Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere

Abstract

Nucleation of aerosol particles from trace atmospheric vapours is thought to provide up to half of global cloud condensation nuclei. Aerosols can cause a net cooling of climate by scattering sunlight and by leading to smaller but more numerous cloud droplets, which makes clouds brighter and extends their lifetimes. Atmospheric aerosols derived from human activities are thought to have compensated for a large fraction of the warming caused by greenhouse gases. However, despite its importance for climate, atmospheric nucleation is poorly understood. Recently, it has been shown that sulphuric acid and ammonia cannot explain particle formation rates observed in the lower atmosphere. It is thought that amines may enhance nucleation, but until now there has been no direct evidence for amine ternary nucleation under atmospheric conditions. Here we use the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN and find that dimethylamine above three parts per trillion by volume can enhance particle formation rates more than 1,000-fold compared with ammonia, sufficient to account for the particle formation rates observed in the atmosphere. Molecular analysis of the clusters reveals that the faster nucleation is explained by a base-stabilization mechanism involving acid-amine pairs, which strongly decrease evaporation. The ion-induced contribution is generally small, reflecting the high stability of sulphuric acid-dimethylamine clusters and indicating that galactic cosmic rays exert only a small influence on their formation, except at low overall formation rates. Our experimental measurements are well reproduced by a dynamical model based on quantum chemical calculations of binding energies of molecular clusters, without any fitted parameters. These results show that, in regions of the atmosphere near amine sources, both amines and sulphur dioxide should be considered when assessing the impact of anthropogenic activities on particle formation.

Figure 1. Plot of experimental, atmospheric and theoretical nucleation rates against H₂SO₄ concentration.

Observations in the atmospheric boundary layer are indicated by small coloured squares^,,. The CLOUD data, recorded at 38% RH and 278 K, show J_gcr with only H₂SO₄, water and contaminants (<0.1 p.p.t.v. DMA and <2 p.p.t.v. NH₃) in the chamber (open black circles, curve 1); J_gcr with <0.1 p.p.t.v. DMA and 2–250 p.p.t.v. NH₃ (coloured triangles, curve 2); and J_n, J_gcr and J_π with 10 p.p.t.v. NH₃ and 3–5 p.p.t.v. DMA (coloured circles, curve 3), 5–13 p.p.t.v. DMA (coloured circles, curve 4) and 13–140 p.p.t.v. DMA (coloured circles, curve 5). The mixing ratios of NH₃ or DMA are indicated by a colour scale. The curves are drawn to guide the eye; the straight sections follow power laws, J ∝ [H₂SO₄]ⁿ, with fitted slopes n of 3.6 ± 0.5 (curve 1), 2.7 ± 0.1 (curve 2), 5.0 ± 0.8 (curve 3), 3.6 ± 0.2 (curve 4) and 3.7 ± 0.1 (curve 5). The flattening of curves 1 and 2 at higher [H₂SO₄] results from saturation of the ion production rate and also a decreasing contribution of ammonia ternary nucleation. The bars indicate 1σ total errors, although the overall factor 2 systematic scale uncertainty on [H₂SO₄] is not shown. Theoretical expectations (ACDC model) are indicated for H₂SO₄ nucleation with 10 p.p.t.v. NH₃ (dashed blue line and blue band) and for 10 p.p.t.v. DMA plus 10 p.p.t.v. NH₃ (dashed red line and orange band, assuming a sticking probability of 0.5 for neutral–neutral collisions and 1.0 for charged–neutral collisions). The bands correspond to the uncertainty range of the theory: +1 and −1 kcal mol⁻¹ binding energy (blue band) and sticking probabilities for neutral–neutral collisions between 0.1 and 1.0 (orange band), for the lower and upper limits, respectively. PowerPoint slide

Figure 2. Contribution of DMA and ions to amine ternary nucleation.

Measurements recorded at 38% RH and 278 K. a, Nucleation rates, J_n, J_gcr and J_π, as a function of DMA mixing ratio. b, Ion-induced fractions, J_iin/J_gcr and J_iin/J_π, as a function of J_gcr or J_π, at DMA = 3–140 p.p.t.v. In a, all nucleation rates are scaled to [H₂SO₄] = 2.0 × 10⁶ cm⁻³ (0.08 p.p.t.v.) using the fitted slopes in Fig. 1. The point at 0.1 p.p.t.v. DMA shows the mean projected J_gcr measurement at contaminant-level DMA and NH₃. The bars indicate 1σ total errors and include correlated systematic contributions. Theoretical expectations are shown by dashed red lines (sticking probability of 0.5 for neutral–neutral collisions and 1.0 for charged–neutral collisions) and uncertainties by orange bands (sticking probabilities for neutral–neutral collisions between 0.1 and 1.0). PowerPoint slide

Figure 3. Mass and molecular composition of charged clusters during a nucleation event with DMA.

Molecular composition of charged clusters measured by the APi-TOF for J_gcr = 1.2 cm⁻³ s⁻¹, 4.0 × 10⁶ cm⁻³ [H₂SO₄], 11 p.p.t.v. NH₃, 9.4 p.p.t.v. DMA, 38% RH and 278 K. a, Negative particles. b, Positive particles. Cluster mass/charge, m/z, defect (difference from integer m/z) is plotted against m/z; each circle represents a distinct molecular composition and its area represents counts s⁻¹. The labels (n, m) indicate the number of sulphuric acid (nSA) and DMA (mDMA) molecules in pure clusters of SA and DMA, including both neutral and charged species. The addition of a single SA (H₂SO₄) or DMA (C₂H₇N) molecule to any cluster displaces it on the plot by a vector distance indicated by the grey arrows in b. Red circles represent pure SA clusters; green circles are clusters containing SA and DMA alone; black circles contain ammonia in addition (only appearing in some clusters above m/z = 900); other clusters (mostly containing light organic contaminants) are grey circles. Water molecules evaporate rapidly in the APi-TOF and are not detected (see Supplementary Information). PowerPoint slide

Figure 4. Plot of neutral H₂SO₄ dimer against monomer concentrations before and after the addition of DMA.

Concentrations were measured by the CIMS in CLOUD without DMA (open circles) and with 3–140 p.p.t.v. DMA and 10 p.p.t.v. NH₃ (coloured circles), at 38% RH and 278 K. Ions are absent from the CLOUD chamber (the clearing field is on). The bars indicate 1σ counting errors. The fitted red curve through the DMA data shows a quadratic dependence on monomer concentration. The other curves show the expected neutral dimer concentrations for the binary H₂SO₄–H₂O system (short-dashed black line), for production in the CIMS ion source (dashed black line and grey uncertainty band) and for 10 p.p.t.v. DMA in the ACDC model, assuming 0.5 sticking probability (dashed red line). The orange band shows the model uncertainty range (sticking probabilities between 0.1 and 1.0). The brown curve indicates the upper limit of the dimer concentration calculated with the ACDC model, which is close to the kinetic limit (unit sticking probability and negligible evaporation). PowerPoint slide

Extended Data Figure 1. Schematic diagram of the CLOUD experiment at the CERN Proton Synchrotron.

Extended Data Figure 2. Theoretical dependence of amine ternary nucleation rates on RH.

Modelled neutral nucleation rates as a function of RH (left-hand scale) at 2.0 × 10⁶ cm⁻³ [H₂SO₄] and 278 K, and either 0.1 p.p.t.v. DMA (purple curve) or 10 p.p.t.v. DMA (red curve). The nucleation rates relative to their value at 38% RH are shown on the right-hand scale (dashed purple and red curves).

Extended Data Figure 3. Theoretical dependence of ammonia ternary and amine ternary nucleation rates on temperature.

Modelled GCR nucleation rates as a function of temperature (left-hand scale) at 2.0 × 10⁶ cm⁻³ [H₂SO₄] and either 2.0 × 10⁸ cm⁻³ [NH₃] (blue curve) or 2.0 × 10⁸ cm⁻³ [DMA] (red curve). (A concentration of 2.0 × 10⁸ cm⁻³ is equivalent to mixing ratios between 7.0 p.p.t.v. at 255 K and 8.2 p.p.t.v. at 300 K.) The sulphuric acid–DMA nucleation rate relative to the value at T = 278 K is shown on the right-hand scale (dashed red line). In the sulphuric acid–DMA system a sticking probability of 0.5 is assumed for all neutral–neutral collisions, and 1.0 for all charged–neutral collisions.

Extended Data Figure 4. Theoretical concentrations of negative, positive and neutral clusters during DMA ternary nucleation.

Modelled steady-state concentrations (mDMA versus nSA) at 4.0 × 10⁶ cm⁻³ [H₂SO₄], 10 p.p.t.v. DMA, 4 ion pairs cm⁻³ s⁻¹ and 278 K. a, Negative clusters. b, Positive clusters. c, Neutral clusters. A sticking probability of 0.5 is assumed for all neutral–neutral collisions and 1.0 for all charged–neutral collisions. The numbers below the centre of each circle show log₁₀C, where C (cm⁻³) is the cluster concentration (the threshold is 0.01 cm⁻³). The circle areas within each panel are proportional to C (with the exception of the DMA monomer in c).