber et 90 al., 2018;Caravan et al., 2020;Lin et al., 2021). For other large SCIs only estimations based on theoretical and indirect studies are available for their reactions with water vapor and their thermal decomposition.
The result of Welz et al. (2012) about the fast reaction of SO 2 with formaldehyde oxide was extended in later direct and indirect studies where similarly fast reactions with SO 2 were confirmed for CH 3 CHOO, (CH 3 ) 2 COO, Z-nopinone oxide (product of β-pinene ozonolysis), syn-MVK-oxide and anti-MACR-oxide (Ahrens et al., 2014;Caravan et al., 2020;Lin et 95 al., 2021;Vereecken et al., 2017 and references therein), with rate coefficients in the range of (3-16) × 10 -11 cm 3 molecule -1 s -1 . Reactions of other SCIs with SO 2 were suggested to be similarly fast on the basis of theoretical results (Kurtén et al., 2011).
Theoretical analysis suggests that the reaction of SCI with SO 2 proceeds via a barrierless cycloaddition of SO 2 to SCI forming a sulfur-bearing secondary ozonide (SOZ) which can either be stabilized or decompose to form SO 3 or other products theory predicts negligible SOZ stabilization and about unity yield of SO 3 (Kuwata et al., 2015). These results are supported by experimental studies for CH 2 OO (Berndt et al., 2014a;Wang et al., 2018) and (CH 3 ) 2 OO, CH 3 CHOO (Berndt et al., 2014b).
For larger SCIs with expected longer SOZ lifetime the SO 3 yield may depend on the SOZ fate in the atmosphere with respect to its decomposition or further reactions, e.g. with H 2 O, (Kuwata et al., 2015;, although a large yield of sulfur trioxide exceeding 80% was observed by Ahrens et al. (2014) for the large SCI formed during the ozonolysis of β-105 pinene Estimations based on the available or evaluated kinetic parameters show that the atmospheric SCI concentrations vary by orders of magnitude depending on conditions specific for different environments, such as the concentrations and composition of alkenes, ozone concentration or humidity. Using chemistry-transport global modeling the highest SCI concentrations of the order of 10 4 -10 5 molecule cm -3 were inferred for the regions with highest isoprene and terpenes emissions, e.g. above the 110 tropical forest (Chhantyal-Pun et al., 2019;Khan et al., 2018;Newland et al., 2018;Vereecken et al., 2017;Novelli et al., 2017). Estimated using steady-state calculations, the concentration of SCI ranges from 2.3×10 3 molecule cm -3 at a rural site to 5.5×10 4 molecule cm -3 in an urban polluted environment . The estimated contribution of SCI to H 2 SO 4 formation is also highly variable: about 7% in rural environments and up to 70% over tropical regions .
At the global scale, the contribution of SCI to SO 2 oxidation was estimated to be negligible, contributing less than 1% (Newland et al., 2018). The uncertainty associated to the predicted SCI concentrations was estimated to be one order of magnitude , due to poorly defined SCI formation and loss rates. Even a higher uncertainty may be expected for the estimated contribution of SCIs to SO 2 oxidation considering not well defined reaction rate coefficients for the reaction of different SCIs with SO 2 .
The adequacy of the mechanism treating the SO 2 oxidation by OH as a predominant source of the atmospheric H 2 SO 4 was 120 tested in a number of field campaigns where simultaneous measurements of OH and H 2 SO 4 were conducted (Supplement, p. 2, Table S1). Selected ion Chemical Ionisation Mass Spectrometry (CIMS) technique for simultaneous measurements of OH and H 2 SO 4 was first introduced by (Eisele and Tanner, 1993) and since then it has been used in a number of field measurements in different environments. In these studies the measurements of OH, H 2 SO 4 , SO 2 and aerosol surface area were used to compare the rate of H 2 SO 4 production in reaction (R1) and the rate of H 2 SO 4 loss on aerosol particles assuming steady state condition 125 between these processes. In a number of measurements campaigns the H 2 SO 4 budget was found to be closed using the uptake coefficient of unity corresponding to upper limit of the H 2 SO 4 loss rate on existing particles. This was observed in different environments including remote marine , forested rural (Birmili et al., 2000;Boy et al., 2013) and forested remote sites (Eisele and Tanner, 1993;Weber et al., 1997). However, in other field studies conducted in various environments the H 2 SO 4 condensation sink calculated using an uptake coefficient of unity was found to significantly exceed its formation rate via SO 2 oxidation by OH indicating either an H 2 SO 4 uptake efficiency lower than unity or the presence of sources of H 2 SO 4 other than reaction (R1) (Bardouki et al., 2003;Berresheim et al., 2002Berresheim et al., , 2014Boy et al., 2013;Jefferson et al., 1998;Mauldin III et al., 2012;Petäjä et al., 2009). Several additional H 2 SO 4 gas phase sources were suggested such as the oxidation of DMS or DMDS in remote coastal environments proceeding with SO 3 formation (Berresheim et al., 2002(Berresheim et al., , 2014Jefferson et al., 1998) or SO 2 oxidation by SCIs in the boreal forest and in moderately polluted environments (Boy et al., 2013;Kim et 135 al., 2015;Mauldin III et al., 2012). A heterogeneous formation of gas phase H 2 SO 4 via the catalytic oxidation of SO 2 on the surface of black carbon aerosols has also been recently shown to be important under polluted conditions (Yao et al., 2020).
In this work, we present an evaluation of the role of SCIs in H 2 SO 4 production at a remote site on Cape Corsica near the

Field site
Measurements were performed at the Ersa site from 18 July to 5 August 2013 during ChArMEx/SAFFMED field campaign . The Ersa station (42. 969°N, 9.380°E) is located at Cape Corsica on the northern edge of Corsica (Michoud et al., 2017;Zannoni et al., 2017). It is situated at an altitude of 533 meters above sea level on the top of a hill dominating the northern part of the cape. On its eastern, northern and western sides it is a few km away from the coast and has a direct view of the sea. The measurement site is isolated by a mountain range from the closest large city, Bastia, situated about 30 km south of the site. The site is surrounded by widespread vegetation such as scrubland typical of the Mediterranean areas, responsible of biogenic VOC emissions Zannoni et al., 2015).

OH and H2SO4 measurements
Concentrations of OH radicals and H 2 SO 4 , as well as total peroxy radicals (HO 2 +RO 2 , not discussed here), were measured using chemical ionization mass spectrometry (CIMS) Eisele and Tanner, 1991). A detailed description of the instrument is presented elsewhere (Kukui et al., 2008(Kukui et al., , 2012. Here we briefly present the measurement technique and essential details about the setup and performance of the instrument during the ChArMEx/SAFMED campaign. A detailed description of the calibration system used during the campaign, which was not presented before, is given in Supplement, pp.  into sulfuric acid and NO for peroxy radicals conversion into OH and their subsequent detection as OH) and the radical quencher (NO 2 ) are introduced into the reactor through a set of injectors. NO 2 used as a scavenger removes not only the OH radicals, but also peroxy radicals converting them into HO 2 NO 2 and RO 2 NO 2 peroxy nitrates. Switching the reactant flows between the different injectors allows measurements in four different modes: the background mode, two different OH radical measurement modes and the peroxy radicals measurement mode (Supplement, p. 3, Fig. S1). The two OH measurement modes differ by the time used for the chemical conversion, 4 ms and 20 ms. Ratio of the signals with the short and the long conversion times may be used as an indicator of an artificial OH formation in the reactor (Kukui et al., 2008).
Measurements were performed by monitoring the peak intensities at m/z=62 (NO 3¯) , m/z=97 (H 32 SO 4¯) , and m/z=99 (H 34 SO 4¯) with the CIMS, respectively noted I 62 , I 97 , and I 99 hereafter. Every measurement of OH was derived from 1 min of OH ion signal count and two 30 s background ion signal counts before and after the OH signal measurement. Peroxy radicals

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were measured at the end of the OH detection sequence by switching on the NO flow to the corresponding injector for a duration of 2 min. To avoid any possible influence of traces of NO on the OH measurements a time delay of 6 min was imposed after switching off the NO flow and before starting the next OH measurement sequence in order to ensure flushing of the CCR.
The OH and the H 2 SO 4 data were averaged resulting either in a sequence of 3 points with a step of 7 min separated by a time gap of 15 min or yielding a sequence with a time step of about 90 min. The latter was used to match the time resolution of the 190 VOCs measurements (Supplement, p. 13, Table S4).  (Faloona et al., 2004). A detailed description of the calibration system with definitions of C R and I R are given in Supplement, pp. 4-12, Sect. S3.
The overall accuracy of the calibration coefficients was estimated taking into account uncertainties of all parameters used ChArMEx/SAFMED campaign the observed level of OH background signal was significantly higher than typical OH background found during calibration or field measurements in air with low VOCs concentrations (see Fig. 7 and discussion in Sect. 4.4). Accordingly, the lower limits of detection at signal-to-noise ratio of 2 and a 15 min integration time were 2×10 5 molecule cm -3 for H 2 SO 4 and 5×10 5 molecule cm -3 and 2×10 5 molecule cm -3 for OH daytime and nighttime measurements, respectively.

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During the ChArMEx/SAFMED campaign the instrument was installed in a dedicated container with the CCR fixed to the roof of the container via an interface cap covered with a PTFE sheet. The sampling aperture of the reactor (3 mm diameter) was positioned 50 cm above the roof and about 3 m above the ground.
To avoid possible contamination of ambient air by the SO 2 , NO and NO 2 reactants added to the CCR, a trap was set up at the pumps exhaust by using two 100 L cylinders containing zeolites. The cylinders were refilled several times during 215 measurements. A flexible exhaust tube of 30 m length was always placed downwind from the container.

Complementary measurements
The aerosol particle size distribution was measured using a scanning mobility particle sizer (SMPS TSI 3080, associated with a CPC TSI 3010) in the range from 10.9 nm to 495.8 nm and with an aerodynamic particle sizer (APS, TSI 3321) in the range from 542 nm to 19.48 μm. As the SMPS measurements were made with dehydrated particles the particle diameters were 220 corrected to ambient humidity using particle growth-factor (GF) of 1.5 at 90% determined with Volatility Hygroscopic-Tandem Differential Mobility Analyzer (VH-TDMA) (Villani et al., 2008). The dependence of the GF on relative humidity (RH) was calculated using the one-parameter approximation from Rissler et al. (2006). An estimated uncertainty of measured particle number densities and GF corrected particle diameters were of 10% and 15%, respectively.
SO 2 concentration was measured by UV fluorescence (Thermo Environmental Instruments (TEI), Model 43C-TLE) with an estimated accuracy of 20%, a lower detection limit of 0.05 ppb and a time resolution of 5 min.
Ozone concentration was measured by means of a CraNOx II (Eco Physics) NOx and O 3 monitor with an estimated accuracy of 10%.
A detailed description of VOCs measurements during ChArMEx/SAFMED campaign is given in Michoud et al. (2017).
The measurements of 23 unsaturated VOCs including alkenes, aldehydes, ketones, isoprene and monoterpenes were used in this work for estimation of SCIs concentrations. Employed measurement techniques and concentration ranges for measured unsaturated VOCs are given in Supplement, p. 13, Table S4 together with associated time resolution, limit of detection and uncertainties. The data were averaged or interpolated with a time step of 90 min.
Wind speed and direction, relative humidity, temperature and photolysis rates were also measured throughout the campaign.

Estimation of H2SO4 steady state concentrations
Concentrations of H 2 SO 4 produced via SO 2 oxidation by OH and sum of SCIs, H 2 SO 4 OH and H 2 SO 4 SCI , respectively, were calculated assuming validity of a steady state between the H 2 SO 4 production and its loss (see discussion in Sect. 4.1), Here  Table S5). It is assumed here that the H 2 SO 4 yield in reaction of SCIs with SO 2 is a unity for all SCIs, giving an upper limit for the contribution to H 2 SO 4 formation of accounted SCIs.

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The CS was calculated using measured particle size distributions and number concentrations by calculating diffusional flux to the aerosol particles assuming an accommodation coefficient of a unity (Hanson, 2005) and using Fuchs-Sutugin transition correction (Jefferson et al., 1998;Seinfeld and Pandis, 2016). Diffusion coefficient of H 2 SO 4 was estimated using its dependence on relative humidity from Hanson and Eisele (2000) giving 0.077 cm 2 s -1 at RH of 60.6% (median value). Without considering an uncertainty on the accommodation coefficient, the accuracy of the calculated CS of 20% was assessed accounting for the uncertainties of particle measurements given in Sect.
where X denotes a specific VOC, are rate coefficients for the thermal decomposition, the reaction with H 2 O and the reaction with water dimer for the specific SCI i , respectively. Concentration of (H 2 O) 2 was calculated using an equilibrium constant for water dimer formation and its temperature dependence from Ruscic (2013).

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The rate coefficients and the yields X and CS data). The night time OH and H 2 SO 4 measurements on the 26 and 28-30 of July were influenced by strong fog event deteriorating the accuracy of the corresponding OH and H 2 SO 4 data.
Comparing the present observations ( Fig. 2) with previous measurements in the Mediterranean region, the mean peak OH concentration during the noon hours was close to the peak OH levels observed during CYPHEX campaign in the summer of peak levels were similar to those observed during ChArMEx (J(O 1 D) was measured during ChArMEx, but not yet published).
The somewhat lower ChArMEx OH noon concentrations compared to CYPHEX are consistent with higher OH reactivity observed during ChArMEx (Zannoni et al., 2017), although the direct comparison of radical chemistry at these two sites is not straightforward considering that, among other differences between these sites, biogenic VOCs at Cyprus site, e.g. isoprene and 290 monoterpenes, were 3-5 times lower compared to ChArMEx. About four times higher OH peak concentrations, 2.1×10 7 molecule cm -3 , were observed during the MINOS campaign in the summer of 2001 in Crete (central Mediterranean) with similar O 3 and J(O 1 D) observed levels (Berresheim et al., 2003). This difference is difficult to explain based on the available data (Mallik et al., 2018). The observed ChArMEx H 2 SO 4 concentrations were about two times higher than the observed OH concentrations. For other sites the observed ratios [H 2 SO 4 ]/[OH] were in the range from 1 to 9 with only one example when this ratio was less than unity (Supplement, p. 2, Table S1). The ratio [H 2 SO 4 ]/[OH] depends on the SO 2 concentration and condensation sink correlating with the aerosol particle surface area concentration. The condensation sink and [SO 2 ] values during ChArMEx,

Comparison of observed H2SO4 with sulfuric acid produced from OH+SO2 (H2SO4 OH )
Time series of the H 2 SO 4 produced in the reaction of OH with SO 2 , H 2 SO 4 OH , calculated according to Eq. (1) and of the observed H 2 SO 4 are presented in Fig. 1b and 2b  measurement uncertainties. Dashed lines represent 1:1 ratios. Inserts, log-plot (upper) and linear plot (lower right), are added for clearer presentation of the night-time data.

Comparison of observed H2SO4 with sulfuric acid produced from SCIs+SO2 (H2SO4 SCI )
Estimated according to Eq. (2) and (3), mean diel profiles of the sulphuric acid H 2 SO 4 SCI produced in the reactions of SO 2 355 with SCIs generated by the ozonolysis of the measured unsaturated VOCs, excluding α-terpinene, are presented in Fig. 4a. The calculated contribution from α-terpinene alone is up to six times larger than observed [H 2 SO 4 ] (Fig. 4b). It is not clear if the reason for this large overestimation can be related to an erroneous α-terpinene measurements and/or to incorrect kinetic parameters used for the calculation of H 2 SO 4 production from α-terpinene. The observed α-terpinene daytime concentrations were similar to the concentrations of α-pinene and β-pinene. Accounting for about 100 times faster α-terpinene consumption 360 in reactions with OH and O 3 compared to other terpenes (Atkinson et al., 2006;IUPAC, 2020) that would imply about 100 times larger α-terpinene emission rate at the measurement site, which is unlikely considering observed compositions of monoterpene emissions of biogenic origin (Geron et al., 2000). Being well outside of the uncertainty of the H 2 SO 4 measurements the contribution from α-terpinene was therefore excluded from consideration in this work.  The calculated concentration of the sum of H 2 SO 4 SCI reaches a maximum of about 7×10 5 molecules cm -3 around midday and goes down to about 1.5×10 5 molecules cm -3 during the night. The largest estimated contribution to the H 2 SO 4 SCI is from α-pinene, limonene and isoprene during the day and from MVK and EVK at the night-time (Fig. 4).
The largest contribution to the calculated sum of SCIs was from the syn form of acetaldehyde oxide, Z-CH 3 CHOO (2), E- (31)   , as well as to an unaccounted 405 contribution from the ozonolysis of some unsaturated compounds not measured during the campaign. The latter explanation is supported by a significant daytime and night-time missing OH reactivity of about 50%, observed by Zannoni et al. (2017) during ChArMEx using the same VOCs data as in the present work. The main unaccounted species were suggested to be reactive biogenic VOCs including sesquiterpenes, oxygenated terpenes and their oxidation products. Ozonolysis of these compounds could be an additional unaccounted source of SCI in the present work. The steady state conditions were hardly disturbed by variations of the concentrations of SO 2 and O 3 , species of non-local origin with the lifetime longer than the lifetime of H 2 SO 4 . Concerning the H 2 SO 4 condensation sink, a variation in the aerosol molecules (HOMs) from the ozonolysis of monoterpenes may initiate new particle formation and its fast initial growth with the growth rates of tens of nm per hour (Stolzenburg et al., 2018;Tröstl et al., 2016). However, these variation are still slow compared to the H 2 SO 4 lifetime of several minutes.

MVK-oxide (10) from isoprene, oxo-substituted E-(C(O)R)CHOO
The time scales for the variations of VOCs, SCI, OH and CS may depend on the distribution of the biogenic emissions,

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wind speed, conditions of turbulent mixing and others. Without information about all these details we can only estimate the maximum time for such variations corresponding to the time of air masses presence over the land. To fulfill the steady state conditions this presence time has to be at least longer than the lifetime of H 2 SO 4 . The prevailing wind directions were from the NE and, predominantly, from the SW. The SW direction corresponds to the shortest presence times because of the shortest distance to the coastline and the highest wind speeds corresponding to this wind directions (Supplement, p. 17, Fig. S11, S12).

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On average, the presence time over the land, t pr , significantly exceeded the H 2 SO 4 lifetime, t H2SO4 , with the median value of the ratio of t pr / t H2SO4 being of 7.7 (4.3 -17.9 interquartile range). were analyzed with a filtered data using criteria t pr / t H2SO4 > 1 and t pr / t H2SO4 > 10. No significant difference were found compared with the unfiltered data, as it is also illustrated in Fig. 6 showing no apparent correlation

OH+SO2 reaction rate constant
In this work the rate coefficient for the reaction of OH with SO 2 , k 1 =8.06×10 -13 cm 3 molecule -1 s -1 (at 760 torr and 298 K), was taken from the last published IUPAC recommendation (Atkinson et al., 2004). The two latest JPL evaluations recommend an about 20% larger rate constant of 9.6×10 -13 cm 3 molecule -1 s -1 (Burkholder et al., 2015(Burkholder et al., , 2019. Using the rate constant from the JPL evaluations in this work would result in about 20% larger H 2 SO 4 production rate via OH+SO 2 . This would yield a 455 closed H 2 SO 4 budget with only OH+SO 2 source during the day and about two times lower contribution from a missing source during the night. In a recent study of Blitz et al. (2017aBlitz et al. ( , 2017b a significantly lower rate constant of 5.8×10 -13 cm 3 molecule -1 s -1 was derived from experiments with vibrationally excited OH (v=1,2,3)+SO 2 and using the master equation analysis of the pressure and temperature dependence of their own and some others experimental OH+SO 2 reaction rate constants. An even lower rate 460 constant of 4.8×10 -13 cm 3 molecule -1 s -1 has been derived by Medeiros et al. (2018) applying more detailed master equation analysis of experimental data from Blitz et al. (2017aBlitz et al. ( , 2017b and some other data. These recent results have not been confirmed by other studies. Also, they have been discussed but not recommended by the latest JPL evaluation (Burkholder et al., 2019).
Using the lower rate constant from Medeiros et al. (2018) in our study would result in about 2 times reduced H 2 SO 4 465 production by oxidation of SO 2 by OH and would invoke either significantly larger contribution from an additional H 2 SO 4 source or a lower H 2 SO 4 uptake coefficient, of about 0.5 instead of unity. As shown in Table 1, the reaction of OH with SO 2 would explain only about 50% and 5% of the observed H 2 SO 4 production during the day and during the night, respectively.
In previous field studies of the H 2 SO 4 budget listed in Supplement, p. 2, Table S1, the OH+SO 2 production rate of H 2 SO 4 was calculated using k 1 rate coefficient in the range of (8.5 -12) × 10 -13 cm 3 molecule -1 s -1 . Reanalysis of these data using the lower k 1 would lead to a conclusion that the H 2 SO 4 budget was never observed to be closed with the uptake coefficient close to unity. On the other hand, the employing the lower k 1 from Blitz et al. (2017aBlitz et al. ( , 2017b in model studies results in a significantly larger relative contribution of SCI to the H 2 SO 4 formation. For example, the SCI contribution of 7% in a rural environment and of 70% in tropical regions, which were estimated assuming the lower k 1 by Vereecken et al. (2017), would be reduced respectively to negligible and to about 30%, if the larger k 1 from IUPAC or JPL was used.

H2SO4 loss
The mass accommodation coefficient of unity used in this work was measured by Hanson (2005) for the H 2 SO 4 uptake on 5 -20 nm diameter particles composed of water and sulfuric acid. The efficient uptake of H 2 SO 4 is supported by other studies where the accommodation coefficients of about 0.7 were determined for the uptake on liquid sulfuric acid (Pöschl et al., 1998) and on ammonium sulfate and sodium chloride particles . Lower mass accommodation coefficients in the range of 0.2-0.3 were determined for the uptake on hydrocarbon coated particles  suggesting that the uptake coefficient may depend on aerosol composition. Considering the measurements uncertainty, the results obtained in this work are consistent with the accommodation coefficient in the range from about 0.8 to 1. At lower uptake values the OH+SO 2 source would significantly override the calculated H 2 SO 4 loss during the day. At the same time, the apparently missing H 2 SO 4 source during the night can be explained by a lower uptake coefficient, down to about 0.5.
Another possible loss mechanism of H 2 SO 4 can be via collisions of sulfuric acid molecules leading to new particles formation (NPF) in the atmosphere. For some atmospheric conditions like in the presence of high concentrations of base atmospheric components, e.g. ammonia and amines (Almeida et al., 2013), stabilizing the H 2 SO 4 dimer and larger clusters, the nucleation may proceed at collisionaly limited rate corresponding to an effective bimolecular H 2 SO 4 loss rate coefficient of about 4×10 -10 cm 3 molecule -1 s -1 (Kürten et al., 2014). Such conditions might have been encountered in highly polluted 490 industrial and urban environments, regions influenced by strong agricultural emissions, and in chamber experiments (Kürten et al., 2016(Kürten et al., , 2018Yao et al., 2018). Removal of H 2 SO 4 with this rate constant would significantly contribute to its loss during ChArMEx, increasing it by about a factor of 2 for the largest observed H 2 SO 4 concentrations. During the ChArMEx/SAFMED experiment, the H 2 SO 4 loss rate assuming sulphuric acid initiated kinetic particle formation can be estimated from several episodes of the NPF observed during the campaign (on days from 29/07 to 2/09; (Berland et al., 2017) the observed NPF on this day could be related to the evolution of the H 2 SO 4 concentration. The growth rates (GR) and 12 nm particle apparent formation rates from the SMPS measurements are 3.1 nm h -1 and 8.2×10 -3 cm -3 s -1 , respectively. GR was calculated using a maximum-concentration method . Using the approach of Kerminen and Kulmala 500 (2002) and Sihto et al. (2006) to derive the rate of formation of critical cluster of size 1 nm, J 1 , we obtain around 1 cm -3 s -1 at H 2 SO 4 concentration of around 10 7 molecule cm -3 . Assuming kinetically limited nucleation mechanism in which the critical cluster contains two sulfuric acid molecules, this corresponds to the bimolecular rate constant of 5×10 -13 for the removal of H 2 SO 4 , which would correspond to a negligible H 2 SO 4 loss compared to the condensation on existing particles. Similar rates, several orders of magnitude below the collision limited rate, were found in other diverse continental and marine atmospheric 505 environments (Kuang et al., 2008). Besides, the GR estimated from the time delay between H 2 SO 4 profile and corresponding increase in N 10-15 particles is about 2 times faster than derived from the particle size growth rate. Using this latter GR would result in an even slower particle formation. Finally, analysis of particles origin and their chemical composition at the measurement site made with ATOMF MS indicate that even if the traces of amines were present they were of remote origin and not present on the measurement site (Arndt et al., 2017).

SCIs interference with OH and H2SO4 measurements
High concentrations of SO 2 used in the chemical conversion reactor of CIMS instruments for the conversion of OH into H 2 SO 4 may lead to an interference with OH and H 2 SO 4 measurements due to an artificial generation of H 2 SO 4 in reactions of SCIs with SO 2 inside the reactor. When the reactant used as a scavenger of OH does not react fast with SCIs, e.g. using propane, the contribution of the artificially formed H 2 SO 4 is the same for the OH measurements in the background (BG) and the OH 515 signal + background modes (OH+BG), if the SO 2 injection position rests unchanged for the measurements in both modes. In this case the OH signal derived from the difference of the OH+BG and the BG signals is free from the artificial H 2 SO 4 formation. However, the presence of an additional SO 2 oxidant not efficiently removed by OH scavenger may lead to a significant increase of the BG level as it was observed in previous field measurements (Berresheim et al., 2014;Mauldin III et al., 2012).

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In the instrument used in this study the OH is scavenged with NO 2 . The rate constant for the reaction of NO 2 with CH 2 OO, CH 3 CHOO and (CH 3 ) 2 COO stabilized CIs was found to be of about 2 × 10 -12 cm 3 molecule -1 s -1 (Chhantyal-Pun et al., 2017;Stone et al., 2014;Taatjes et al., 2013), which is about 10 -100 times smaller than typical rate constants of the reactions of SCIs with SO 2 (Supplement, p. 15,  Fig. S1 for details), the OH signal derived from the OH+BG and the BG difference may also be influenced by the artificial H 2 34 SO 4 production in reactions of SCIs with SO 2 .
As presented in Fig. 7, the BG signal observed during ChArMEx showed a diel profile similar to that of VOCs or OH ( Fig.   1 and 2) with maximum at noon and minimum during the night.  Figure 7. Interference with OH measurements from the H 2 SO 4 produced in the reactor by reactions of SO 2 with SCIs: calculated contribution to measured OH (red line, left axis) and comparison of the observed background (BG) (blue line, right axis) and the BG calculated assuming its origin from the reactions of SCIs with SO 2 inside the reactor (black line, right axis).

540
To examine if the observed high BG levels can be explained by the SO 2 reaction with SCIs the H 2 SO 4 concentration produced in the reactor in reactions of the sum of SCIs with SO 2 , [H 2 34 SO 4 ] SCI/R , was calculated for BG and OH+BG modes using Eq. (4), which is similar to Eq. (2) but neglects the H 2 SO 4 losses in the reactor: where the index R j corresponds to different parts of the reactor, SCI inside the reactor were calculated using the ambient measurements with accounting for a dilution inside the reactor.
The concentration of O 3 in the IMR was corrected by accounting for ozone generated in the corona discharge ion source 550 and added to the IMR with the flow of primary ions. Amount of O 3 generated by the ion source depends on the ion source operating configuration, i.e., flow rates of injected mixtures, composition of the mixtures and potentials of the ion source electrodes. The concentration of O 3 in the IMR was not monitored during the campaign, but according to later checks this concentration was of 2 ± 1 ppm and this value was used for the estimation of [H 2 34 SO 4 ] SCI/R . As shown in Fig. 7, the calculated and the observed BGs exhibit similar variability and correspond to comparable 555 concentration levels allowing us to suggest that the observed elevated BG levels were related to the SO 2 oxidation by SCIs in the reactor. This hypothesis is supported by results of later experiments on the ozonolysis of terpenes conducted in an environmental chamber where a strong dependence of the observed BG level on the SCIs production rate was confirmed, as shown in Supplement, p. 19, Fig. S14 for the ozonolysis of α-pinene.
Concerning a possible interference of the SO 2 +SCIs reaction with the OH measurements, Fig. 7 shows that during the 560 campaign, this interference was negligible because the difference of [H 2 34 SO 4 ] SCI/R produced in the OH+BG and the BG modes corresponded to OH concentration lower than 2×10 4 molecule cm -3 , about 10 times lower than the OH lower detection limit.
Notably, this contribution is independent on the [O 3 ] in the IMR influencing equally the OH+BG and BG measurements.

Conclusions
The formation of H 2 SO 4 was observed at the Ersa station in northern Corsica, a site influenced by local emissions of the night. Based on the OH, H 2 SO 4 , SO 2 and particle number density measurements and assuming validity of a steady state between H 2 SO 4 production and its loss by condensation on existing aerosol particles with a unity accommodation coefficient, we have found that the contribution of the SO 2 + OH reaction accounts for (86 ± 4) % and only for (9 ± 2) % of the H 2 SO 4 production during the day and night, respectively. The given accuracy of these values has been estimated without accounting 570 for the large uncertainty in the OH + SO 2 reaction rate coefficient, which results in a larger uncertainty in the derived here contribution to H 2 SO 4 formation from the OH+SO 2 source, from about a factor of 1.5 its overestimation to about 20% its underestimation.
Estimating the H 2 SO 4 production from the SO 2 oxidation by SCIs, we conclude that despite the low calculated SCIs concentrations ((1 -3) × 10 3 molecule cm -3 for the sum of SCIs), this source may explain about 10% of the H 2 SO 4 formation 575 during the day and represents a major source of H 2 SO 4 accounting for about 40% of its formation during the night. The sum of the H 2 SO 4 production rates via SO 2 +OH and SO 2 +SCIs correspond to a closure of the H 2 SO 4 budget during the day, but seem to underestimate by 50% the H 2 SO 4 production during the night, with the latter being possibly related to uncertainties in the used in this work kinetic parameters, an unaccounted contribution from the ozonolysis of some unsaturated compounds not measured during the campaign, as well as to some yet unidentified H 2 SO 4 production mechanisms during night-time.
LSCE (now at CyI) for managing the campaign site and providing meteorology data, Eric Hamonou and François Dulac from LSCE for organizing the ChArMEx campaign and for managing and coordinating the ChArMEx project, François Dulac for 595 editorial corrections and suggestions.

Financial support
This research has received funding from the French National Research Agency (ANR) projects SAFMED (grant ANR-12-BS06-0013). This work is part of the ChArMEx project supported by ADEME, CEA, CNRS-INSU and Météo-France through the multidisciplinary program MISTRALS (Mediterranean Integrated Studies aT Regional And Local Scales). The station at