The primary vapour responsible for atmospheric nucleation is thought to be sulphuric acid (H 2 SO 4 ), derived from the oxidation of sulphur dioxide. However, peak daytime H 2 SO 4 concentrations in the atmospheric boundary layer are about 106 to 3 × 107 cm−3 (0.04–1.2 parts per trillion by volume (p.p.t.v.)), which results in negligible binary homogeneous nucleation of H 2 SO 4 –H 2 O (ref. 3). Additional species such as ammonia or amines4,5 are therefore necessary to stabilize the embryonic clusters and decrease evaporation. However, ammonia cannot account for particle formation rates observed in the boundary layer3 and, despite numerous field and laboratory studies6,7,8,9,10,11,12,13,14,15,16, amine ternary nucleation has not yet been observed under atmospheric conditions. Amine emissions are dominated by anthropogenic activities (mainly animal husbandry), but about 30% of emissions are thought to arise from the breakdown of organic matter in the oceans, and 20% from biomass burning and soil8,17. Atmospheric measurements of gas-phase amines are sparse, but typical values range between negligible and a few tens of p.p.t.v. per amine species17,18,19,20.

Here we report results from the CLOUD experiment at CERN (for experimental details see Methods, Extended Data Fig. 1 and Supplementary Information). The data were obtained during three campaigns at the CERN Proton Synchrotron between October 2010 and November 2012, and comprise measurements of sulphuric acid–amine nucleation at atmospheric concentrations. Dimethylamine (DMA; C 2 H 7 N) was selected for this study because it is expected to have cluster binding energies representative of other light alkyl amines4.

Nucleation rates J were measured under neutral (J n ), galactic cosmic ray (J gcr ) and π+ beam (J π ) conditions, corresponding to ion-pair concentrations of about 0, 650 and 3,000 cm−3, respectively. Both J gcr and J π comprise the sum of neutral and ion-induced nucleation rates, whereas J n measures the neutral rate alone. Figure 1 shows the nucleation rates at 1.7 nm mobility diameter (1.4 nm mass diameter) as a function of [H 2 SO 4 ] for ‘binary’ (H 2 SO 4 –H 2 O), ammonia ternary (H 2 SO 4 –NH 3 –H 2 O) and amine ternary (H 2 SO 4 –DMA–H 2 O) nucleation at 278 K and 38% relative humidity (RH). Here ‘binary’ includes previous measurements made in the presence of NH 3 and DMA contaminants3, estimated from later campaigns to be <2 p.p.t.v. and <0.1 p.p.t.v., respectively, for the conditions of ref. 3. Nucleation rates with 5 p.p.t.v. DMA are enhanced more than 1,000-fold compared with 250 p.p.t.v. ammonia (Fig. 1). Additional DMA up to 140 p.p.t.v. results in a less than threefold further rate increase, indicating that amine levels of about 5 p.p.t.v. are sufficient to reach the rate limit for amine ternary nucleation under atmospheric conditions ([H 2 SO 4 ] ≤ 3 × 107 cm−3, or 1.2 p.p.t.v.).

Figure 1: Plot of experimental, atmospheric and theoretical nucleation rates against H 2 SO 4 concentration. Observations in the atmospheric boundary layer are indicated by small coloured squares21,22,23. The CLOUD data, recorded at 38% RH and 278 K, show J gcr with only H 2 SO 4 , water and contaminants (<0.1 p.p.t.v. DMA and <2 p.p.t.v. NH 3 ) 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. NH 3 (coloured triangles, curve 2); and J n , J gcr and J π with 10 p.p.t.v. NH 3 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 NH 3 or DMA are indicated by a colour scale. The curves are drawn to guide the eye; the straight sections follow power laws, J ∝ [H 2 SO 4 ]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 [H 2 SO 4 ] 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 [H 2 SO 4 ] is not shown. Theoretical expectations (ACDC model) are indicated for H 2 SO 4 nucleation with 10 p.p.t.v. NH 3 (dashed blue line and blue band) and for 10 p.p.t.v. DMA plus 10 p.p.t.v. NH 3 (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 Full size image

The amine ternary nucleation rates pass through the band of atmospheric observations (Fig. 1). However, the latter reveal distinct regional differences, with some environments showing nucleation rates both above and below the amine limit (boreal forest and mountain21,22), whereas others are only below the limit (agricultural, livestock, industrial and urban21,23). This suggests that nucleation in different regions of the boundary layer may be controlled by different ternary vapours. In regions where amines are likely to be present (livestock farming and urban), the atmospheric rates are compatible with amine nucleation. However, the atmospheric data show considerable variability, probably resulting from variations in ternary vapour concentrations and particle coagulation sinks. When growth rates are low, the measured nucleation rates are highly sensitive to particle coagulation sinks, which influence particle losses both above and below the quoted formation threshold sizes. Losses below the threshold size are uncorrected, implying higher variability in the atmosphere, where conditions are less well defined than in the laboratory.

Figure 1 shows the theoretical expectations for NH 3 (blue band) and DMA ternary nucleation (orange band), obtained with the Atmospheric Cluster Dynamics Code model (ACDC)24 (see Methods and Supplementary Information for further details). The model uses cluster evaporation and fragmentation rates calculated from quantum chemistry, with no fitted parameters25. The agreement is quite good, although the model predicts somewhat higher DMA ternary nucleation rates than measured experimentally. Part of this discrepancy is due to the smaller size—and hence higher formation rate—of the modelled clusters (up to four acid and four base molecules per cluster, corresponding to mobility diameters of 1.2–1.4 nm). Computational studies (see Supplementary Information and Extended Data Figs 2 and 3) indicate that DMA ternary nucleation is rather insensitive to RH or temperature, reflecting the strong acid–base binding. The experimental measurements obtained at 38% RH and 278 K may therefore be considered representative of a wide range of boundary layer conditions.

Plots of the nucleation rates J n , J gcr and J π against DMA mixing ratio are shown in Fig. 2a. Here, all measurements have been scaled to [H 2 SO 4 ] = 2.0 × 106 cm−3 using the fitted slopes, n, from Fig. 1. The addition of only 5 p.p.t.v. DMA enhances the nucleation rate of sulphuric acid particles by more than six orders of magnitude, but the addition of further DMA up to 140 p.p.t.v. produces a negligible further increase. The measured neutral, galactic cosmic ray (GCR) and beam nucleation rates are indistinguishable, within experimental uncertainties. However, a more sensitive determination of the ion-induced nucleation rate, , is obtained from direct ion measurements with the neutral cluster and air ion spectrometer. The ion-induced fractions, J iin /J gcr or J iin /J π (Fig. 2b), are found to average about 20% at 0.5 cm−3 s−1 but grow in relative importance as the total nucleation rate decreases. This indicates that the influence of galactic cosmic rays on the nucleation of sulphuric acid–amine particles is only significant at low overall formation rates. No difference is measured for the ion-induced fraction under GCR or beam conditions (Fig. 2b). This follows, because the ion–ion recombination lifetimes are below 10 min and are comparable to the monomer arrival rate on the cluster (one molecule per 12 min for H 2 SO 4 ·HSO 4 − at 106 cm−3 [H 2 SO 4 ]). Consequently, although the ion pair concentration is larger for beam conditions, it is compensated for by a shorter ion lifetime, which decreases the time available for nucleation before the ion cluster is neutralized.

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 [H 2 SO 4 ] = 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 NH 3 . 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 Full size image

Figure 3 shows the molecular composition of nucleating charged clusters in the presence of DMA for negative ions (Fig. 3a) and positive ions (Fig. 3b), measured with atmospheric-pressure interface time-of-flight mass spectrometers (APi-TOFs). The predominant negatively charged clusters include an HSO 4 − or HSO 5 − ion. The latter is deprotonated peroxysulphuric acid, whose presence varies with the ozone concentration in the chamber (it is absent when no ozone is present). We found no indication that the nucleation rates are sensitive to the relative contribution of these ion species. Contaminant NO 3 − ions are also detected, but at much lower concentrations. The predominant positively charged clusters contain a protonated DMA ion, DMA·H+ (C 2 H 7 N·H+), in association with H 2 SO 4 and DMA. The remaining positive ions are largely protonated light organic contaminants, mostly also nitrogen-containing.

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 [H 2 SO 4 ], 11 p.p.t.v. NH 3 , 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 (H 2 SO 4 ) or DMA (C 2 H 7 N) 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 Full size image

Amine ternary nucleation is observed to proceed by the same base-stabilization mechanism as that found previously for ammonia ternary nucleation3. We will use the label (n, m) to indicate the number of sulphuric acid (nSA) and DMA (mDMA) molecules in pure SA·DMA clusters, where n and m include both neutral and charged species. Negatively charged nucleation (Fig. 3a) proceeds as follows. The first step is dimer (2, 0) formation: HSO 4 −·H 2 SO 4 (for simplicity the ‘HSO 4 −’ ion implies either HSO 4 − or HSO 5 −). This constitutes an acid–base pair because HSO 4 − is a Lewis base (an electron pair donor). Consequently the first negatively charged cluster to which DMA can bind to form an acid–base pair is the acid trimer. The most abundant acid trimer contains two DMA molecules (3, 2). Thereafter, each additional acid molecule is stabilized by one additional DMA molecule, following a sequence of acid–base pairs: (3, 2) → (4, 3) → (5, 4) → (n, n − 1). Our calculations suggest that the process involves mainly the accretion of SA·DMA (dimethylaminium bisulphate) clusters, but it may also involve the stepwise addition of an SA molecule followed by a DMA molecule. Beyond (7, 6) clusters, there is evidence for further neutralization of the acid by additional DMA (partial formation of dimethylaminium sulphate). Positively charged nucleation (Fig. 3b) proceeds similarly. Here DMA·H+ is a Lewis acid and so binds only weakly with H 2 SO 4 . Hence the first positively charged cluster is a DMA·H+ ion together with a single SA·DMA acid–base pair (1, 2). Thereafter, the cluster grows by the accretion of SA·DMA pairs, exactly as seen for negatively charged clusters. No DMA·H+ monomer is detected because its mass-to-charge ratio (m/z), 46, is below the APi-TOF cutoff, as configured for these experiments.

Because both HSO 4 − and DMA are Lewis bases, each can form an acid–base pair with H 2 SO 4 . In fact HSO 4 − is the stronger base, as demonstrated by its much stronger binding energy with H 2 SO 4 (Supplementary Table 1)4. The only fundamental difference is that not more than one HSO 4 − ion can be present in the cluster because of electrostatic repulsion. So, although the APi-TOF measures only charged clusters in the CLOUD chamber, this suggests that neutral nucleation proceeds by the same mechanism, namely the initial formation of an acid–base pair (SA·DMA)—equivalent to the acid–base pair (SA·HSO 4 −) seen in charged nucleation (Fig. 3a)—and subsequently the accretion of additional SA·DMA pairs. This is also indicated by the Atmospheric Cluster Dynamics Code (ACDC) model (see Supplementary Information and Extended Data Fig. 4).

There is direct experimental evidence to support this picture of the neutral nucleation mechanism. Figure 4 shows a plot of the concentration of the neutral acid dimer against that of the neutral acid monomer, measured with the chemical ionization mass spectrometer (CIMS) before and after the addition of DMA, when the clearing field was present (implying that there were only neutral clusters in the CLOUD chamber). We infer from the observed absence of DMA on the negatively charged monomer or dimer (Fig. 3a) that, after charging in the CIMS, clusters containing one H 2 SO 4 molecule will be detected as DMA-free charged monomers, and clusters containing two H 2 SO 4 molecules will be detected as DMA-free charged dimers—regardless of whether or not they were originally clustered with DMA. Before adding any DMA, the dimer concentrations are consistent with the expected production in the CIMS ion source. However, with 5 p.p.t.v. DMA or more, the dimer concentrations are about six orders of magnitude higher than those expected for a pure binary system26. The concentration of neutral acid dimer with DMA approaches the kinetic limit, indicating highly stable clusters with negligible evaporation, and supporting the neutral nucleation mechanism inferred above.

Figure 4: Plot of neutral H 2 SO 4 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. NH 3 (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 H 2 SO 4 –H 2 O system (short-dashed black line)26, 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 Full size image

A previous experiment27 measured unexpectedly high dimer concentrations in a laminar flow tube and concluded that a stabilizing contaminant must be present, although none was measured. This was proposed27 to explain the high ‘binary’ nucleation rates previously measured in ref. 28 in the same flow tube. Another experiment15 measured high dimer formation rates linked to amine mixing ratios of about 1 part per billion by volume and above. Our results directly link high concentrations of neutral H 2 SO 4 dimer with amines at atmospheric levels.

The results reported here show that nucleation in the atmospheric boundary layer is highly sensitive to trace amine levels of only a few p.p.t.v. Sulphuric acid–amine nucleation is found to proceed by the same base-stabilization mechanism as that previously observed for ammonia, in which each additional acid molecule in the cluster is stabilized by one (or occasionally, two) base molecules3. However, the acid–base pairs that sulphuric acid forms with amines are more tightly bound than with ammonia, resulting in cluster formation rates that approach the kinetic limit. Little increase is seen above 5 p.p.t.v. DMA, indicating that nucleation at atmospheric H 2 SO 4 concentrations (below 3 × 107 cm−3 or 1.2 p.p.t.v.) is then limited by the availability of H 2 SO 4 and not that of DMA. Our experimental rate and molecular measurements are well reproduced by a dynamical model based on quantum chemical calculations of binding energies of molecular clusters.

Although measurements of ambient gas-phase amines are rare, mixing ratios of a few p.p.t.v. in the continental boundary layer have been reported17,19,20, suggesting that sulphuric acid–amine nucleation is likely to be an important atmospheric process. However, atmospheric observations indicate both the presence10,11,16 and the absence22 of a significant amine fraction in newly formed particles, which suggests considerable variability of ambient amine levels. Although amines are volatile vapours, our measurements show that sulphate particles constitute an almost perfect sink (negligible evaporation). However, unlike H 2 SO 4 , amine vapours are directly emitted from sources in their chemically active form and so they will be localized to source regions, with highly variable concentrations that depend on ambient sulphate particle sinks and OH radical levels (the DMA oxidation lifetime is about 4 h at 106 cm−3 [OH]). Amines can therefore explain only a part of atmospheric nucleation. Indeed, our measurements leave open the possibility that nucleation may also proceed with other atmospheric vapours, such as highly oxidized organic species of very low volatility. In such cases, extremely low amine concentrations may still enhance nucleation by forming stable acid–base pairs with some fraction of the sulphuric acid molecules in an embryonic cluster (constituting at least four-component nucleation).

The ion-induced contribution to amine ternary nucleation is generally small, except at low overall formation rates. Ions can enhance nucleation either by an increased collision rate between a charged cluster and polar molecules (such as H 2 SO 4 or H 2 SO 4 ·DMA) or by an increased cluster binding energy (and hence decreased evaporation rate). Because neutral clusters of H 2 SO 4 and DMA are highly stable, charge offers little competitive advantage. Taken together with previous CLOUD measurements3, this suggests that ions can be significant in atmospheric particle formation provided that the associated neutral particles have appreciable evaporation and provided that the overall nucleation rates are low and below the ion-pair production rate.