Fabian Mahrt

Last Name

Mahrt

First Name

Fabian

ORCID

0000-0002-7059-6765

Organisational unit

01709 - Lehre Umweltsystemwissenschaften

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Publications1 - 10 of 19

The Role of Cloud Processing for the Ice Nucleating Ability of Organic Aerosol and Coal Fly Ash Particles
Kilchhofer, Kevin; Mahrt, Fabian; Kanji, Zamin (2021)
Journal of Geophysical Research: Atmospheres
Ice nucleating particles are a minor fraction of tropospheric aerosol, yet they play a key role for cloud microphysical processes. One poorly understood process is the impact of atmospheric aging of aerosol particles on ice nucleation. Here we study the impact of cloud processing on the ice nucleation abilities of two physicochemically different aerosol particles by taking two model systems for atmospheric organic aerosol (OA), as well as coal fly ash (CFA) particles representing an inorganic aerosol type. The ice nucleation activity of the unprocessed particles is compared to aerosol particles that are first exposed to conditions mimicking trajectories though cirrus clouds (CC) and mixed-phase clouds (MPC) prior to testing their ice nucleation activity at temperatures below 243 K. We observed that unprocessed OA do not exhibit heterogeneous ice nucleation, requiring homogeneous freezing conditions of solution droplets to form ice. However, after CC processing raffinose particles showed heterogeneous ice nucleation activity at 218 K and a water saturation ratio of 0.68–0.82, reaching activated fractions of up to 0.3. This enhancement compared to unprocessed raffinose particles results from an increase in particle viscosity upon CC processing. We also present new results of unprocessed CFA particles exhibiting strong heterogeneous ice nucleation activity at temperatures below 235 K in the deposition and/or pore condensation and freezing mode. In contrast to the OA, the CFA show a decrease in ice nucleation activity after both MPC and CC processing. Furthermore, cloud processing and generating CFA particles from aqueous suspensions do not have the same effect on their ice nucleation ability.
Future warming exacerbated by aged-soot effect on cloud formation
Lohmann, Ulrike; Friebel, Franz; Kanji, Zamin; et al. (2020)
Nature Geoscience
Clouds play a critical role in modulating the Earth’s radiation balance and climate. Anthropogenic aerosol particles that undergo aging processes, such as soot, aid cloud droplet and ice crystal formation and thus influence the microphysical structure of clouds. However, the associated changes in cloud radiative properties and climate effects remain uncertain and are largely omitted in climate models. Here we present global climate simulations of past and future effects of both ozone-aged soot particles acting as cloud condensation nuclei and sulfuric acid-aged soot particles acting as ice-nucleating particles on the structure and radiative effects of clouds. Under pre-industrial conditions, soot aging led to an increase in thick, low-level clouds that reduced negative shortwave effective radiative forcing by 0.2 to 0.3 W m−2. In the simulations of a future, warmer climate under double pre-industrial atmospheric carbon dioxide concentrations, soot aging and compensating cloud adjustments led to a reduction in low-level clouds and enhanced high-altitude cirrus cloud thickness, which influenced the longwave radiative balance and exacerbated the global mean surface warming by 0.4 to 0.5 K. Our findings suggest that reducing emissions of soot particles is beneficial for future climate, in addition to air quality and human health.
Process-oriented analysis of aircraft soot-cirrus interactions constrains the climate impact of aviation
Kaercher, Bernd; Mahrt, Fabian; Marcolli, Claudia (2021)
Communications Earth & Environment
Fully accounting for the climate impact of aviation requires a process-level understanding of the impact of aircraft soot particle emissions on the formation of ice clouds. Assessing this impact with the help of global climate models remains elusive and direct observations are lacking. Here we use a high-resolution cirrus column model to investigate how aircraft-emitted soot particles, released after ice crystals sublimate at the end of the lifetime of contrails and contrail cirrus, perturb the formation of cirrus. By allying cloud simulations with a measurement-based description of soot-induced ice formation, we find that only a small fraction (<1%) of the soot particles succeeds in forming cloud ice alongside homogeneous freezing of liquid aerosol droplets. Thus, soot-perturbed and homogeneously-formed cirrus fundamentally do not differ in optical depth. Our results imply that climate model estimates of global radiative forcing from interactions between aircraft soot and large-scale cirrus may be overestimates. The improved scientific understanding reported here provides a process-based underpinning for improved climate model parametrizations and targeted field observations.
Aging induced changes in ice nucleation activity of combustion aerosol as determined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy
Mahrt, Fabian; Alpert, Peter A.; Dou, Jing; et al. (2020)
Environmental Science: Processes & Impacts
Fresh soot particles are generally hydrophobic, however, particle hydrophilicity can be increased through atmospheric aging processes. At present little is known on how particle chemical composition and hydrophilicity change upon atmospheric aging and associated uncertainties governing the ice cloud formation potential of soot. Here we sampled two propane flame soots referred to as brown and black soot, characterized as organic carbon rich and poor, respectively. We investigated how the ice nucleation activity of these particles changed through aging in water and aqueous acidic solutions, using a continuous flow diffusion chamber operated at cirrus cloud temperatures (T ≤ 233 K). Single aggregates of both unaged and aged soot were chemically characterized by scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (STXM/NEXAFS) measurements. Particle wettability was determined through water sorption measurements. Unaged black and brown soot particles exhibited significantly different ice nucleation activities. Our experiments revealed significantly enhanced ice nucleation activity of the aged soot particles compared to the fresh samples, lowering the required relative humidities at which ice formation can take place at T = 218 K by up to 15% with respect to water (ΔRHi ≈ 25%). We observed an enhanced water uptake capacity for the aged compared to the unaged samples, which was more pronounced for the black soot. From these measurements we concluded that there is a change in ice nucleation mechanism when aging brown soot. Comparison of the NEXAFS spectra of unaged soot samples revealed a unique spectral feature around 287.5 eV in the case of black soot that was absent for the brown soot, indicative of carbon with hydroxyl functionalities. Comparison of the NEXAFS spectra of unaged and aged soot particles indicates changes in organic functional groups, and the aged spectra were found to be largely similar across soot types, with the exception of the water aged brown soot. Overall, we conclude that atmospheric aging is important to representatively assess the ice cloud formation activity of soot particles.
Pore condensation and freezing is responsible for ice formation below water saturation for porous particles
David, Robert O.; Marcolli, Claudia; Fahrni, Jonas; et al. (2019)
Proceedings of the National Academy of Sciences of the United States of America
Ice nucleation in the atmosphere influences cloud properties, altering precipitation and the radiative balance, ultimately regulating Earth’s climate. An accepted ice nucleation pathway, known as deposition nucleation, assumes a direct transition of water from the vapor to the ice phase, without an intermediate liquid phase. However, studies have shown that nucleation occurs through a liquid phase in porous particles with narrow cracks or surface imperfections where the condensation of liquid below water saturation can occur, questioning the validity of deposition nucleation. We show that deposition nucleation cannot explain the strongly enhanced ice nucleation efficiency of porous compared with nonporous particles at temperatures below −40 °C and the absence of ice nucleation below water saturation at −35 °C. Using classical nucleation theory (CNT) and molecular dynamics simulations (MDS), we show that a network of closely spaced pores is necessary to overcome the barrier for macroscopic ice-crystal growth from narrow cylindrical pores. In the absence of pores, CNT predicts that the nucleation barrier is insurmountable, consistent with the absence of ice formation in MDS. Our results confirm that pore condensation and freezing (PCF), i.e., a mechanism of ice formation that proceeds via liquid water condensation in pores, is a dominant pathway for atmospheric ice nucleation below water saturation. We conclude that the ice nucleation activity of particles in the cirrus regime is determined by the porosity and wettability of pores. PCF represents a mechanism by which porous particles like dust could impact cloud radiative forcing and, thus, the climate via ice cloud formation.
Photochemistry of iron-containing secondary organic aerosol is impacted by relative humidity during formation
Garner, Natasha M.; Mahrt, Fabian; Top, Jens; et al. (2025)
npj Climate and Atmospheric Science
Secondary organic aerosol (SOA) comprises most of the submicron atmospheric particle mass, and often becomes internally mixed with other particles. When SOA mixes with transition metal (e.g., iron) containing particles, metal-organic complexes can form, enabling photochemical reactions that change aerosol physicochemical properties. We studied the photochemistry of alpha-pinene SOA formed on iron-containing ammonium sulfate seed particles at varying relative humidities (RH). Chemical composition and photochemical reduction of particles were analyzed by X-ray spectromicroscopy and infrared spectroscopy. SOA formed at low vs. high RH had different chemical functionality, including abundant carboxylic acids and alcohols. Following photolysis, carboxylic acids and unsubstituted alkanes decreased, and alcohols increased, consistent with decarboxylation reactions. Iron in SOA formed at high RH was readily photochemically reduced, but iron in SOA formed at low RH was not. Overall, RH conditions at SOA formation affect not only chemical composition but also iron-complex formation and hence photochemical processing of aerosols.
A burning issue - Soot particles acting as ice cloud seeds
Mahrt, Fabian (2019)
The ice phase in tropospheric clouds plays a key role for many atmospheric processes. Ice crystals are important as surfaces for heterogeneous chemical reactions, influence the hydrological cycle through initiation of precipitation formation and determine climate by impacting Earth's energy balance. Ice crystals can form in cirrus clouds at temperatures below 235 K and in mixed-phase clouds (MPCs), where the temperature (T) range is between 235-273 K and supercooled cloud droplets and ice crystals can co-exists in a thermodynamically metastable state. The initial formation of an ice crystal is usually catalyzed by aerosols that can act as ice nucleating particles (INPs), through lowering of the thermodynamic energy barrier that is associated with forming the ice phase. Despite significant advances in understanding atmospheric ice formation over the past decades, representation of the ice phase in global climate models remains associated with major uncertainties in terms of projected radiative forcing. This mainly results from a lack of understanding the aerosol-cloud interactions and their feedback processes, such as impacts on cloud phase, lifetime and albedo effects. While some aerosol species, such as mineral dust, have been identified as important INPs and environmental conditions required for dust particles to nucleate ice seem to be well understood, research on the ice nucleation activity of soot particles remains inconclusive. This despite soot being considered the second most important radiative forcing agent and constituting a major anthropogenic pollutant from fossil fuel and biomass combustion. The contradictory results of the ice formation ability of soot are mainly based on an insufficient understanding of the relationship between the physicochemical particle properties and ice nucleation activity and mechanism, respectively. In addition, the link of the particle properties to how these can change upon atmospheric aging of the soot particles is largely unknown. As such, understanding of these processes is required to quantify the impacts of anthropogenic climate change. The goal of this thesis is to improve the understanding of soot ice nucleation through an experimental investigation of the cloud formation abilities of different combustion particles in a controlled laboratory setup. Laboratory measurements were performed on six different soot types. The ice nucleation activity was studied using a continuous flow diffusion chamber (CFDC) setup over a temperature range from 218-253 K, covering the MPC and cirrus regime, as a function of relative humidity (RH) and soot particle size. Particle physicochemial properties were characterized by a suite of auxiliary measurements, including water vapor sorption and thermogravimetric analysis. A strong dependence on the temperature regime was found, with ice nucleation being absent for T > 235 K, where only supercooled liquid cloud droplets formed on the soot particles. At cirrus temperatures, a dependence of ice nucleation on the particle size was observed, with larger particles showing ice nucleation activity at lower ice supersaturations at a given T. The ice nucleation was strongly linked to the particle properties, in particular the wettability of the soot. This, along with the dependence on the temperature regime revealed a pore condensation and freezing (PCF) mechanism to be the responsible ice nucleation mechanism. The impact of atmospheric aging on the ice nucleation activity of soot was investigated through two independent studies, addressing the effect of cloud processing and aging of particles in acidic aqueous solutions, respectively. To test the role of cloud processing on soot ice nucleation activity, a new experimental platform was developed, where two CFDCs are coupled in series. The setup allows to mimic different cloud processing scenarios in the first CFDC and subsequently test the ice nucleation activity of the cloud processed aerosol particles within the second CFDC. Upon cloud processing, the soot properties changed with processed particles showing enhanced cloud formation potential at T < 233 K. In particular, the cloud processed particles were found to show a change in wettability and particle morphology, with processed soot aggregates forming more compact clusters with increased total pore volume and wettability. An increased ice nucleation activity was found, independent of the cloud processing scenario. Furthermore, the formation of a cloud hydrometeor was identified to be the key factor for enhancing the ice nucleation activity. Overall the better ice nucleation ability was attributed to an enhanced ice formation via PCF, resulting from the changes in particle properties upon cloud processing. Moreover, the effect on soot ice nucleation by aging of soot particles in acidic aqueous solutions, mimicking aging in cloud and haze droplets was investigated. Aging was achieved through exposure of the soots to slightly acidic and aqueous solutions containing sulfuric acid and pure water only. Aged soot samples showed a significantly enhanced ice nucleation activity, which could be linked to distinct chemical functionalities present on the particles by characterization using near edge X-ray fine structure spectroscopy. Finally, to further the understanding on cloud phase fractionation, a new instrument, the high speed particle phase discriminator (PPD-HS) was tested and characterized. PPD-HS is shown to accurately discriminate the shape of spherical cloud droplets and aspherical ice crystals based on symmetry analysis of the spatial intensity distribution of near-forward scattered light. The lower size limit for particle shape discrimination using PPD-HS was experimentally found to be approximately 3 micrometer, with detection rates of a few hundred particles per second, thus showing enhanced capabilities to previous devices. Deployment of PPD-HS within a CFDC setup was successfully tested and provides an alternative to the previous deployment of a particle size threshold for discriminating cloud droplets and ice crystals at MPC conditions. Hence, phase discrimination can be achieved largely independent of particle size when using of PPD-HS.
Quantifying and improving the optical performance of the laser ablation aerosol particle time of flight mass spectrometer (LAAPToF) instrument
Zawadowicz, Maria A.; Lance, Sara; Jayne, John T.; et al. (2020)
Aerosol Science and Technology
Physicochemical properties of charcoal aerosols derived from biomass pyrolysis affect their ice-nucleating abilities at cirrus and mixed-phase cloud conditions
Mahrt, Fabian; Rösch, Carolin; Gao, Kunfeng; et al. (2023)
Atmospheric Chemistry and Physics
Atmospheric aerosol particles play a key role in air pollution, health, and climate. Particles from biomass burning emissions are an important source of ambient aerosols, have increased over the past few decades, and are projected to further surge in the future as a result of climate and land use changes. Largely as a result of the variety of organic fuel materials and combustion types, particles emitted from biomass burning are often complex mixtures of inorganic and organic materials, with soot, ash, and charcoal having previously been identified as main particle types being emitted. Despite their importance for climate, their ice nucleationactivities remain insufficiently understood, in particular for charcoalparticles, whose ice nucleation activity has not been reported. Here, wepresent experiments of the ice nucleation activities of 400 nm size-selected charcoal particles, derived from the pyrolysis of two different biomass fuels, namely a grass charcoal and a wood charcoal. We find that the pyrolysis-derived charcoal types investigated do not contribute to ice formation via immersion freezing in mixed-phase cloud conditions. However, our results reveal considerable heterogeneous ice nucleation activity of both charcoal types at cirrus temperatures. An inspection of the ice nucleation results together with dynamic vapor sorption measurements indicates that cirrus ice formation proceeds via pore condensation and freezing. We find wood charcoal to be more ice-active than grass charcoal at cirrus temperatures. We attribute this to the enhanced porosity and water uptake capacity of the wood compared to the grass charcoal. In support of the results, we found a positive correlation of the ice nucleation activity of the wood charcoal particles and their chemical composition, specifically the presence of (inorganic) mineral components, based on single-particle mass spectrometry measurements. Even though correlational in nature, our results corroborate recent findings that ice-active minerals could largely govern the aerosol-cloud interactions of particles emitted from biomass burning emissions.
Soot aerosols from commercial aviation engines are poor ice-nucleating particles at cirrus cloud temperatures
Testa, Baptiste; Durdina, Lukas; Alpert, Peter A.; et al. (2024)
Atmospheric Chemistry and Physics
Ice-nucleating particles catalyze ice formation in clouds, affecting climate through radiative forcing from aerosol–cloud interactions. Aviation directly emits particles into the upper troposphere where ice formation conditions are favorable. Previous studies have used proxies of aviation soot to estimate their ice nucleation activity; however, investigations with commercial aircraft soot from modern in-use aircraft engines have not been quantified. In this work, we sample aviation soot particles at ground level from different commercial aircraft engines to test their ice nucleation ability at temperatures ≤ 228 K as a function of engine thrust and soot particle size. Additionally, soot particles were catalytically stripped to reveal the impact of mixing state on their ice nucleation ability. Particle physical and chemical properties were further characterized and related to the ice nucleation properties. The results show that aviation soot nucleates ice at or above relative humidity conditions required for homogeneous freezing of solution droplets (RHhom). We attribute this to a mesopore paucity inhibiting pore condensation and the sulfur content which suppresses freezing. Only large soot aggregates (400 nm) emitted under 30 %–100 % thrust conditions for a subset of engines (2 out of 10) nucleate ice via pore condensation and freezing. For those specific engines, the presence of hydrophilic chemical groups facilitates the nucleation. Aviation soot emitted at thrust ≥ 100 % (sea level thrust) nucleates ice at or above RHhom. Overall, our results suggest that aviation soot will not contribute to natural cirrus formation and can be used in models to update impacts of soot–cirrus clouds.

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Fabian Mahrt

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