About
The Grassian research group focuses on the chemistry and impacts of environmental interfaces including atmospheric aerosols, aqueous microdroplets, engineered and geochemical nanomaterials, and indoor surfaces.
Research topics
- Chemistry
- Nanotechnology
- Environmental chemistry
- Environmental science
- Geography
- Geology
- Engineering
- Chemical engineering
- Materials science
- Meteorology
- Organic chemistry
- Oceanography
- Biochemical engineering
Selected publications
ChemRxiv · 2026-02-11
articleOpen accessSenior authorSulfur speciation and pH are important parameters in aerosol chemistry that influence the oxidation of S(IV) to S(VI), a critical process that contributes to haze pollution. Here, we monitor the evolution of the composition and pH of aqueous aerosols (radii ~4 μm) prepared from bulk solutions containing sulfite/bisulfite through quantitative spectroscopic measurements utilizing an aerosol optical tweezer coupled to cavity enhanced Raman spectroscopy. Under N 2 at ~80% relative humidity, individual, trapped aqueous aerosols are generated from solutions ranging from ca. pH 2 to 6 with a vibrating mesh nebulizer. Aerosols show a significant loss of sulfur relative to bulk solutions due to SO 2 evaporation with a concomitant increase in aerosol pH. An ambient pressure mass spectrometer confirms SO 2 partitioning from the aerosol into the gas phase. Most interesting is that the loss of sulfur from the aerosol due to the partitioning of gas-phase SO 2 occurs at a higher solution pH compared to bulk solutions, suggesting an enrichment of protons at the aerosol surface that plays a role in the protonation of bisulfite, a precursor to the formation of gasphase SO 2 . An electrical low-pressure impactor was used to determine that the aerosols have a slight positive charge. In air (21% O 2 in N 2 ), S(IV) is completely oxidized to S(VI), as sulfate, on the time scale of minutes and occurs several hundred times faster than for bulk solutions. In addition to soluble S(IV) oxidation, we also establish the time scales for gas-phase partitioning and aerosol pH changes. Overall, this study shows the need to quantify aqueous aerosol composition, pH, and charge following production from bulk solutions and the role of aqueous surfaces in driving compositional changes, highlighting the role of surfaces in multiphase sulfur chemistry for accurate representation in air quality models.
The Journal of Physical Chemistry Letters · 2026-04-02
articleOpen accessSenior authorCorrespondingDNA, a highly negatively charged polyelectrolyte biomolecule, shows negligible propensity for the air–water interface, as DNA prefers to remain fully hydrated in aqueous solutions. Previous studies have shown that cationic surfactants can complex with DNA at the air–water interface through electrostatic interactions and drive DNA to the interface. Here we show a new mechanism, involving DNA complexes to palmitic acid monolayers in the presence of divalent cations Mg2+ and Ca2+, suggesting that the interaction occurs through a divalent cation bridge that stabilizes DNA at the air–water interface. Interestingly, we find that the surface propensity of DNA depends on the base sequence due to differential nucleotide affinities for interfacial complexation. Oligomers with higher AT content were the most surface-active. Overall, this study provides new insights into DNA at the air–water interface under environmentally and biologically relevant conditions.
The Journal of Physical Chemistry Letters · 2026-01-08 · 1 citations
articleOpen accessSenior authorCorrespondingSea spray aerosols (SSAs), generated through oceanic bubble bursting, are chemically complex particles that significantly influence climate processes and ecosystem health. These aerosols are enriched with biological macromolecules such as enzymes and proteins, whose structure and activity at the air–water interface remain poorly understood, particularly under the highly variable pH conditions of SSAs. In this study, we investigate the pH-dependent surface activity of Burkholderia cepacia lipase (BCL), a model extracellular enzyme commonly found in marine environments. Using surface tension and infrared reflection–absorption spectroscopy (IRRAS) measurements, we observe that BCL exhibits increased surface propensity at higher pH compared to acidic conditions. All-atom molecular dynamics simulations further reveal molecular-level insight into these observations, showing structural changes in BCL at the interface in acidic environments with new, highly atmosphere exposed conformations. Additionally, we explore the heterogeneous reactivity of BCL-containing aerosol particles with gaseous nitric acid to identify potential reactive sites relevant to interactions with atmospheric trace gases. Understanding these heterogeneous reaction pathways of biological macromolecules not only may be relevant for SSAs but also has broad implications for the atmospheric reactivity of bioaerosols.
Size-Dependent Reaction Pathways in Multiphase SO2 Oxidation Chemistry
ChemRxiv · 2026-02-11
articleSenior authorThe paradigm for multiphase SO 2 chemistry in the atmosphere is traditionally based on bulk studies and involves a multistep process of uptake and hydrolysis of SO 2 followed by oxidation. Here we have investigated several processes relevant to SO 2 chemistry, including uptake, evaporation, hydrolysis, and oxidation, as a function of size in aqueous microdroplets. Our results reveal contrasting size dependencies for some of these processes. Most notably, SO 2 uptake is suppressed as the microdroplet radius ( r ) decreases, while oxidation to sulfate is strongly enhanced. The suppressed SO 2 uptake is explained by a size-dependent SO 2 evaporation rate scaling as 1/ r , driven by high interfacial proton activity that accelerates HSO 3 - protonation and SO 2 volatilization. The size dependence shows that oxidation proceeds over 100 times faster than uptake for typical atmospheric aerosols ( r ≤ 5 μm). Consequently, larger cloud and fog microdroplets follow the conventional multiphase pathway (i.e. surface SO 2 hydration and subsequent S(IV) (aq) oxidation) while smaller aqueous aerosols are dominated by direct oxidation at the surface. Overall, microdroplet size dictates the different reaction pathway, essentially acting as a mechanistic switch between them. This study highlights the complexity of multiphase chemistry and the critical roles of droplet size and the air/water interface, emphasizing how different size-dependencies in multistep processes can reshape our understanding of multiphase atmospheric processes.
The Journal of Physical Chemistry A · 2026-04-01
articleOpen accessSenior authorCorrespondingSulfur speciation and pH are important parameters in aerosol chemistry that influence the oxidation of S(IV) to S(VI), a critical process that contributes to haze pollution. Here, we monitor the evolution of the composition and pH of aqueous aerosols (radii ∼ 4 μm) prepared from bulk solutions containing sulfite/bisulfite through quantitative spectroscopic measurements utilizing an aerosol optical tweezer coupled to cavity-enhanced Raman spectroscopy. Under N2 at ∼80% relative humidity, individual, trapped aqueous aerosols are generated from solutions ranging from ca. pH 2–6 with a vibrating mesh nebulizer. Aerosols show a significant loss of sulfur relative to bulk solutions due to SO2 evaporation with a concomitant increase in the aerosol pH. An ambient pressure mass spectrometer confirms SO2 partitioning from the aerosol into the gas phase. Most interesting is that the loss of sulfur from the aerosol due to the partitioning of gas-phase SO2 occurs at a higher solution pH compared to bulk solutions, suggesting an enrichment of protons at the aerosol surface that plays a role in the protonation of bisulfite, a precursor to the formation of gas-phase SO2. An electrical low-pressure impactor was used to determine whether the aerosols have a slight positive charge. In air (21% O2 in N2), S(IV) is completely oxidized to S(VI), as sulfate, on the time scale of minutes and occurs several hundred times faster than for bulk solutions. In addition to soluble S(IV) oxidation, we also establish the time scales for gas-phase partitioning and aerosol pH changes. Overall, this study shows the need to quantify aqueous aerosol composition, pH, and charge following production from bulk solutions and the role of aqueous surfaces in driving compositional changes, highlighting the role of surfaces in multiphase sulfur chemistry for accurate representation in air quality models.
Environmental Science & Technology · 2026-03-21
articleSenior authorCorrespondingAtmospheric processing of Biomass Burning Aerosols (BBAs) changes the chemical composition of these aerosols, potentially affecting iron (Fe) solubility. To probe these changes, we conducted laboratory experiments using maghemite nanoparticles as a representative iron oxide phase. Dissolved Fe(II) concentrations were approximately 3 times higher, and total dissolved Fe was 1.4 times higher, when both catechol and oxalate were present compared to experiments with one organic compound (48 h, pH 2, HCl). In situ attenuated total reflectance Fourier transform infrared spectroscopy of the maghemite-aqueous interface shows that oxalate initially outcompetes catechol for surface binding, but as the reaction progresses, catechol forms inner-sphere complexes, promoting reductive dissolution. Iron isotope measurements indicate that oxalate-assisted dissolution accounts for most of the iron released during the first 6 h of the reaction, while catechol-assisted dissolution contributes substantially in later stages. The observed synergy in Fe(II) and total dissolved Fe arises from a combination of solution phase and surface reactions that occur when both compounds are present. These results suggest that the dark atmospheric processing of BBA can increase dissolved Fe(II) and total dissolved Fe, potentially contributing to the adverse health effects of smoke. These findings also highlight that mixtures produce outcomes not predictable from single compounds alone.
Size Dependent Uncatalyzed Sulfite Oxidation in Aqueous Microdroplets
Environmental Science & Technology · 2025-09-11 · 7 citations
articleOpen accessSenior authorCorrespondingAqueous aerosols and microdroplets exhibit unique chemical kinetics relative to the bulk phase, impacting air quality and climate via multiphase atmospheric processes. Here, we investigate uncatalyzed oxidation of sulfite to sulfate by O2 in aqueous microdroplets deposited on a superhydrophobic substrate, as a function of size, gas-phase composition, and temperature utilizing in situ micro-Raman spectroscopy. We show that the uncatalyzed sulfite oxidation is size-dependent across varying O2 concentrations and temperatures. Reaction rates scale with the surface-area to volume ratio (1/radius) of the microdroplet. We use a resistor-based approach to model multiphase mass transfer and reactions in the experimental system, confirming that the observed size-dependent kinetics reflect slow bulk kinetics coupled with an efficient reaction at the interface─k2 = 9.43 × 10–3 (+2.26 × 10–3/–2.38 × 10–3) M–1 s–1, γs,0 = 9.27 × 10–10 (+9.30 × 10–11/–1.70 × 10–10) at 298 K and 21% O2. Above a critical droplet radius, bulk kinetics dominate, but for sufficiently small atmospherically relevant sizes, rates are accelerated due to the role of the interfacial reaction. These results provide insights into chemistry in microcompartments and provide an outlook for improved representations of sulfate formation in atmospheric droplets and aerosols in large-scale atmospheric chemistry models.
Autobiography of Vicki H. Grassian
The Journal of Physical Chemistry A · 2025-08-28
articleOpen access1st authorCorrespondingEffects of Wind Speed on Water Uptake, Phase State, and Viscosity of Sea Spray Aerosols
ACS Earth and Space Chemistry · 2025-10-21
articleOpen accessThis study investigates the effects of wind speed on physicochemical properties such as water uptake, phase state, and viscosity at varying relative humidity (RH) of individual nascent sea spray aerosols (SSAs). We examined SSA sized within 0.1-0.6 μm generated from a wind-wave channel at two wind speeds: 10 m/s representing a wind lull scenario over the ocean and 19 m/s corresponding to wind speeds encountered in stormy conditions. Atomic force microscopy (AFM) was utilized to study two predominant SSA morphologies: core-shell and rounded. AFM phase state measurements at 60% RH revealed that shells of core-shells at 19 m/s were largely liquid, while those at 10 m/s were mostly semisolid or liquid with similar proportions, where semisolid shells exhibited higher viscosities at lower wind speed. Rounded SSAs were predominantly liquid or semisolid at 60% RH, with similar semisolid viscosities for both wind speeds. Increased water uptake was observed for core-shells at 19 m/s, while rounded SSA had similar hygroscopicity between the two wind conditions. Collectively, we observed a variation in the physicochemical properties of SSA generated at two wind speeds, which can be attributed to the impact of elevated wind speed on disrupting the sea surface microlayer film structure and composition.
Cell Reports Physical Science · 2025-07-24 · 4 citations
articleOpen accessSenior author<h2>Summary</h2> Environmental DNA (eDNA) can interact with various geochemical surfaces, including mineral surfaces. These interactions may alter the structure, stability, and reactivity of eDNA, impacting the cycling of genetic information and the accuracy of species identification. Here, we report on pH-dependent surface interactions between herring testes DNA adsorbed on titanium dioxide (TiO<sub>2</sub>) particle surfaces. Utilizing vibrational spectroscopic probes of surface-adsorbed DNA, DNA retains its native B-form at pH values from 6.5 to 8, whereas at pH 3–5, the structure of adsorbed DNA changes to the Z-form, which can undergo dehybridization. These changes are consistent with electrostatic and hydrogen-bonding interactions at the surface that drive these large structural changes within DNA. Overall, this study provides evidence for mineral-specific DNA-surface interactions that depend on pH and shows the role that interactions with geochemical surfaces may play in eDNA structure and stability.
Recent grants
NSF · $540k · 2010–2014
NSF · $100k · 2014–2016
NSF · $450k · 2016–2020
NSF · $115k · 2016–2017
NSF · $39k · 2016–2017
Frequent coauthors
- 130 shared
Kimberly A. Prather
Scripps Institution of Oceanography
- 55 shared
Christopher Lee
University of British Columbia
- 53 shared
Mark A. Young
University of California, San Diego
- 50 shared
Sarah C. Larsen
University of Houston
- 46 shared
P. D. Kleiber
University of Iowa
- 41 shared
Jonas Baltrušaitis
Lehigh University
- 38 shared
Francesca Malfatti
National Institute of Oceanography and Applied Geophysics
- 37 shared
Olga Laskina
West Pharmaceutical Services (United States)
Labs
Education
- 1987
PhD, Chemistry
UC Berkeley College of Chemistry
- 1982
M.S, Chemistry
Rensselaer Polytechnic Institute
- 1981
BS, Chemistry
University at Albany State University of New York
Awards & honors
- 2021 American Chemical Society Award in Surface Chemistry
- Sustainability Nanotechnology Award (2020)
- IUPAC Distinguished Woman in Chemistry or Chemical Engineeri…
- William H. Nichols Medal (2019)
- American Institute of Chemists Chemical Pioneer Award (2018)
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