This presentation will be an invitation to think differently about the possible oxidation pathways occurring in secondary organic aerosols.

Despite the importance of aerosols in atmospheric chemistry, climate and air pollution, our ability to assess their impact on atmospheric physics and chemistry is still limited due to insufficient understanding of many processes associated with the sources of particles, their chemical composition and morphology, and evolution of their composition and properties during their atmospheric lifetime. Indeed, atmospheric aerosols can be viewed as a complex conglomerate of thousands of chemical compounds forming a system that evolves in the atmosphere by chemical and dynamical processing including chemical interaction with oxidants and sunlight.

A significant body of literature on photo-induced charge or energy transfer in organic molecules from other fields of science (biochemistry and water waste treatment) exists. Such organic molecules are aromatics, substituted carbonyls and/or nitrogen containing compounds – all ubiquitous in tropospheric aerosols. Multiphase processes have also been shown to produce light absorbing compounds in the particle phase. The formation of such light absorbing species could induce new photochemical processes within the aerosol particles and/or at the gas/particle interface. Therefore, while bulk phase aquatic photochemistry has recognized several of these processes that accelerate degradation of dissolved organic matter, only little is known about such processes in/on atmospheric particles.

This presentation will discuss photosensitization in the troposphere as having a significant role in SOA formation and ageing as studied by means of laser transient absorption spectroscopy, flow tube and simulation chamber experiments, all coupled to advanced analytical techniques. We will provide kinetic and mechanistic information on how photosensitization may introduce new chemical pathways, so far unconsidered, which can impact both the chemical composition of the atmosphere and might thus contribute to close the current SOA underestimation.

How to cite: George, C.: Photosensitization is in the air and impacts the multiphase on oxidation capacity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2093, https://doi.org/10.5194/egusphere-egu23-2093, 2023.

Aerosols have a net cooling effect in our atmosphere, mainly due to their ability to act as cloud condensation nuclei (CCN) and to reflect incoming solar radiation [1]. In aerosols the surface is particularly important as it is the site for many processes which are central to cloud formation, in particular evaporation and condensation. Aerosol surfaces contain a wide variety of organic and inorganic chemical compounds. Understanding the resulting surface structure and composition is a necessary first step in gaining a better understanding of the role aerosols play in the atmosphere.

In particular, the surface composition affects the hygroscopicity of the particle and thereby its ability to act as a CCN [2]. While the surface propensity of individual compounds has been studied widely, much less is known about cooperative effects in more complex mixed systems [3]. Here we use sum-frequency-generation (SFG) spectroscopy in conjunction with surface tension data to study whether surface competition or cooperative enhancement can be observed in mixed solutions of halides and acids or alcohols of different chain length.

We find that in certain cases the presence of different ions can affect the surface structure in a unique way. However, these effects are not systematic and appear to depend on the specific nature of the interaction between the ions and the respective organic compound. This further highlights that the surface structure of mixed systems cannot be extrapolated from studies of simple solutions.

[1] IPCC. Climate Change 2013: The Physical Science Basis. [2] P. Zieger et al., Nat. Commun. 2017, 8, 15883. [3] M.T. Lee et al., Phys. Chem. Chem. Phys. 2019, 21, 8418

How to cite: Saak, C. M., Mika, S. M., and Backus, E. H. G.: Ion specific Interactions and Surface Enrichment on Mixed Aerosol Surfaces, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14850, https://doi.org/10.5194/egusphere-egu23-14850, 2023.

Effect of pH on pyruvic acid in bulk and at the air/liquid interface
Veronika Wank and Ellen H. G. Backus
University of Vienna, Faculty of Chemistry, Department of Physical Chemistry,Währinger Straße 42, 1090 Vienna, Austria
Abstract:
Pyruvic acid, known as a relevant carboxylic acid for plant metabolic processes is also a contributor to the formation of secondary organic aerosols (SOA) and thus of great importance for the atmospheric cycle, especially in aerosols. At the interface, the behavior of relevant atmospheric molecules in aerosols is a major topic, as the surface is one of the first areas where chemical processes take place and thus determines the main reactivity of the aerosol. For acidic molecules, the acid/base behavior, especially at the interface, is relevant for understanding the chemical interaction of organic matter in atmospheric aerosols, where reaction rates and product distributions change due to different pH conditions, e.g. chemical processing and molecule transport.
Since the pH of aqueous aerosols can vary widely, it is particularly important to understand changes in the surface structure due to the resulting (de)protonation reactions.
This encourages us to take a closer look at the interfacial region of pyruvic acid in aqueous solution by using a complex surface-specific spectroscopic technique, so called sum frequency generation spectroscopy (SFG). For comparison, infrared bulk measurements utilizing ATR spectroscopy were completed. By combining ATR and SFG, the protonation state of pyruvic acid for bulk and interface was determined by probing the vibrational signatures of the carboxylic acid groups.
Our results show that pyruvic acid at the water interface is more alkaline than in the bulk, which indicates that the carboxylic acid group deprotonates at a higher pH value at the surface than in the bulk. It is also evident from the SFG spectra that at lower pH the water molecules on the surface are displaced by PA molecules, whereas at higher pH the water molecules return to the surface and the PA molecules tend to go into the bulk.
This implies that the protonation state of carboxylic acids like PA can thus affect the molecular orientation, conformation and function of molecules on aqueous surfaces, which likely has a significant impact on the chemical processes taking place at the aerosol surface in the atmosphere.

How to cite: Wank, V.: Effect of pH on pyruvic acid in bulk and at the air/liquid interface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15680, https://doi.org/10.5194/egusphere-egu23-15680, 2023.

The formation and impact of ice crystals in clouds remain a challenge to understand and thus to be predicted in models. In recent years, measurements of ice-nucleating particles (INPs) in ambient air have become more frequent, using well-established and novel techniques such as mobile cloud chambers and filter-based freezing assays. To assure that these techniques are working as intended, validation and intercomparison measurements are required. This is especially relevant due to ongoing efforts for the establishment of INP monitoring networks at the European level (ACTRIS; Aerosol, Clouds and Trace Gases Research Infrastructure) and in the United States (ARM; Atmospheric Radiation Measurement).

Here we present results from PICNIC (The Puy de Dôme ICe Nucleation Intercomparison Campaign), conducted at a mountain site in Central France (1465 m a.s.l.) in October 2018. INP concentrations relevant in the mixed-phase cloud regime were determined using three online INP techniques (Colorado State University-Continuous Flow Diffusion Chamber, CSU-CFDC; Spectrometer for Ice Nuclei, SPIN; Portable Ice Nucleation Experiments, PINE) and seven filter-based offline freezing devices (FRankfurt Ice Nuclei Deposition FreezinG Experiment, FRIDGE; Ice Nucleation Droplet Array INDA; Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology, INSEKT; CSU Ice Spectrometer, IS; Leipzig Ice Nucleation Array, LINA; LED based Ice Nucleation Detection Apparatus LINDA; Micro-Orifice Uniform Deposit Impactor–Droplet Freezing Technique, MOUDI-DFT). The campaign focused on INP concentration measurements performed at the same time and at comparable nucleation temperatures, which is why filter sampling for offline techniques was started and stopped simultaneously, and online INP measurements were conducted at similar thermodynamic conditions. While the ice chambers yielded reasonable agreements within factors of 2 to 5, with lower concentrations found by SPIN, a systematic deviation between filter samples collected directly outside on the station’s rooftop and those sampled downstream from a whole air inlet is observed. A potential loss of larger aerosol particles via the inlet and an impact of the disaggregation of larger aerosol particles in solution might cause these differences, which needs to be investigated in future studies.

How to cite: Lacher, L. and Freney, E. and the PICNIC team: The Puy de Dôme Ice Nucleation Intercomparison Campaign, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8694, https://doi.org/10.5194/egusphere-egu23-8694, 2023.

Pollen belong to a subset of atmospheric aerosol particles that enable the glaciation of supercooled liquid water, which is present in clouds down to an ambient temperature of -38°C. While the ice nucleating properties of pollen are widely researched in laboratory studies, it is challenging to evaluate their effect on clouds in observations at a large scale. Using a combination of ground-based measurements of pollen concentration and satellite observations of cloud properties during springtime, we show that the ice nucleating properties of pollen promote the glaciation of supercooled liquid water. We further establish the link between the pollen-induced increase in cloud ice to a higher precipitation frequency. In light of anthropogenic climate change, the extended and strengthened pollen season and alterations in biodiversity can, therefore, introduce a localized climate forcing and a modification of the precipitation frequency and intensity, especially during springtime.

How to cite: Kretzschmar, J., Wirth, C., Pöhlker, M., Stratmann, F., Wex, H., and Quaas, J.: Enhancement of cloud glaciation and rain frequency by airborne pollen, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11479, https://doi.org/10.5194/egusphere-egu23-11479, 2023.

Primary biological aerosol particles (PBAPs) are emitted from Earth’s biosphere, including pollen, fungal spores, virus, bacteria, and plant debris. PBAPs are linked to adverse health effects and have the potential to influence ice nucleation at warmer temperatures. Anemophilous (or wind-driven) pollen is one type of PBAP, and the emitted pollen grains can rupture under high humidity to form smaller sub-pollen particles (SPP). Both pollen and SPP can be lifted to the upper troposphere under convective conditions, readily take up water and serve as cloud condensation nuclei (CCN) and ice nucleating particles (INPs), and therefore impact cloud formation and reflectivity. Although these biological aerosol have proven to be effective INPs in previous studies, they are typically not included in emission inventories. Therefore, it is difficult to quantify their effects on cloud formation and local climate.

Here, we include the emission and rupture of pollen in WRF-Chem simulations and investigate the impacts of pollen and SPP on both warm and ice clouds in the United States South Great Plains (SGP) from April 11-20, 2013, a period with high pollen emission and convective events. We update the Morrison microphysics scheme inside WRF-Chem using aerosol-aware INP parameterizations, considering different freezing mechanisms including heterogeneous freezing (immersion, contact, and deposition freezing) and homogeneous freezing. We further incorporate heterogeneous ice nucleation from pollen and SPP in the model to evaluate pollen effects on ice cloud formation. The corresponding pollen and SPP INP parameterizations are obtained by laboratory experiments that indicate pollen grains are more efficient INPs than SPP and could contribute to ice cloud formation. The model simulation results are evaluated using observational data from Atmospheric Radiation Measurement (ARM) SGP sites.  We conducted a suite of sensitivity tests to examine the impacts of pollen and SPP on one convective event (April 17-18, 2013), and compare the newly developed pollen and SPP INP parameterizations with those developed in previous literature. Our results highlight that the addition of PBAPs such as pollen could shift the convective event onset timing and vertical structure.

How to cite: Zhang, Y., Subba, T., Hendrickson, B. N., Brooks, S. D., and Steiner, A. L.: Simulating the impacts of pollen on cloud formation by heterogeneous ice nucleation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10480, https://doi.org/10.5194/egusphere-egu23-10480, 2023.

With raising temperatures in the Arctic, the extent of sea ice is decreasing dramatically resulting in a larger fraction of the Arctic ocean surface being exposed to the atmosphere. Therefore, the ice-free ocean and in particular the sea surface microlayer (SML), which represents the upper 1 mm of the water column is becoming of increasing interest as a source of bioaerosols with ice nucleating properties. These biological ice nucleating particles (INPs) can be aerosolized by wave breaking and bubble bursting. In the atmosphere, they may trigger the freezing of cloud droplets and thus affect the lifetime of clouds as well as their radiative properties. Recent studies proposed a link between biological ice nucleating aerosols in the Arctic sea water and phytoplanktonic blooms.

Thus, we examined the concentration and characteristics of INPs in both, the sea bulk water, and the surface microlayer for two locations in southwest Greenland throughout a phytoplankton bloom. Further, we investigated possible links between INP concentrations in the sea water, the abundance and community composition of bacteria and algae, as well as the phytoplanktonic growth season derived from satellite data and in-situ chlorophyll concentrations. Preliminary results will be presented.

How to cite: Wieber, C., Jensen, L. Z., A. Ferreira, A. S., Vergeynst, L., Finster, K., and Šantl-Temkiv, T.: Dynamics of biological ice nucleating particles during a phytoplanktonic bloom in the Arctic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11673, https://doi.org/10.5194/egusphere-egu23-11673, 2023.

Soot particles emitted by aircraft in the troposphere can serve as ice nucleating particles (INPs) in the cirrus regime, which can compete with natural cirrus formation by homogeneous freezing of solution droplets and influence cirrus cloudiness and microphysics. Soot particles show varying ice nucleation (IN) abilities, depending on the particle properties that regulate the IN pathway and effectiveness. Pore condensation and freezing (PCF) is an important pathway for soot IN, which first forms supercooled water in soot mesopores (2-50 nm width) via capillary condensation and then freezes below homogeneous nucleation temperature. Soot PCF shows dependence on the particle mesopore abundance and soot-water contact angle (θ) according to the Kelvin equation. However, the relative importance of θ and mesopore abundance in soot PCF has not been disentangled.

In this study, the θ of organic-lean soot was changed after exposure to a high (20 ppmv) and a low (2 ppmv) O₃ concentration condition to mimic a long- (~20 h) and short-term (~2 h) aging in the real atmosphere respectively, without changing the soot mesopore abundance. Secondary organic formation was avoided by establishing a volatile free experimental environment. The IN activities of both fresh and O₃-aged soot particles, with aggregate size of 200 and 400 nm, were tested by the Horizontal Ice Nucleation Chamber (HINC) under cirrus cloud conditions for T≤233 K. The size and mass of soot particles for IN experiments were also monitored. The θ of bulk samples exposed to the same O₃ concentration conditions (20, 2 or 0 ppmv) as used for IN experiments was directly measured by the Sessile-drop method. The O₃-aging effects on soot chemical composition, soot-water interaction ability and porosity were also characterized by using thermogravimetric analysis (TGA), dynamic vapor sorption (DVS) and N₂ Brunauer–Emmett–Teller (BET) techniques, respectively.

The adsorption of O₃ onto soot particles was demonstrated by a particle mass increase after O₃-exposure. The unchanged chemical functional group abundance and increased water-uptake ability at low relative humidity (RH) conditions after O₃-aging implies that the soot-water interaction increase is likely due to direct O₃-H₂O binding rather than surface oxidation, which explains the θ decrease of O₃-aged soot. Compared to unaged soot of the same size, a higher O₃ exposure level leads to a larger decrease in soot-water θ and a more significant IN enhancement for O₃-aged soot. However, O₃-aged soot presents unchanged pore size distribution and particle size, irrespective of O₃ exposure concentration. According to the Kelvin equation, the θ decrease induced by O₃-aging can alone contribute to the IN enhancement of soot, given that the lower the θ is, the lower the RH condition required to trigger capillary condensation. In conclusion, this study highlights the single importance of θ in soot PCF in the cirrus regime, by modifying the soot-water θ through O₃-aging while remaining the porosity.

How to cite: Gao, K. and Kanji, Z.: The importance of soot-water contact angle in soot ice nucleation ability in the cirrus regime, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-138, https://doi.org/10.5194/egusphere-egu23-138, 2023.

Mineral dust is the main responsible ice nucleating particle (INP) type for mixed-phase clouds and has an influence over regions far from the dust source. Parameterizations of the dust ice nucleation (IN) ability used in models are normally obtained from experimental characterization of dust collected from the surface at origin (Ullrich et al., 2017). However, IN properties of long-range transported dust may differ from the dust at origin due to different mineralogy and physical or chemical transformations along the transport route. Here we present preliminary results of ice nucleation properties of Saharan dust transported to Spain and Finland in the temperature range relevant for mixed-phase cloud formation.

In this work, the IN ability in immersion mode is evaluated using an Mk-1 droplet-freezing cold stage (Sikora Scientific Instrumentation). The drop-freezing assays are performed by depositing a set of dust-containing droplets (1 μL) onto a hydrophobic glass that rests atop the cold stage, whose temperature can be varied at a specific cooling rate. A high-resolution camera captures images tracking the freezing events, while the temperature is decreased. Dilutions of the original suspensions are prepared to explore the colder temperature range. Heat treatments are applied to the samples to investigate the contribution of the sample’s biological-origin components to its ice nucleation ability. The BET surface area is measured for selected samples.

This work was supported by the Academy of Finland Flagship ACCC (grant no. 337552) and MEDICEN project (grant no. 345125).

Ullrich, R. et al. (2017). A New Ice Nucleation Active Site Parameterization for Desert Dust and Soot J. Atmos. Sci., 74, 699-717.

How to cite: Piedehierro, A. A., Welti, A., Mustonen, L., Meinander, O., Viisanen, Y., and Laaksonen, A.: Long-range transport effect on the ice nucleation properties of Saharan dust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12590, https://doi.org/10.5194/egusphere-egu23-12590, 2023.