13 Search Results

The processes of new particle formation (NPF) and growth are important contributors to cloud condensation nuclei (CCN) concentrations, and CCN are important for climate from their impact on planetary radiative forcing. While the general ubiquity and importance of NPF is understood, the vertical extent and governing mechanisms of NPF and growth in the lower troposphere are uncertain. We present an analysis of four NPF events and two non-NPF events during the HI-SCALE field campaign at the Southern Great Plains observatory in Oklahoma, USA. Firstly, we analyzed airborne and ground-based observations of aerosol and gas-phase properties. Secondly, we used a columnmore » aerosol chemistry and microphysics model to probe factors that influence the vertical profile of NPF. During HI-SCALE, we found several instances of enhanced NPF occurring several hundred meters above the surface; however, the spatio-temporal characteristics of the observed NPF made comparisons between airborne- and ground-based observations difficult. The model represented the observed NPF (or lack of NPF) and particle growth at the surface to final diameters within 10 nm. The model predicted enhanced NPF rates in the upper mixed layer, and this enhancement is primarily due to the temperature dependence in the NPF schemes, but this was also dependent on the vertical profile of gas-phase precursors measured during HI-SCALE. We found vertical mixing in the model either enhanced or suppressed NPF rates, aerosol number concentrations, and particle growth rates at the surface. Here, our analysis provides insights for future field campaigns and modeling efforts investigating the vertical profile of NPF.« less

The evolution of organic aerosol (OA) and aerosol size distributions within smoke plumes is uncertain due to the variability in rates of coagulation and OA condensation/evaporation between different smoke plumes and at different locations within a single plume. We use aircraft data from the FIREX-AQ campaign to evaluate differences in evolving aerosol size distributions, OA, and oxygen to carbon ratios (O : C) between and within smoke plumes during the first several hours of aging as a function of smoke concentration. The observations show that the median particle diameter increases faster in smoke of a higher initial OA concentration (>more » 1000 µg m^-3), with diameter growth of over 100 nm in 8 h – despite generally having a net decrease in OA enhancement ratios – than smoke of a lower initial OA concentration (< 100 µg m^-3), which had net increases in OA. Observations of OA and O V C suggest that evaporation and/or secondary OA formation was greater in less concentrated smoke prior to the first measurement (5–57 min after emission). We simulate the size changes due to coagulation and dilution and adjust for OA condensation/evaporation based on the observed changes in OA. We found that coagulation explains the majority of the diameter growth, with OA evaporation/condensation having a relatively minor impact. We found that mixing between the core and edges of the plume generally occurred on timescales of hours, slow enough to maintain differences in aging between core and edge but too fast to ignore the role of mixing for most of our cases.« less

Wildfires are a source of primary aerosols and precursors for secondary aerosols to the atmosphere. In this work, we discover that the evolution of these aerosols depends strongly on the coupled effects of dilution, photooxidation, and partitioning.

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Secondary organic aerosols (SOA) constitute an important fraction of fine-mode atmospheric aerosol mass. Frameworks used to develop SOA parameters from laboratory experiments and subsequently used to simulate SOA formation in atmospheric models make many simplifying assumptions about the processes that lead to SOA formation in the interest of computational efficiency. These assumptions can limit the ability of the model to predict the mass, composition, and properties of SOA accurately. In this work, we developed a computationally efficient, process-level model named simpleSOM to represent the chemistry, thermodynamic properties, and microphysics of SOA. simpleSOM simulates multigenerational gas-phase chemistry, phase-state-influenced kinetic gas/particle partitioning,more » heterogeneous chemistry, oligomerization reactions, and vapor losses to the walls of Teflon chambers. As a case study, we used simpleSOM to simulate SOA formation from the photooxidation of α-pinene. This was done to demonstrate the ability of the model to develop parameters that can reproduce environmental chamber data, to highlight the chemical and microphysical processes within simpleSOM, and discuss implications for SOA formation in chambers and in the real atmosphere. SOA parameters developed from experiments performed in the chamber at the California Institute of Technology (Caltech) reproduced observations of SOA mass yield, O:C, and volatility distribution gathered from other experiments. Sensitivity simulations suggested that multigenerational gas-phase aging contributed to nearly half of all SOA and that in the absence of vapor wall losses, SOA production in the Caltech chamber could be nearly 50% higher. Heterogeneous chemistry did not seem to affect SOA formation over the short timescales for oxidation experienced in the chamber experiments. Simulations performed under atmospherically relevant conditions indicated that the SOA mass yields were sensitive to whether and how oligomerization reactions and the particle phase state were represented in the chamber experiment from which the parameters were developed. simpleSOM provides a comprehensive, process-based framework to consistently model the SOA formation and evolution in box and 3D models.« less

Biomass burning emits vapors and aerosols into the atmosphere that can rapidly evolve as smoke plumes travel downwind and dilute, affecting climate- and health-relevant properties of the smoke. To date, theory has been unable to explain observed variability in smoke evolution. Here, we use observational data from the Biomass Burning Observation Project (BBOP) field campaign and show that initial smoke organic aerosol mass concentrations can help predict changes in smoke aerosol aging markers, number concentration, and number mean diameter between 40–262 nm. Because initial field measurements of plumes are generally >10 min downwind, smaller plumes will have already undergone substantialmore » dilution relative to larger plumes and have lower concentrations of smoke species at these observations closest to the fire. The extent to which dilution has occurred prior to the first observation is not a directly measurable quantity. We show that initial observed plume concentrations can serve as a rough indicator of the extent of dilution prior to the first measurement, which impacts photochemistry, aerosol evaporation, and coagulation. Cores of plumes have higher concentrations than edges. By segregating the observed plumes into cores and edges, we find evidence that particle aging, evaporation, and coagulation occurred before the first measurement. We further find that on the plume edges, the organic aerosol is more oxygenated, while a marker for primary biomass burning aerosol emissions has decreased in relative abundance compared to the plume cores. Finally, we attempt to decouple the roles of the initial concentrations and physical age since emission by performing multivariate linear regression of various aerosol properties (composition, size) on these two factors.« less

Secondary organic aerosol formation via condensation of organic vapors onto existing aerosol transforms the chemical composition and size distribution of ambient aerosol, with implications for air quality and Earth’s radiative balance. Gas-to-particle conversion is generally thought to occur on a continuum between equilibrium-driven partitioning of semivolatile molecules to the pre-existing mass size distribution and kinetic-driven condensation of low volatility molecules to the pre-existing surface area size distribution. However, we offer experimental evidence in contrast to this framework. When catechol is sequentially oxidized by O₃ and NO₃ in the presence of (NH₄)₂SO₄ seed particles with a single size mode, we observemore » a bimodal organic aerosol mass size distribution with two size modes of distinct chemical composition with nitrocatechol from NO₃ oxidation preferentially condensing onto the large end of the pre-existing size distribution (~750 nm). A size-resolved chemistry and microphysics model reproduces the evolution of the two distinct organic aerosol size modes-heterogeneous nucleation to an independent, nitrocatechol-rich aerosol phase.« less

Particle phase state is a property of atmospheric aerosols that has important implications for the formation, evolution, and gas/particle partitioning of secondary organic aerosol (SOA). In this work, we use a size- resolved chemistry and microphysics model (Statistical Oxidation Model coupled to the TwO Moment Aerosol Sectional (SOM-TOMAS)), updated to include an explicit treatment of particle phase state, to constrain the bulk diffusion coefficient (D_b) of SOA produced from α-pinene ozonolysis. By leveraging data from laboratory experiments performed in the absence of a seed and under dry conditions, we find that the D_b for SOA can be constrained ((1–7) ×more » 10^–15 cm² s^–1 in these experiments) by simultaneously reproducing the time-varying SOA mass concentrations and the evolution of the particle size distribution. Another version of our model that used the predicted SOA composition to calculate the glass-transition temperature, viscosity, and, ultimately, Db (~10^–15 cm² s^–1) of the SOA was able to reproduce the mass and size distribution measurements when we included oligomer formation (oligomers accounted for about a fifth of the SOA mass). Furthermore, our work highlights the potential of a size-resolved SOA model to constrain the particle phase state of SOA using historical measurements of the evolution of the particle size distribution« less

With an ongoing interest in displacing petroleum-based sources of energy with biofuels, we measure and model the formation and composition of secondary organic aerosol (SOA) from organic compounds present in biofuels.

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