Ion-induced nucleation of pure biogenic particles

Abstract

Atmospheric aerosols and their effect on clouds are thought to be important for anthropogenic radiative forcing of the climate, yet remain poorly understood. Globally, around half of cloud condensation nuclei originate from nucleation of atmospheric vapours. It is thought that sulfuric acid is essential to initiate most particle formation in the atmosphere, and that ions have a relatively minor role. Some laboratory studies, however, have reported organic particle formation without the intentional addition of sulfuric acid, although contamination could not be excluded. Here we present evidence for the formation of aerosol particles from highly oxidized biogenic vapours in the absence of sulfuric acid in a large chamber under atmospheric conditions. The highly oxygenated molecules (HOMs) are produced by ozonolysis of α-pinene. We find that ions from Galactic cosmic rays increase the nucleation rate by one to two orders of magnitude compared with neutral nucleation. Our experimental findings are supported by quantum chemical calculations of the cluster binding energies of representative HOMs. Ion-induced nucleation of pure organic particles constitutes a potentially widespread source of aerosol particles in terrestrial environments with low sulfuric acid pollution.

Figure 1. Evolution of HOMs and particles during a typical run.

a, Evolution of selected HOM monomers (E₁), dimers (E₂) and peroxy radicals (RO₂·) at 300 p.p.t.v. α-pinene, 33 p.p.b.v. O₃, zero H₂ or HONO, 38% relative humidity, 278 K and [H₂SO₄] < 5 × 10⁴ cm⁻³ (the same run as shown in Extended Data Fig. 4). The HOMs start to appear soon after the first injection of α-pinene into the chamber at 21:22, 23 October 2013. A HOM monomer is a highly oxygenated molecule derived from α-pinene (C₁₀H₁₆), and a HOM dimer is a covalently bound pair of monomers. Peroxy radicals are identified by an odd H number. The HOMs are charged with an ion in the CI-APi-TOF mass spectrometer. The systematic scale uncertainty on the HOM concentrations is +80%/−45%. b, Evolution of the particle number concentrations measured in the PSM1.8 (red curve) and CPC2.5 (blue curve) particle counters. The high-voltage clearing field (HVCF) was switched off at 05:16, 24 October 2013, marking the transition from neutral (ion-free) to GCR conditions in the chamber. A sharp increase in the rate of particle formation is seen, due to ion-induced nucleation of pure biogenic particles. However, no change occurs in the HOM concentrations (a), because these are predominantly neutral gas-phase molecules. The dotted and dashed curves in b show the PSM1.8 and CPC2.5 distributions, respectively, simulated for this run with the AEROCLOUD kinetic model, which is used to derive the experimental nucleation rates (see Methods). PowerPoint slide

Figure 2. Molecular composition and mass spectra of charged clusters during GCR nucleation events without sulfuric acid.

a, b, Cluster mass defect (difference from integer mass) versus m/z of negatively (a) and positively (b) charged clusters measured with the APi-TOF at 240 p.p.t.v. α-pinene, 34 p.p.b.v. O₃, zero H₂ or HONO, 38% relative humidity, 278 K and [H₂SO₄] below the detection limit (5 × 10⁴ cm⁻³). The values of J_gcr and total HOMs concentration are, respectively, 3.4 cm⁻³ s⁻¹ and 1.7 × 10⁷ cm⁻³ (a), and 3.3 cm⁻³ s⁻¹ and 2.4 × 10⁷ cm⁻³ (b). The mass bands are labelled according to the number of HOM monomer units in the cluster, E_n. Each circle represents a distinct molecular composition and its area represents the counts per second. The most highly oxidized compounds are located at the lower right-hand edge of each band. The dark blue circle represents ions; the light blue circle represents ions. Clusters with fully identified molecular composition are coloured according to their core ion: purple (), green (E⁻) or orange (). Grey circles are unidentified clusters. c, d, Mass spectra from the same events for negative (c) and positive (d) clusters up to m/z = 3,000 Th. A particle of 1.7-nm mobility diameter has a mass of about 1,200 Th. The ‘Nessie’ plot (d) shows that positive-ion-induced nucleation involves HOM dimers alone (E₁. clusters are not seen owing to instrument tuning). The decreasing signal amplitude at larger masses is due to the lower concentration and decreasing detection efficiency of the APi-TOF mass spectrometer (the efficiency versus m/z depends on the instrument tune and polarity). PowerPoint slide

Figure 3. Pure biogenic nucleation rates versus HOM concentration.

Neutral (J_n; circles) and GCR (J_gcr; triangles) nucleation rates versus total HOMs concentration (RO₂· + E₁ + E₂). The fraction of total HOMs that participate in nucleation (ELVOCs) is about 36% (ref. 21). The experimental conditions are 10–1,300 p.p.t.v. α-pinene (for measurements below J_1.7 = 10 cm⁻³ s⁻¹), 30–35 p.p.b.v. O₃, zero H₂ or HONO, 38% relative humidity, 278 K and <8 × 10⁵ cm⁻³ H₂SO₄. The colour scale shows [H₂SO₄]; purple and blue points correspond to contaminant level (below the detection threshold); other colours correspond to measurements after SO₂ was added to the chamber. The fitted curves show parameterizations (described in Methods) for J_n (dashed), J_gcr (solid) and ion-induced nucleation (J_iin = J_gcr − J_n; dot-dashed). The J_iin parameterization assumes that the nucleation rate falls steeply at HOM concentrations below the experimental measurements, following a similar slope to that for J_n. The bars indicate 1σ total errors, although the overall systematic scale uncertainty of +80%/−45% on the HOM concentration is not shown. PowerPoint slide

Figure 4. Experimental and atmospheric nucleation rates versus H₂SO₄ concentration.

CLOUD measurements of the neutral (J_n; circles), GCR (J_gcr; triangles) and π beam (J_π; diamonds) biogenic nucleation rates at 1.7 nm (J_1.7) versus [H₂SO₄]. The CLOUD experimental conditions are 10–1,300 p.p.t.v. α-pinene (for measurements below J_1.7 = 10 cm⁻³ s⁻¹), 25–35 p.p.b.v. O₃, zero H₂ or HONO, 20%–40% relative humidity and 278 K. Measurements below 1 × 10⁵ cm⁻³ for [H₂SO₄] are near to the detection limit of the CI-APi-TOF and should be considered as upper-estimates (to avoid overlap, some data points at the H₂SO₄ detection limit are displaced by up to 1 × 10⁴ cm⁻³). The total HOMs concentration from α-pinene oxidation is indicated by the colour scale. Observations of particle formation in the atmospheric boundary layer (mainly at 3-nm threshold size) are indicated by small grey circles^,,,. Following convention, the H₂SO₄ concentration refers to monomers alone; that is, H₂SO₄ bound in molecular clusters is not included. The kinetic upper limit on sulfuric acid nucleation is indicated by the blue band, which is bounded by dashed lines indicating J_1.7 and J₃. This band assumes the CLOUD condensation sink, which is comparable to that of a pristine atmosphere. The upper limit on J_iin from the GCR ion-pair production rate at ground level is indicated by the dot-dashed line. The bars indicate 1σ total errors, although the overall +50%/−33% systematic scale uncertainty on [H₂SO₄] is not shown. PowerPoint slide

Extended Data Figure 1. Small-ion mass spectra.

a, b, Composition of positive (a) and negative (b) small ions measured by the APi-TOF under GCR conditions and before adding any SO₂ to the chamber. The experimental conditions are zero α-pinene, 35 p.p.b.v. O₃, zero H₂ or HONO, 38% relative humidity, 278 K and [H₂SO₄] < 5 × 10⁴ cm⁻³. Collisions will transfer positive charge to contaminant molecules having the highest proton affinity (a), and negative charge to contaminant molecules with the lowest proton affinity, that is, highest gas-phase acidity (b). From molecular cluster measurements, the positive ions also include ammonium (), but its mass is below the set acceptance cut-off of the APi-TOF. c, The negative small-ion spectrum at [H₂SO₄] = 1.2 × 10⁵ cm⁻³, after adding 32 p.p.t.v. SO₂ to the chamber, showing that the dominant ions species shift from nitrate to sulfur-containing. The experimental conditions are 340 p.p.t.v. α-pinene, 35 p.p.b.v. O₃, zero H₂ or HONO, 38% relative humidity and 278 K. Water molecules evaporate rapidly from most hydrated ions in the APi-TOF and so are not detected.

Extended Data Figure 2. HOM yields versus α-pinene oxidation rates with O₃ and OH.

Total HOM mixing ratios versus α-pinene reaction rate with (i) O₃ plus OH· (ozone without H₂ scavenger; circles and solid line), (ii) O₃ alone (ozone with 0.1% H₂ scavenger; triangles and dashed line) and (iii) OH· alone (produced by ultraviolet photolysis of nitrous acid, HONO, in the absence of O₃; squares and dotted line). The yields are shown for total HOMs = RO₂· + E₁ + E₂. The experimental conditions are 38% relative humidity, 278 K and (i) 70–440 p.p.t.v. α-pinene, 21–35 p.p.b.v. O₃, zero H₂ or HONO, 0%–100% ultraviolet, (ii) 80–1,230 p.p.t.v. α-pinene, 21–35 p.p.b.v. O₃, 0.1% H₂, zero HONO, 0%–100% ultraviolet, and (iii) 840–910 p.p.t.v. α-pinene, zero O₃ or H₂, 0.5–3 p.p.b.v. HONO, 0%–100% ultraviolet. The bars indicate 1σ point-to-point errors. Overall systematic scale uncertainties of ±40% for the reaction rates and +80%/−45% for the HOM mixing ratios are not shown. The combined errors on the HOM molar yields for either ozonolysis or hydroxyl chemistry are +100%/−60% (±1σ).

Extended Data Figure 3. Proposed mechanism for the formation of the E₁ and E₂ surrogates via peroxy radical formation.

The proposed scheme for the formation of the ELVOC monomer (C₁₀H₁₄O₇) and dimer (C₂₀H₃₀O₁₄) surrogates selected for quantum chemical calculations (Extended Data Fig. 7) is based on recently established autoxidation mechanisms for a series of cycloalkane + O₃ systems^,,,,,. Peroxy radicals in the figure are indicated by a green label, E₁ by a red label and E₂ by a blue label. Addition of ozone to the double bond of α-pinene produces two carbonyl-substituted Criegee biradicals. The energy-rich Criegee biradical is either collisionally stabilized, or isomerizes via 1,4-H-shift to a vinylhydroperoxide (VHP), which then decomposes to yield an OH· and an alkenoxy radical. The alkenoxy radical reacts with O₂, leading to a peroxy radical, which is the potential precursor to a sequence of autoxidation reactions leading to the formation of HOMs. Here the peroxy radical C₁₀H₁₅O₄· is chosen as the starting point for HOM formation. The first intramolecular hydrogen abstraction is likely to take place at the aldehydic carbon from the opposite side of the peroxy group, although the rigid four-carbon-atom ring could hinder bending of the structure. For the cis configuration where the peroxy group and the aldehydic hydrogen are on the same side of the cyclobutyl ring, the 1,7-H shift rate is calculated to be 0.14 s⁻¹, which initiates the autoxidation chemistry on a fast timescale compared to the HOM lifetime resulting from loss to the CLOUD chamber walls (about 900 s). The resultant acylic radical undergoes rapid O₂ addition, leading to an -OOH functionalized peroxyacyl radical (C₁₀H₁₅O₆·). The second intramolecular hydrogen abstraction is expected to proceed at the carbon atom in the α position of the peroxyacyl group via 1,4-H isomerization. The resultant C₁₀H₁₅O₈· terminates by known reactions of peroxy radicals (HO₂· or RO₂· under the present experimental conditions), producing a spectrum of HOM monomers that includes the E₁ surrogate, C₁₀H₁₄O₇. The homogeneous recombination of two peroxy radicals via elimination of O₂ produces the covalently bound dimer C₂₀H₃₀O₁₄ chosen as the E₂ surrogate. Alternatively, C₁₀H₁₅O₈· can undergo further autoxidation, if sufficiently labile hydrogen atoms are available, leading to the observed closed-shell monomers with ≥9 O (Fig. 1). The self/cross-reaction of the C₁₀H₁₅O₄· peroxy radical produces an alkoxy radical, which decomposes rapidly, leading to a carbonyl-functionalized peroxy radical (C₁₀H₁₅O₅·). This peroxy radical is another potential starting structure for HOM formation. The carbon-ring-opening reaction pathway, while increasing the steric availability of the H atom, might be a slow step. The effective formation rate of the C=O-functionalized peroxy radical is calculated to be less than about 10⁻³ s⁻¹, which is comparable to its wall deposition rate. The timescale with respect to the subsequent autoxidation reaction, on the other hand, is expected to be of the order of seconds, by analogy with that for branched-chain peroxy radicals. The unbalanced sources and sinks potentially account for the low signals of peroxy radicals with odd oxygen numbers (for example, C₁₀H₁₅O₅, C₁₀H₁₅O₇ and C₁₀H₁₅O₉). The autoxidation process of C₁₀H₁₅O₅· is presumed to proceed by an autoxidative reaction pathway similar to that for C₁₀H₁₅O₄·, eventually leading to the spectrum of HOM monomers and dimers observed in the CLOUD chamber. Except for the autoxidation channel, all the peroxy radicals are still subject to well-established reactions such as R(′)O₂ + RO₂/HO₂, which are potentially important if the reaction rate is comparable to that for the autoxidation channel.

Extended Data Figure 4. Typical nucleation run sequence.

Example of a typical measurement sequence of the neutral and GCR nucleation rates as a function of coordinated universal time (utc), at zero H₂ or HONO, 38% relative humidity and 278 K. a, The run began at 21:22, 23 October 2013, by starting the α-pinene flow into the chamber to reach a chosen equilibrium value near 300 p.p.t.v., which produced an equilibrium total HOMs concentration near 2 × 10⁷ cm⁻³ (0.8 p.p.t.v.). b, Particles (red curve) formed at a slow rate in the chamber without ions present (‘neutral’ conditions). The clearing field high voltage (HV) was turned off at 05:16, 24 October 2013, and the subsequent presence of ions in the chamber from GCRs caused a sharp increase in the particle formation rate by about one order of magnitude (as seen by the increase in the gradient of the red curve). The nucleation rates are measured under constant gas conditions in the period before (J_n = 0.14 cm⁻³ s⁻¹) and after (J_gcr = 3.3 cm⁻³ s⁻¹) turning off the clearing field high voltage. c, d, Ion-induced nucleation is observed both for positive (c) and negative (d) charged particles, followed by rapid particle growth to sizes above 10 nm. e, The nucleated particles grew over a period of several hours to diameters approaching 50 nm, where they begin to constitute cloud condensation nuclei. A sharp increase in the formation rate of particles above the SMPS detection threshold of 5 nm can be seen when GCR ions are present. The colour scale in c–e indicates dN/dlog(D_p), where N (in cm⁻³) is the particle number concentration and D_p (in nm) is the particle diameter. The concentrations of ozone and contaminant H₂SO₄ were essentially constant during the run, which ended at 09:30 when the α-pinene flow to the chamber was turned off. The H₂SO₄ measurement near 5 × 10⁴ cm⁻³ corresponds to the instrumental background level of the CI-APi-TOF mass spectrometer and so represents an upper limit on the actual concentration. Further characteristics of this run can be seen in Fig. 1.

Extended Data Figure 5. Ion-induced nucleation event without H₂SO₄, measured in the NAIS.

a, b, Example of a nucleation event showing the growth versus time of positive (a) and negative (b) charged particles at 530 p.p.t.v. α-pinene, 35 p.p.b.v. O₃, zero H₂ or HONO, 3.4 × 10⁷ cm⁻³ HOM, 38% relative humidity, 278 K and [H₂SO₄] < 5 × 10⁴ cm⁻³. The colour scale shows the concentration of ions and charged particles. The clearing field high voltage was turned off at 06:48, marking the start of GCR ionization conditions in the chamber, and the α-pinene flow into the chamber was stopped at 10:52. Ion-induced nucleation can be seen for positive and negative charged particles, followed by rapid growth to sizes above 10 nm. Ion–ion recombination progressively neutralizes the charged particles as they grow, but some reappear at larger sizes, owing to diffusion charging.

Extended Data Figure 6. Nucleation rates versus relative humidity.

Neutral (J_n; circles) and GCR (J_gcr; triangles) nucleation rates versus relative humidity. The experimental conditions are 250–800 p.p.t.v. α-pinene, 30–35 p.p.b.v. O₃, zero H₂ or HONO, (1.1−2.9) × 10⁷ cm⁻³ HOM, 278 K and (0.5−1.5) × 10⁵ cm⁻³ H₂SO₄. All measurements have been corrected to the same total HOMs concentration (2.05 × 10⁷ cm⁻³) using the curves shown in Fig. 3. The bars indicate 1σ total errors, although these are not shown in the x direction because they are smaller than the symbols.

Extended Data Figure 7. Surrogate molecules chosen for quantum chemical calculations.

a, b, Structures of the surrogate molecules chosen for quantum chemical calculations to represent the ELVOC monomer, E₁, C₁₀H₁₄O₇ (a) and the covalently bound dimer, E₂, C₂₀H₃₀O₁₄ (b). Grey spheres represent carbon atoms, red are oxygen atoms and white are hydrogen atoms. We show their proposed formation mechanisms in Extended Data Fig. 3.

Extended Data Figure 8. Relationship between cluster formation energies and equilibrium evaporation/condensation rates.

Estimated ELVOC vapour mixing ratios versus the ΔG_{278 K} at which the condensation and evaporation rates of the cluster at 278 K are in equilibrium^,. For example, a formation free energy of −15.3 kcal mol⁻¹ corresponds to equal rates for particle evaporation and vapour condensation at 278 K and 1 p.p.t.v. ELVOC vapour mixing ratio (2.6 × 10⁷cm⁻³). The evaporation rate increases by a factor of 10 for each 1.27 kcal mol⁻¹ reduction of the cluster formation energy.