Insect Infestation Increases Viscosity of Biogenic Secondary Organic Aerosol

Plant stress alters emissions of volatile organic compounds. However, little is known about how this could influence climate-relevant properties of secondary organic aerosol (SOA), particularly from complex mixtures such as real plant emissions. In this study, the chemical composition and viscosity were examined for SOA generated from real healthy and aphid-stressed Canary Island pine (Pinus canariensis) trees, which are commonly used for landscaping in Southern California. Healthy Canary Island pine (HCIP) and stressed Canary Island pine (SCIP) aerosols were generated in a 5 m³ environmental chamber at 35-84% relative humidity and room temperature via OH-initiated oxidation. Viscosities of the collected particles were measured using an offline poke-flow method, after conditioning the particles in a humidified air flow. SCIP particles were consistently more viscous than HCIP particles. The largest differences in particle viscosity were observed in particles conditioned at 50% relative humidity where the viscosity of SCIP particles was an order of magnitude larger than that of HCIP particles. The increased viscosity for the aphid-stressed pine tree SOA was attributed to the increased fraction of sesquiterpenes in the emission profile. The real pine SOA particles, both healthy and aphid-stressed, were more viscous than α-pinene SOA particles, demonstrating the limitation of using a single monoterpene as a model compound to predict the physicochemical properties of real biogenic SOA. However, synthetic mixtures composed of only a few major compounds present in emissions (<10 compounds) can reproduce the viscosities of SOA observed from the more complex real plant emissions.

Lethal heat stress-dependent volatile emissions from tobacco leaves: what happens beyond the thermal edge?

Natural vegetation is predicted to suffer from extreme heat events as a result of global warming. In this study, we focused on the immediate response to heat stress. Photosynthesis and volatile emissions were measured in the leaves of tobacco (Nicotiana tabacum cv. Wisconsin 38) after exposure to heat shock treatments between 46 °C and 55 °C. Exposure to 46 °C decreased photosynthetic carbon assimilation rates (A) by >3-fold. Complete inhibition of A was observed at 49 °C, together with a simultaneous decrease in the maximum quantum efficiency of PSII, measured as the Fv/Fm ratio. A large increase in volatile emissions was observed at 52 °C. Heat stress resulted in only minor effects on the emission of monoterpenes, but volatiles associated with membrane damage such as propanal and (E)-2-hexenal+(Z)-3-hexenol were greatly increased. Heat induced changes in the levels of methanol and 2-ethylfuran that are indicative of modification of cell walls. In addition, the oxidation of metabolites in the volatile profiles was strongly enhanced, suggesting the acceleration of oxidative processes at high temperatures that are beyond the thermal tolerance limit.

Using Ionic Liquids To Study the Migration of Semivolatile Organic Vapors in Smog Chamber Experiments

Atmospheric organic aerosols comprise complex mixtures of a myriad of compounds with a wide range of structures and volatilities. To understand the fate of atmospheric organic aerosols and their contribution to particulate matter pollution, we need to study the relative portion divided between semivolatile organic compounds (SVOCs) and low-volatility organic compounds (LVOCs). SVOCs can effectively migrate and exchange between aerosol populations and thus are more accessible for further reactions and removal processes, while LVOCs will essentially stay in the particle phase. Here, we introduce using ionic liquid droplets as novel sorbents for organic vapors in smog chamber experiments to study the transfer of constituents between aerosol populations and to separate SVOCs and LVOCs from chamber-produced secondary organic aerosols (SOAs). SOA was formed and condensed on the ammonium-sulfate seeds, and later ionic liquid droplets were introduced into the chamber. We show that there are considerable yields of both LVOCs and SVOCs produced from α-pinene ozonolysis, and the uptake of SVOCs into the ionic liquid increases as the amount of reacted α-pinene increases. We also show that the SVOCs absorbed into the ionic liquid re-evaporate more readily compared to SOA originally condensed on the ammonium-sulfate seeds. We are thus able to differentiate the semivolatile components that partition into the extremely polar ionic liquid aerosols from the demonstrably less volatile components also condensed on the ammonium-sulfate seeds. Combined with previous studies using other organic aerosols as solvents to probe SVOC transfer between aerosol populations, we provide a wide set of measurements to probe and constrain the physical and thermodynamic properties of chamber-produced SOA complex.

Cloud Activation Potentials for Atmospheric α-Pinene and β-Caryophyllene Ozonolysis Products

The formation of atmospheric cloud droplets due to secondary organic aerosol (SOA) particles is important for quantifying the Earth's radiative balance under future, possibly warmer, climates, yet is only poorly understood. While cloud activation may be parametrized using the surface tension depression that coincides with surfactant partitioning to the gas-droplet interface, the extent to which cloud activation is influenced by both the chemical structure and reactivity of the individual molecules comprising this surfactant pool is largely unknown. We report herein considerable differences in the surface tension depression of aqueous pendant droplets that contain synthetically prepared ozonolysis products derived from α-pinene and β-caryophyllene, the most abundant of the monoterpenes and sesquiterpenes, respectively, that are emitted over the planet's vast forest ecosystems. Oxidation products derived from β-caryophyllene were found to exhibit significantly higher surface activity than those prepared from α-pinene, with the critical supersaturation required for cloud droplet activation reduced by 50% for β-caryophyllene aldehyde at 1 mM. These considerable reductions in the critical supersaturation were found to coincide with free energies of adsorption that exceed ∼25 kJ/mol, or just one hydrogen bond equivalent, depending on the ammonium sulfate and oxidation product concentration in the solution. Additional experiments showed that aldehyde-containing oxidation products exist in equilibrium with hydrated forms in aqueous solution, which may modulate their bulk solubility and surface activity. Equilibration time scales on the order of 10-5 to 10-4 s calculated for micrometer-sized aerosol particles indicate instantaneous surface tension depression in the activation processes leading to cloud formation in the atmosphere. Our findings highlight the underlying importance of molecular structure and reactivity when considering cloud condensation activity in the presence of SOA particles.

Temperature dependencies of Henry's law constants for different plant sesquiterpenes

Sesquiterpenes are plant-produced hydrocarbons with important ecological functions in plant-to-plant and plant-to-insect communication, but due to their high reactivity they can also play a significant role in atmospheric chemistry. So far, there is little information of gas/liquid phase partition coefficients (Henry's law constants) and their temperature dependencies for sesquiterpenes, but this information is needed for quantitative simulation of the release of sesquiterpenes from plants and modeling atmospheric reactions in different phases. In this study, we estimated Henry's law constants (Hpc) and their temperature responses for 12 key plant sesquiterpenes with varying structure (aliphatic, mono-, bi- and tricyclic sesquiterpenes). At 25 °C, Henry's law constants varied 1.4-fold among different sesquiterpenes, and the values were within the range previously observed for monocyclic monoterpenes. Hpc of sesquiterpenes exhibited a high rate of increase, on average ca. 1.5-fold with a 10 °C increase in temperature (Q10). The values of Q10 varied 1.2-fold among different sesquiterpenes. Overall, these data demonstrate moderately high variation in Hpc values and Hpc temperature responses among different sesquiterpenes. We argue that these variations can importantly alter the emission kinetics of sesquiterpenes from plants.

Water diffusion in atmospherically relevant α-pinene secondary organic material

We report the first direct measurements of water diffusion coefficients in secondary organic aerosol.

Secondary organic material (SOM) constitutes a large mass fraction of atmospheric aerosol particles. Understanding its impact on climate and air quality relies on accurate models of interactions with water vapour. Recent research shows that SOM can be highly viscous and can even behave mechanically like a solid, leading to suggestions that particles exist out of equilibrium with water vapour in the atmosphere. In order to quantify any kinetic limitation we need to know water diffusion coefficients for SOM, but this quantity has, until now, only been estimated and has not yet been measured. We have directly measured water diffusion coefficients in the water soluble fraction of α-pinene SOM between 240 and 280 K. Here we show that, although this material can behave mechanically like a solid, at 280 K water diffusion is not kinetically limited on timescales of 1 s for atmospheric-sized particles. However, diffusion slows as temperature decreases. We use our measured data to constrain a Vignes-type parameterisation, which we extend to lower temperatures to show that SOM can take hours to equilibrate with water vapour under very cold conditions. Our modelling for 100 nm particles predicts that under mid- to upper-tropospheric conditions radial inhomogeneities in water content produce a low viscosity surface region and more solid interior, with implications for heterogeneous chemistry and ice nucleation.

Viscosity of α-pinene secondary organic material and implications for particle growth and reactivity

Particles composed of secondary organic material (SOM) are abundant in the lower troposphere. The viscosity of these particles is a fundamental property that is presently poorly quantified yet required for accurate modeling of their formation, growth, evaporation, and environmental impacts. Using two unique techniques, namely a "bead-mobility" technique and a "poke-flow" technique, in conjunction with simulations of fluid flow, the viscosity of the water-soluble component of SOM produced by α-pinene ozonolysis is quantified for 20- to 50-μm particles at 293-295 K. The viscosity is comparable to that of honey at 90% relative humidity (RH), similar to that of peanut butter at 70% RH, and at least as viscous as bitumen at ≤30% RH, implying that the studied SOM ranges from liquid to semisolid or solid across the range of atmospheric RH. These data combined with simple calculations or previous modeling studies are used to show the following: (i) the growth of SOM by the exchange of organic molecules between gas and particle may be confined to the surface region of the particles for RH ≤ 30%; (ii) at ≤30% RH, the particle-mass concentrations of semivolatile and low-volatility organic compounds may be overpredicted by an order of magnitude if instantaneous equilibrium partitioning is assumed in the bulk of SOM particles; and (iii) the diffusivity of semireactive atmospheric oxidants such as ozone may decrease by two to five orders of magnitude for a drop in RH from 90% to 30%. These findings have possible consequences for predictions of air quality, visibility, and climate.

Potential climatic impact of organic haze on early Earth

We have explored the direct and indirect radiative effects on climate of organic particles likely to have been present on early Earth by measuring their hygroscopicity and cloud nucleating ability. The early Earth analog aerosol particles were generated via ultraviolet photolysis of an early Earth analog gas mixture, which was designed to mimic possible atmospheric conditions before the rise of oxygen. An analog aerosol for the present-day atmosphere of Saturn's moon Titan was tested for comparison. We exposed the early Earth aerosol to a range of relative humidities (RHs). Water uptake onto the aerosol was observed to occur over the entire RH range tested (RH=80-87%). To translate our measurements of hygroscopicity over a specific range of RHs into their water uptake ability at any RH < 100% and into their ability to act as cloud condensation nuclei (CCN) at RH > 100%, we relied on the hygroscopicity parameter κ, developed by Petters and Kreidenweis. We retrieved κ=0.22 ±0.12 for the early Earth aerosol, which indicates that the humidified aerosol (RH < 100 %) could have contributed to a larger antigreenhouse effect on the early Earth atmosphere than previously modeled with dry aerosol. Such effects would have been of significance in regions where the humidity was larger than 50%, because such high humidities are needed for significant amounts of water to be on the aerosol. Additionally, Earth organic aerosol particles could have activated into CCN at reasonable-and even low-water-vapor supersaturations (RH > 100%). In regions where the haze was dominant, it is expected that low particle concentrations, once activated into cloud droplets, would have created short-lived, optically thin clouds. Such clouds, if predominant on early Earth, would have had a lower albedo than clouds today, thereby warming the planet relative to current-day clouds.

Organic aerosol formation in citronella candle plumes

Citronella candles are widely used as insect repellants, especially outdoors in the evening. Because these essential oils are unsaturated, they have a unique potential to form secondary organic aerosol (SOA) via reaction with ozone, which is also commonly elevated on summer evenings when the candles are often in use. We investigated this process, along with primary aerosol emissions, by briefly placing a citronella tealight candle in a smog chamber and then adding ozone to the chamber. In repeated experiments, we observed rapid and substantial SOA formation after ozone addition; this process must therefore be considered when assessing the risks and benefits of using citronella candle to repel insects.

Aging of organic aerosol: bridging the gap between laboratory and field studies

The oxidation of organics in aerosol particles affects the physical properties of aerosols through a process known as aging. Atmospheric particles compose a huge set of specific organic compounds, most of which have not been identified in field measurements. Laboratory experiments inevitably address model systems of reduced complexity to isolate critical chemical phenomena, but growing evidence suggests that composition effects may play a central role in the atmospheric aging of organic particles. In this review we seek to address the connections between recent laboratory studies and recent field campaigns addressing the aging of organic aerosols. We review laboratory studies on the uptake of oxidants, the evolution of particle-water interactions, and the evolution of particle density with aging. Finally, we review field data addressing condensed-phase lifetimes of organic tracers. These data suggest that although matrix effects identified in the laboratory have taken a step toward reconciling laboratory-field disagreements, further work is needed to understand the actual aging rates of organics in ambient particles.

Secondary organic aerosol production from terpene ozonolysis. 1. Effect of UV radiation

We report secondary organic aerosol (SOA) yields from the ozonolysis of alpha-pinene under both dark and UV-illuminated conditions. Exposure to UV light reduces SOA yield by 20-40%, with a maximum reduction in yield coinciding with a minimum in the amount of terpene consumed (15-30 ppb). The data are consistent with a constant absolute reduction in the yield of approximately 0.03. Gas chromatography mass spectrometry analysis of filter samples indicates that the major products found in alpha-pinene SOA include organic acids (e.g., pinic acid), keto acids (e.g., pinonic acid), and hydroxy keto acids (e.g., 10-hydroxypinonic acid). Analysis of filter-based results suggests that yield reduction is a result of the formation of a more volatile product distribution when experiments are conducted in the presence of UV light. These results implythat previous "dark bag" experiments may overestimate SOA generation from monoterpenes and also that SOA generation in the atmosphere may depend significantly on actinic flux.