Search for: All records

When the phenomena of smog and acid deposition were first recognized, it was largely gas phase chemists and photochemists who leapt into the fray to untangle the sources and chemistry involved. Over time, the importance of multiphase chemistry was recognized, as illustrated in a dramatic manner with the discovery of the Antarctic ozone hole which is driven by heterogeneous chemistry on polar stratospheric clouds. Since then, it has become clear that multiphase chemistry is central to both the lower and upper atmosphere and that this deeply intertwines interactions between the gas and condensed phases in the atmosphere. As a result, it can be argued that multiphase atmospheric chemistry begins … and ends… with gases. This paper is based on the 2018 Polanyi Medal award presentation at the 25th International Symposium on Gas Kinetics & Related Phenomena and traces research carried out in the author's laboratory on multiphase chemistry over a number of decades. While a great deal has been learned about these processes, they remain one of the areas of greatest uncertainty in understanding atmospheric composition, air quality, chemistry, and climate change.

 

Both ambient and laboratory-generated particles can have a surface composition different from the bulk, but there are currently few analytical techniques available to probe these differences. Easy ambient sonic-spray ionization mass spectrometry (EASI-MS) was applied to solid, laboratory-generated particles with core–shell morphologies formed from a variety of dicarboxylic acids. The soft ionization facilitated parent peak detection for the two compounds, from which the depth probed could be determined from the relative signal intensities. Two different configurations of a custom-made nebulizer are reported that yield different probe depths. In the “orthogonal mode,” with the nebulizer ∼10 centimeters away from the particle stream and at a 90° angle to the MS inlet, evaporation of the nebulizer droplets forms ions before interaction with the particles. The probe depth for orthogonal mode EASI-MS is shown to be 2–4 nm in these particle systems. In the “droplet mode”, the nebulizer and particle streams are in close proximity to each other and the MS inlet so that the particles interact with charged liquid droplets. This configuration resulted in full dissolution of the particles and gives particle composition similar to that from collection on filters and extraction of the particles (bulk). These studies establish that EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles. 

Secondary organic aerosol (SOA) particles are ubiquitous in air and understanding the mechanism by which they grow is critical for predicting their effects on visibility and climate. The uptake of three organic nitrates into semi-solid SOA particles formed by α-pinene ozonolysis either with or without an OH scavenger was investigated. Four types of experiments are presented here. In Series A, uptake of the selected organic nitrates (2-ethylhexyl nitrate (2EHN); β-hydroxypropyl nitrate (HPN); β-hydroxyhexyl nitrate (HHN)) into impacted SOA particles was interrogated by attenuated total reflectance (ATR)-FTIR. In this case, equilibrium was reached and partition coefficients ( K SOA = [–ONO 2 ] SOA /[–ONO 2 ] air ) were measured to be K 2EHN = (3.2–11) × 10 4 , K HPN = (4.4–5.4) × 10 5 , and K HHN = (4.9–9.0) × 10 6 . In Series B, SOA particles were exposed on-the-fly to gas phase organic nitrates for comparison to Series A, and uptake of organic nitrates was quantified by HR-ToF-AMS analysis, which yielded similar results. In Series C (AMS) and D (ATR-FTIR), each organic nitrate was incorporated into the SOA as the particles formed and grew. The incorporation of the RONO 2 was much larger in Series C and D ( during growth ), exceeding equilibrium values determined in Series A and B ( after growth ). This suggests that enhanced uptake of organic nitrates during SOA formation and growth is due to a kinetically controlled “burying” mechanism, rather than equilibrium partitioning. This has important implications for understanding SOA formation and growth under conditions where the particles are semi-solid, which is central to accurately predicting properties for such SOA. 

Understanding impacts of secondary organic aerosol (SOA) in air requires a molecular-level understanding of particle growth via interactions between gases and particle surfaces. The interactions of three gaseous organic nitrates with selected organic substrates were measured at 296 K using attenuated total reflection Fourier transform infrared spectroscopy. The organic substrates included a long chain alkane (triacontane, TC), a keto-acid (pinonic acid, PA), an amorphous ester oligomer (poly(ethylene adipate) di-hydroxy terminated, PEA), and laboratory-generated SOA from α-pinene ozonolysis. There was no uptake of the organic nitrates on the non-polar TC substrate, but significant uptake occurred on PEA, PA, and α-pinene SOA. Net uptake coefficients ( γ ) at the shortest reaction times accessible in these experiments ranged from 3 × 10 −4 to 9 × 10 −6 and partition coefficients ( K ) from 1 × 10 7 to 9 × 10 4 . Trends in γ did not quantitatively follow trends in K , suggesting that the intermolecular forces involved in gas–surface interactions are not the same as those in the bulk, which is supported by theoretical calculations. Kinetic modeling showed that nitrates diffused throughout the organic films over several minutes, and that the bulk diffusion coefficients evolved as uptake/desorption occurred. A plasticizing effect occurred upon incorporation of the organic nitrates, whereas desorption caused decreases in diffusion coefficients in the upper layers, suggesting a crusting effect. Accurate predictions of particle growth in the atmosphere will require knowledge of uptake coefficients, which are likely to be several orders of magnitude less than one, and of the intermolecular interactions of gases with particle surfaces as well as with the particle bulk. 

Real-time in situ mass spectrometry analysis of airborne particles is important in a number of applications, including exposure studies in ambient air, industrial settings, and assessing impacts on visibility and climate. However, obtaining molecular and 3D structural information is more challenging, especially for heterogeneous solid or semi-solid particles. We report a study of extractive electrospray ionization mass spectrometry (EESI-MS) for the analysis of solid particles with an organic coating. The goal is to elucidate how much of the overall particle content is sampled, and the sensitivity of this technique to the surface layers. It is shown that for NaNO3 particles coated with glutaric acid (GA), very little of the solid NaNO3 core is sampled compared to the GA coating, while for GA particles coated with malonic acid (MA), significant signals from both the MA coating and the GA core are observed. However, conventional ESI-MS of the same samples collected on a Teflon filter and extracted detects much more core material compared to EESI-MS in both cases. These results show that for the experimental conditions used here, EESI-MS does not sample the entire particle, but instead is more sensitive to surface layers. Separate experiments on single component particles of NaNO3, glutaric acid or citric acid show that there must be a kinetics limitation to dissolution that is important in determining EESI-MS sensitivity. We propose a new mechanism of EESI solvent droplet interaction with solid particles that is consistent with the experimental observations. In conjunction with previous EESI-MS studies of organic particles, these results suggest EESI does not necessarily sample the entire particle when solid, and that not only solubility but also surface energies and the kinetics of dissolution play an important role. 

While atmospheric particles affect health, visibility and climate, the details governing their formation and growth are poorly understood on a molecular level. A simple model system for understanding the interactions between the gas and particle phases is the reaction of bases with acids, both of which are common constituents of atmospheric particles. In the present study, uptake coefficients for the reactions of gas phase ammonia, methylamine, ethylamine, dimethylamine and trimethylamine with a series of solid dicarboxylic acids (diacids) were measured at 296 ± 1 K using a Knudsen cell interfaced to a quadrupole mass spectrometer. The uptake coefficients ( γ ) for a given amine follow an odd–even trend in carbon number of the diacid, and are larger for the odd carbon diacids. Values range from γ = 0.4 for ethylamine on malonic acid (C3) to less than ∼10 −6 for ammonia and all amines on adipic (C6) and pimelic (C7) acids. Basicity or structure of the amines/ammonia alone do not explain the effect of the base on uptake. The crystal structures of the diacids also play a key role, which is especially evident for malonic acid (C3). Evaporation of aqueous mixtures of amines/ammonia with odd carbon diacids show the formation of ionic liquids (ILs) or in some cases, metastable ILs that revert back to a stable solid salt upon complete evaporation of water. The trends with amine and diacid structure provide insight into the mechanisms of uptake and molecular interactions that control it, including the formation of ionic liquid layers in some cases. The diversity in the kinetics and mechanisms involved in this relatively simple model system illustrate the challenges in accurately representing such processes in atmospheric models.