This work presents experimental data on the cloud condensation nuclei (CCN) activity of two‐component mixtures containing surfactants. Nine binary systems were tested combining strong ionic (sodium dodecyl sulfate) and nonionic surfactants (Zonyl FS‐300 and Triton X‐100) with nonsurfactant compounds (glucose, ammonium sulfate, or sodium chloride). Control tests were performed for systems combining organic (glucose) and inorganic compounds (ammonium sulfate or sodium chloride). Results show that CCN activity deviates strongly relative to predictions made from measurements of bulk surface tension. Köhler theory accounting for surface tension reduction and surface partitioning underpredicts the CCN activity of particles containing Zonyl FS‐300 and Triton X‐100. Partitioning theory better describes data for Zonyl FS‐300 and Triton X‐100 when limiting surface adsorption to 1.5 monolayers of the growing drop. Deviations from predictions were observed. Likely explanations include solute‐solute interactions and nonspherical particle shape. The findings presented here examine in detail the perturbation of CCN activity by surfactants and may offer insight into both the success and limitations of physical models describing CCN activity of surface active molecules.

Abstract. We present in situ measurements of particle-phase liquid water. Measurements were conducted from 3 June to 15 July 2013 during the Southern Oxidant and Aerosol Study (SOAS) in the southeastern US. The region is photochemically active, humid, dominated by biogenic emissions, impacted by anthropogenic pollution, and known to contain high concentrations of organic aerosol mass. Measurements characterized mobility number size distributions of ambient atmospheric aerosols in three states: unperturbed, dry, and dry-humidified. Unperturbed measurements describe the aerosol distribution at ambient temperature and relative humidity. For the dry state, the sample was routed through a cold trap upstream of the inlet then reheated, while for the dry-humidified state the sample was rehumidified after drying. The total volume of water and semi-volatile compounds lost during drying was quantified by differencing dry and unperturbed volumes from the integrated size spectra, while semi-volatile volumes lost during drying were quantified differencing unperturbed and dry-humidified volumes. Results indicate that particle-phase liquid water was always present. Throughout the SOAS campaign, median water mass concentrations at the relative humidity (RH) encountered in the instrument typically ranged from 1 to 5 μg m−3 but were as high as 73 μg m−3. On non-raining days, morning time (06:00–09:00) median mass concentrations exceeded 15 μg m−3. Hygroscopic growth factors followed a diel cycle and exceed 2 from 07:00 to 09:00 local time. The hygroscopicity parameter kappa ranged from 0.14 to 0.46 and hygroscopicity increased with increasing particle size. An observed diel cycle in kappa is consistent with changes in aerosol inorganic content and a dependency of the hygroscopicity parameter on water content. Unperturbed and dry-humidified aerosol volumes did not result in statistically discernible differences, demonstrating that drying did not lead to large losses in dry particle volume. We anticipate that our results will help improve the representation of aerosol water content and aqueous-phase-mediated partitioning of atmospheric water-soluble gases in photochemical models.

Organic aerosols in the atmosphere are composed of a wide variety of species, reflecting the multitude of sources and growth processes of these particles. Especially challenging is predicting how these particles act as cloud condensation nuclei (CCN). Previous studies have characterized the CCN efficiency for organic compounds in terms of a hygroscopicity parameter, κ. Here we extend these studies by systematically testing the influence of the number and location of molecular functional groups on the hygroscopicity of organic aerosols. Organic compounds synthesized via gas-phase and liquid-phase reactions were characterized by high-performance liquid chromatography coupled with scanning flow CCN analysis and thermal desorption particle beam mass spectrometry. These experiments quantified changes in κ with the addition of one or more functional groups to otherwise similar molecules. The increase in κ per group decreased in the following order: hydroxyl ≫ carboxyl > hydroperoxide > nitrate ≫ methylene (where nitrate and methylene produced negative effects, and hydroperoxide and nitrate groups produced the smallest absolute effects). Our results contribute to a mechanistic understanding of chemical aging and will help guide input and parametrization choices in models relying on simplified treatments such as the atomic oxygen:carbon ratio to predict the evolution of organic aerosol hygroscopicity.

Abstract. The second phase of the Fifth International Ice Nucleation Workshop (FIN-02) involved the gathering of a large number of researchers at the Karlsruhe Institute of Technology's Aerosol Interactions and Dynamics of the Atmosphere (AIDA) facility to promote characterization and understanding of ice nucleation measurements made by a variety of methods used worldwide. Compared to the previous workshop in 2007, participation was doubled, reflecting a vibrant research area. Experimental methods involved sampling of aerosol particles by direct processing ice nucleation measuring systems from the same volume of air in separate experiments using different ice nucleating particle (INP) types, and collections of aerosol particle samples onto filters or into liquid for sharing amongst measurement techniques that post-process these samples. In this manner, any errors introduced by differences in generation methods when samples are shared across laboratories were mitigated. Furthermore, as much as possible, aerosol particle size distribution was controlled so that the size limitations of different methods were minimized. The results presented here use data from the workshop to assess the comparability of immersion freezing measurement methods activating INPs in bulk suspensions, methods that activate INPs in condensation and/or immersion freezing modes as single particles on a substrate, continuous flow diffusion chambers (CFDCs) directly sampling and processing particles well above water saturation to maximize immersion and subsequent freezing of aerosol particles, and expansion cloud chamber simulations in which liquid cloud droplets were first activated on aerosol particles prior to freezing. The AIDA expansion chamber measurements are expected to be the closest representation to INP activation in atmospheric cloud parcels in these comparisons, due to exposing particles freely to adiabatic cooling. The different particle types used as INPs included the minerals illite NX and potassium feldspar (K-feldspar), two natural soil dusts representative of arable sandy loam (Argentina) and highly erodible sandy dryland (Tunisia) soils, respectively, and a bacterial INP (Snomax®). Considered together, the agreement among post-processed immersion freezing measurements of the numbers and fractions of particles active at different temperatures following bulk collection of particles into liquid was excellent, with possible temperature uncertainties inferred to be a key factor in determining INP uncertainties. Collection onto filters for rinsing versus directly into liquid in impingers made little difference. For methods that activated collected single particles on a substrate at a controlled humidity at or above water saturation, agreement with immersion freezing methods was good in most cases, but was biased low in a few others for reasons that have not been resolved, but could relate to water vapor competition effects. Amongst CFDC-style instruments, various factors requiring (variable) higher supersaturations to achieve equivalent immersion freezing activation dominate the uncertainty between these measurements, and for comparison with bulk immersion freezing methods. When operated above water saturation to include assessment of immersion freezing, CFDC measurements often measured at or above the upper bound of immersion freezing device measurements, but often underestimated INP concentration in comparison to an immersion freezing method that first activates all particles into liquid droplets prior to cooling (the PIMCA-PINC device, or Portable Immersion Mode Cooling chAmber–Portable Ice Nucleation Chamber), and typically slightly underestimated INP number concentrations in comparison to cloud parcel expansions in the AIDA chamber; this can be largely mitigated when it is possible to raise the relative humidity to sufficiently high values in the CFDCs, although this is not always possible operationally. Correspondence of measurements of INPs among direct sampling and post-processing systems varied depending on the INP type. Agreement was best for Snomax® particles in the temperature regime colder than −10 ∘C, where their ice nucleation activity is nearly maximized and changes very little with temperature. At temperatures warmer than −10 ∘C, Snomax® INP measurements (all via freezing of suspensions) demonstrated discrepancies consistent with previous reports of the instability of its protein aggregates that appear to make it less suitable as a calibration INP at these temperatures. For Argentinian soil dust particles, there was excellent agreement across all measurement methods; measures ranged within 1 order of magnitude for INP number concentrations, active fractions and calculated active site densities over a 25 to 30 ∘C range and 5 to 8 orders of corresponding magnitude change in number concentrations. This was also the case for all temperatures warmer than −25 ∘C in Tunisian dust experiments. In contrast, discrepancies in measurements of INP concentrations or active site densities that exceeded 2 orders of magnitude across a broad range of temperature measurements found at temperatures warmer than −25 ∘C in a previous study were replicated for illite NX. Discrepancies also exceeded 2 orders of magnitude at temperatures of −20 to −25 ∘C for potassium feldspar (K-feldspar), but these coincided with the range of temperatures at which INP concentrations increase rapidly at approximately an order of magnitude per 2 ∘C cooling for K-feldspar. These few discrepancies did not outweigh the overall positive outcomes of the workshop activity, nor the future utility of this data set or future similar efforts for resolving remaining measurement issues. Measurements of the same materials were repeatable over the time of the workshop and demonstrated strong consistency with prior studies, as reflected by agreement of data broadly with parameterizations of different specific or general (e.g., soil dust) aerosol types. The divergent measurements of the INP activity of illite NX by direct versus post-processing methods were not repeated for other particle types, and the Snomax® data demonstrated that, at least for a biological INP type, there is no expected measurement bias between bulk collection and direct immediately processed freezing methods to as warm as −10 ∘C. Since particle size ranges were limited for this workshop, it can be expected that for atmospheric populations of INPs, measurement discrepancies will appear due to the different capabilities of methods for sampling the full aerosol size distribution, or due to limitations on achieving sufficient water supersaturations to fully capture immersion freezing in direct processing instruments. Overall, this workshop presents an improved picture of present capabilities for measuring INPs than in past workshops, and provides direction toward addressing remaining measurement issues.