Viscous Secondary Organic Aerosol Research Articles

Characterization and Quantification of Particle-Bound Criegee Intermediates in Secondary Organic Aerosol.

The ozonolysis of alkenes contributes substantially to the formation of secondary organic aerosol (SOA), which are important modulators of air quality and the Earth’s climate. Criegee intermediates (CIs) are abundantly formed through this reaction. However, their contributions to aerosol particle chemistry remain highly uncertain. In this work, we present the first application of a novel methodology, using spin traps, which simultaneously quantifies CIs produced from the ozonolysis of volatile organic compounds in the gas and particle phases. Only the smallest CI with one carbon atom was detected in the gas phase of a β-caryophyllene ozonolysis reaction system. However, multiple particle-bound CIs were observed in β-caryophyllene SOA. The concentration of the most abundant CI isomer in the particle phase was estimated to constitute ∼0.013% of the SOA mass under atmospherically relevant conditions. We also demonstrate that the lifetime of CIs in highly viscous SOA particles is at least on the order of minutes, substantially greater than their gas-phase lifetime. The confirmation of substantial concentrations of large CIs with elongated lifetimes in SOA raises new questions regarding their influence on the chemical evolution of viscous SOA particles, where CIs may be a previously underestimated source of reactive species.

Diurnal and Seasonal Variations in the Phase State of Secondary Organic Aerosol Material over the Contiguous US Simulated in CMAQ

Secondary organic aerosol (SOA) accounts for a substantial portion of atmospheric particulate matter. Phase state plays an important role in the formation and evolution of SOA, while current air quality models usually assume that SOA particles are homogeneous and well-mixed liquids. In this study, we simulate glass transition temperature (Tg) and particle viscosity of SOA based on organic molecular composition over the contiguous US in 2016 using the Community Multiscale Air Quality (CMAQ) model. Simulations show that oligomers from anthropogenic and biogenic SOA and acid-catalyzed isoprene SOA are large contributors to Tg of dry SOA, and the dominant species that regulates the dry Tg variation is dependent on location and season. At the surface, the Tg of dry SOA is higher in the western than in the eastern US, which is due to higher mass fractions of accretion products in the western US. Taking into account the water uptake by SOA, the estimated SOA viscosity shows a prominent geospatial gradient, which is nearly a mirror image of relative humidity. SOA viscosity exhibits a strong diel cycle, and the phase state tends to be more viscous in daytime. The seasonal variations in SOA viscosity are substantially smaller than the diurnal variations. Simulations for four diverse field sites show that Tg and SOA viscosity exhibit significant vertical variations that increase with the altitude. SOA in winter undergoes glass transition at lower altitude (∼3 km) than the other three seasons, and SOA occurs as a non-liquid phase at lower altitude at night than during the daytime. This suggests that chemical transport models may need to consider the bulk diffusion limitations in partitioning into viscous SOA in dry western areas of US and aloft in the humid eastern US as well as dry periods during the day to accurately predict SOA formation, size distribution dynamics, and subsequent impacts.

Evidence for a kinetically controlled burying mechanism for growth of high viscosity secondary organic aerosol.

Secondary organic aerosol (SOA) particles are ubiquitous in air and understanding the mechanism by which they grow is critical for predicting their effects on visibility and climate. The uptake of three organic nitrates into semi-solid SOA particles formed by α-pinene ozonolysis either with or without an OH scavenger was investigated. Four types of experiments are presented here. In Series A, uptake of the selected organic nitrates (2-ethylhexyl nitrate (2EHN); β-hydroxypropyl nitrate (HPN); β-hydroxyhexyl nitrate (HHN)) into impacted SOA particles was interrogated by attenuated total reflectance (ATR)-FTIR. In this case, equilibrium was reached and partition coefficients (KSOA = [-ONO2]SOA/[-ONO2]air) were measured to be K2EHN = (3.2-11) × 104, KHPN = (4.4-5.4) × 105, and KHHN = (4.9-9.0) × 106. In Series B, SOA particles were exposed on-the-fly to gas phase organic nitrates for comparison to Series A, and uptake of organic nitrates was quantified by HR-ToF-AMS analysis, which yielded similar results. In Series C (AMS) and D (ATR-FTIR), each organic nitrate was incorporated into the SOA as the particles formed and grew. The incorporation of the RONO2 was much larger in Series C and D (during growth), exceeding equilibrium values determined in Series A and B (after growth). This suggests that enhanced uptake of organic nitrates during SOA formation and growth is due to a kinetically controlled "burying" mechanism, rather than equilibrium partitioning. This has important implications for understanding SOA formation and growth under conditions where the particles are semi-solid, which is central to accurately predicting properties for such SOA.

Carbon Dioxide Diffusivity in Single, Levitated Organic Aerosol Particles.

The diffusivity of molecules relevant to condensed-phase chemistry within viscous secondary organic aerosol (SOA) remains highly uncertain. Whereas there has been an effort to characterize water diffusivity as well as the diffusivity of larger compounds, data are lacking almost entirely for small molecules, such as carbon dioxide (CO2). Here we use photochemically generated CO2 in single particles of aqueous citric acid as a SOA proxy, levitated in an electrodynamic balance, to deduce CO2 diffusivity in the particle with unprecedented accuracy. For medium viscosities at intermediate relative humidities (∼25-40% RH), we find CO2 diffusivities DCO2 ≈ 10-14 m2 s-1, agreeing with the Stokes-Einstein relationship based on current viscosity data but 10 times lower than that for water. Conversely, under dry high-viscosity conditions, we find that DCO2 ≈ 10-16 m2 s-1, which is 10 times higher than for water. We infer that the chemical degradation of atmospheric SOA particles will likely not be limited by CO2 diffusivity.

Photochemical Formation of Nitrite and Nitrous Acid (HONO) upon Irradiation of Nitrophenols in Aqueous Solution and in Viscous Secondary Organic Aerosol Proxy.

Irradiated nitrophenols can produce nitrite and nitrous acid (HONO) in bulk aqueous solutions and in viscous aqueous films, simulating the conditions of a high-solute-strength aqueous aerosol, with comparable quantum yields in solution and viscous films (10-5-10-4 in the case of 4-nitrophenol) and overall reaction yields up to 0.3 in solution. The process is particularly important for the para-nitrophenols, possibly because their less sterically hindered nitro groups can be released more easily as nitrite and HONO. The nitrophenols giving the highest photoproduction rates of nitrite and HONO (most notably, 4-nitrophenol and 2-methyl-4-nitrophenol) could significantly contribute to the occurrence of nitrite in aqueous phases in contact with the atmosphere. Interestingly, dew-water evaporation has shown potential to contribute to the gas-phase HONO levels during the morning, which accounts for the possible importance of the studied process.

Condensed-phase biogenic-anthropogenic interactions with implications for cold cloud formation.

Anthropogenic and biogenic gas emissions contribute to the formation of secondary organic aerosol (SOA). When present, soot particles from fossil fuel combustion can acquire a coating of SOA. We investigate SOA-soot biogenic-anthropogenic interactions and their impact on ice nucleation in relation to the particles' organic phase state. SOA particles were generated from the OH oxidation of naphthalene, α-pinene, longifolene, or isoprene, with or without the presence of sulfate or soot particles. Corresponding particle glass transition (Tg) and full deliquescence relative humidity (FDRH) were estimated using a numerical diffusion model. Longifolene SOA particles are solid-like and all biogenic SOA sulfate mixtures exhibit a core-shell configuration (i.e. a sulfate-rich core coated with SOA). Biogenic SOA with or without sulfate formed ice at conditions expected for homogeneous ice nucleation, in agreement with respective Tg and FDRH. α-pinene SOA coated soot particles nucleated ice above the homogeneous freezing temperature with soot acting as ice nuclei (IN). At lower temperatures the α-pinene SOA coating can be semisolid, inducing ice nucleation. Naphthalene SOA coated soot particles acted as ice nuclei above and below the homogeneous freezing limit, which can be explained by the presence of a highly viscous SOA phase. Our results suggest that biogenic SOA does not play a significant role in mixed-phase cloud formation and the presence of sulfate renders this even less likely. However, anthropogenic SOA may have an enhancing effect on cloud glaciation under mixed-phase and cirrus cloud conditions compared to biogenic SOA that dominate during pre-industrial times or in pristine areas.

Heterogeneous ice nucleation of viscous secondary organic aerosol produced from ozonolysis of <i>α</i>-pinene

Abstract. There are strong indications that particles containing secondary organic aerosol (SOA) exhibit amorphous solid or semi-solid phase states in the atmosphere. This may facilitate heterogeneous ice nucleation and thus influence cloud properties. However, experimental ice nucleation studies of biogenic SOA are scarce. Here, we investigated the ice nucleation ability of viscous SOA particles. The SOA particles were produced from the ozone initiated oxidation of α-pinene in an aerosol chamber at temperatures in the range from −38 to −10 °C at 5–15 % relative humidity with respect to water to ensure their formation in a highly viscous phase state, i.e. semi-solid or glassy. The ice nucleation ability of SOA particles with different sizes was investigated with a new continuous flow diffusion chamber. For the first time, we observed heterogeneous ice nucleation of viscous α-pinene SOA for ice saturation ratios between 1.3 and 1.4 significantly below the homogeneous freezing limit. The maximum frozen fractions found at temperatures between −39.0 and −37.2 °C ranged from 6 to 20 % and did not depend on the particle surface area. Global modelling of monoterpene SOA particles suggests that viscous biogenic SOA particles are indeed present in regions where cirrus cloud formation takes place. Hence, they could make up an important contribution to the global ice nucleating particle budget.

Phase transition observations and discrimination of small cloud particles by light polarization in expansion chamber experiments

Abstract. Cloud microphysical processes involving the ice phase in tropospheric clouds are among the major uncertainties in cloud formation, weather, and general circulation models. The detection of aerosol particles, liquid droplets, and ice crystals, especially in the small cloud particle-size range below 50 μm, remains challenging in mixed phase, often unstable environments. The Cloud Aerosol Spectrometer with Polarization (CASPOL) is an airborne instrument that has the ability to detect such small cloud particles and measure the variability in polarization state of their backscattered light. Here we operate the versatile Cosmics Leaving OUtdoor Droplets (CLOUD) chamber facility at the European Organization for Nuclear Research (CERN) to produce controlled mixed phase and other clouds by adiabatic expansions in an ultraclean environment, and use the CASPOL to discriminate between different aerosols, water, and ice particles. In this paper, optical property measurements of mixed-phase clouds and viscous secondary organic aerosol (SOA) are presented. We report observations of significant liquid–viscous SOA particle polarization transitions under dry conditions using CASPOL. Cluster analysis techniques were subsequently used to classify different types of particles according to their polarization ratios during phase transition. A classification map is presented for water droplets, organic aerosol (e.g., SOA and oxalic acid), crystalline substances such as ammonium sulfate, and volcanic ash. Finally, we discuss the benefits and limitations of this classification approach for atmospherically relevant concentrations and mixtures with respect to the CLOUD 8–9 campaigns and its potential contribution to tropical troposphere layer analysis.