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A large source of low-volatility secondary organic aerosol

Abstract

Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol1,2, which is known to affect the Earth’s radiation balance by scattering solar radiation and by acting as cloud condensation nuclei3. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non-volatile organic vapours4,5,6, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies2. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere–aerosol–climate feedback mechanisms6,7,8, and the air quality and climate effects of biogenic emissions generally.

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Figure 1: Mass spectrum of ELVOCs produced by α-pinene ozonolysis.
Figure 2: ELVOC chamber experiments.
Figure 3: Comparison of measured particle growth rates with those predicted from measured ELVOC concentrations in Hyytiälä forest.
Figure 4: The importance of precursor vapours for aerosol growth at different sizes.

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Acknowledgements

M.E. was supported by the Emil Aaltonen foundation; J.A.T. was supported by the US Department of Energy, Office of Science (DE-SC0006867). This work was supported by the ERC Advanced Grant EU-FP7-ATMNUCLE (project no. 227463), the EU-FP7 project PEGASOS (project no. 265148), the Academy of Finland (project no. 251427 and 266388) and by the Academy of Finland Center of Excellence programme (project no. 1118615). We thank M. Kajos, S. Schallhart and T. Ruuskanen for measurement support, O. Kupiainen for collision rate calculations, and the tofTools team for analysis tools for mass spectra.

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Contributions

M.E., J.A.T., E.K., I.P., M. Springer, F.R., R.T., B.L., F.L.-H., S.A., I.-H.A., M.R., T.J., J. Kangasluoma, T.B. and J.W. conducted the data collection and analysis. H.J. and M.C. provided data analysis tools. J. Kontkanen and T.N. analysed the ambient data. M.E., J.A.T., T.K., L.B.N., S.J., H.G.K. and T.F.M. provided model calculations and developed the formation mechanism. M.E. and J.A.T. wrote the manuscript. M. Sipilä, M.D.M., T.P., A.W., V.-M.K., M.K., D.W. and T.F.M. did data interpretation and editing of the manuscript. All authors discussed the results and commented on the paper.

Corresponding author

Correspondence to Mikael Ehn.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Comparisons of ELVOC spectra.

a, b, Comparison of CI-APi-TOF spectra from Hyytiälä and JPAC, during night (a) and day (b). The JPAC spectra (light red) show ELVOCs clustered with 15NO3 whereas Hyytiälä data are clustered with 14NO3, and therefore identical ELVOCs will be found shifted by 1 Th. The features of the night-time spectrum measured in Hyytiälä are very similar to those in the JPAC α-pinene ozonolysis (2 p.p.b. α-pinene, 17 p.p.b. ozone) spectrum (a), with the same pattern of peaks, as well as most major single peaks in good agreement. The day-time Hyytiälä ELVOC spectrum is also replicated quite well, but here using conditions where α-pinene is mainly oxidized by OH, in the presence of NOx (0.7 p.p.b. α-pinene, 20 p.p.b. ozone, 107 cm−3 OH, 30 p.p.b. NOx, 3 p.p.b. NO). The patterns overlap, and although some single peaks are only found in one of the spectra, the major peaks such as the organic nitrates at 339 Th and 371 Th (C10H15NO9 and C10H15NO11) in the Hyytiälä spectrum are found at 340 Th and 372 Th in the JPAC spectrum, as expected. The fraction of α-pinene reacting with ozone in b is probably higher in Hyytiälä than under our chamber conditions. The use of labelled nitrate enables us to assign the N-atom to the ELVOCs and not to clustered HNO3. The peak at 402 Th in the Hyytiälä spectrum in b, on the other hand, corresponds to the same ELVOC as 339 Th, with one additional HNO3 adduct. The corresponding peak in the JPAC spectrum is now shifted to 404 Th, due to the presence of two 15N-atoms. c, d, ELVOC peak shifts due to isotopically labelled ozone during α-pinene ozonolysis measurements in the Tropos flow tube at 25% relative humidity, using 80 p.p.b. α-pinene. Ozone concentrations, both labelled and non-labelled, were roughly 15 p.p.b. c.p.s., counts per second.

Extended Data Figure 2 OH oxidation experiments.

a, Formation of ELVOCs by hydroxyl radical (OH) oxidation of α-pinene. As in Fig. 2a, the yield is estimated on the basis of the amount of monoterpene (MT) reacted. The α-pinene reactions with ozone and the ELVOCs produced from ozonolysis have been accounted for, and they were always <40% of the total ELVOCs and <10% of the reacting MT. At higher OH concentrations, the apparent yield increases, suggesting that multiple OH oxidation steps may produce ELVOCs. At the lowest OH concentrations used (dark blue data points), the ELVOC yield is about 1%, and we take this as an upper limit for the prompt formation of ELVOCs by OH oxidation of α-pinene. If the influence of ozone was not accounted for, the maximum OH yield estimate would still be <1.5%. Therefore, the OH formed during ozonolysis, when the O3/OH ratio was 10–1,000 times higher than in the experiments depicted here (5–45 p.p.b. ozone, 8 × 106 to 4 × 108 cm−3 OH, 0.1–17 p.p.b. α-pinene), will only make a minor contribution to ELVOC formation. This was further supported by only a minor effect on ELVOCs observed when 30 p.p.m. CO was added to scavenge OH in the chamber during ozonolysis experiments. b, Gas phase oxidation products versus particle surface area during OH oxidation of α-pinene (0.7 p.p.b. α-pinene, 53 p.p.b. ozone, 108 cm−3 OH). The amount of ammonium sulphate seed aerosol in the chamber was varied, and ELVOCs decreased as the particle surface area (that is, condensation sink) increased. At the same time, the gas phase concentrations of semi-volatile oxidation products like pinic and pinonic acid measured by the UW HR-ToF-CIMS remained largely unperturbed. This is further supported by the filter sampling, which showed that the SOA explained by pinic acid always remained <3%. The fraction of these vapours entering the particle phase is thus either very small, or the equilibration process extremely slow (on the order of hours), and as we see a clear decrease in ELVOCs, and this loss explains the majority of the measured SOA, the observed SVOC behaviour is consistent with our other findings. During the ozonolysis seed addition experiments reported in Fig. 2, the UW HR-ToF-CIMS was focused on measuring particle phase composition with the filters, and therefore reliable data for the SVOCs can only be presented for OH oxidation experiments. However, the condensation dynamics are not expected to change considerably between the two cases. c, Formation of ELVOCs from pinonic acid oxidation by hydroxyl radicals (OH). Pinonic acid was added to the chamber and once stable (at 13:50), the ultraviolet lights were turned on and ELVOC concentrations increased rapidly, in accordance with prompt formation of ELVOCs from the pinonic acid + OH reactions. The ELVOCs quickly decrease after this, due to a large condensation sink produced by the formed aerosol particles. The ELVOCs increase again as the amount of aerosol surface decreases, in line with the expected ELVOC behaviour. The starting ozone concentration was 80 p.p.b., and this concentration dropped to 40 p.p.b. after the ultraviolet light was turned on. Once the light had been turned on, the resulting OH concentrations were estimated to be 2 × 108 cm−3. The apparent increase before ultraviolet lights were turned on is due to the low time resolution of the data. All concentrations stayed stable until the ultraviolet lights were turned on.

Extended Data Figure 3 Elemental ratios measured by the AMS during ammonium sulphate seed addition.

The period corresponds to that shown in Fig. 2b. At 09:00 ammonium sulphate was added, and there is a slight decrease in the O/C ratio, consistent with more SVOCs being able to condense at higher SOA loading. Before the seed addition, the data are noisier owing to the relatively low SOA loading, but during this period we expect only ELVOCs to be able to condense onto the particles. The calculated O/C and H/C ratios for the gas phase ELVOCs are shown in shaded pink and grey, respectively. Again, at higher loadings the values start to diverge, consistent with an increased contribution from SVOCs to SOA mass. The O/C and H/C were calculated according to an improved elemental analysis methodology (M.C. et al., manuscript in preparation). The light orange dots depict the SOA O/C calculated using the old methodology commonly used until now, yielding values that are lower by roughly 0.15–0.2. These values are consistent with previous α-pinene ozonolysis experiments16 as well as average O/C values measured in Hyytiälä76. It should, however, be noted that the ability of the AMS to provide accurate O/C is dependent on the specific compounds, and together with possible particle-phase evolution, the gas and particle phase O/C is not expected to match perfectly even if completely error-free measurements were possible.

Extended Data Figure 4 Nano-CN and particle dependence on ELVOCs.

a, The concentration of particles smaller than 1.5 nm in diameter, as measured by the particle size magnifier, PSM (PSM<1.5nm), correlates extremely well with ELVOC concentrations. The black squares correspond to the same period as plotted in Fig. 2a; coloured squares were measured during SO2 addition and correspond to the same period as in Fig. 2d. These data show that PSM<1.5nm is independent of H2SO4, in line with the PSM directly detecting some fraction of the ELVOCs. The slope of the relationship is consistent with reported PSM<1.5nm detection efficiencies of <1% for organic ions of this size44. The linear correlation, which is independent of H2SO4 concentrations, links the large ELVOC molecules formed by α-pinene oxidation and measured by mass spectrometric methods to nano-CN24. During these experiments, ELVOC monomers and dimers showed a clear correlation, and therefore no conclusions could be drawn about which ELVOCs were actually detected by the PSM. be, PSM<1.5nm dependence on different ELVOCs during NOx additions. The data correspond to the same time period as in Extended Data Fig. 10b. We find that monomers (b), peroxy radicals (c) and organic nitrates (d), all with masses around 300 Da, are still found in the chamber after PSM<1.5nm has reached zero. However, the dimers, that is, the largest ELVOCs observed, with masses around 500 Da, show a linear dependence with PSM<1.5nm (e), indicating that ELVOC dimers can act as nano-CN. This suggests that dimers may also act as condensation nuclei for vapours in the atmosphere under certain conditions. fi, Particles larger than 3 nm as a function of different combinations of ELVOC and H2SO4 concentrations. Panel f shows the combination plotted in Fig. 2d, which gives a near-linear dependence. Also [ELVOC] × [H2SO4] gives a relatively linear slope (h), though low-H2SO4 points become more offset. The agreement becomes much worse when neglecting either compound completely (g, i). Bases such as ammonia and amines may influence the particle formation in addition to H2SO4 and ELVOCs, but any background levels of these compounds in JPAC probably stayed constant enough during these experiments to not influence the observed slopes.

Extended Data Figure 5 Structures and ELVOC yields of different VOCs.

a, Structures of the main compounds studied in this work. b, ELVOC molar yields of selected VOCs during ozonolysis. The reported yields are subject to the absolute ELVOC concentration uncertainty (±50%), but the relative concentrations are more precise. No OH scavenger was used during these experiments.

Extended Data Figure 6 Instrumental and chamber details influencing ELVOC yield calculations.

a, Estimated uncertainty sources, and corresponding ranges, in the quantification of ELVOCs using the CI-APi-TOF. The transmission efficiency (TE) of ions in the CI-APi-TOF can vary depending on the ions’ mass/charge. If the transmission in the mass range where ELVOCs are measured (300–700 Th) is different from that for the nitrate ions (62–188 Th) used for normalizing, the ratio A (see Methods section ‘ELVOC detection and quantification using the CI-APi-TOF’) will be influenced. We estimate a maximum influence of a factor of 2 from this source. The nominal residence time for ion–molecule collisions in the charger is 200 ms, but if the mixing of ions into the sample flow is not instantaneous, or either the sample or sheath flows are offset, the residence time will change. We estimate a maximum error in the residence time of 50 ms. b, Time series of three OH-produced ELVOC molecules during switching of ultraviolet lights on and off in JPAC. When the ultraviolet lights were turned off, the OH concentration quickly dropped, and we consequently observed the decay of OH-produced ELVOCs. Once sufficient decay had been observed, the ultraviolet lights were turned back on, and both OH and the plotted ELVOCs quickly increased to their original levels. This experiment was repeated three times. c, The concentrations are plotted relative to the time when the ultraviolet was switched off. The exponential ELVOC decay corresponded to lifetimes (τ) of 75–90 s. The much longer decay times corresponding to ELVOC losses to particles (condensation sink, CS, grey dotted line) and flush out (grey dashed line) are also plotted. d, Steady-state chamber conditions during experiments presented in Figs 1 and 2. See Methods for details of parameters that were kept constant.

Extended Data Figure 7 Kinetic box model details.

a, The key reactions used to model the ELVOC system, with species coloured according to Extended Data Fig. 10. ΣRO2 indicates all non-ELVOC peroxy radical species (RO2, R′O2, R′′O2 and R′′′O2). b, Parameters used in the kinetic model for runs A and B, corresponding to results in Extended Data Fig. 10d and e, respectively. *Suggested by MCM; †for simplicity, OH reactions were excluded from the model, and although these do not produce a significant fraction of ELVOCs, their contribution to RO2 is non-negligible. Thus, our model will underpredict RO2, which is partly compensated by a relatively high RELVOCO2 + RO2 rate coefficient; ‡Half that suggested by MCM.

Extended Data Figure 8 Particle formation and growth event on 26–27 March 2011 in Hyytiälä.

The colour-coded data in this figure depict the aerosol particle concentration as a function of particle diameter and time. A high concentration, as signified by the dark red colour, of small particles appears around noon on 26 March, and this particle mode grows throughout the night. The nucleation mode geometric mean diameters obtained by log-normal fits are shown by black dots.

Extended Data Figure 9 Schematic illustrations of ELVOC formation.

a, Internal hydrogen abstraction by an RO2 (‘H-shift’), followed by oxygen addition at the alkyl radical site, forming a more oxidized peroxy radical. Depending on the exact structure of the molecule, this new RO2 can perform subsequent H-shifts/O2-additions to increase the oxygen content even further. b, c, Simplified diagram of ELVOC formation. The general form is shown in b, and a specific example pathway from α-pinene (C10H16) ozonolysis in c. For clarity, all reactions not leading to ELVOCs are omitted. The first reaction yields a peroxy radical (b: RO2, c: C10H15O4), that can undergo several fast H-shift reactions followed by O2-addition, resulting in more oxidized peroxy radicals (b: RELVOCO2, c: C10H15O10). RELVOCO2 can react through well-established pathways with either HO2, RO2 or NO, all of which can form ELVOCs. The relative abundance of these species as well as the rates of the unimolecular decomposition reactions determines the overall ELVOC mass yield, while not necessarily affecting the molar yield. Only reactions of RELVOCO2 with other RO2 can form dimers (RELVOCOOR), whereas organic nitrates (RELVOCONO2) only form in reactions with NO. All compounds in bold font were directly measured.

Extended Data Figure 10 Responses of ELVOC sub-groups to varying chamber conditions.

ac, Measured ELVOC behaviour during low-NOx (a) and high-NOx (b, c) experiments. Data in a are from the same experiments as shown in Fig. 2a, conditions in b varied within the following ranges: 0–70 p.p.b. NOx, 80–90 p.p.b. ozone, 6–7 p.p.b. α-pinene. The RO2 radicals increase roughly as the square root of the α-pinene reaction rate, whereas the closed shell molecules show a more linear dependence, as expected. In b, α-pinene and ozone are kept constant, while adding different amounts of NOx. Panel c contains the same data as b, but with the data plotted against the fitted NO concentration acquired from b. d–f, Modelled ELVOC behaviour during low-NOx (d) and high-NOx (e, f) conditions, simulating the experiments plotted in ac. For the monomers and dimers, one-fifth (20%) of the total products is plotted, for a better comparison with a–c, where only specific molecules are shown.

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Ehn, M., Thornton, J., Kleist, E. et al. A large source of low-volatility secondary organic aerosol. Nature 506, 476–479 (2014). https://doi.org/10.1038/nature13032

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