New Particle Formation from the Vapor Phase: From Barrier-Controlled Nucleation to the Collisional Limit.

Studies of vapor phase nucleation have largely been restricted to one of two limiting cases-nucleation controlled by a substantial free energy barrier or the collisional limit where the barrier is negligible. For weakly bound systems, exploring the transition between these regimes has been an experimental challenge, and how nucleation evolves in this transition remains an open question. We overcome these limitations by combining complementary Laval expansion experiments, providing new particle formation data for carbon dioxide over a uniquely broad range of conditions. Our experimental data together with a kinetic model using rate constants from high-level quantum chemical calculations provide a comprehensive picture of new particle formation as nucleation transitions from a barrier-dominated process to the collisional limit.

How volatile components catalyze vapor nucleation.

Gas phase nucleation is a ubiquitous phenomenon in planetary atmospheres and technical processes, yet our understanding of it is far from complete. In particular, the enhancement of nucleation by the addition of a more volatile, weakly interacting gaseous species to a nucleating vapor has escaped molecular-level experimental investigation. Here, we use a specially designed experiment to directly measure the chemical composition and the concentration of nucleating clusters in various binary CO2-containing vapors. Our analysis suggests that CO2 essentially catalyzes nucleation of the low vapor pressure component through the formation of transient, hetero-molecular clusters and thus provides alternative pathways for nucleation to proceed more efficiently. This work opens up new avenues for the quantitative assessment of nucleation mechanisms involving transient species in multicomponent vapors.

Carbon dioxide and propane nucleation: the emergence of a nucleation barrier.

We investigate homogeneous gas-phase nucleation of CO2 and C3H8 in the uniform postnozzle flow of Laval expansions in the temperature range of 31.2 K to 62.9 K and 32.0 K to 42.1 K, respectively. Time-dependent cluster size distributions are recorded with mass spectrometry after single-photon ionization with vacuum ultraviolet light. Net monomer-cluster forward rate constants and experimental nucleation rates J are retrieved from the time-dependent cluster size distributions. The comparison of experimental enhancement factors derived from these net forward rates with calculated enhancement factors provides an indication for the transition from barrier-limited to barrierless nucleation. Our data suggest such a transition for CO2, but not for C3H8. The values of J lie in the range from 9 × 1014 cm-3 s-1 to 6 × 1015 cm-3 s-1. For CO2, the comparison of J with a modeled nucleation rate JQM based on quantum chemical calculations of the free energy barrier also hints at a transition from barrierless condensation to barrier-limited nucleation. Furthermore, we address the influence of the carrier gas pressure on the nucleation rate.

Extraction of monomer-cluster association rate constants from water nucleation data measured at extreme supersaturations.

We utilize recently reported data for water nucleation in the uniform postnozzle flow of pulsed Laval expansions to derive water monomer association rates with clusters. The nucleation experiments are carried out at flow temperatures of 87.0 K and 47.5 K and supersaturations of lnS ∼ 41 and 104, respectively. The cluster size distributions are measured at different nucleation times by mass spectrometry coupled with soft single-photon ionization at 13.8 eV. The soft ionization method ensures that the original cluster size distributions are largely preserved upon ionization. We compare our experimental data with predictions by a kinetic model using rate coefficients from a previous ab initio calculation with a master equation approach. The prediction and our experimental data differ, in particular, at the temperature of 87.0 K. Assuming cluster evaporation to be negligible, we derive association rate coefficients between monomer and clusters purely based on our experimental data. The derived dimerization rate lies 2-3 orders of magnitude below the gas kinetic collision limit and agrees with the aforementioned ab initio calculation. Other than the dimerization rate, however, the derived rate coefficients between monomer and cluster j (j ≥ 3) are on the same order of magnitude as the kinetic collision limit. A kinetic model based on these results confirms that coagulation is indeed negligible in our experiments. We further present a detailed analysis of the uncertainties in our experiments and methodology for rate derivation and specify the dependency of the derived rates on uncertainties in monomer and cluster concentrations.

Infrared Spectroscopy and Mass Spectrometry of CO2 Clusters during Nucleation and Growth.

CO2 clusters with 2 to 4300 molecules are characterized with mass spectrometry and infrared spectroscopy in the uniform postnozzle flow of Laval expansions at constant temperatures of ∼29 and ∼43 K. The mass spectra provide independent, accurate information on the cluster size distributions and through magic numbers also on cluster structures. The experimental results are complemented with force field, quantum chemical, and vibrational exciton calculations. We find our data to be consistent with predominantly fcc cuboctahedral structures for clusters with more than about 50 molecules. Infrared spectra of cluster size distributions with average sizes above 140-220 molecules are completely dominated by the features from the larger cuboctahedral clusters in the distribution. For very small clusters, exciton simulations predict a pronounced broadening of the infrared band as soon as the average cluster size exceeds about five molecules. The nucleation behavior of CO2 under the present conditions is found to be barrierless in agreement with similar trends previously observed for other compounds at very high supersaturation.

Water nucleation at extreme supersaturation.

We report water cluster formation in the uniform postnozzle flow of a Laval nozzle at low temperatures of 87.0 and 47.5 K and high supersaturations of lnS ∼ 41 and 104, respectively. Cluster size distributions were measured after soft single-photon ionization at 13.8 eV with mass spectrometry. Critical cluster sizes were determined from cluster size distributions recorded as a function of increasing supersaturation, resulting in critical sizes of 6-15 and 1, respectively. Comparison with previous data for propane and toluene reveals a systematic trend in the nucleation behavior, i.e., a change from a steplike increase to a gradual increase of the maximum cluster size with increasing supersaturation. Experimental nucleation rates of 5 · 1015 cm-3 s-1 and 2 · 1015 cm-3 s-1 for lnS ∼ 41 and 104, respectively, were retrieved from cluster size distributions recorded as a function of nucleation time. These lie 2-3 orders of magnitude below the gas kinetic collision limit assuming unit sticking probability, but they agree very well with a recent prediction by a master equation model based on ab initio transition state theory. The experimental observations are consistent with barrierless growth at 47.5 K, but they hint at a more complex nucleation behavior for the measurement at 87.0 K.

Toluene Cluster Formation in Laval Expansions: Nucleation and Growth.

Toluene cluster formation has been investigated in the postnozzle flows of Laval expansions at flow temperatures between ∼48 and 73 K, toluene number concentrations between ∼1013 and 1015 cm-3, and for growth times of up to ∼170 µs. The clusters were detected by soft ionization mass spectrometry to ensure minimum cluster fragmentation upon ionization. The optimum conditions were achieved with single-photon ionization using vacuum ultraviolet (VUV) photons of 13.3 eV energy and low fluences. The nature of the onset of toluene cluster formation hints at barrierless nucleation, which seems a likely scenario for the high supersaturations (>1019) of the present experiments. This contrasts with the onset behavior observed for propane in earlier studies, which suggested nucleation in the presence of a barrier. Subsequent cluster growth has been studied as a function of the growth time for various toluene partial pressures. Size-resolved growth data have been recorded for all cluster sizes from the dimer to aggregates composed of ∼2400 monomers (∼4.4 nm in size), revealing general trends in the growth behavior. The current experiments provide systematic size- and time-resolved data on cluster formation at high supersaturations as a possible benchmark for the understanding of cluster formation under such conditions.