Stabilized Criegee Intermediates Research Articles

Direct Kinetic Measurements of a Cyclic Criegee Intermediate; Unimolecular Decomposition of c-(CH2)5COO.

We report the first direct kinetic measurements of a cyclic stabilized Criegee Intermediate. We have measured the unimolecular reaction rate coefficient of cyclohexanone oxide (c-(CH2)5COO) in the temperature 213-296 K and pressure 7-50 Torr ranges using absorption spectrometry. The c-(CH2)5COO was produced by the photolysis of c-(CH2)5CIBr at 213 nm in the presence of O2. We compare the measured fast c-(CH2)5COO unimolecular rate coefficient, 1998 ± 147 s-1 at 296 K, with the literature calculations for the structurally similar E-nopinone oxide formed in β-pinene ozonolysis. The kuni(c-(CH2)5COO)/kuni(E-nopinone oxide) ratio calculated using transition-state theory and density functional theory agrees well with this comparison. We have also measured the bimolecular rate coefficient of the reaction between c-(CH2)5COO and trifluoroacetic acid at 253 K and 10 Torr and obtained the value (8.7 ± 1.0) × 10-10 cm3 molecule-1 s-1. This very large value agrees with previous kinetic measurements for reactions between stabilized Criegee intermediates and halogenated organic acids.

Synthesis and Characterization of Organic Peroxides from Monoterpene-Derived Criegee Intermediates in Secondary Organic Aerosol.

Ozonolysis of alkenes is known to produce reactive intermediates─stabilized Criegee intermediates (SCIs), and their subsequent bimolecular reactions with various carboxylic acids can form α-acyloxyalkyl hydroperoxides (AAHPs), which is considered a major class of organic peroxides in secondary organic aerosol (SOA). Despite their atmospheric and health importance, the molecular-level identification of organic peroxides in atmospheric aerosols is highly challenging, preventing further assessment of their environmental fate. Here, we synthesize 20 atmospherically relevant AAHPs through liquid-phase ozonolysis, in which two types of monoterpene-derived SCIs from either α-pinene or 3-carene are scavenged by 10 different carboxylic acids to form AAHPs with diverse structures. These AAHPs are identified individually by liquid chromatography coupled with high-resolution mass spectrometry. AAHPs were previously thought to decompose quickly in an aqueous environment such as cloud droplets, but we demonstrate here that AAHPs hydrolysis rates are highly compound-dependent with rate constants differing by 2 orders of magnitude. In contrast, the aqueous-phase formation rate constants between SCI and various carboxylic acids vary only within a factor of 2-3. Finally, we identified two of the 20 synthesized AAHPs in α-pinene SOA and two in 3-carene SOA, contributing ∼0.3% to the total SOA mass. Our results improve the current molecular-level understanding of organic peroxides and are useful for a more accurate assessment of their environmental fate and health impact.

Impact of temperature on the role of Criegee intermediates and peroxy radicals in dimer formation from β-pinene ozonolysis

Abstract. Stabilized Criegee intermediates (SCIs) and organic peroxy radicals (RO2) are critical in atmospheric oxidation processes and secondary organic aerosol (SOA) formation. However, the influence of temperature on their corresponding reaction mechanisms in SOA formation is unclear. Through utilizing formic acid as a SCI scavenger and regulating the ratio of hydroperoxyl radials (HO2) to RO2 ([HO2]/[RO2]) from ∼ 0.3 to ∼ 1.9 using different concentrations of CO, the roles of RO2 and SCIs in SOA formation were investigated from 248 to 298 K, particularly for dimer formation in β-pinene ozonolysis. The SOA yield increased by 21 % from 298 to 273 K, while it decreased by 40 % from 273 to 248 K. Both changing [HO2]/[RO2] and scavenging SCIs significantly affect SOA yield and composition. SCI reactions accounted for more than 40 % of dimer and SOA mass formation for all temperatures. Increasing [HO2]/[RO2] inhibited dimer and SOA formation, and this inhibition became larger with decreasing temperature. Compared to low [HO2]/[RO2] (0.30–0.34), the dimer abundance at high [HO2]/[RO2] (1.53–1.88) decreased by about 31 % at 298 K and 70 % at 248 K. [HO2]/[RO2] has a specific impact on SCI-controlled dimers at lower temperatures by especially influencing the C9–SCI reactions with RO2. The dimers formed from C9–SCI reactions with RO2 were estimated to decrease by 61 % at high [HO2]/[RO2] compared to low [HO2]/[RO2] at 248 K. The high reactivity and substantial contribution to SOA of β-pinene-derived SCIs at lower temperatures observed in this study suggest that monoterpene-derived SCI reactions should be accounted for in describing colder regions of the atmosphere.

Roaming in the Unimolecular Decay of syn-Methyl-Substituted Criegee Intermediates.

Alkene ozonolysis generates transient carbonyl oxide species, known as Criegee intermediates, which are a significant nonphotolytic source of OH radicals in the troposphere. This study demonstrates that unimolecular decay of syn-methyl-substituted Criegee intermediates proceeds via 1,4 H atom transfer to vinyl hydroperoxides, resulting in OH fission to O-O products or, alternatively, OH roaming to hydroxycarbonyl products. Newly generated Criegee intermediates are shown to yield hydroxycarbonyls with sufficient internal excitation to dissociate via C-C fission to acyl and hydroxymethyl (CH2OH) radicals. The stabilized Criegee intermediates and unimolecular products are rapidly cooled in a pulsed supersonic expansion for photoionization detection with time-of-flight mass spectrometry. CH2OH products are identified by 2 + 1 resonance-enhanced multiphoton ionization via the 3pz Rydberg state upon unimolecular decay of CH3CHOO, (CH3)2COO, (CH3)(CH3CH2)COO, and (CH3)(CH2═CH)COO (methyl vinyl ketone oxide). The stabilized Criegee intermediates are separately detected using 10.5 eV photoionization. This study provides the first experimental evidence of roaming in the unimolecular decay of isoprene-derived methyl vinyl ketone oxide and extends earlier studies that reported stabilized hydroxycarbonyl products.

Oligomer formation from cross-reaction of Criegee intermediates in the styrene-isoprene-O3 mixed system

Alkene ozonolysis can produce stabilized Criegee intermediates (SCIs), which play a key role in oligomers' formation. Though styrene and isoprene coexist in the ambient atmosphere as important anthropogenic and biogenic secondary organic aerosol (SOA) precursors, respectively, their cross-reactions have not received attention. This study investigated the interactions of SCIs from styrene and isoprene ozonolysis for the first time. The high-resolution Orbitrap mass spectrometer was used to determine the unique ion mass spectra of the isoprene-styrene-O3 mixture. The results show that the signal intensities of new ions account for >8.4% of total ions in the mass spectra of the styrene-isoprene-O3 mixed system. Styrene and isoprene ozonolysis can produce characteristic C7–SCI and C4–SCI, respectively. C7–SCI and C4–SCI can be involved in the cross-reactions, and the results of tandem mass spectra directly confirmed both C7–SCI and C4–SCI as chain units. The O/C and H/C ratios of cross-products are in the range of 0.38–1.07 and 1.00–1.50, respectively, which are consistent with cross-reaction products. Adding a C7–SCI unit reduces the oligomer's volatility by 1.3–1.4 orders of magnitude lower than adding a C4–SCI unit. Thus, C4–SCI can compete with C7–SCI to react with styrene-derived RO2/RC(O)OH to produce more volatile cross-products, while the less volatile cross-products can be formed when isoprene-derived RO2/RC(O)OH reacted with C7–SCI instead of C4–SCI. The SOA yield of the mixed system is lower than that of the single styrene-O3 system but higher than that of the single isoprene-O3 system. Ambient particles were also collected, and 5 possible SCI-related cross-products were identified. This study illustrates the effects of SCI-related cross-reactions on SOA components and physicochemical properties, providing a basis for future research on SCI-related cross-reactions that frequently occur in the ambient atmosphere.

Effect of industrial alkene ozonolysis on atmospheric H2SO4 formation

This study has employed the master chemical mechanism (MCM) to investigate the influence of the ozone oxidation pathways in the atmospheric formation of H2SO4 from short-chain olefins in industrialized areas. In-situ H2SO4 formation data were obtained using a high-resolution chemical ionization time-of-flight mass spectrometer, and the simulated H2SO4 concentrations calculated using updated parameters for the MCM model exhibited good agreement with observations. In the simulation analysis of different reaction pathways involved in H2SO4 formation, hydroxyl radicals were found to dominate H2SO4 production during the daytime, while olefin ozone oxidation contributed up to 65% of total H2SO4 production during the night-time. A sensitivity analysis of the H2SO4 production parameters has revealed a high sensitivity to changes in sulfur dioxide, and a relatively high sensitivity to olefins with fast ozonolysis reaction rates and bimolecular reaction rates of resulting stabilized Criegee Intermediates. A high relative humidity promotes daytime H2SO4 formation, but has an inhibiting effect during the night-time due to the different dominant reaction pathways.

Sources, sinks, and chemistry of Stabilized Criegee Intermediates in the Indo-Gangetic Plain

Night-time oxidation significantly affects the atmospheric concentration of primary and secondary air pollutants but is poorly constrained over South Asia. Here, using a comprehensively measured and unprecedented set of precursors and sinks of Stabilized Criegee Intermediates (SCI), in the summertime air of the Indo-Gangetic Plain (IGP), we investigate the chemistry, and abundance in detail. This study reports the first summertime levels from the IGP of ethene, propene, 1-butene, cis-2-butene, trans-2-butene, 1-pentene, cis-2-pentene, trans-2-pentene, and 1-hexene and their possible roles in SCI chemistry. Ethene, propene, and 1-butene were the highest ambient alkenes in both the summer and winter seasons. Applying chemical steady-state to the measured precursors, the average calculated SCI concentrations were 4.4 (±3.6) × 103 molecules cm−3, with Z-CH3CHOO (55 %) as the major SCI. Z-RCHOO (35 %) and α-pinene derived PINOO (34 %) were identified as the largest contributors to SCI with a 7.8 × 105 molecules cm−3 s−1 production rate. The peak SCI occurred during the evenings. For all SCI species, the loss was dominated (>50 %) by unimolecular decomposition or reactions with water vapor or water vapor dimer. Pollution events influenced by crop burning resulted in significantly elevated SCI production (2.1 times higher relative to non-polluted periods) reaching as high as (7.4 ± 2.5) × 105 molecules cm−3 s−1. Among individual SCI species, Z-CH3CHOO was highest in all the plume events measured accounting for at least ~41 %. Among alkenes, trans-2-butene was the highest contributor to P(SCI) in plume events ranging from 22 to 32 %. SCIs dominated the night-time oxidation of sulfur dioxide with rates as high as 1.4 (±1.1) × 104 molecules cm−3 s−1 at midnight, suggesting that this oxidation pathway could be a significant source of fine mode sulfate aerosols over the Indo-Gangetic Plain, especially during summertime biomass burning pollution episodes.

Organic acid formation in the gas-phase ozonolysis of α,β-unsaturated ketones.

Organic acids are key species in determining the radiative properties of the atmosphere due to their contribution to particle formation. Reported discrepancies between field measurements and modelling suggest significant missing sources. Herein, we present a mechanistic investigation on the gas-phase ozonolysis of ethyl vinyl ketone (EVK, 1-penten-3-one), which we chose as a model compound for α,β-unsaturated ketones. Experiments were performed in a 1080 L quartz-glass reaction chamber (QUAREC) at 990 ± 15 mbar and 298 ± 2 K (r. h. ≪ 0.1%) and analysed via long-path FTIR spectrometry and PTR-ToF-MS. The experiments were performed in the presence of an excess of CO to suppress the chemistry of OH radicals. For a comprehensive picture, in selected experiments, SO2 was also added to the reaction system to scavenge the stabilized Criegee intermediates (sCIs) and to investigate their formation yield. Combining the results of both set-ups allowed us to quantify 2-oxobutanal, for which we report vapour-phase FTIR spectra. In addition, we introduce the first-ever infrared spectra of perpropionic acid, which was also positively identified in the EVK + O3 system. A detailed analysis of the experimental findings allowed us to link the identified reaction products (acetaldehyde, ethyl hydroperoxide, and perpropionic acid) to known bimolecular reactions of RO2 radicals. Thereby, it is shown that the EVK + O3 reaction yields formic acid, HC(O)OH, and propionic acid, C2H5C(O)OH, and their formation is not covered by mechanisms reported in the literature. Three different pathways accounting for their formation from chemically activated CIs are proposed and possible implications for the ozonolysis of α,β-unsaturated ketones in the atmosphere are discussed.

Low-pressure and nascent yields of stabilized Criegee intermediates CH2OO and CH3CHOO in ozonolysis of propene.

The yields of stabilized Criegee intermediates (sCIs), both CH2OO and CH3CHOO, produced from ozonolysis of propene at low pressures (7-16 Torr) were measured indirectly using cavity ringdown spectroscopy (CRDS) and chemical titration with an excess amount of sulfur dioxide (SO2). The method of monitoring the consumption of SO2 as a scavenger and the production of secondary formaldehyde (HCHO) allows characterization of the total sCI and the stabilized CH2OO yields at low pressure and in a short residence time. Both the total sCI and the stabilized CH2OO yields in the propene ozonolysis were found to decrease with decreasing pressure. By extrapolating the 7-16 Torr measurements to the zero-pressure limit, the nascent yield of the total sCIs was determined to be 25 ± 2%. The ranges of nascent yields of stabilized CH2OO and stabilized CH3CHOO were estimated to be 20-25% and 0-5%, respectively. The branching ratios of the stabilized and high-energy CH2OO* and CH3CHOO* were also determined.

Oligomer formation from the gas-phase reactions of Criegee intermediates with hydroperoxide esters: mechanism and kinetics

Abstract. Hydroperoxide esters, formed in the reactions of carbonyl oxides (also called Criegee intermediates, CIs) with formic acid, play a crucial role in the formation of secondary organic aerosol (SOA) in the atmosphere. However, the transformation mechanism of hydroperoxide esters in the presence of stabilized Criegee intermediates (SCIs) is not well understood. Herein, the oligomerization reaction mechanisms and kinetics of distinct SCI (CH2OO, syn-CH3CHOO, anti-CH3CHOO, and (CH3)2COO) reactions, with their respective hydroperoxide esters and with hydroperoxymethyl formate (HPMF), are investigated in the gas phase using quantum chemical and kinetics modeling methods. The calculations show that the addition reactions of SCIs with hydroperoxide esters proceed through successive insertion of SCIs into hydroperoxide ester to form oligomers that involve SCIs as the repeated chain unit. The saturated vapor pressure and saturated concentration of the formed oligomers decrease monotonically as the number of SCIs is increased. The exothermicity of oligomerization reactions decreases significantly when the number of methyl substituents increases, and the exothermicity of anti-methyl substituted carbonyl oxides is obviously higher than that of syn-methyl substituted carbonyl oxides. The −OOH insertion reaction is energetically more feasible than the −CH insertion pathway in the SCI oligomerization reactions, and the barrier heights increase with increasing the number of SCIs added to the oligomer, except for syn-CH3CHOO. For the reactions of distinct SCIs with HPMF, the barrier of the −OOH insertion pathway shows a dramatic decrease when a methyl substituent occurs at the anti-position, while it reveals a significant increase when a methyl group is introduced at the syn-position and dimethyl substituent. Compared with the rate coefficients of the CH2OO + HPMF reaction, the rate coefficients increase by about 1 order of magnitude when a methyl substituent occurs at the anti-position, whereas the rate coefficients decrease by 1–2 orders of magnitude when a methyl group is introduced at the syn-position. These new findings advance our current understanding of the influence of Criegee chemistry on the formation and growth processes and the chemical compositions of SOA.

Acylperoxy Radicals as Key Intermediates in the Formation of Dimeric Compounds in α-Pinene Secondary Organic Aerosol.

High molecular weight dimeric compounds constitute a significant fraction of secondary organic aerosol (SOA) and have profound impacts on the properties and lifecycle of particles in the atmosphere. Although different formation mechanisms involving reactive intermediates and/or closed-shell monomeric species have been proposed for the particle-phase dimers, their relative importance remains in debate. Here, we report unambiguous experimental evidence of the important role of acyl organic peroxy radicals (RO2) and a small but non-negligible contribution from stabilized Criegee intermediates (SCIs) in the formation of particle-phase dimers during ozonolysis of α-pinene, one of the most important precursors for biogenic SOA. Specifically, we find that acyl RO2-involved reactions explain 50-80% of total oxygenated dimer signals (C15-C20, O/C ≥ 0.4) and 20-30% of the total less oxygenated (O/C < 0.4) dimer signals. In particular, they contribute to 70% of C15-C19 dimer ester formation, likely mainly via the decarboxylation of diacyl peroxides arising from acyl RO2 cross-reactions. In comparison, SCIs play a minor role in the formation of C15-C19 dimer esters but react noticeably with the most abundant C9 and C10 carboxylic acids and/or carbonyl products to form C19 and C20 dimeric peroxides, which are prone to particle-phase transformation to form more stable dimers without the peroxide functionality. This work provides a clearer view of the formation pathways of particle-phase dimers from α-pinene oxidation and would help reduce the uncertainties in future atmospheric modeling of the budget, properties, and health and climate impacts of SOA.

Large Pressure Effects Caused by Internal Rotation in the s-cis-syn-Acrolein Stabilized Criegee Intermediate at Tropospheric Temperature and Pressure.

Criegee intermediates are important atmospheric oxidants, and quantitative kinetics for stabilized Criegee intermediates are key parameters for atmospheric modeling but are still limited. Here we report barriers and rate constants for unimolecular reactions of s-cis-syn-acrolein oxide (scsAO), in which the vinyl group makes it a prototype for Criegee intermediates produced in the ozonolysis of isoprene. We find that the MN15-L and M06-2X density functionals have CCSD(T)/CBS accuracy for the unimolecular cyclization and stereoisomerization of scsAO. We calculated high-pressure-limit rate constants by the dual-level strategy that combines (a) high-level wave function-based conventional transition-state theory (which includes coupled-cluster calculations with quasiperturbative inclusion of quadruple excitations because of the strongly multiconfigurational character of the electronic wave function) and (b) canonical variational transition-state theory with small-curvature tunneling based on a validated density functional. We calculated pressure-dependent rate constants both by system-specific quantum Rice-Ramsperger-Kassel theory and by solving the master equation. We report rate constants for unimolecular reactions of scsAO over the full range of atmospheric temperature and pressure. We found that the unimolecular reaction rates of this larger-than-previously studied Criegee intermediate depend significantly on pressure. Particularly, we found that falloff effects decrease the effective unimolecular cyclization rate constant of scsAO by about a factor of 3, but the unimolecular reaction is still the dominant atmospheric sink for scsAO at low altitudes. The large falloff caused by the inclusion of the stereoisomerization channel in the master equation calculations has broad implications for mechanistic analysis of reactions with competitive internal rotations that can produce stable rotamers.

Year-round modeling of sulfate aerosol over Asia through updates of aqueous-phase oxidation and gas-phase reactions with stabilized Criegee intermediates

Sulfate aerosol (SO42−) is a main component of particulate matter in Asian countries. Accurate numerical modeling is important for appropriate policy-making aimed at reducing SO42− concentrations. The modeling performance for SO42− is superior among aerosol components, however, current models underestimate SO42− concentrations during winter over Asia. Previous studies have proposed a heterogeneous process for winter haze events, but these kinds of studies are limited spatially and temporally because they cover only mainland China in winter. Underestimation has also been reported in other regions where the aerosol concentration is much lower than in China. In this study, the aqueous- and gas-phase oxidation processes in the current modeling were reconsidered, and their roles were evaluated over Asia using a year-round simulation. The existing aqueous-phase oxidation of O2 with Fe and Mn as catalysts was refined, and oxidation with NO2 was added due to the neutralized atmosphere over Asia. For gas-phase oxidation, three stabilized Criegee intermediates (SCIs; formaldehyde oxide (CH2OO), acetaldehyde oxide (CH3CHOO), and propionaldehyde oxide ((CH3)2COO)) were introduced. Considering the uncertainty of the reaction of CH2OO with water, the upper and lower limits of the rate constant were applied. The updated oxidation processes led to an increase in the modeled SO42− concentration. The model performance over Asia in winter was effectively improved, and the updates did not degrade the model performance in other seasons. The improvements in biases were approximately 3% during winter, whereas the deterioration in biases were within 1% in spring to summer and 2% in autumn, when the lower limit of the CH2OO rate constant with water was used. The role of SCIs depended strongly on the rate constants of the reaction of CH2OO with water. The simulated concentration of SCIs agreed well with the estimated levels, and the lower limit of the rate constant of CH2OO with water fitted within the estimated SCIs concentration levels. SCIs may play an important role in SO42− production over Asia in winter, especially the downwind region of China. The approach taken in this study has the potential to improve modeling performance in other regions as well.

Molecular composition of secondary organic aerosol from styrene under different NOx and humidity conditions

Styrene is highly reactive in the atmosphere, which has been identified as a hazardous air pollutant by the Clean Air Act, and as an important secondary organic aerosol (SOA) precursor. In this study, the SOA formation from styrene under different NOx and relative humidity (RH) conditions were explored in an indoor chamber. The chemical composition of SOA was characterized using a Fourier transform infrared spectroscopy (FTIR) and a modified atmospheric pressure chemical ionization mass spectrometry (APCI-MS). The results show that the SOA yields decrease from 11.4% to 7.2% at about 70 μg m−3 SOA as RH increases from 8.5% to 62.4% in the H2O2 system; the SOA yields decrease from 1.1% to 0.7% at about 10 μg m−3 SOA as RH increases from 8.4% to 60.9% in the NOx system. In the H2O2 system, C7 and C8 series compounds are major components of SOA at both RHs. The total signal ratio of less oxidized C7 and C8 compounds (nO < 4) to the total SOA at 8.8% RH is 2.2 times higher than that at 63.3% RH. The OSc values of SOA were determined to be −0.2 at 8.8% RH and 0.04 at 63.3% RH based on MS data. It is proposed that high RH can accelerate the heterogeneous uptake of H2O2 onto particles and promote the further oxidation of less oxidized compounds in the particle phase. In the NOx system, SOA formation is significantly suppressed by the competitive reaction of RO2 with NO. The FTIR and MS results show that the main composition of SOA is organonitrates. At 8.3% RH, OH, O3 and NO3 are oxidants in the styrene-NOx irritation system. Among these oxidants, NO3 plays a minor role due to its photolysis. While at 60.9% RH, SOA formation from O3 is significantly inhibited by the reaction of stabilized Criegee intermediates (SCIs) with H2O, and SOA from the NO3 pathway is suppressed by the heterogeneous uptake of N2O5 to humid particles.

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

Abstract. Reaction of stabilized Criegee intermediates (SCIs) with SO2 was proposed as an additional pathway of gaseous sulfuric acid (H2SO4) formation in the atmosphere, supplementary to the conventional mechanism of H2SO4 production by oxidation of SO2 in reaction with OH radicals. However, because of a large uncertainty in mechanism and rate coefficients for the atmospheric formation and loss reactions of different SCIs, the importance of this additional source is not well established. In this work, we present an estimation of the role of SCIs in H2SO4 formation at a western Mediterranean (Cape Corsica) remote site, where comprehensive field observations including gas-phase H2SO4, OH radicals, SO2, volatile organic compounds (VOCs) and aerosol size distribution measurements were performed in July–August 2013 as a part of the project ChArMEx (Chemistry-Aerosols Mediterranean Experiment). The measurement site was under strong influence of local emissions of biogenic volatile organic compounds, including monoterpenes and isoprene generating SCIs in reactions with ozone, and, hence, presenting an additional source of H2SO4 via SO2 oxidation by the SCIs. Assuming the validity of a steady state between H2SO4 production and its loss by condensation on existing aerosol particles with a unity accommodation coefficient, about 90 % of the H2SO4 formation during the day could be explained by the reaction of SO2 with OH. During the night the oxidation of SO2 by OH radicals was found to contribute only about 10 % to the H2SO4 formation. The accuracy of the derived values for the contribution of OH + SO2 reaction to the H2SO4 formation is limited mostly by a large, at present factor of 2, uncertainty in the OH + SO2 reaction rate coefficient. The contribution of the SO2 oxidation by SCIs to the H2SO4 formation was evaluated using available measurements of unsaturated VOCs and steady-state SCI concentrations estimated by adopting rate coefficients for SCI reactions based on structure–activity relationships (SARs). The estimated concentration of the sum of SCIs was in the range of (1–3) × 103 molec. cm−3. During the day the reaction of SCIs with SO2 was found to account for about 10 % and during the night for about 40 % of the H2SO4 production, closing the H2SO4 budget during the day but leaving unexplained about 50 % of the H2SO4 formation during the night. Despite large uncertainties in used kinetic parameters, these results indicate that the SO2 oxidation by SCIs may represent an important H2SO4 source in VOC-rich environments, especially during nighttime.

Relative Humidity Changes the Role of SO2 in Biogenic Secondary Organic Aerosol Formation.

SO2 influences secondary organic aerosol (SOA) and organosulfates (OSs) formation but mechanisms remain elusive. This study focuses on this topic by investigating biogenic γ-terpinene ozonolysis under various SO2 and relative humidity (RH) conditions. With a constant SO2 concentration (∼110 ppb), the increase in RH transformed SO2 sinks from stabilized Criegee intermediates (sCIs) to peroxides in aerosol particles. The associated changes in particle acidity and liquid water content may collectively first lead to decreased and then increased SOA yield with increasing RH, with the turning point appearing at ∼30% RH. The abundance of most OSs formed under 45% RH was more than 5 times higher than that of OSs formed under 10% RH, possibly due to interactions of dissolved SO2 with hydroperoxides (ROOH) in SOA. ROOHs formed from the autoxidation processes of alkylperoxy radicals were proposed to be precursors for highly oxidized OSs (HOOSs) that decreased SOA volatility and showed a certain abundance in ambient aerosols. This study highlights that high RH potentially enhances the contribution of SO2 to OSs formation, and particularly, HOOSs formation during monoterpene ozonolysis in the atmosphere.

Formation of nighttime sulfuric acid from the ozonolysis of alkenes in Beijing

Abstract. Gaseous sulfuric acid (SA) has received a lot of attention for its crucial role in atmospheric new particle formation (NPF). And for this reason, studies until now have mainly focused on daytime SA when most NPF events occur. While daytime SA production is driven by SO2 oxidation of OH radicals of photochemical origin, the formation of SA during nighttime and its potential influence on particle formation remains poorly understood. Here we present evidence for significant nighttime SA production in urban Beijing during winter, yielding concentrations between 1.0 and 3.0 × 106 cm−3. We found a high frequency (∼ 30 %) of nighttime SA events, which are defined by the appearance of a distinct SA peak observed between 20:00 and 04:00 local time, with the maximum concentration exceeding 1.0 × 106 cm−3. These events mostly occurred during unpolluted nights with a low vapor condensation sink. Furthermore, we found that under very clean conditions (visibility &gt; 16.0 km) with abundant ozone (concentration &gt; 2.0 × 1011 cm−3, ∼ 7 ppb), the overall sink of SA was strongly correlated with the products of O3, alkenes and SO2 concentrations, suggesting that the ozonolysis of alkenes played a major role in nighttime SA formation under such conditions. This is in light of previous studies showing that the ozonolysis of alkenes can form OH radicals and stabilized Criegee intermediates (SCIs), both of which are able to oxidize SO2 and thus lead to SA formation. However, we also need to point out that there exist additional sources of SA under more polluted conditions, which are not investigated in this study. Moreover, there was a strong correlation between SA concentration and the number concentration of sub-3 nm particles on both clean and polluted nights. Different from forest environments, where oxidized biogenic vapors are the main driver of nighttime clustering, our study demonstrates that the formation of nighttime cluster mode particles in urban environments is mainly driven by nighttime SA production.

Application of chemical derivatization techniques combined with chemical ionization mass spectrometry to detect stabilized Criegee intermediates and peroxy radicals in the gas phase

Abstract. Short-lived highly reactive atmospheric species, such as organic peroxy radicals (RO2) and stabilized Criegee intermediates (SCIs), play an important role in controlling the oxidative removal and transformation of many natural and anthropogenic trace gases in the atmosphere. Direct speciated measurements of these components are extremely helpful for understanding their atmospheric fate and impact. We describe the development of an online method for measurements of SCIs and RO2 in laboratory experiments using chemical derivatization and spin trapping techniques combined with H3O+ and NH4+ chemical ionization mass spectrometry (CIMS). Using chemical derivatization agents with low proton affinity, such as electron-poor carbonyls, we scavenge all SCIs produced from a wide range of alkenes without depleting CIMS reagent ions. Comparison between our measurements and results from numeric modeling, using a modified version of the Master Chemical Mechanism, shows that the method can be used for the quantification of SCIs in laboratory experiments with a detection limit of 1.4×107 molecule cm−3 for an integration time of 30 s with the instrumentation used in this study. We show that spin traps are highly reactive towards atmospheric radicals and form stable adducts with them by studying the gas-phase kinetics of the reaction of spin traps with the hydroxyl radical (OH). We also demonstrate that spin trap adducts with SCIs and RO2 can be simultaneously probed and quantified under laboratory conditions with a detection limit of 1.6×108 molecule cm−3 for an integration time of 30 s for RO2 species with the instrumentation used in this study. Spin trapping prevents radical secondary reactions and cycling, ensuring that measurements are not biased by chemical interferences, and it can be implemented for detecting RO2 species in laboratory studies and potentially in the ambient atmosphere.

Quantification of the role of stabilized Criegee intermediates in the formation of aerosols in limonene ozonolysis

Abstract. Stabilized Criegee intermediates (SCIs) have the potential to oxidize trace species and to produce secondary organic aerosols (SOAs), making them important factors in tropospheric chemistry. This study quantitatively investigates the performance of SCIs in SOA formation at different relative humidity (RH) levels, and the first- and second-generation oxidations of endo- and exocyclic double bonds ozonated in limonene ozonolysis are studied separately. Through regulating SCI scavengers, the yields and rate constants of SCIs in a reaction system were derived, and the quantities of SCIs were calculated. The quantity of SOAs decreased by more than 20 % under low-humidity conditions (10 % RH–50 % RH), compared to that under dry conditions, due to the reactions of SCIs with water, while the inhibitory effect of water on SOA formation was not observed under high-humidity conditions (60 % RH–90 % RH). When using excessive SCI scavengers to exclude SCI reactions, it was found that the effect of water on SOA formation with the presence of SCIs was different from that without the presence of SCIs, suggesting that SCI reactions were relevant to the non-monotonic impact of water. The fractions of the SCI contribution to SOAs were similar between dry and high-humidity conditions, where the SCI reactions accounted for ∼ 63 % and ∼ 73 % in SOA formation in the first- and second-generation oxidation; however, marked differences in SOA formation mechanisms were observed. SOA formation showed a positive correlation with the quantity of SCIs, and the SOA formation potential of SCIs under high-humidity conditions was more significant than that under dry and low-humidity conditions. It was estimated that 20 %–30 % of SCIs could be converted into SOAs under high-humidity conditions, while this value decreased by nearly half under dry and low-humidity conditions. The typical contribution of limonene-derived SCIs to SOA formation is calculated to be (8.21 ± 0.15) × 10−2 µg m−3 h−1 in forest, (4.26 ± 0.46) × 10−2 µg m−3 h−1 in urban areas, and (2.52 ± 0.28) × 10−1 µg m−3 h−1 in indoor areas. Water is an uncertainty in the role SCIs play in SOA formation, and the contribution of SCIs to SOA formation needs consideration even under high RH in the atmosphere.

Sources and sinks driving sulfuric acid concentrations in contrasting environments: implications on proxy calculations

Abstract. Sulfuric acid has been shown to be a key driver for new particle formation and subsequent growth in various environments, mainly due to its low volatility. However, direct measurements of gas-phase sulfuric acid are oftentimes not available, and the current sulfuric acid proxies cannot predict, for example, its nighttime concentrations or result in significant discrepancies with measured values. Here, we define the sources and sinks of sulfuric acid in different environments and derive a new physical proxy for sulfuric acid to be utilized in locations and during periods when it is not measured. We used H2SO4 measurements from four different locations: Hyytiälä, Finland; Agia Marina, Cyprus; Budapest, Hungary; and Beijing, China, representing semi-pristine boreal forest, rural environment in the Mediterranean area, urban environment and heavily polluted megacity, respectively. The new proxy takes into account the formation of sulfuric acid from SO2 via OH oxidation and other oxidation pathways, specifically via stabilized Criegee intermediates. The sulfuric acid sinks included in the proxy are its condensation sink (CS) and atmospheric clustering starting from H2SO4 dimer formation. Indeed, we found that the observed sulfuric acid concentration can be explained by the proposed sources and sinks with similar coefficients in the four contrasting environments where we have tested it. Thus, the new proxy is a more flexible and an important improvement over previous proxies. Following the recommendations in this paper, a proxy for a specific location can be derived.