Co-condensation of nonane and D2O in a supersonic nozzle

Understanding the Formation and Growth of New Atmospheric Particles at the Molecular Level through Laboratory Molecular Beam Experiments

Atmospheric new particle formation (NPF), which exerts comprehensive implications for climate, air quality and human health, has received extensive attention. From molecule to cluster is the initial and most important stage of the nucleation process of atmospheric new particles. However, due to the complexity of the nucleation process and limitations of experimental characterization techniques, there is still a great uncertainty in understanding the nucleation mechanism at the molecular level. Laboratory-based molecular beam methods can experimentally implement the generation and growth of typical atmospheric gas-phase nucleation precursors to nanoscale clusters, characterize the key physical and chemical properties of clusters such as structure and composition, and obtain a series of their physicochemical parameters, including association rate coefficients, electron binding energy, pickup cross section and pickup probability and so on. These parameters can quantitatively illustrate the physicochemical properties of the cluster, and evaluate the effect of different gas phase nucleation precursors on the formation and growth of atmospheric new particles. We review the present literatures on atmospheric cluster formation and reaction employing the experimental method of laboratory molecular beam. The experimental apparatuses were classified and summarized from three aspects of cluster generation, growth and detection processes. Focus of this review is on the properties of nucleation clusters involving different precursor molecules of water, sulfuric acid, nitric acid and NxOy, respectively. We hope this review will provide a deep insight for effects of cluster physicochemical properties on nucleation, and reveal the formation and growth mechanism of atmospheric new particle at the molecular level.

Mutual independence of water and n-nonane nucleation at low temperatures

The interaction of water with different substances in the earth's atmosphere lies at the heart of many processes that influence our climate. However, it is still unclear how different species interact with water on the molecular level and in which ways this interaction contributes to the water vapor phase transition. Here, we report the first measurements of water-nonane binary nucleation in the 50-110 K temperature range, along with unary nucleation data of both. The time-dependent cluster size distribution in a uniform post-nozzle flow was measured by time-of-flight mass spectrometry coupled with single-photon ionization. From these data, we extract experimental rates and rate constants for both nucleation and cluster growth. The observed mass spectra of water/nonane clusters are not or only slightly affected by the introduction of the other vapor, and the formation of mixed clusters was not observed during nucleation of the mixed vapor. Additionally, the nucleation rate of either substance is not much affected by the presence (or absence) of the other species, i.e., the nucleation of water and nonane proceeds independently, indicating that hetero-molecular clusters do not play a role during nucleation. Only at the lowest temperature of our experiment (i.e., 51 K) do the measurements suggest that interspecies interaction slows water cluster growth. The findings here are in contrast to our earlier work in which we showed that vapor components in other mixtures, e.g., CO₂ and toluene/H₂O, can interact to promote nucleation and cluster growth in a similar temperature range.

Vapor-phase nucleation of n-pentane, n-hexane, and n-heptane: Critical cluster properties

The first and second nucleation theorems provide a way to determine the molecular content and excess internal energies of critical clusters, which rely solely on experimental nucleation rates measured at constant temperatures and supersaturations, respectively. Here, we report the size n^* and excess internal energy E_x(n^*) of n-pentane, n-hexane, and n-heptane critical clusters when particles form under the highly supersaturated conditions present in supersonic expansions. In summary, critical clusters contain from ∼2 to ∼11 molecules and exhibit the expected increase in the critical cluster size with increasing temperature and decreasing supersaturation. Surprisingly, the n^* values for all three alkanes appear to lie along a single line when plotted as a function of supersaturation. Within the framework of the capillarity approximation, the excess internal energies determined for the n-heptane critical clusters formed under the low temperature (∼150 K) conditions in our supersonic nozzle are reasonably consistent with those determined under higher temperature (∼250 K) conditions in the thermal diffusion cloud chamber by Rudek et al. [J. Chem. Phys. 105, 4707 (1996)].

An effective experimental method and apparatus for unsteady water vapor condensation investigation in high speed expansion flow

This paper presents an effective experimental method and system for mechanism study of unsteady water vapor condensation encountered in high speed expansion flow. We proposed an experimental method and designed a simplified expansion system to fulfill the study. Tunable diode laser absorption spectroscopy (TDLAS) and light sheet technique are integrated in the system to monitor the unsteady condensation process in a high speed expansion flow generated by the expansion system. Two near infrared water vapor absorption transitions (1395.0 nm and 1409.27 nm) and one near infrared methane absorption transition (1653.73 nm) are applied in the TDLAS measurement to measure the transient flow parameters during the condensation process. Using the experimental method, time dependent condensation processes are monitored with different expansion time scales. The light sheet results visually reveal the condensation phenomena during the expansion process, while TDLAS results quantitatively follow the condensation process. The experimental results are compared with computational fluid dynamics simulations and a good agreement between them is observed, which indicates that the presented experimental method and system is effective in investigating unsteady water vapor condensation in high speed expansion flow.

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.

CO2 condensation onto alkanes: unconventional cases of heterogeneous nucleation

The classical picture invoked for heterogeneous nucleation is frequently that of a liquid condensing onto an immiscible solid particle. Here, we examine heterogeneous nucleation of CO2 onto particles comprised of n-pentane or n-hexane under conditions where CO2 should be a solid and the seed particles may be liquid or solid. Although CO2 condensed under all but one of the six conditions investigated, these experiments do not easily fit into the framework of standard heterogeneous nucleation experiments. Rather they explore unconventional regimes of heterogeneous nucleation in which the state of the seed particle may both affect whether deposition can proceed, and, in turn, be influenced by the presence of the condensing species. The work complements the earlier work of Tanimura et al. [RSC Adv., 2015, 5, 105537-105550] that investigated CO2 condensation onto ice nanoparticles, by using seed particles comprised of non-polar compounds that form and freeze under conditions where CO2 is already supersaturated with respect to the solid ice. In some cases, the conditions for seed formation approach the limit of homogeneous CO2 nucleation. Vibrational spectroscopy measurements help pinpoint where CO2 starts to condense. Furthermore, these IR measurements suggest that the n-alkanes never freeze in the presence of CO2, even if the temperatures are well below those required for them to freeze when CO2 is absent. Over the temperature range 65 < T/K < 140, the conditions corresponding to the onset of CO2 heterogeneous nucleation on pre-existing seed particle almost all lie very close to the extrapolated vapor-liquid equilibrium line of CO2 for a broad range of seed materials.