43 research outputs found
Water activity as the determinant for homogeneous ice nucleation in aqueous solutions
No full textKoop T, Luo BP, Tsias A, Peter T. Water activity as the determinant for homogeneous ice nucleation in aqueous solutions. NATURE. 2000;406(6796):611-614.The unique properties of water in the supercooled (metastable) state are not fully understood(1). In particular, the effects of solutes and mechanical pressure on the kinetics of the liquid-to-solid phase transition of supercooled water and aqueous solutions to ice have remained unresolved. Here we show from experimental data that the homogeneous nucleation of ice from supercooled aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution-that is, the ratio between the water vapour pressures of the solution and of pure water under the same conditions. In addition, we show that the presence of solutes and the application of pressure have a very similar effect on ice nucleation. We present a thermodynamic theory for homogeneous ice nucleation, which expresses the nucleation rate coefficient as a function of water activity and pressure. Recent observations from clouds containing ice are in good agreement with our theory and our results should help to overcome one of the main weaknesses of numerical models of the atmosphere, the formulation of cloud processes
Corrigendum to "A thermodynamic model of mixed organic–inorganic aerosols to predict activity coefficients" published in Atmos. Chem. Phys., 8, 4559–4593, 2008
No full textNo abstract available
Measurements of Surrogate Respiratory Sessile Droplet pH and Implications for Exhaled Respiratory Aerosol and Airborne Disease Transmission
No full textRespiratory aerosol pH has been proposed as a key factor driving the infectivity loss of SARS-CoV-2 viruses and influenza A virus in exhaled aerosols, thus affecting the airborne transmission of respiratory diseases. Sodium bicarbonate acts as a principal buffer in biological systems, regulating blood pH and the CO2 balance between gas and liquid phases. Upon exhalation, changes in gas-phase conditions alter aerosol composition and pH. Despite Raman spectroscopy being used to quantify atmospherically relevant aerosol pH, the kinetics of CO2 partitioning and pH variability in respiratory droplets remain poorly understood. In this paper, a method to investigate the HCO3–/CO32– equilibrium in a surrogate respiratory fluid system within sessile droplets is proposed to elucidate the pH evolution of an exhaled respiratory aerosol. The enzymatic catalysis of CO2 hydration and H2CO3 dehydration is explored. Experimental results were used to benchmark the ResAM model, which simulates respiratory aerosol droplet thermodynamics and pH evolution. Simulated pH evolution profiles of picoliter droplets show size independence. Simulations for both sessile droplets and respiratory aerosols show that carbonic anhydrase significantly increases the rate of pH increase, and gas-phase CO2 levels are important for determining the final droplet pH. Consequences for understanding the aerobiological pathways for virus transmission are considered.<br/
Time evolution of steep diffusion fronts in highly viscous aerosol particles measured with Mie resonance spectroscopy
No full textISSN:0021-9606ISSN:1089-7690ISSN:1089-769
A novel model to predict the physical state of atmospheric H2SO4/NH3/H2O aerosol particles
No full textISSN:1680-7324ISSN:1680-7375ISSN:1680-737
SAGE II Measurements of Stratospheric Aerosol Properties at Non-Volcanic Levels
No full textSince 2000, stratospheric aerosol levels have been relatively stable and at the lowest levels observed in the historical record. Given the challenges of making satellite measurements of aerosol properties at these levels, we have performed a study of the sensitivity of the product to the major components of the processing algorithm used in the production of SAGE II aerosol extinction measurements and the retrieval process that produces the operational surface area density (SAD) product. We find that the aerosol extinction measurements, particularly at 1020 nm, remain robust and reliable at the observed aerosol levels. On the other hand, during background periods, the SAD operational product has an uncertainty of at least a factor of 2 during due to the lack of sensitivity to particles with radii less than 100 nm
Internal mixing of the organic aerosol by gas phase diffusion of semivolatile organic compounds
No full textThis paper shows that most of the so far identified constituents of the tropospheric organic particulate matter belong to a semivolatile fraction for which gas phase diffusion in the lower troposphere is sufficiently fast to establish thermodynamic equilibrium between aerosol particles. For the first time analytical expressions for this process are derived. Inspection of vapor pressure data of a series of organic substances allows a rough estimate for which substances this mixing process must be considered. As general benchmarks we conclude that for typical aerosol radii between 0.1 and 1 µm this mixing process is efficient at 25°C for polar species with molecular weights up to 200 and for non-polar species up to 320. At −10°C, these values are shifted to 150 for polar and to 270 for non-polar substances. The extent of mixing of this semivolatile fraction is governed by equilibrium thermodynamics, leading to a selectively, though not completely, internally mixed aerosol. The internal mixing leads to a systematic depression of melting and deliquescence points of organic and mixed organic/inorganic aerosols, thus leading to an aerosol population in the lower troposphere which is predominantly liquid
Microspectroscopy reveals dust derived apatite grains in acidic, highly weathered Hawaiian soils
No full textDust deposition is an important source of phosphorus P to many ecosystems. However, there is little evidence of dust derived P containing minerals in soils. Here we studied P forms along a well described climatic gradient on Hawaii, which is also a dust deposition gradient. Soil mineralogy and soil P forms from six sites along the climatic gradient were analyzed with bulk X ray diffraction and P K edge X ray absorption near edge structure and microscale X ray fluorescence, P K edge X ray absorption near edge structure, and Raman analysis methods. In the wettest soils, apatite grains ranging from 5 to 30 m in size were co located at the micro scale with quartz, a known continental dust indicator suggesting recent atmospheric deposition. In addition to co location with quartz, further evidence of dust derived P included backward trajectory modeling indicating that dust particles could be brought to Hawaii from the major global dust loading areas in central Asia and northern Africa. Although it is not certain whether the individual observed apatite grains were derived from long distance transport of dust, or from local dust sources such as volcanic ash or windblown fertilizer, these observations offer direct evidence that P containing minerals have reached surface layers of highly weathered grassland soils through atmospheric depositio
Oxalic acid as a heterogeneous ice nucleus in the upper troposphere and its indirect aerosol effect
No full textZobrist B, Marcolli C, Koop T, et al. Oxalic acid as a heterogeneous ice nucleus in the upper troposphere and its indirect aerosol effect. ATMOSPHERIC CHEMISTRY AND PHYSICS. 2006;6(10):3115-3129.Heterogeneous ice freezing points of aqueous solutions containing various immersed solid dicarboxylic acids (oxalic, adipic, succinic, phthalic and fumaric) have been measured with a differential scanning calorimeter. The results show that only the dihydrate of oxalic acid (OAD) acts as a heterogeneous ice nucleus, with an increase in freezing temperature between 2 and 5 K depending on solution composition. In several field campaigns, oxalic acid enriched particles have been detected in the upper troposphere with single particle aerosol mass spectrometry. Simulations with a microphysical box model indicate that the presence of OAD may reduce the ice particle number density in cirrus clouds by up to ~50% when compared to exclusively homogeneous cirrus formation without OAD. Using the ECHAM4 climate model we estimate the global net radiative effect caused by this heterogeneous freezing to result in a cooling as high as −0.3 Wm−2
