Spatially distributed atmospheric boundary layer properties in Houston - A value-added observational dataset.

In 2022, Houston, TX became a nexus for field campaigns aiming to further our understanding of the feedbacks between convective clouds, aerosols and atmospheric boundary layer (ABL) properties. Houston's proximity to the Gulf of Mexico and Galveston Bay motivated the collection of spatially distributed observations to disentangle coastal and urban processes. This paper presents a value-added ABL dataset derived from observations collected by eight research teams over 46 days between 2 June - 18 September 2022. The dataset spans 14 sites distributed within a ~80-km radius around Houston. Measurements from three types of instruments are analyzed to objectively provide estimates of nine ABL parameters, both thermodynamic (potential temperature, and relative humidity profiles and thermodynamic ABL depth) and dynamic (horizontal wind speed and direction, mean vertical velocity, updraft and downdraft speed profiles, and dynamical ABL depth). Contextual information about cloud occurrence is also provided. The dataset is prepared on a uniform time-height grid of 1 h and 30 m resolution to facilitate its use as a benchmark for forthcoming numerical simulations and the fundamental study of atmospheric processes.

Effect of Aggregation and Molecular Size on the Ice Nucleation Efficiency of Proteins.

Aerosol acts as ice-nucleating particles (INPs) by catalyzing the formation of ice crystals in clouds at temperatures above the homogeneous nucleation threshold (-38 °C). In this study, we show that the immersion mode ice nucleation efficiency of the environmentally relevant protein, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), occurs at temperatures between -6.8 and -31.6 °C. Further, we suggest that this range is controlled by the RuBisCO concentration and protein aggregation. The warmest median nucleation temperature (-7.9 ± 0.8 °C) was associated with the highest concentration of RuBisCO (2 × 10-1 mg mL-1) and large aggregates with a hydrodynamic diameter of ∼103 nm. We investigated four additional chemically and structurally diverse proteins, plus the tripeptide glutathione, and found that each of them was a less effective INP than RuBisCO. Ice nucleation efficiency of the proteins was independent of the size (molecular weight) for the five proteins investigated in this study. In contrast to previous work, increasing the concentration and degree of aggregation did not universally increase ice nucleation efficiency. RuBisCO was the exception to this generalization, although the underlying molecular mechanism determining why aggregated RuBisCO is such an effective INP remains elusive.

Effect of Acidity on Ice Nucleation by Inorganic-Organic Mixed Droplets.

Aerosol acidity significantly influences heterogeneous chemical reactions and human health. Additionally, acidity may play a role in cloud formation by modifying the ice nucleation properties of inorganic and organic aerosols. In this work, we combined our well-established ice nucleation technique with Raman microspectroscopy to study ice nucleation in representative inorganic and organic aerosols across a range of pH conditions (pH -0.1 to 5.5). Homogeneous nucleation was observed in systems containing ammonium sulfate, sulfuric acid, and sucrose. In contrast, droplets containing ammonium sulfate mixed with diethyl sebacate, poly(ethylene glycol) 400, and 1,2,6-hexanetriol were found to undergo liquid-liquid phase separation, exhibiting core-shell morphologies with observed initiation of heterogeneous freezing in the cores. Our experimental findings demonstrate that an increased acidity reduces the ice nucleation ability of droplets. Changes in the ratio of bisulfate to sulfate coincided with shifts in ice nucleation temperatures, suggesting that the presence of bisulfate may decrease the ice nucleation efficiency. We also report on how the morphology and viscosity impact ice nucleation properties. This study aims to enhance our fundamental understanding of acidity's effect on ice nucleation ability, providing context for the role of acidity in atmospheric ice cloud formation.

Glass Transition Temperatures of Organic Mixtures from Isoprene Epoxydiol-Derived Secondary Organic Aerosol.

The phase states and glass transition temperatures (Tg) of secondary organic aerosol (SOA) particles are important to resolve for understanding the formation, growth, and fate of SOA as well as their cloud formation properties. Currently, there is a limited understanding of how Tg changes with the composition of organic and inorganic components of atmospheric aerosol. Using broadband dielectric spectroscopy, we measured the Tg of organic mixtures containing isoprene epoxydiol (IEPOX)-derived SOA components, including 2-methyltetrols (2-MT), 2-methyltetrol-sulfate (2-MTS), and 3-methyltetrol-sulfate (3-MTS). The results demonstrate that the Tg of mixtures depends on their composition. The Kwei equation, a modified Gordon-Taylor equation with an added quadratic term and a fitting parameter representing strong intermolecular interactions, provides a good fit for the Tg-composition relationship of complex mixtures. By combining Raman spectroscopy with geometry optimization simulations obtained using density functional theory, we demonstrate that the non-linear deviation of Tg as a function of composition may be caused by changes in the extent of hydrogen bonding in the mixture.

Pollen Emissions of Subpollen Particles and Ice Nucleating Particles.

Pollen grains significantly contribute to the aerosol population, and levels are predicted to increase in the future. Under humid atmospheric conditions, pollen grains can rupture creating pollen grain fragments referred to as subpollen particles (SPPs) which are dispersed into the atmosphere with wind. In this laboratory study, SPP emission factors were determined for ryegrass, Lolium sp., and giant ragweed,Ambrosia trifida, in terms of the number of SPPs produced per pollen grain and the number of SPPs produced per m2, which were compared to previously measured live oak,Quercus virginiana, emission factors. The SPP emission factors were 4.9 × 1013 ± 4.3 × 1013 SPPs per m2 for ryegrass, 1.3 × 1015 ± 1.1 × 1015 SPPs per m2 for giant ragweed, and 1.1 × 1015 ± 1.6 × 1015 SPPs per m2 for live oak. SPPs and whole pollen grains from these species were evaluated for their ice nucleation efficiency in immersion and contact mode freezing. Measurements of the ice nucleation efficiency indicate that SPPs are weakly effective INPs in immersion mode, but that pollen grains represent a source of moderately efficient INPs in immersion and contact modes.

Ocean Aerobiology.

Ocean aerobiology is defined here as the study of biological particles of marine origin, including living organisms, present in the atmosphere and their role in ecological, biogeochemical, and climate processes. Hundreds of trillions of microorganisms are exchanged between ocean and atmosphere daily. Within a few days, tropospheric transport potentially disperses microorganisms over continents and between oceans. There is a need to better identify and quantify marine aerobiota, characterize the time spans and distances of marine microorganisms' atmospheric transport, and determine whether microorganisms acclimate to atmospheric conditions and remain viable, or even grow. Exploring the atmosphere as a microbial habitat is fundamental for understanding the consequences of dispersal and will expand our knowledge of biodiversity, biogeography, and ecosystem connectivity across different marine environments. Marine organic matter is chemically transformed in the atmosphere, including remineralization back to CO2. The magnitude of these transformations is insignificant in the context of the annual marine carbon cycle, but may be a significant sink for marine recalcitrant organic matter over long (∼104 years) timescales. In addition, organic matter in sea spray aerosol plays a significant role in the Earth's radiative budget by scattering solar radiation, and indirectly by affecting cloud properties. Marine organic matter is generally a poor source of cloud condensation nuclei (CCN), but a significant source of ice nucleating particles (INPs), affecting the formation of mixed-phase and ice clouds. This review will show that marine biogenic aerosol plays an impactful, but poorly constrained, role in marine ecosystems, biogeochemical processes, and the Earth's climate system. Further work is needed to characterize the connectivity and feedbacks between the atmosphere and ocean ecosystems in order to integrate this complexity into Earth System models, facilitating future climate and biogeochemical predictions.

Marine Aerosols and Clouds.

The role of marine bioaerosols in cloud formation and climate is currently so uncertain that even the sign of the climate forcing is unclear. Marine aerosols form through direct emissions and through the conversion of gas-phase emissions to aerosols in the atmosphere. The composition and size of aerosols determine how effective they are in catalyzing the formation of water droplets and ice crystals in clouds by acting as cloud condensation nuclei and ice nucleating particles, respectively. Marine organic aerosols may be sourced both from recent regional phytoplankton blooms that add labile organic matter to the surface ocean and from long-term global processes, such as the upwelling of old refractory dissolved organic matter from the deep ocean. Understanding the formation of marine aerosols and their propensity to catalyze cloud formation processes are challenges that must be addressed given the major uncertainties associated with aerosols in climate models.

Modeling the single and multiple scattering properties of soot-laden mineral dust aerosols.

Fractal particle morphologies are employed to study the light scattering properties of soot-laden mineral dust aerosols. The applicability of these models is assessed in comparison with measurements and other numerical studies. To quantify the dust-soot mixing effects on the single and multiple scattering properties, a parameterization of the effective bulk properties is developed. Based on the parameterized bulk properties, polarized one-dimensional radiative transfer simulations are performed. The results indicate that small uncertainties in conjunction with soot contamination parameters may lead to large uncertainties in both forward and inverse modeling involving mineral dust contaminated with soot.

Role of Organic Hydrocarbons in Atmospheric Ice Formation via Contact Freezing.

An optical ice microscope apparatus equipped with a sealed cooling stage and a CCD camera was used to examine contact freezing events between a water droplet and ice nucleating particles (INP) containing organic hydrocarbons including octacosane, squalane, and squalene. Sample viscosities were measured with a capillary viscometer and compositions were characterized using Fourier transfer infrared spectroscopy with horizontal attenuated total reflectance and Raman microspectroscopy. All of the samples proved to be moderately efficient ice nuclei that induced freezing between -23 and -26 °C, regardless of whether the INP was solid or liquid. At their ice nucleating temperatures, the viscosity of the liquid samples (squalane and squalene) was 0.6 P or greater. Oxidation increased the viscosity of squalene to over 1330 P, but decreased the viscosity of squalane to 0.07 P at room temperature. Most importantly, our results demonstrate that even moderately viscous liquids in contact with water droplets can act to catalyze freezing, plausibly by providing a flexible template which decreases the energy barrier to ice nucleation. The simple soccer ball model of nucleation theory was used to derive the probability of freezing and nucleation rate coefficients as a function of temperature for each type of INP.

Effects of chemical aging on the ice nucleation activity of soot and polycyclic aromatic hydrocarbon aerosols.

The role of soot particles as ice nuclei (IN) in heterogeneous freezing processes in the atmosphere remains uncertain. Determination of the freezing efficiency of soot is complicated by the changing properties of soot particles undergoing atmospheric aging processes. In this study, the heterogeneous freezing temperatures of droplets in contact with fresh and oxidized soot particles were determined using an optical microscope apparatus equipped with a sealed cooling stage and a CCD video camera. Experiments were also conducted using fresh and oxidized polycyclic aromatic hydrocarbons (PAHs), including anthracene, pyrene, and phenanthrene, as potential ice nuclei. Chemical changes at the surface of the aerosols caused by exposure to ozone were characterized using Fourier transform infrared spectroscopy with horizontal attenuated total reflectance (FTIR-HATR). In addition, Brunauer-Emmett-Teller (BET) measurements were used to determine the specific surface areas of the soot particles. Mean freezing temperatures on fresh particles ranged from -19 to -24 °C, depending on the IN composition and size. In all cases, exposure to ozone facilitated ice nucleation at warmer temperatures, by 2-3 °C, on average. In addition, nucleation rate coefficients for a single temperature and IN type increased by as much as 4 orders of magnitude because of oxidation. Furthermore, a fraction of the oxidized soot particles froze at temperatures above -10 °C. A modified version of classical nucleation theory that accounts for a range of contact angles on nucleation sites within an IN population was used to derive the probability of freezing as a function of temperature for each type of IN. In summary, our results suggest that atmospheric oxidation processes may both extend the range over which particles can act as ice nuclei to warmer temperatures and increase heterogeneous nucleation rates on soot and pollutant aerosols.

Raman spectroscopy of glyoxal oligomers in aqueous solutions.

Raman microscopy and Attenuated Total Reflection infrared spectroscopy were utilized to facilitate investigations of equilibria between various hydrated and oligomeric forms of glyoxal in aqueous glyoxal solution droplets. The assignment of spectra is obtained with the assistance of B3LYP density functional quantum chemical calculations of vibrational wavenumbers, Raman activities, and infrared intensities. Several forms of glyoxal derivatives with similar functional groups, e.g., hydroxyl and dioxolane rings, are found to be present. The absence of a Raman spectral peak corresponding to the vibrational carbonyl stretch provides evidence that both carbonyl groups of a glyoxal molecule become hydrated in solutions of a broad concentration range. The presence of bands corresponding to deformation vibrations of the dioxolane ring indicates that dihydrated glyoxal oligomers are formed in glyoxal solutions with concentrations of 1 M and higher. Under typical ambient temperature and humidity conditions, concentrated glyoxal solution droplets undergo evaporation with incomplete water loss. Our results suggest that formation of crystalline glyoxal trimer dihydrate from concentrated solutions droplets is hindered by the high viscosity of the amorphous trimer and requires dry conditions that could rarely be achieved in the atmosphere. However, crystallization may be possible for droplets of low initial glyoxal concentrations, such as those produced by evaporating cloud droplets.

Atmospheric ammonia mixing ratios at an open-air cattle feeding facility.

Mixing ratios of total and gaseous ammonia were measured at an open-air cattle feeding facility in the Texas Panhandle in the summers of 2007 and 2008. Samples were collected at the nominally upwind and downwind edges of the facility. In 2008, a series of far-field samples was also collected 3.5 km north of the facility. Ammonium concentrations were determined by two complementary laboratory methods, a novel application of visible spectrophotometry and standard ion chromatography (IC). Results of the two techniques agreed very well, and spectrophotometry is faster, easier, and cheaper than chromatography. Ammonia mixing ratios measured at the immediate downwind site were drastically higher (approximately 2900 parts per billion by volume [ppbv]) than thos measured at the upwind site (< or = 200 ppbv). In contrast, at 3.5 km away from the facility, ammonia mixing ratios were reduced to levels similar to the upwind site (< or = 200 ppbv). In addition, PM10 (particulate matter < 10 microm in optical diameter) concentrations obtained at each sampling location using Grimm portable aerosol spectrometers are reported. Time-averaged (1-hr) volume concentrations of PM10 approached 5 x 10(12) nm3 cm(-3). Emitted ammonia remained largely in the gas phase at the downwind and far-field locations. No clear correlation between concentrations of ammonia and particles was observed. Overall, this study provides a better understanding of ammonia emissions from open-air animal feeding operations, especially under the hot and dry conditions present during these measurements.