Currently, the physical state of mixed organic/inorganic aerosol particles is not well characterized, largely because of the still unclear chemical composition of the organic fraction and of its properties with respect to mixing with the inorganic fraction. To obtain insight in the possible phases and phase transitions of such aerosol particles, we investigated the ternary poly(ethylene glycol)-400/ammonium sulfate/water system as a representative model system with partially immiscible constituents. For this purpose, we used optical microscopy and micro-Raman spectroscopy on micrometer-sized particles deposited on a hydrophobically coated substrate. The particles show liquid-liquid phase separations both upon decreasing (approximately 90-85%) and increasing (during ammonium sulfate deliquescence) relative humidities. In dependence upon the organic-to-inorganic ratio, OIR (i.e., poly(ethylene glycol)-400 to ammonium sulfate dry mass), phase separation is observed to occur by fundamentally different mechanisms, namely, nucleation-and-growth (OIR = 8:1 to 2:1), spinodal decomposition (OIR = 1.5:1 to 1:1.5) and growth of a second phase at the surface of the particle (OIR = 1:2 to 1:8). For each of these mechanisms, after completion of the phase separation, the resulting morphology of the particles is an aqueous ammonium sulfate inner phase surrounded by a mainly poly(ethylene glycol)-400 containing outer phase. We depict the various physical states of the ternary system in the relative humidity/composition phase diagram, constructed from bulk data and single particle measurements. Given the complex chemical composition of the organic fraction in tropospheric aerosols, it is expected that repulsive forces between the organic and inorganic aerosol constituents exist and that liquid-liquid phase separations commonly occur. The presence of liquid-liquid phase separations may change the partitioning of semivolatile species between the gas and the condensed phase, whereas the predominantly organic shell is likely to influence heterogeneous chemical reactions, such as N(2)O(5) hydrolysis.

Using optical microscopy, we investigated the efflorescence of ammonium sulfate (AS) in aqueous AS and in aqueous 1:1 and 8:1 (by dry weight) poly(ethylene glycol)-400 (PEG-400)/AS particles deposited on a hydrophobically coated slide. Aqueous PEG-400/AS particles exposed to decreasing relative humidity (RH) exhibit a liquid-liquid phase separation below approximately 90% RH with the PEG-400-rich phase surrounding the aqueous AS inner phase. Pure aqueous AS particles effloresced in the RH range from 36.3% to 43.7%, in agreement with literature data (31-48% RH). In contrast, aqueous 1:1 (by dry weight) PEG-400/AS particles with diameters of the AS phase from 7.2 to 19.2 mum effloresced between 26.8% and 33.9% RH and aqueous 8:1 (by dry weight) PEG-400/AS particles with diameters of the AS phase from 1.8 to 7.3 mum between 24.3% and 29.3% RH. Such low efflorescence relative humidity (ERH) values have never been reached before for AS particles of this size range. We show that these unprecedented low ERHs of AS in PEG-400/AS particles could not possibly be explained by the presence of low amounts of PEG-400 in the aqueous AS phase, by a potential inhibition of water evaporation via anomalously slow diffusion through the PEG coating, or by different time scales between various experimental techniques. High-speed photography of the efflorescence process allowed the development of the AS crystallization fronts within the particles to be monitored with millisecond time resolution. The nucleation sites were inferred from the initial crystal growth sites. Analysis of the probability distribution of initial sites of 31 and 19 efflorescence events for pure AS and 1:1 (by dry weight) PEG-400/AS particles, respectively, showed that the particle volume can be excluded as the preferred nucleation site in the case of pure AS particles. For aqueous 1:1 (by dry weight) PEG-400/AS particles preferential AS nucleation in the PEG phase and at the PEG/AS/substrate contact line can be excluded. On the basis of this probability analysis of efflorescence events together with the AS ERH values of pure aqueous AS and aqueous PEG-400/AS particles aforementioned, we suggest that in pure aqueous AS particles nucleation starts at the surface of the particles and attribute the lower ERH values observed for aqueous PEG-400/AS particles to the suppression of the surface-induced nucleation process. Our results suggest that surface-induced nucleation is likely to also occur during the efflorescence of atmospheric AS aerosol particles, possibly constituting the dominating nucleation pathway.

Abstract. In 2013, an ice core was recovered from Roosevelt Island in the Ross Sea, Antarctica, as part of the Roosevelt Island Climate Evolution (RICE) project. Roosevelt Island is located between two submarine troughs carved by paleo-ice-streams.The RICE ice core provides new important information about the past configuration of the West Antarctic Ice Sheet and its retreat during the most recent deglaciation. In this work, we present the RICE17 chronology and discuss preliminary observations from the new records of methane, the isotopic composition of atmospheric molecular oxygen (δ 18O atm ), the isotopic 5 composition of atmospheric molecular nitrogen (δ 15 N-N 2 ), and total air content (TAC). RICE17 is a composite chronology combining annual layer interpretations, gas synchronization, and firn modeling strategies in different sections of the core. An automated matching algorithm is developed for synchronizing the high-resolution section of the RICE gas records (60-720 m, 1971 CE to 30 ka) to corresponding records from the WAIS Divide ice core, while deeper sections are manually matched. Ice age for the top 343 m (2635 yr BP, before 1950 C.E.) is derived from annual layer interpretations and described in the accom-10 panying paper by Winstrup et al. (2017). For deeper sections, the RICE17 ice age scale is based on the gas age constraints and the ice age-gas age offset estimated by a firn densification model. Novel aspects of this work include: 1) stratigraphic matching of centennial-scale variations in methane for pre-anthropogenic time periods, a strategy which will be applicable for developing precise chronologies for future ice cores, 2) the observation of centennial-scale variability in methane throughout the Holocene which suggests that similar variations during the late pre-15 industrial period need not be anthropogenic, and 3) the observation of continuous climate records dating back to ∼65 ka which provide evidence that the Roosevelt Island Ice Dome was a constant feature throughout the last glacial period.

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