About

Shabnam Akhtari is a professor at the Pennsylvania State University, based in the McAllister Building. Her research interests include Number Theory, Geometry of Numbers, and Diophantine Analysis. Her work focuses on these areas, contributing to the understanding of their underlying mathematical structures and properties.

Research topics

• Chemistry
• Computer Science
• Organic chemistry
• Environmental chemistry
• Meteorology
• Materials science
• Thermodynamics
• Chromatography
• Geology
• Physics
• Chemical engineering
• Chemical physics

Selected publications

• Immersion Freezing Efficiency of ZnAl ₂ O ₄ and MgAl ₂ O ₄ Spinels, ZnO, and MgO: The Role of Oxygen Vacancies

  The Journal of Physical Chemistry A · 2026-02-09

  articleOpen accessSenior author

  is more stable on the surfaces of the Zn-containing oxides. The study suggests that oxygen vacancies, which are common defects on metal oxide surfaces that affect their adsorption and catalytic properties, can influence the efficiency with which mineral dust aerosol particles activate ice formation and affect cloud radiative forcing.

  PublisherDOI

• Immersion Freezing Efficiency of ZnAl2O4 and MgAl2O4 Spinels, ZnO, and MgO: The Role of Oxygen Vacancies

  ChemRxiv · 2026-01-28

  articleOpen accessSenior author

  Aerosol particles that catalyze ice nucleation alter the optical properties and precipitation cycles of clouds. Although mineral dust aerosol particles containing metal oxides are susceptible to the formation of oxygen vacancies (VO) on their surfaces, the impact of these defects on ice nucleation activity has not been addressed. To investigate the impact of VO sites, we conducted a droplet immersion freezing assay on zinc aluminate (ZnAl2O4) and magnesium aluminate (MgAl2O4) spinels annealed under air, nitrogen, and oxygen atmospheres. We observe that samples annealed under nitrogen promote ice nucleation at warmer temperatures compared to those treated in oxidizing atmospheres, with the effecting being most pronounced for ZnAl2O4. To further understand these results, we investigated the immersion freezing of zinc oxide (ZnO) and magnesium oxide (MgO). Here, we observe that ZnO nucleates ice at substantially warmer temperatures than MgO after annealing under nitrogen. We hypothesize the trends in ice nucleation activity are due to the varying concentrations of VO that form during the annealing process on the oxide surfaces, which tend to be higher in the absence of O2. Density functional theory (DFT) calculations support our hypothesis, indicating VO are more stable on the surfaces of the Zn-containing oxides. The study suggests that oxygen vacancies, which are common defects on metal oxide surfaces that affect their adsorption and catalytic properties, can influence the efficiency with which mineral dust aerosol particles activate ice formation and affect cloud radiative forcing.

  PublisherOA PDFDOI

• Immersion Freezing Efficiency of ZnAl2O4 and MgAl2O4 Spinels, ZnO, and MgO: The Role of Oxygen Vacancies

  ChemRxiv · 2025-09-23

  articleSenior author

  Aerosol particles that catalyze ice nucleation alter the optical properties and precipitation cycles of clouds. Although mineral dust aerosol particles containing metal oxides are susceptible to the formation of oxygen vacancies (VO) on their surfaces, the impact of these defects on ice nucleation activity has not been addressed. To investigate the impact of VO sites, we conducted a droplet immersion freezing assay on zinc aluminate (ZnAl2O4) and magnesium aluminate (MgAl2O4) spinels annealed under air, nitrogen, and oxygen atmospheres. We observe that samples annealed under nitrogen promote ice nucleation at warmer temperatures compared to those treated in oxidizing atmospheres, with the effecting being most pronounced for ZnAl2O4. To further understand these results, we investigated the immersion freezing of zinc oxide (ZnO) and magnesium oxide (MgO). Here, we observe that ZnO nucleates ice at substantially warmer temperatures than MgO after annealing under nitrogen. We hypothesize the trends in ice nucleation activity are due to the varying concentrations of VO that form during the annealing process on the oxide surfaces, which tend to be higher in the absence of O2. Density functional theory (DFT) calculations support our hypothesis, indicating VO are more stable on the surfaces of the Zn-containing oxides. The study suggests that oxygen vacancies, which are common defects on metal oxide surfaces that affect their adsorption and catalytic properties, can influence the efficiency with which mineral dust aerosol particles activate ice formation and affect cloud radiative forcing.

  PublisherDOI

• Immersion Freezing Efficiency of ZnAl2O4 and MgAl2O4 Spinels, ZnO, and MgO: The Role of Oxygen Vacancies

  ChemRxiv · 2025-12-09

  articleSenior author

  Aerosol particles that catalyze ice nucleation alter the optical properties and precipitation cycles of clouds. Although mineral dust aerosol particles containing metal oxides are susceptible to the formation of oxygen vacancies (VO) on their surfaces, the impact of these defects on ice nucleation activity has not been addressed. To investigate the impact of VO sites, we conducted a droplet immersion freezing assay on zinc aluminate (ZnAl2O4) and magnesium aluminate (MgAl2O4) spinels annealed under air, nitrogen, and oxygen atmospheres. We observe that samples annealed under nitrogen promote ice nucleation at warmer temperatures compared to those treated in oxidizing atmospheres, with the effecting being most pronounced for ZnAl2O4. To further understand these results, we investigated the immersion freezing of zinc oxide (ZnO) and magnesium oxide (MgO). Here, we observe that ZnO nucleates ice at substantially warmer temperatures than MgO after annealing under nitrogen. We hypothesize the trends in ice nucleation activity are due to the varying concentrations of VO that form during the annealing process on the oxide surfaces, which tend to be higher in the absence of O2. Density functional theory (DFT) calculations support our hypothesis, indicating VO are more stable on the surfaces of the Zn-containing oxides. The study suggests that oxygen vacancies, which are common defects on metal oxide surfaces that affect their adsorption and catalytic properties, can influence the efficiency with which mineral dust aerosol particles activate ice formation and affect cloud radiative forcing.

  PublisherDOI

• Salting out and nitrogen effects on cloud-nucleating ability of amino acid aerosol mixtures

  Environmental Science Atmospheres · 2025-01-01 · 4 citations

  articleOpen access

  We investigate the water uptake ability of amino acid ternary mixtures.

  PublisherOA PDFDOI

• Influence of Salting Out and Organic Nitrogen on Mixed Amino Acid Aerosol Cloud-Nucleating Ability

  2025-03-14

  preprintOpen access

  Aerosols are present as complex organic-inorganic mixtures within our atmosphere, resulting in particles presenting phase separated morphology. Mixed organic-inorganic aerosols can be predominantly found in nascent sea spray aerosols (SSA). When these aerosols are exposed to supersaturated conditions (>100% RH), the water uptake ability of the aerosols vary based on the composition of the mixture. Previous studies have characterized phase separated systems through the determination of an average oxygen to carbon (O/C) ratio where liquid-liquid phase separation (LLPS) reaches its limit. The hygroscopicity of complex mixtures presenting LLPS was previously studied through the measurement of CCN activity within a 2-methylglutaric (2-MGA)/ammonium sulfate (AS) binary system and a 2-MGA/AS/sucrose ternary system; both studies correlated water-uptake abilities to O/C and surface tension. However, little is known about the influence of solubility of the third component on phase separation of a ternary mixture containing 2-MGA/AS. Additionally, the water-uptake properties of mixtures containing nitrogen containing compounds, such as amino acids, are not well defined. Amino acids are a major component of SSA and can contribute to aerosol hygroscopicity. Therefore, it is undetermined if O/C alone is an acceptable parameter for the estimation of solubility and hygroscopicity of complex amino acid mixtures. To improve our understanding of LLPS within aerosol mixtures and factors influencing its presence, three ternary systems were studied – a leucine system (2-MGA/AS/leucine), valine system (2-MGA/AS/valine), and proline system (2-MGA/AS/proline). For each system, the CCN activity of mixture compositions with varying O/C ratios and compositions was measured using a Cloud Condensation Nuclei Counter (CCNC) at 0.375% to 1.667% SS. For all mixtures, the single hygroscopic parameter κ was calculated. Experimental κ-results demonstrated increased hygroscopic activity as the amino acid became more soluble in the order of leucine

  PublisherDOI

• Predicting Liquid-Liquid Phase Separation of Submicron Proxies for Atmospheric Secondary Aerosol

  ChemRxiv · 2025-01-20

  preprintOpen accessSenior author

  Liquid–liquid phase separation (LLPS) of atmospheric aerosols can significantly impact climate, air quality, and human health. However, their complex composition, small size, and history-dependent properties result in great uncertainty in the modeling of aerosol phase state and atmospheric processes. Herein, using cryogenic transmission electron microscopy (cryo-TEM), we examined model submicron aerosols composed of organic compounds and ammonium sulfate, and established a parameterization for the separation relative humidity (SRH) that accounts for chemical composition, particle size, and equilibration time. We evaluated different variables that describe chemical composition: O/C ratio, partition coefficient, solubility, molar mass, and polarizability. The O/C ratio fits the SRH of micrometer droplets best, and by using a scaling factor to translate the micrometer SRH parameterization to submicron aerosols, we incorporate the effects of size and equilibration time. The measured scaling factor for the submicron mean SRH (30nm – 1𝜇m, 20 min equilibration times) is 0.80, the factor becomes 1 with equilibration time over 1 hour, and is equal to 0, meaning that SRH is absent, when the aerosol dry diameter is smaller than 30 nm. Our parameterization will aid in universal SRH modeling, potentially leading to more accurate predictions of aerosol mass, optical properties, hygroscopicity, and heterogeneous chemistry.

  PublisherOA PDFDOI

• Predicting Liquid–Liquid Phase Separation of Submicrometer Proxies for Atmospheric Secondary Aerosol

  ACS ES&T Air · 2025-03-04 · 1 citations

  articleOpen accessSenior authorCorresponding

  Liquid–liquid phase separation (LLPS) of atmospheric aerosols can significantly impact climate, air quality, and human health. However, their complex composition, small size, and history-dependent properties result in great uncertainty in the modeling of aerosol phase state and atmospheric processes. Herein, using cryogenic transmission electron microscopy (cryo-TEM), we examined model submicron aerosols composed of organic compounds and ammonium sulfate and established a parametrization for the separation relative humidity (SRH) that accounts for chemical composition, particle size, and equilibration time. We evaluated different variables that describe chemical composition: O/C ratio, partition coefficient, solubility, molar mass, and polarizability. The O/C ratio fits the SRH of micrometer droplets best, and by using a scaling factor to translate the micrometer SRH parametrization to submicron aerosols, we incorporate the effects of size and equilibration time. The measured scaling factor for the submicron mean SRH (30 nm–1 μm, 20 min equilibration times) is 0.80, and the factor becomes 1 with equilibration time over 1 h and is equal to 0, meaning that SRH is absent, when the aerosol dry diameter is smaller than 30 nm. Our parametrization will aid in universal SRH modeling, potentially leading to more accurate predictions of aerosol mass, optical properties, hygroscopicity, and heterogeneous chemistry.

  PublisherDOI

• Pristine and Aged Microplastics Can Nucleate Ice through Immersion Freezing

  ACS ES&T Air · 2024-11-07 · 17 citations

  articleOpen accessSenior authorCorresponding

  Microplastics (MP) are ubiquitous in the environment; their atmospheric relevance is being increasingly recognized. Because of their atmospheric concentrations, there is the question of whether MP can act as ice nucleating particles in the atmosphere. This study investigates the immersion freezing activity of lab-prepared MP of four different compositions─low density polyethylene (LDPE), polypropylene (PP), poly(vinyl chloride) (PVC), and poly(ethylene terephthalate) (PET)─using droplet freezing assays. The MP are also exposed to ultraviolet light, ozone, sulfuric acid, and ammonium sulfate to mimic environmental aging of the plastics to elucidate the role that these processes play in the ice nucleating activity of MP. Results show that all studied MP act as immersion nuclei, and aging processes can modify this ice nucleating activity, leading, primarily, to decreases in ice nucleating activity for LDPE, PP, and PET. The ice nucleating activity of PVC generally increased following aging, which we attribute to a cleaning of chemical defects present on the surface of the stock material. Chemical changes were monitored with infrared spectroscopy (ATR-FTIR), and the growth of a peak at 1650–1800 cm–1 was associated with a decrease in ice nucleating activity while loss of an existing peak in that region was associated with an increase in ice nucleating activity. The studied MP have ice nucleating activities sufficient to be a non-negligible source of ice nucleating particles in the atmosphere if present in sufficiently high concentrations.

  PublisherOA PDFDOI

• Pristine and Aged Microplastics Can Nucleate Ice Through Immersion Freezing

  ChemRxiv · 2024-06-17 · 3 citations

  preprintOpen accessSenior author

  Microplastics (MP) are ubiquitous in the environment; their atmospheric relevance is increasingly recognized. Because of their atmospheric concentrations, a question exists as to whether MP can act as ice nucleating particles in the atmosphere. This study investigates the immersion freezing activity of lab-prepared MP of four different compositions—low density polyethylene (LDPE), polypropylene (PP), poly(vinyl chloride) (PVC), and polyethylene terephthalate (PET)—using droplet freezing assays. The MP are also exposed to ultraviolet light, ozone, sulfuric acid, and ammonium sulfate to mimic environmental aging of the plastics to elucidate the role that these processes play in the ice nucleating activity of MP. Results show that all studied MP act as immersion nuclei and aging processes can modify this ice nucleating activity, leading, primarily, to decreases in ice nucleating activity for LDPE, PP, and PET. The ice nucleating activity of PVC generally increased following aging which we attribute to a cleaning of chemical defects present on the surface of the stock material. Chemical changes were monitored with infrared spectroscopy (ATR-FTIR) and the growth of a peak at 1650-1800 cm-1 was associated with a decrease in ice nucleating activity while loss of an existing peak in that region was associated with an increase in ice nucleating activity. The studied MP have ice nucleating activities sufficient to be a non-negligible source of ice nucleating particles in the atmosphere if present in sufficiently high concentrations.

  PublisherOA PDFDOI

Recent grants

• CAREER: Molecular Scale Characterization of Water Adsorption on Surfaces Relevant to Cirrus Cloud Formation

  NSF · $534k · 2014–2020

• Collaborative Research: Effects of Aerosol Phase, Morphology, and Mixing State on Droplet Formation

  NSF · $299k · 2017–2021

• Surface Characteristics that Drive Heterogeneous Ice Nucleation

  NSF · $493k · 2019–2023

Frequent coauthors

• Margaret A. Tolbert

  University of Colorado Boulder

  42 shared

• C. A. Hasenkopf

  25 shared

• Daniel P. Veghte

  The Ohio State University

  20 shared

• M. R. Beaver

  Environmental Protection Agency

  18 shared

• Muhammad Altaf

  University of Education

  17 shared

• K. J. Baustian

  15 shared

• Matthew E. Wise

  Maryland Aerospace (United States)

  13 shared

• Esther Chong

  Pennsylvania State University

  12 shared

Labs

• Freedman Research GroupPI

Awards & honors

• Fellow, Penn State Institutes for Energy and the Environment…
• ACS Physical Chemistry Division Early-Career Award for Exper…
• Penn State Eberly College of Science Dean’s Climate and Dive…
• NSF Career Award 2014-2020
• NOAA Climate and Global Change Postdoctoral Fellowship 2008-…