Stratospheric aerosol injection could mitigate harmful effects of global warming, but could have undesirable side effects, such as warming the stratosphere and depleting the ozone layer. We explore the potential benefits of solid alumina and calcite particles as alternatives to sulfate aerosols by using an experimentally informed aerosol-chemistry-climate model. Compared to sulfur dioxide, injection of solids reduces stratospheric warming by up to 70% and diffuse radiation by up to 40%, highlighting their potential benefits. Achieving −1 W m⁻² of radiative forcing would likely result in very small ozone changes, but sizable uncertainties remain. These arise from poorly understood heterogeneous chemical and microphysical processes, which, under less likely assumptions, could lead to larger global ozone column changes between −14% and +4%. Our work provides recommendations for improving the understanding of stratospheric aerosol injection using materials other than sulfur dioxide, and underscores the need for kinetic laboratory studies.

Chemical nucleation of carbonaceous nanoparticles is investigated during pyrolysis of n-heptane at high temperature (2200–2600 K) by reactive molecular dynamics (MD) simulations. The MD-derived n-heptane consumption rate is in agreement with kinetic modeling, validating the present work at high temperature and high fuel concentration conditions. The critical nucleus size is quantified by the free formation energy at 2200–2600 K for n-heptane concentrations ranging from 3 × 10²⁰−9 × 10²⁰ #/cm³. Increasing temperature leads to smaller critical size, starting from 59 ± 7 carbon atoms at 2200 K that decreases down to 33 ± 3 carbon atoms at 2600 K, while the fuel concentration hardly affects the critical nucleus size. The onset time of nucleation decreases exponentially with temperature, consistent with previous shock tube pyrolysis experiments. The nucleation rate is obtained by the rate of formation of critical and supercritical hydrocarbon molecules. An Arrhenius-type relationship between the nucleation rate and the process temperature is proposed, exhibiting a first-order dependency to the initial fuel concentration. The number density of carbon nuclei derived by this nucleation rate is four to five orders of magnitude higher than that obtained by kinetic models for soot nucleation by reactive polyaromatic hydrocarbon (PAH) dimerization. The present MD-derived nucleation rate provides a computationally efficient pathway to model carbonaceous nanoparticle formation dynamics without relying on individual chemical reaction rate constants or on computationally expensive PAH-based models.

Here, the light absorption of brown carbon (BrC) emitted by wood combustion and denuded from volatile organic carbon (VOC) at 300 °C is elucidated using a recently developed thermal decomposition platform coupled with a suite of real-time aerosol instrumentation and time-integrated sampling systems. The BrC particle size distribution, morphology and optical properties are closely controlled by increasing the combusted wood mass from 50 to 600 mg to emulate those measured for “real world” wildfire particulate matter (PM) emissions. Size-fractionation of such wildfire-like BrC reveals that the PM_0.1–2.5 fraction contains high molecular weight, carcinogenic polycyclic aromatic hydrocarbons (PAHs) and absorbs up to five times more light compared to the PM_0.1 fraction. Thus, increasing the combusted wood mass from 50 to 600 mg increases the PM_0.1–2.5 concentration by a factor of about eight and enhances the overall BrC mass absorption cross-section, MAC, up to a factor of two at a wavelength of 405 nm. Condensation of VOC on BrC reduces its MAC up to 40 %. Still, the particle size seems to largely determine the BrC light absorption, as large VOC-rich particles absorb more light compared to small VOC-lean ones. The size-resolved BrC MAC measured here can be interfaced with climate models to estimate the climate impact of wildfire PM emissions.

Wildfire particulate matter from Canadian forest fires significantly impacted the air quality in the northeastern United States during the summer of 2023. Here, we used real-time and time-integrated instrumentation to characterize the physicochemical properties and radiative effects of wildfire particulate matter reaching the metropolitan areas of New Jersey/ New York during this extreme incident. The radiative forcing of −352.4 W/m² derived here based on the measured optical properties of wildfire particulate matter explains, to some extent, the ground level temperature reduction of about 3 °C observed in New Jersey/ New York City during this incident. Such negative radiative forcing in densely populated megacities may limit natural ventilation, increase the residence time of wildfire particulate matter and background air pollutants, exacerbating public health risks. This study highlights the importance of radiative effects from wildfire particulate matter in densely populated areas and their potential implications for climate, air quality and public health.

The toxicity, climate impact, as well as the physical and chemical properties of ultra-fine soot particles emitted from combustion systems are strongly dependent on their size and morphology. Research attention has been paid in the last three decades to developing more accurate and capable methods to model soot particle coagulation in the presence of inception, surface growth, and oxidation, to predict particle size distribution as well as the detailed aggregate morphology of soot. While soot particle concentrations in hydrocarbon flames are primarily governed by soot kinetics, the morphology of soot particles is controlled by both soot kinetics and particle dynamics. Flame-generated soot particles are fractal aggregates formed by polydisperse and nearly spherical primary particles with a certain degree of overlapping. The properties of fractal aggregates, nanoparticle coagulation, and soot formation chemistry all play important roles in soot formation. This article reviews all these aspects but the focus is on recent progress in macro- and meso-scale modeling of soot particle aggregation in laminar sooting flames to avoid the complexities of turbulence. The reviewed macro-scale methods based on the population balance equation include the commonly used sectional methods and methods of moments. The main features of three recently developed state-of-the-art meso-scale methods, namely the event-driven Discrete Element Method, Monte Carlo Aggregation Code, and detailed stochastic population balance model are reviewed. To highlight the complexities of modeling the particle size distribution and detailed particle morphology without and with surface growth, numerical simulations of three test cases were conducted using the event-driven Discrete Element Method, the Monte Carlo Aggregation Code, and the two macro-scale methods. A detailed analysis of the results was presented to understand how different treatments of particle coagulation and surface growth in the two meso-scale methods affect the predicted particle size and morphology. The remaining challenges in modeling detailed soot particle morphology are outlined.

Recent studies have suggested that injection of solid particles such as alumina and calcite particles for stratospheric aerosol injection (SAI) instead of sulfur-based injections could reduce some of the adverse side effects of SAI such as ozone depletion and stratospheric heating. Here, we present a version of the global aerosol-chemistry-climate model SOCOL-AERv2 and the Earth system model (ESM) SOCOLv4 which incorporate a solid-particle microphysics scheme for assessment of SAI of solid particles. Microphysical interactions of the solid particle with the stratospheric sulfur cycle were interactively coupled to the heterogeneous chemistry scheme and the radiative transfer code (RTC) for the first time within an ESM. Therefore, the model allows simulation of heterogeneous chemistry at the particle surface as well as feedbacks between microphysics, chemistry, radiation and climate. We show that sulfur-based SAI results in a doubling of the stratospheric aerosol burden compared to the same mass injection rate of calcite and alumina particles with a radius of 240 nm. Most of the sulfuric acid aerosol mass resulting from SO₂ injection does not need to be lifted to the stratosphere but is formed after in situ oxidation and subsequent water uptake in the stratosphere. Therefore, to achieve the same radiative forcing, larger injection rates are needed for calcite and alumina particle injection than for sulfur-based SAI. The stratospheric sulfur cycle would be significantly perturbed, with a reduction in stratospheric sulfuric acid burden by 53 %, when injecting 5 Mtyr^-1 (megatons per year) of alumina or calcite particles of 240 nm radius. We show that alumina particles will acquire a sulfuric acid coating equivalent to about 10 nm thickness if the sulfuric acid is equally distributed over the whole available particle surface area in the lower stratosphere. However, due to the steep contact angle of sulfuric acid on alumina particles, the sulfuric acid coating would likely not cover the entire alumina surface, which would result in available surface for heterogeneous reactions other than the ones on sulfuric acid. When applying realistic uptake coefficients of 1.0, 10^-5 and 10^-4 for H₂SO₄, HCl and HNO₃, respectively, the same scenario with injections of calcite particles results in 94 % of the particle mass remaining in the form of CaCO₃. This likely keeps the optical properties of the calcite particles intact but could significantly alter the heterogeneous reactions occurring on the particle surfaces. The major process uncertainties of solid-particle SAI are (1) the solid-particle microphysics in the injection plume and degree of agglomeration of solid particles on the sub-ESM grid scale, (2) the scattering properties of the resulting agglomerates, (3) heterogeneous chemistry on the particle surface, and (4) aerosol-cloud interactions. These uncertainties can only be addressed with extensive, coordinated experimental and modelling research efforts. The model presented in this work offers a useful tool for sensitivity studies and incorporating new experimental results on SAI of solid particles.

Here, commercial carbon black (CB) grades are characterized in detail to determine the link between their physicochemical properties and solar steam generation performance. The CB nanoparticles used here have surface mean primary particle diameters of 11–406 nm resulting in specific surface areas of 8–300 m²/g. Thermogravimetric analysis, dynamic light scattering, Raman spectroscopy, and x-ray diffraction reveal that fine CB nanoparticles form large agglomerates, have a more disordered nanostructure and larger organic carbon content than coarse CB grades. Most importantly, UV–vis spectroscopy and Mie theory show that increasing the particle size from 14 to 406 nm reduces the light absorption of CB dispersed in water up to 86%. So, the water evaporation flux of suspensions containing 11–14 nm CB nanoparticles is up to 25% larger than that obtained for suspensions of 406 nm particles. Thus, good control of particle size is essential to optimize the solar steam generation enabled by CB.