A new atmospherically relevant oxidant of sulphur dioxide

Atmospheric oxidation is a key phenomenon that connects atmospheric chemistry with globally challenging environmental issues, such as climate change, stratospheric ozone loss, acidification of soils and water, and health effects of air quality. Ozone, the hydroxyl radical and the nitrate radical are generally considered to be the dominant oxidants that initiate the removal of trace gases, including pollutants, from the atmosphere. Here we present atmospheric observations from a boreal forest region in Finland, supported by laboratory experiments and theoretical considerations, that allow us to identify another compound, probably a stabilized Criegee intermediate (a carbonyl oxide with two free-radical sites) or its derivative, which has a significant capacity to oxidize sulphur dioxide and potentially other trace gases. This compound probably enhances the reactivity of the atmosphere, particularly with regard to the production of sulphuric acid, and consequently atmospheric aerosol formation. Our findings suggest that this new atmospherically relevant oxidation route is important relative to oxidation by the hydroxyl radical, at least at moderate concentrations of that radical. We also find that the oxidation chemistry of this compound seems to be tightly linked to the presence of alkenes of biogenic origin.

Atmospheric oxidation is a key phenomenon that connects atmospheric chemistry with globally challenging environmental issues, such as climate change 1 , stratospheric ozone loss 2 , acidification of soils and water 3 , and health effects of air quality 4 . Ozone, the hydroxyl radical and the nitrate radical are generally considered to be the dominant oxidants that initiate the removal of trace gases, including pollutants, from the atmosphere. Here we present atmospheric observations from a boreal forest region in Finland, supported by laboratory experiments and theoretical considerations, that allow us to identify another compound, probably a stabilized Criegee intermediate (a carbonyl oxide with two freeradical sites) or its derivative, which has a significant capacity to oxidize sulphur dioxide and potentially other trace gases. This compound probably enhances the reactivity of the atmosphere, particularly with regard to the production of sulphuric acid, and consequently atmospheric aerosol formation. Our findings suggest that this new atmospherically relevant oxidation route is important relative to oxidation by the hydroxyl radical, at least at moderate concentrations of that radical. We also find that the oxidation chemistry of this compound seems to be tightly linked to the presence of alkenes of biogenic origin.
Oxidation of trace gases drives atmospheric chemistry and influences thereby both air quality and climate, and their interaction with each other and the biosphere 1,5-7 . The main gas-phase oxidants under consideration (so far) are the OH radical (OH . , referred to here as OH for simplicity), O 3 , the nitrate radical (NO 3 . , referred to here as NO 3 for simplicity) and Cl atoms, of which OH is important only during the daytime and NO 3 during night time. Recently, an increasing number of investigations have focused on atmospheric reactivity, more specifically the missing reactivity 8,9 and sources 5,10 of OH and its temporal variability 11 , as well as missing HONO sources 7 . Sulphuric acid (H 2 SO 4 ) is a key compound, connecting atmospheric oxidation chemistry with the formation and growth of new aerosol particles 12 . Until now, the general consensus has been that the rate at which sulphur dioxide (SO 2 ) is converted to gaseous H 2 SO 4 is determined by the OH concentration. Here we show that there is another important source of gaseous H 2 SO 4 that is not directly related to OH.
Our investigation into this uncharted oxidation chemistry is based on simultaneous observations of OH and H 2 SO 4 using chemical ionization mass spectrometry 13  represents a yet unexplored oxidant X (or sum of several such oxidants) which, similarly to OH, is capable of converting SO 2 to gaseous sulphuric acid in the atmosphere.
We started our investigation using field observations performed at the SMEAR II station in the Finnish boreal forest region (Supplementary Information). Figure 1a and SO 4 ] is remarkably high, up to about 10 6 molecules cm -3 . This observation indicates the presence of a non-OH source for H 2 SO 4 production, and further suggests that there might be a connection between this source and the oxidant X. We calculated the H 2 SO 4 concentration resulting from the reaction of SO 2 with OH (green line in Fig. 1b). The difference between the H 2 SO 4 concentration measured by the CIMS and that due to the reaction of SO 2 with OH is our best estimate of the H 2 SO 4 concentration resulting from the non-OH source, [H 2 SO 4 ] non-OH . Figure 1c shows that the value of [H 2 SO 4 ] non-OH increases with increasing [X], reaching values as high as (2-3) 3 10 6 molecules cm -3 during our measurements. Sulphuric acid originating from this non-OH source may contribute up to 50% of the total H 2 SO 4 budget ( Fig. 1b and Supplementary Information), demonstrating the important role of this H 2 SO 4 formation route.
The dominance of [X] over [OH] particularly during evenings and nights suggests that the compound X might be related to surface emissions and subsequent ozone chemistry taking place in the boundary layer. In order to investigate this chemistry, we carried out laboratory experiments using two systems with different flow characteristics (Supplementary Information). In these experiments, SO 2 was exposed to mixtures of ozone and various alkenes, and the resulting H 2 SO 4 concentration was measured with CIMS and modelled using a scheme based on known OH chemistry (Supplementary Information). Because alkene-ozone reactions are known to produce OH, the experiments were conducted both with and without an OH-scavenger (H 2 or CO). Figure 2a shows the measured and modelled H 2 SO 4 concentration as a function of the amount of monoterpene (limonene and a-pinene) reacted with ozone. As in the field study, we observed H 2 SO 4 that cannot be explained by the reaction of SO 2 with OH alone. In these experiments, the production of H 2 SO 4 from this non-OH source appeared to be more efficient for monoterpenes than for other alkenes (for example, MCH, 1-methyl-cyclohexene; see Supplementary Information). The role of the new H 2 SO 4 production chemistry   SO 4 ]. These are absolute uncertainties, based on the stated uncertainties of the values used in the calculation (such as CIMS calibration lamp intensity, water photolysis reaction rate coefficient and OH losses). The statistical uncertainties are less important in comparison with these uncertainties. The shaded green area in b depicts the range of additional uncertainty in calculated [H 2 SO 4 ] OH obtained by taking into account the uncertainties in the measured values of the SO 2 concentration (60.05 p.p.b.) and condensation sink. The upper limit of each error estimate was calculated by assuming that the SO 2 concentration was 0.05 p.p.b. larger than the measured value and that the condensation sink was at its minimum value at the measured relative humidity. The minimum value of the condensation sink, CS, was obtained by assuming that there were no super-micrometre particles and that the hygroscopicity of the submicrometre particles was at its minimum. Consequently, the lower limit of each error estimate was calculated by assuming that the SO 2 concentration was 0.05 p.p.b. lower than the measured value, and that the condensation sink was at its maximum value at the measured relative humidity. The maximum value of CS was obtained by assuming that the super-micrometre particles contributed 5% to total CS, and that the hygroscopicity of the sub-micrometre particles was at its maximum (see Supplementary Information and references therein).   figure). Cutting of branches results in enlarged emissions of monoterpenes from trees with large storage pools, such as spruce (Picea abies) and pine (Pinus sylvestris) 15 .

RESEARCH LETTER
becomes dominant at high monoterpene concentrations, as shown by the convergence of the data series taken in the absence and in the presence of the OH scavenger. Monoterpenes, including limonene and a-pinene used in our experiments, are emitted effectively by trees, and these compounds are abundantly present at our field measurement site during the summertime 15,16 . To confirm vegetation as a source of the alkenes responsible for X formation in the boreal forest environment, we performed an additional experiment where branches of different trees were cut and placed in the immediate vicinity of the CIMS inlet (Fig. 2b, see also Supplementary Information). The production of OH from ozonolysis of branch emissions during this experiment was minor in comparison to production of X. This experiment indisputably substantiates our conclusion, demonstrating the role of trees in producing compound X and, consequently, affecting gaseous sulphuric acid production.
The field and laboratory measurements presented above give strong evidence of the existence of a previously unknown oxidant X, but do not reveal its identity. Experiments 17,18 and quantum chemical calculations 19,20 have demonstrated that the reactions of SO 2 with the most common non-OH oxidants (O 3 and NO 3 ) and with peroxy radicals (HO 2 , H 3 COO and larger analogues) are extremely slow. Stabilized Criegee intermediates (sCIs), formed in the ozonolysis of all alkenes, are known to oxidize SO 2 (ref. 21) but the rate constant of the SO 2 1 sCI reaction has been assumed to be fairly low, of the order of 10 215 cm 3 s -1 (ref. 22). However, recent theoretical quantum mechanical studies 20,23 , as well as laboratory experiments 24 , have found the SO 2 1 sCI reaction to be significantly faster than previously thought. Another property that influences the oxidation capacity of sCIs is their lifetime against unimolecular decomposition reactions. The sCIs formed in ozonolysis of larger alkenes, such as monoterpenes, may have longer lifetimes than those formed from lower-molecular-weight alkenes. We note here that besides sCIs, other ozonolysis intermediates might also be responsible for the observed additional oxidation of SO 2 (refs 25, 26). Figure 3 summarizes schematically this new mechanism of atmospheric oxidation chemistry.
We finalize our analysis by investigating whether the proposed chemistry is consistent with field observations. For this purpose, we estimated the reaction rate between sCI and SO 2 from the laboratory measurement data presented in Fig. 2a (see Supplementary Information for theoretical considerations and assumptions made in deriving the reaction rate). The resulting rate coefficient was about 6 3 10 213 cm 3 s 21 for a-pinene 1 sCI and about 8 3 10 213 cm 3 s -1 for limonene 1 sCI. In Fig. 4a and b we have calculated the H 2 SO 4 concentration originating from the two SO 2 oxidation pathways, OH and non-OH, during the field measurement period reported in Fig. 1  Criegee intermediates (CI) formed during ozonolysis, ,50% decompose to produce OH on a subsecond timescale while the other 50% are stabilized, producing stabilized Criegee radicals, sCI. These sCI can then decompose over a much longer lifetime, t. Our observations suggest that sCI, or non-OH derivatives of sCI, can also oxidize at least SO 2 (red arrows), thus altering the known view of oxidation chemistry in the atmosphere. Typical OH chemistry is depicted in blue.   Fig. 1a, now showing, in addition to [H 2 SO 4 ] measured (black line) and calculated from OH reaction (blue area), the concentration calculated from the X reaction as in a (red area). Between 30 July and 2 August, the SO 2 concentration was mostly below the detection limit during which time the calculated concentration is not depicted. The error bars, not shown for clarity, are essentially the same as in Fig. 1a.

LETTER RESEARCH
during some evenings and nights, addition of the new SO 2 oxidant significantly improves the overall agreement between measured and calculated H 2 SO 4 concentrations. The chemistry investigated here is tightly connected with the presence of biogenic volatile organic compounds (BVOCs), and thereby with forest emissions. Covering vast areas of the Earth's surface, forests play an important role in global cycles of carbon, water and energy 6 . BVOCs emitted by forests dominate the global secondary organic aerosol loading 27 , and contribute significantly to the global budget of cloud condensation nuclei 28 . Our findings add to the already substantial significance of forests in the Earth system by introducing a previously unknown oxidant, probably an sCI, capable of oxidizing at least SO 2 and possibly also other atmospheric trace gases relevant to atmospheric chemistry. Because gaseous sulphuric acid is formed in this process, the new chemistry is likely to affect the formation of new atmospheric particles, the production of secondary cloud condensation nuclei and ultimately climate. Our findings demonstrate a new connection between anthropogenic activities (SO 2 emissions), natural ecosystems (BVOC emissions and secondary organic aerosol formation) and climate (from cloud properties to radiative forcing). This connection is likely to change in the future as a result of changing SO 2 and BVOC emissions due to air quality regulations and warming climate 29 . More detailed experimental and theoretical investigations are clearly needed to find out the importance of the new oxidant in atmospheric chemistry and climate at present and under future conditions.

METHODS SUMMARY
The results of the laboratory experiments described here were obtained from two different experimental systems. Both systems used the flow tube (that is, continuous flow) technique, where gases are added to a continuous stream and allowed to react for a known period of time. Sulphuric acid was then measured at the exit of the flow tube using nitrate-ion-based chemical ionization mass spectrometry (CIMS). One experimental apparatus (referred to as IfT-LFT) was located at the Leibniz-Institute for Tropospheric Research in Leipzig, Germany, and the other at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. The two systems differ from each other in their geometries, residence times and the method by which the reagent gases are introduced. The CIMS instrument was also used in the field measurements performed at the SMEAR II station located in a Finnish boreal forest for the detection of H 2 SO 4 , OH and X. A more detailed description of the methods is given in Supplementary Information. Full Methods and any associated references are available in the online version of the paper.

METHODS
CIMS measurements. Measurements of OH and H 2 SO 4 were performed using the Chemical Ionization Mass Spectrometer (CIMS) technique. The technique has been described elsewhere 13,31,32 therefore only details relevant to the present work will be discussed here. Briefly, sample air is drawn through the 1.9 cm stainless steel inlet, and a small amount (,10 14 molecules cm 23 ) of isotopically labelled 34 Isotopically labelled SO 2 is used to discriminate between H 2 SO 4 derived from OH and ambient H 2 SO 4 . To prevent cycling of HO 2 and RO 2 back into OH, propane is added on a continuous basis through a second pair of injectors located ,5 cm downstream of the first pair, after the ambient OH initially present has been converted into H 2 SO 4 . Propane is added through these injectors at sufficient concentrations to remove more than 99% of the OH which has been cycled back from HO 2 or RO 2 via reactions with NO or O 3 . To account for other unknown processes which can convert SO 2 into H 2 SO 4 an 'OH background' is performed, in which propane is added along with the 34 SO 2 through the front injectors at a concentration sufficient to remove .98% of the OH present. These OH background values are used in this work to describe the measurement of X. More details concerning the injectors and the sampling port chemistry are described elsewhere 31 . Once formed, the H 2 34 SO 4 is measured in the same manner as H 2 SO 4 via chemical ionization. IfT-LFT. Experiments were carried out in the atmospheric pressure flow-tube IfT-LFT (i.d. 8 cm; length 505 cm) at 293 6 0.5 K (ref. 33). The flow tube consists of a first section (56 cm) that includes the inlet system for gas input (air premixed with SO 2 from a calibration gas mixture (1 p.p.m.v. or 10 p.p.m.v. SO 2 in N 2 (Messer)), O 3 from an ozone generator outside the flow tube (UVP OG-2), the OH scavenger H 2 and the olefin premixed from a metering device). At the end of the tube, all sampling outlets are attached. O 3 and SO 2 concentrations were measured by means of gas monitors (Thermo Environmental Instruments: 49C and 43C) or by long-path ultraviolet absorption spectroscopy (Perkin-Elmer: Lambda 800) using a gas cell with a White-mirror optics adjusted at a path-length of 512 cm. The organics were followed by proton transfer reaction-mass spectrometry (PTR-MS) or by on-line gas chromatography-flame ionization detection (GC-FID) connected via a cryo-enrichment device. Sulphuric acid in the IfT-LFT was measured with a chemical ionization mass spectrometer, CIMS, in the same way as described for the NCAR experiments. The flow was set at 15 l min 21 (STP) resulting in a residence time of about 95 s. H 2 (99.999%, Messer) was directly added to the carrier gas flow. As the carrier gas we used high-purity synthetic air (99.9999999%, Linde and further purification with GateKeeper CE-500KF-O-4R, Aeronex). All gas flows were set by means of calibrated gas flow controllers (MKS 1259/1179) and the pressure in the tube was measured using a capacitive manometer (Baratron). NCAR. The reaction system consists of a glass flow tube with a movable stainless steel injector. A stream of hydrocarbon-free air (also called 'zero air' below) containing SO 2 and the alkene being studied is added to the glass flow tube. Ozone is produced inside the stainless steel injector by passing a flow of O 2 over a mercury Pen-Ray lamp located inside the injector. The ozone is then introduced into the main flow at the end of the injector. The gases then can react as the flow proceeds towards the exit of the tube, where the flow is sampled. The reaction time can be varied by either adjusting the amount of the main flow, or by changing the position of the end of the injector.
The flow tube consists of a 71-cm-long Pyrex tube connected to a 20.3-cm-long Pyrex Y. Both pieces have 3.38 cm i.d. and are connected via no. 40 O-ring joints sealed with a silicone O-ring. The Pyrex Y allows access of the movable injector into the flow tube as well as providing a means to introduce the main flow. The injector consisted of a thin walled 100-cm-long, 1.27-cm-i.d. stainless steel tube with one end sealed and 24 0.2-mm holes drilled radially 0.5 cm from the sealed end. Inside the injector is a mercury Pen-Ray lamp. Ozone was produced by passing a flow of O 2 over the lamp. The lamp was situated such that the end of the lamp is ,5 cm from the sealed end to prevent radiation from the lamp photolysing the main flow. The injector was inserted into the main glass flow tube by means of a no. 40 O-ring joint reduced to a 1.9-cm o.d. tube, and sealed via a Swagelok fitting modified to use silicone O-rings.
The zero air used in this system was produced by filtering ambient air via a zero air generator (Adco). The UHP (ultra-high purity) oxygen was provided by General Air and had a stated purity of 99.9999%. The SO 2 used was a 0.5% mixture of SO 2 in UHP N 2 and was provided by Scott Speciality Gases. Alkene mixtures were made 'in house' at NCAR, and their concentrations determined via gas chromatography. All flows into the flow tube were controlled by means of mass flow controllers (MKS).
The flow was sampled at the exit of the flow tube by a CIMS 13,14 (Chemical Ionization Mass Spectrometer) measuring H 2 SO 4 , a PTR-MS measuring various hydrocarbon products, and an O 3 analyser (2B Technologies). Field measurements. Field measurements were conducted at the SMEAR II field station in Finnish boreal forest 34 . The station (61u 519 N, 24u 179 E) is situated in southern Finland about 60 km northeast of the city of Tampere. The nearest village with some industrial activity is approximately 10 km away, and the nearest buildings are by a small lake 500 m away from the measurement station. The station is surrounded by a coniferous Scots pine dominated forest. Other major species include spruce and birch. All field measurements discussed in this Letter were done in a container located in a small open area surrounded by the forest. The SMEAR II station is equipped with extensive meteorological and gas and aerosol instrumentation.
The calculated concentration of sulphuric acid resulting from the reaction of SO 2 with either OH or X was obtained by assuming a steady state between the sulphuric acid production and its loss by condensation onto pre-existing aerosol particles. Detailed descriptions of calculations are given in Supplementary  Information