Hygroscopic properties of aerosol particles in the northeastern Atlantic during ACE-2

Measurements of the hygroscopic properties of sub-micrometer atmospheric aerosol particles were performed with hygroscopic tandem differential mobility analysers (H-TDMA) at 5 sites in the subtropical north-eastern Atlantic during the second Aerosol Characterization Experiment (ACE-2) from 16 June to 25 July 1997. Four of the sites were in the marine boundary layer and one was, at least occasionally, in the lower free troposphere. The hygroscopic diameter growth factors of individual aerosol particles in the dry particle diameter range 10−440 nm were generally measured for changes in relative humidity (RH) from <10% to 90%. In the marine boundary layer, growth factors at 90% RH were dependent on location, air mass type and particle size. The data was dominated by a unimodal growth distribution of more-hygroscopic particles, although a bimodal growth distribution including less-hygroscopic particles was observed at times, most often in the more polluted air masses. In clean marine air masses the more-hygroscopic growth factors ranged from about 1.6 to 1.8 with a consistent increase in growth factor with increasing particle size. There was also a tendency toward higher growth factors as sodium to sulphate molar ratio increased with increasing sea-salt contribution at higher wind speeds. During outbreaks of European pollution in the ACE-2 region, the growth factors of the largest particles were reduced, but only slightly. Growth factors at all sizes in both clean and polluted air masses were markedly lower at the Sagres, Portugal site due to more proximate continental influences. The frequency of occurrence of less-hygroscopic particles with a growth factor of ca. 1.15 was greatest during polluted conditions at Sagres. The free tropospheric 50 nm particles were predominately less-hygroscopic, with an intermediate growth factor of 1.4, but more-hygroscopic particles with growth factors of about 1.6 were also frequent. While these particles probably originate from within the marine boundary layer, the less-hygroscopic particles are probably more characteristic of lower free tropospheric air masses. For those occasions when measurements were made at 90% and an intermediate 60% or 70% RH, the growth factor G(RH) of the more-hygroscopic particles could be modelled empirically by a power law expression. For the ubiquitous more-hygroscopic particles, the expressions G(RH)=(1-RH/100)-0.210 for 50 nm Aitken mode particles and G(RH)=(1-RH/100)-0.233 for 166 nm accumulation mode particles are recommended for clean marine air masses in the north-eastern Atlantic within the range 0<RH<95%, and for wind speeds for which the local sea-salt production is small (<ca. 8 m s-1)

Optical bulk-boundary dichotomy in a quantum spin Hall insulator

The bulk-boundary correspondence is a key concept in topological quantum materials. For instance, a quantum spin Hall insulator features a bulk insulating gap with gapless helical boundary states protected by the underlying Z2 topology. However, the bulk-boundary dichotomy and distinction are rarely explored in optical experiments, which can provide unique information about topological charge carriers beyond transport and electronic spectroscopy techniques. Here, we utilize mid-infrared absorption micro-spectroscopy and pump-probe micro-spectroscopy to elucidate the bulk-boundary optical responses of Bi4Br4, a recently discovered room-temperature quantum spin Hall insulator. Benefiting from the low energy of infrared photons and the high spatial resolution, we unambiguously resolve a strong absorption from the boundary states while the bulk absorption is suppressed by its insulating gap. Moreover, the boundary absorption exhibits a strong polarization anisotropy, consistent with the one-dimensional nature of the topological boundary states. Our infrared pump-probe microscopy further measures a substantially increased carrier lifetime for the boundary states, which reaches one nanosecond scale. The nanosecond lifetime is about one to two orders longer than that of most topological materials and can be attributed to the linear dispersion nature of the helical boundary states. Our findings demonstrate the optical bulk-boundary dichotomy in a topological material and provide a proof-of-principal methodology for studying topological optoelectronics.Comment: 26 pages, 4 figure

Hygroscopic Properties of Atmospheric Aerosol Particles in Various Environments

Atmospheric aerosol particles affect our living environment in many ways. Their effects are influenced by the interaction between the aerosol particles and the ubiquitous water vapour. The number-size distribution is a primary important parameter for aerosol particles and can be measured using the differential mobility particle sizer (DMPS). Programs have been developed to invert DMPS measurement data to actual aerosol number-size distributions and further fit it to multimodal lognormal distributions. The hygroscopic tandem differential mobility analyser (H-TDMA) is the essential instrument used in studying aerosol hygroscopic properties. The H-TDMA has been developed continuously regarding measurement control and data acquisition system, as well as in the subsequent data interpretation and quality assurance. Four field experiments have been performed in this work in order to study hygroscopic properties of atmospheric aerosol particles. In a polluted continental environment, the aerosol hygroscopic growth was observed in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. In a remote marine background environment, ship-based hygroscopic measurements were carried out during the Arctic Ocean Expedition in 1996. In a moderately polluted marine environment, six H-TDMA instruments were operated simultaneously by four research groups at five sites in the subtropical north-eastern Atlantic, during the second Aerosol Characterization Experiment in 1997. More recently, hygroscopic growth measurements were performed in the Amazon rain forest, a remote tropical continental site, during the first Cooperative LBA (Large-scale Biosphere-Atmosphere) Airborne Regional Experiment in 1998. The measured hygroscopic growth data can be used in an aerosol hygroscopic growth model, together with measurements of aerosol size distributions and chemical composition. Closure studies were performed to verify whether the independent measurements were consistent, and to what extent the model was able to describe aerosol hygroscopic growth and aerosol-cloud interaction. The sub-micrometer atmospheric aerosol particles observed by several research groups in various environments exhibit a modal hygroscopic structure, indicating that atmospheric aerosol particles are, to some extent, externally mixed regarding their hygroscopic properties and hence also chemically