Snomax® is often used as a surrogate for biological ice nucleating particles (INPs) and has recently been proposed as an INP standard for evaluating ice nucleation methods. We have found the ...immersion freezing properties of Snomax particles to be substantially unstable, observing a loss of ice nucleation ability over months of repeated droplet freezing measurements of the same batch of Snomax stored as dry pellets in a freezer. This reflects the fragility of the most ice active large protein aggregates and presents issues for the use of Snomax as an INP standard. The ice nucleation properties we determined using a fresh Snomax batch agreed well with the recent method intercomparison from the Ice Nucleation UnIT (UNIT) project, while an older batch did not. Using an oil immersion droplet freezing technique, repeated freezes of Snomax droplets resulted in a decrease in ice nucleation ability after successive refreezes. We attribute this to the disruption or displacement of the most ice active protein aggregates that are thought to contain the ice nucleants. Partitioning of the protein aggregates from the droplet into the immersion oil that is accelerated by droplet freezing events could explain the observed decrease in freezing ability. Droplets in mineral oil or low viscosity silicone oil experienced a smaller reduction in freezing temperature than when squalene oil was used. The effect of the immersion oil may be specific to proteinaceous biological particles, and we have not observed it in nonproteinaceous materials. Caution is warranted in the use of oil immersion droplet freezing methods to determine immersion freezing properties.
Key Points
Very ice active Snomax protein aggregates are fragile and their ice nucleation ability decreases over months of freezer storage
Partitioning of ice active protein aggregates into the immersion oil reduces the droplet's measured freezing temperature
Caution is warranted in the use of Snomax as an ice nucleating particle standard
Atmospheric aerosols exert a substantial influence on climate, ecosystems, visibility, and human health. Although secondary organic aerosols (SOA) dominate fine-particle mass, they comprise myriad ...compounds with uncertain sources, chemistry, and interactions. SOA formation involves absorption of vapors into particles, either because gas-phase chemistry produces low-volatility or semivolatile products that partition into particles or because morevolatile organics enter particles and react to form lower-volatility products. Thus, SOA formation involves both production of low-volatility compounds and their diffusion into particles. Most chemical transport models assume a single well-mixed phase of condensing organics and an instantaneous equilibrium between bulk gas and particle phases; however, direct observations constraining diffusion of semivolatile organics into particles containing SOA are scarce. Here we perform unique mixing experiments between SOA populations including semivolatile constituents using quantitative, single-particle mass spectrometry to probe any mass-transfer limitations in particles containing SOA. We show that, for several hours, particles containing SOA from toluene oxidation resist exchange of semivolatile constituents at low relative humidity (RH) but start to lose that resistance above 20% RH. Above 40% RH, the exchange of material remains constant up to 90% RH. We also show that dry particles containing SOA from α-pinene ozonolysis do not appear to resist exchange of semivolatile compounds. Our interpretation is that in-particle diffusion is not rate-limiting to mass transfer in these systems above 40% RH. To the extent that these systems are representative of ambient SOA, we conclude that diffusion limitations are likely not common under typical ambient boundary layer conditions.
We demonstrate the first capture and analysis of secondary organic aerosol (SOA) on a droplet suspended in an aerosol optical tweezers (AOT). We examine three initial chemical systems of aqueous ...NaCl, aqueous glycerol, and squalane at ∼75% relative humidity. For each system we added α-pinene SOA-generated directly in the AOT chamber-to the trapped droplet. The resulting morphology was always observed to be a core of the original droplet phase surrounded by a shell of the added SOA. We also observed a stable emulsion of SOA particles when added to an aqueous NaCl core phase, in addition to the shell of SOA. The persistence of the emulsified SOA particles suspended in the aqueous core suggests that this metastable state may persist for a significant fraction of the aerosol lifecycle for mixed SOA/aqueous particle systems. We conclude that the α-pinene SOA shell creates no major diffusion limitations for water, glycerol, and squalane core phases under humid conditions. These experimental results support the current prompt-partitioning framework used to describe organic aerosol in most atmospheric chemical transport models and highlight the prominence of core-shell morphologies for SOA on a range of core chemical phases.
Winter storms in California's Sierra Nevada increase seasonal snowpack and provide critical water resources and hydropower for the state. Thus, the mechanisms influencing precipitation in this region ...have been the subject of research for decades. Previous studies suggest Asian dust enhances cloud ice and precipitation, whereas few studies consider biological aerosols as an important global source of ice nuclei (IN). Here, we show that dust and biological aerosols transported from as far as the Sahara were present in glaciated high-altitude clouds coincident with elevated IN concentrations and ice-induced precipitation. This study presents the first direct cloud and precipitation measurements showing that Saharan and Asian dust and biological aerosols probably serve as IN and play an important role in orographie precipitation processes over the western United States.
The production, size, and chemical composition of sea spray aerosol (SSA) particles strongly depend on seawater chemistry, which is controlled by physical, chemical, and biological processes. Despite ...decades of studies in marine environments, a direct relationship has yet to be established between ocean biology and the physicochemical properties of SSA. The ability to establish such relationships is hindered by the fact that SSA measurements are typically dominated by overwhelming background aerosol concentrations even in remote marine environments. Herein, we describe a newly developed approach for reproducing the chemical complexity of SSA in a laboratory setting, comprising a unique ocean-atmosphere facility equipped with actual breaking waves. A mesocosm experiment was performed in natural seawater, using controlled phytoplankton and heterotrophic bacteria concentrations, which showed SSA size and chemical mixing state are acutely sensitive to the aerosol production mechanism, as well as to the type of biological species present. The largest reduction in the hygroscopicity of SSA occurred as heterotrophic bacteria concentrations increased, whereas phytoplankton and chlorophyll- a concentrations decreased, directly corresponding to a change in mixing state in the smallest (60–180 nm) size range. Using this newly developed approach to generate realistic SSA, systematic studies can now be performed to advance our fundamental understanding of the impact of ocean biology on SSA chemical mixing state, heterogeneous reactivity, and the resulting climate-relevant properties.
Droplet freezing techniques (DFTs) have been used for half a century to
measure the concentration of ice-nucleating particles (INPs) in the atmosphere
and determine their freezing properties to ...understand the effects of INPs on
mixed-phase clouds. The ice nucleation community has recently adopted droplet
freezing assays as a commonplace experimental approach. These droplet
freezing experiments are often limited by contamination that causes
nonhomogeneous freezing of the “pure” water used to generate the droplets
in the heterogeneous freezing temperature regime that is being measured.
Interference from the early freezing of water is often overlooked and not
fully reported, or measurements are restricted to analyzing the more
ice-active INPs that freeze well above the temperature of the background
water. However, this avoidance is not viable for analyzing the freezing
behavior of less active INPs in the atmosphere that still have potentially
important effects on cold-cloud microphysics. In this work we review a number
of recent droplet freezing techniques that show great promise in reducing these
interferences, and we report our own extensive series of measurements using
similar methodologies. By characterizing the performance of different
substrates on which the droplets are placed and of different pure water
generation techniques, we recommend best practices to reduce these
interferences. We tested different substrates, water sources, droplet
matrixes, and droplet sizes to provide deeper insight into what methodologies
are best suited for DFTs. Approaches for analyzing droplet freezing
temperature spectra and accounting and correcting for the background “pure”
water control spectrum are also presented. Finally, we propose experimental
and data analysis procedures for future homogeneous and heterogeneous ice
nucleation studies to promote a more uniform and reliable methodology that
facilitates the ready intercomparison of ice-nucleating particles measured by
DFTs.
Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested ...in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratory-generated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0 °C, averaging an order of magnitude increase per 5 °C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using “dry” geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.
Ice nucleation and the resulting cloud glaciation are significant atmospheric processes that affect the evolution of clouds and their properties including radiative forcing and precipitation, yet the ...sources and properties of atmospheric ice nucleants are poorly constrained. Heterogeneous ice nucleation caused by ice-nucleating particles (INPs) enables cloud glaciation at temperatures above the homogeneous freezing regime that starts near −35 °C. Biomass burning is a significant global source of atmospheric particles and a highly variable and poorly understood source of INPs. The nature of these INPs and how they relate to the fuel composition and its combustion are critical gaps in our understanding of the effects of biomass burning on the environment and climate. Here we show that the combustion process transforms inorganic elements naturally present in the biomass (not soil or dust) to form potentially ice-active minerals in both the bottom ash and emitted aerosol particles. These particles possess ice-nucleation activities high enough to be relevant to mixed-phase clouds and are active over a wide temperature range, nucleating ice at up to −13 °C. Certain inorganic elements can thus serve as indicators to predict the production of ice nucleants from the fuel. Combustion-derived minerals are an important but under-studied source of INPs in natural biomass-burning aerosol emissions in addition to lofted primary soil and dust particles. These discoveries and insights should advance the realistic incorporation of biomass-burning INPs into atmospheric cloud and climate models. These mineral components produced in biomass-burning aerosol should also be studied in relation to other atmospheric chemistry processes, such as facilitating multiphase chemical reactions and nutrient availability.
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