Peter A. Alpert

A marine biogenic source of atmospheric ice-nucleating particles

The amount of ice present in clouds can affect cloud lifetime, precipitation and radiative properties. The formation of ice in clouds is facilitated by the presence of airborne ice-nucleating particles. Sea spray is one of the major global sources of atmospheric particles, but it is unclear to what extent these particles are capable of nucleating ice. Sea-spray aerosol contains large amounts of organic material that is ejected into the atmosphere during bubble bursting at the organically enriched sea–air interface or sea surface microlayer. Here we show that organic material in the sea surface microlayer nucleates ice under conditions relevant for mixed-phase cloud and high-altitude ice cloud formation. The ice-nucleating material is probably biogenic and less than approximately 0.2 micrometres in size. We find that exudates separated from cells of the marine diatom Thalassiosira pseudonana nucleate ice, and propose that organic material associated with phytoplankton cell exudates is a likely candidate for the observed ice-nucleating ability of the microlayer samples. Global model simulations of marine organic aerosol, in combination with our measurements, suggest that marine organic material may be an important source of ice-nucleating particles in remote marine environments such as the Southern Ocean, North Pacific Ocean and North Atlantic Ocean.

A water activity based model of heterogeneous ice nucleation kinetics for freezing of water and aqueous solution droplets

Immersion freezing of water and aqueous solutions by particles acting as ice nuclei (IN) is a common process of heterogeneous ice nucleation which occurs in many environments, especially in the atmosphere where it results in the glaciation of clouds. Here we experimentally show, using a variety of IN types suspended in various aqueous solutions, that immersion freezing temperatures and kinetics can be described solely by temperature, T, and solution water activity, a(w), which is the ratio of the vapour pressure of the solution and the saturation water vapour pressure under the same conditions and, in equilibrium, equivalent to relative humidity (RH). This allows the freezing point and corresponding heterogeneous ice nucleation rate coefficient, J(het), to be uniquely expressed by T and a(w), a result we term the a(w) based immersion freezing model (ABIFM). This method is independent of the nature of the solute and accounts for several varying parameters, including cooling rate and IN surface area, while providing a holistic description of immersion freezing and allowing prediction of freezing temperatures, J(het), frozen fractions, ice particle production rates and numbers. Our findings are based on experimental freezing data collected for various IN surface areas, A, and cooling rates, r, of droplets variously containing marine biogenic material, two soil humic acids, four mineral dusts, and one organic monolayer acting as IN. For all investigated IN types we demonstrate that droplet freezing temperatures increase as A increases. Similarly, droplet freezing temperatures increase as the cooling rate decreases. The log10(J(het)) values for the various IN types derived exclusively by Tand a(w), provide a complete description of the heterogeneous ice nucleation kinetics. Thus, the ABIFM can be applied over the entire range of T, RH, total particulate surface area, and cloud activation timescales typical of atmospheric conditions. Lastly, we demonstrate that ABIFM can be used to derive frozen fractions of droplets and ice particle production for atmospheric models of cirrus and mixed phase cloud conditions.

Microspectroscopic imaging and characterization of individually identified ice nucleating particles from a case field study

The effect of anthropogenic and biogenic organic particles on atmospheric glaciation processes is poorly understood. We use an optical microscopy setup to identify the ice nuclei (IN) active in immersion freezing (IMF) and deposition ice nucleation within a large population of particles collected on a substrate from an ambient environment in central California dominated by urban and marine aerosols. Multimodal microspectroscopy methods are applied to characterize the physicochemical properties and mixing state of the individual IN and particle populations to identify particle-type classes. The temperature onsets of water uptake occurred between 235 and 257 K at subsaturated conditions, and the onsets of IMF proceeded at subsaturated and saturated conditions for 235–247 K, relevant for ice nucleation in mixed-phase clouds. Particles also took up water and nucleated ice between 226 and 235 K and acted as deposition IN with onset temperatures below 226 K, a temperature range relevant to cirrus cloud formation. The identified IN belong to the most common particle-type classes observed in the field samples: organic coated sea salt and Na-rich, secondary, and refractory carbonaceous particles. Based on these observations, we suggest that the IN are not always particles with unique chemical composition and exceptional ice nucleation propensity; rather, they are common particles in the ambient particle population. The results suggest that particle-type abundance and total particle surface area are also crucial factors, in addition to particle-type ice nucleation efficiency, in determining ice formation within the particle population.

Chemical imaging of ambient aerosol particles: Observational constraints on mixing state parameterization

A new parameterization for quantifying the mixing state of aerosol populations has been applied for the first time to samples of ambient particles analyzed using spectro-microscopy techniques. Scanning transmission X-ray microscopy/near edge X-ray absorption fine structure (STXM/NEXAFS) and computer-controlled scanning electron microscopy/energy dispersive X-ray spectroscopy (CCSEM/EDX) were used to probe the composition of the organic and inorganic fraction of individual particles collected on 27 and 28 June during the 2010 Carbonaceous Aerosols and Radiative Effects study in the Central Valley, California. The first field site, T0, was located in downtown Sacramento, while T1 was located near the Sierra Nevada Mountains. Mass estimates of the aerosol particle components were used to calculate mixing state metrics, such as the particle-specific diversity, bulk population diversity, and mixing state index, for each sample. The STXM data showed evidence of changes in the mixing state associated with a buildup of organic matter confirmed by collocated measurements, and the largest impact on the mixing state was due to an increase in soot dominant particles during this buildup. The mixing state from STXM was similar between T0 and T1, indicating that the increased organic fraction at T1 had a small effect on the mixing state of the population. The CCSEM/EDX analysis showed the presence of two types of particle populations: the first was dominated by aged sea-salt particles and had a higher mixing state index (indicating a more homogeneous population); the second was dominated by carbonaceous particles and had a lower mixing state index.

Ice cloud formation potential by free tropospheric particles from long‐range transport over the Northern Atlantic Ocean

Long-range transported free tropospheric particles can play a significant role on heterogeneous ice nucleation. Using optical and electron microscopy we examine the physicochemical characteristics of ice nucleating particles (INPs). Particles were collected on substrates from the free troposphere at the remote Pico Mountain Observatory in the Azores Islands, after long-range transport and aging over the Atlantic Ocean. We investigate four specific events to study the ice formation potential by the collected particles with different ages and transport patterns. We use single-particle analysis, as well as bulk analysis to characterize particle populations. Both analyses show substantial differences in particle composition between samples from the four events; in addition, single-particle microscopy analysis indicates that most particles are coated by organic material. The identified INPs contained mixtures of dust, aged sea salt and soot, and organic material acquired either at the source or during transport. The temperature and relative humidity (RH) at which ice formed, varied only by 5% between samples, despite differences in particle composition, sources, and transport patterns. We hypothesize that this small variation in the onset RH may be due to the coating material on the particles. This study underscores and motivates the need to further investigate how long-range transported and atmospherically aged free tropospheric particles impact ice cloud formation.

Fatty Acid Surfactant Photochemistry Results in New Particle Formation

Organic interfaces that exist at the sea surface microlayer or as surfactant coatings on cloud droplets are highly concentrated and chemically distinct from the underlying bulk or overlying gas phase. Therefore, they may be potentially unique locations for chemical or photochemical reactions. Recently, photochemical production of volatile organic compounds (VOCs) was reported at a nonanoic acid interface however, subsequent secondary organic aerosol (SOA) particle production was incapable of being observed. We investigated SOA particle formation due to photochemical reactions occurring at an air-water interface in presence of model saturated long chain fatty acid and alcohol surfactants, nonanoic acid and nonanol, respectively. Ozonolysis of the gas phase photochemical products in the dark or under continued UV irradiation both resulted in nucleation and growth of SOA particles. Irradiation of nonanol did not yield detectable VOC or SOA production. Organic carbon functionalities of the SOA were probed using X-ray microspectroscopy and compared with other laboratory generated and field collected particles. Carbon-carbon double bonds were identified in the condensed phase which survived ozonolysis during new particle formation and growth. The implications of photochemical processes occurring at organic coated surfaces are discussed in the context of marine SOA particle atmospheric fluxes.

A continuous flow diffusion chamber study of sea salt particles acting as cloud nuclei: deliquescence and ice nucleation

Abstract Phase changes of sea salt particles alter their physical and chemical properties, which is significant for Earth’s chemistry and energy budget. In this study, a continuous flow diffusion chamber is used to investigate deliquescence, homogeneous and heterogeneous ice nucleation between 242 K and 215 K, of four salts: pure NaCl, pure MgCl2, synthetic sea water salt, and salt distilled from sampled sea water. Anhydrous particles, aqueous droplets and ice particles were discriminated using a polarisation-sensitive optical particle counter coupled with a machine learning analysis technique. The measured onset deliquescence relative humidities agree with previous studies, where sea water salts deliquescence at lower humidities than pure NaCl. Deliquesced salt droplets homogenously freeze when the relative humidity reaches a sufficiently high value at temperatures below 233 K. From 224 K and below, deposition nucleation freezing on a fraction of NaCl particles was observed at humidities lower than the deliquescence relative humidity. At these low temperatures, otherwise unactivated salt particles deliquesced at the expected deliquescence point, followed by homogeneous freezing at temperatures as low as 215 K. Thus, the observed sea salt particles exhibit a triad of temperature-dependent behaviours. First, they act as cloud condensation particles (CCNs) > 233 K, second they can be homogeneous freezing nuclei (HFNs) < 233 K and finally they act as ice nucleating particles (INPs) for heterogeneous nucleation <224 K.

Imaging Molecular Reaction and Diffusion in Organic Aerosol Particles

1. Laboratory of Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland. 2. Institute for Atmospheric and Climate Science, ETH Zürich, 8092 Zürich, Switzerland 3. Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, United Kingdom 4. Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany 5. Aix Marseille Université, CNRS, LCE UMR 7376, 13331 Marseille, France 6. Department of Physics, University of Helsinki, 00014 Helsinki, Finland 7. Université Lyon 1, CNRS, UMR 5256, IRCELYON, 69626 Villeurbanne, France 8. School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States 9. Department of Chemistry, University of California, Irvine, CA 92617, United States

Particle-Phase Photosensitized Radical Production and Aerosol Aging

Atmospheric aerosol particles may contain light absorbing (brown carbon, BrC), triplet forming organic compounds that can sustain catalytic radical reactions and thus contribute to oxidative aerosol aging. We quantify UVA induced radical production initiated by imidazole-2-carboxaldehyde (IC), benzophenone (BPh). and 4-benzoylbenzoic acid (BBA) in the presence of the nonabsorbing organics citric acid (CA), shikimic acid (SA), and syringol (Syr) at varying mixing ratios. We observed a maximum HO2 release of 1013 molecules min-1 cm-2 at a mole ratio XBPh < 0.02 for BPh in CA. Mixtures of either IC or BBA with CA resulted in 1011-1012 molecules min-1 cm-2 of HO2 at mole ratios ( XIC and XBBA) between 0.01 and 0.15. HO2 release was affected by relative humidity ( RH) and film thickness suggesting coupled photochemical reaction and diffusion processes. Quantum yields of HO2 formed per absorbed photon for IC, BBA and BPh were between 10-7 and 5 × 10-5. The nonphotoactive organics, Syr and SA, increased HO2 production due to the reaction with the triplet excited species ensuing ketyl radical production. Rate coefficients of the triplet of IC with Syr and SA measured by laser flash photolysis experiments were kSyr = (9.4 ± 0.3) × 108 M-1 s-1 and kSA = (2.7 ± 0.5) × 107 M-1 s-1. A simple kinetic model was used to assess total HO2 and organic radical production in the condensed phase and to upscale to ambient aerosol, indicating that BrC induced radical production may amount to an upper limit of 20 and 200 M day-1 of HO2 and organic radical respectively, which is greater or in the same order of magnitude as the internal radical production from other processes, previously estimated to be around 15 M per day.