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The physical processes and time scales underlying the evolution of surface tension in atmospheric solution droplets are largely unaccounted for in present models describing cloud droplet formation. Adsorption of surface-active molecules at the surface of a solution droplet depresses the droplet surface tension but also depletes solute from the droplet bulk, which have opposing and sometimes canceling effects in cloud droplet formation. In this work, we study the effect of time-evolving surface tension for cloud droplet activation of particles composed of Nordic Aquatic Fulvic Acid (NAFA) mixed with sodium chloride (NaCl). We model the formation of cloud droplets using Köhler theory with surface tension depression and bulk/surface partitioning evaluated from two different thermodynamic surface models. Continuous ternary parameterizations were constructed from surface tension measurements of macroscopic droplets at different time steps after the formation of a droplet surface. The predicted results are compared to previous measurements of mixed NAFA–NaCl cloud condensation nuclei (CCN) activity and a bulk solution model that does not take the NAFA bulk/surface partitioning equilibrium into account. Whereas the bulk model shows a trend in cloud droplet formation following that of macroscopic surface tension depression with time, the variation with time essentially disappears when bulk/surface partitioning is taken explicitly into account during droplet activation. For all equilibrium time steps considered, the effect of surface tension depression in the NAFA–NaCl system is counteracted by the depletion of solute from the finite-sized droplet bulk phase. Our study highlights that a comprehensive data set is necessary to obtain continuous parameterizations of surface tension and other solution properties required to fully account for the bulk/surface partitioning in growing droplets. To our knowledge, no similar data set currently exists for other aqueous organic systems of atmospheric interest. Additional work is necessary to deconvolve the effects of bulk/surface partitioning in the context of time-evolution on cloud droplet activation and to determine whether the results presented here can be further generalized. Environmental significance The time-evolution of aqueous surface tension is a well-known result of partitioning of surface-active compounds from the bulk to the surface. Surfactants are ubiquitous components of atmospheric organic aerosol. Equilibration time scales potentially impact interpretation of all measurements of cloud condensation nuclei (CCN) activity and modeled effects taking surface tension into account, including closure studies between instruments with different residence times and analysis using κ-Köhler theory. Using continuous surface tension parameterizations at different times, we provide a thermodynamically consistent analysis of the impact of time-dependency for predictions of CCN activity. We show that although time-evolution is clearly seen in surface partitioning, its signature decreases in droplet surface tension and we find no trends in CCN activity with time.

The SPectrometer for Ice Nuclei (SPIN) is a new commercially available ice nuclei counter manufactured by Droplet Measurement Technologies in Boulder, CO. SPIN is a continuous flow diffusion chamber with parallel plate geometry similar to the Zurich Ice Nucleation Chamber (ZINC) (Stetzer et al., 2007) and Portable Ice Nuclei Chamber (PINC) (Chou et al., 2011). It creates supersaturation conditions by coating two parallel plates with ice, holding them at different temperatures (both below 0ºC), and flowing an aerosol stream in the center of a sheath flow between the walls. After flowing through the main chamber, the air stream enters an isothermal evaporation section to evaporate liquid droplets. It then passes through a linear depolarization optical particle counter for particle, droplet, and ice counting. The instrument can be operated at aerosol temperatures as low as -55ºC and ice supersaturations exceeding 60%. This study characterizes the behavior of the SPIN chamber and reports data that validate its performance. In particular, ammonium sulfate experiments are used to compare the deliquescence and homogeneous freezing points reported by SPIN to literature values. Also, silver iodide and NX illite particles are used to investigate the heterogeneous ice nucleation onset conditions (with an ice fraction of 1%) for comparison to literature., American Meteorological Society Annual Meeting, 6-11 January, 2014, Boston, MA, USA

Humic-like substances (HULIS) are a complex group of relatively high molecular weight organic compounds which contribute considerably to the mass of organic carbon (OC) and influence the light-absorbing properties of aerosols. In this work, HULIS were investigated for the first time in the high-Arctic atmosphere, focusing on the chemical characterization and mass contribution of HULIS to the total suspended particle (TSP) mass using weekly aerosol samples collected at Station Nord, northeast Greenland every fourth week during 2010. Average HULIS-C concentration was 11ng C m(-3) during the darker months (November-April) and 4ng C m(-3) during the other months (May-October) with an annual mass concentration of 0.020.01 mu gm(-3). HULIS-C contributed to 3-16% of water-soluble organic carbon (WSOC), whereas HULIS accounted for 0.7-4.1% of TSP mass, with TSP typically below 1.0 mu gm(-3). Concentrations of OC, WSOC, HULIS, selected HULIS functional groups (carboxylic acids, aromatic carboxylic acids, and organosulfates) and levoglucosan overlapped with the typical Arctic haze pattern with elevated concentrations during winter to early spring. The aromatic carboxylic acid portion accounted for a larger share of total carboxylic acid of HULIS during the darker months (7%) compared to the brighter months (3%). The more abundant aromatic carboxylic acid functional groups and the moderate correlation between HULIS and levoglucosan concentrations during the darker months both indicate that biomass burning aerosols and thereby emissions of aromatic compounds could contribute to HULIS in the Arctic, especially during late winter. During the brighter months, relatively higher average molecular weight of HULIS was observed.

Under certain conditions, secondary organic aerosol (SOA) particles can exist in the atmosphere in an amorphous solid or semi-solid state. To determine their relevance to processes such as ice nucleation or chemistry occurring within particles requires knowledge of the temperature and relative humidity (RH) range for SOA to exist in these states. In the CLOUD experiment at CERN, we deployed a new in-situ optical method to detect the viscosity of α-pinene SOA particles and measured their transition from the amorphous viscous to liquid state. The method is based on the depolarising properties of laboratory-produced non-spherical SOA particles and their transformation to non-depolarising spherical liquid particles during deliquescence. We found that particles formed and grown in the chamber developed an asymmetric shape through coagulation. A transition to spherical shape was observed as the RH was increased to between 35 % at −10 °C and 80 % at −38 °C, confirming previous calculations of the viscosity transition conditions. Consequently, α-pinene SOA particles exist in a viscous state over a wide range of ambient conditions, including the cirrus region of the free troposphere. This has implications for the physical, chemical and ice-nucleation properties of SOA and SOA-coated particles in the atmosphere.

The growth of aerosol due to the aqueous phase oxidation of sulfur dioxide by ozone was measured in laboratory-generated clouds created in the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber at the European Organization for Nuclear Research (CERN). Experiments were performed at 10 and -10° C, on acidic (sulfuric acid) and on partially to fully neutralised (ammonium sulfate) seed aerosol. Clouds were generated by performing an adiabatic expansion – pressurising the chamber to 220 hPa above atmospheric pressure, and then rapidly releasing the excess pressure, resulting in a cooling, condensation of water on the aerosol and a cloud lifetime of approximately 6 min. A model was developed to compare the observed aerosol growth with that predicted using oxidation rate constants previously measured in bulk solutions. The model captured the measured aerosol growth very well for experiments performed at 10 and -10° C, indicating that, in contrast to some previous studies, the oxidation rates of SO$_{2}$ in a dispersed aqueous system can be well represented by using accepted rate constants, based on bulk measurements. To the best of our knowledge, these are the first laboratory-based measurements of aqueous phase oxidation in a dispersed, supercooled population of droplets. The measurements are therefore important in confirming that the extrapolation of currently accepted reaction rate constants to temperatures below 0° C is correct.