Relationship between Coating-Induced Soot Aggregate Restructuring and Primary Particle Number

Quantifying the effects of the microphysical hygroscopic restructuring of soot on ensemble optical properties and satellite aerosol optical depth retrievals

Black carbon, or soot, significantly contributes to atmospheric light absorption due to its low single scattering albedo (SSA). This study investigates the impact of soot's hygroscopic restructuring on satellite remote sensing, focusing on radiative forcing, top-of-atmosphere (TOA) reflectance, and aerosol optical depth (AOD) retrievals. We characterized soot aging using relative humidity (RH) growth factor functions and modeled fresh and aging soot aggregates using a cluster-cluster aggregation algorithm. Bulk optical properties for each RH level were simulated using core-mantle generalized multi-sphere Mie-solution, weighted by the probability density function of soot monomer numbers. We incorporated soot aging models into the 6S radiative transfer model by adjusting the geometric mean radius of the soot component to match bulk SSA values at each aging degree. Extensive radiative transfer simulations were conducted, complemented by an analysis of 16 actual soot-containing cases in China from 2016 to 2019. Results showed that identical soot loads correspond to lower reflectance in dry environments and higher reflectance in humid environments. MODIS AOD retrieval errors approached zero when RH was close to 70 %, likely due to the similarity between the 6S default soot and the aging model at this humidity. Neglecting RH led to overestimations in AOD for cases with RH < 65 % and underestimations in high humidity cases (RH > 75 %). The average MODIS AOD retrieval errors for RH < 65 %, RH between 65 % and 75 %, and RH > 75 % were -33.15 %, -1.80 %, and +42.42 %, respectively. This study underscores the necessity of incorporating RH into satellite AOD retrieval algorithms to enhance accuracy and reduce uncertainties.

Enhanced Light Absorption and Radiative Forcing by Black Carbon Agglomerates

The climate models of the Intergovernmental Panel on Climate Change list black carbon (BC) as an important contributor to global warming based on its radiative forcing (RF) impact. Examining closely these models, it becomes apparent that they might underpredict significantly the direct RF for BC, largely due to their assumed spherical BC morphology. Specifically, the light absorption and direct RF of BC agglomerates are enhanced by light scattering between their constituent primary particles as determined by the Rayleigh-Debye-Gans theory interfaced with discrete dipole approximation and recent relations for the refractive index and lensing effect. The light absorption of BC is enhanced by about 20% by the multiple light scattering between BC primary particles regardless of the compactness of their agglomerates. The resulting light absorption agrees very well with the observed absorption aerosol optical depth of BC. ECHAM-HAM simulations accounting for the realistic BC morphology and its coatings reveal high direct RF = 3-5 W/m² in East, South Asia, sub-Sahara, western Africa, and the Arabian peninsula. These results are in agreement with satellite and AERONET observations of RF and indicate a regional climate warming contribution by 0.75-1.25 °C, solely due to BC emissions.

Smog chamber simulation on heterogeneous reaction of O3 and NO2 on black carbon under various relative humidity conditions

In this study, heterogeneous formation of nitrate from O₃ reaction with NO₂ on black carbon (BC) and KCl-treated BC surface in the presence of NH₃ was simulated under 30-90% RH conditions by using a laboratory smog chamber. We found that O₃ and NO₂ in the chamber quickly reacted into N₂O₅ in the gas phase_, which subsequently hydrolyzed into HNO₃ and further neutralized with NH₃ into NH₄NO₃ on the BC surface, along with a small amount of N₂O₅ decomposed into NO and NO₂ through a reaction with the BC surface active site. Meanwhile, the fractal BC aggregates restructured and condensed to spherical particles during the NH₄NO₃ coating process. Compared to that during the exposure to NO₂ or O₃ alone, the presence of strong signals of CH₂O⁺, CH₂O₂⁺ and CH₄NO⁺ during the simultaneous exposure to both NO₂ and O₃ suggested a synergetic oxidizing effect of NO₂ and O₃, which significantly activated the BC surface by forming carbonyl, carboxylic and nitro groups, promoted the adsorption of water vapor onto the BC surface and enhanced the NH₄NO₃ formation. Under <75 ± 2% RH conditions the coating process of NH₄NO₃ on the BC surface consisted of a diffusion of N₂O₅ onto the surface and a subsequent hydrolysis, due to the limited number of water molecules adsorbed. However, under 90 ± 2% RH conditions N₂O₅ directly hydrolyzed on the aqueous phase of the BC surface due to the multilayer water molecules adsorbed, which caused an instant NH₄NO₃ formation on the surface without any delay. The coating rate of NH₄NO₃ on KCl-treated BC particles was 3-4 times faster than that on the pure BC particles at the initial stage, indicating an increasing formation of NH₄NO₃, mainly due to an enhanced hygroscopicity of BC by KCl salts.

Single Parameter for Predicting the Morphology of Atmospheric Black Carbon

Black carbon (BC) from fuel combustion is an effective light absorber that contributes significantly to direct climate forcing. The forcing is altered when BC combines with other substances, which modify its mixing state and morphology, making the evaluation of its atmospheric lifetime and climate impact a challenge. To elucidate the associated mechanisms, we exposed BC aerosol to supersaturated vapors of different chemicals to form thin coatings and measured the coating mass required to induce the restructuring of BC aggregates. We found that studied chemicals fall into two distinct groups based on a single dimensionless parameter, χ, which depends on the diameter of BC monomer spheres and the coating material properties, including vapor supersaturation, molar volume, and surface tension. We show that when χ is small (low-volatility chemicals), the highly supersaturated vapor condenses uniformly over aggregates, including convex monomers and concave junctions in between monomers, but when χ is large (intermediate-volatility chemicals), junctions become preferred. The aggregates undergo prompt restructuring when condensation in the junctions dominates over condensation on monomer spheres. For a given monomer diameter, the coating distribution is mostly controlled by vapor supersaturation. The χ factor can be incorporated straightforwardly into atmospheric models to improve simulations of BC aging.

Impact of Humidity on Silica Nanoparticle Agglomerate Morphology and Size Distribution

The effect of humidity on flame-made metal oxide agglomerate morphology and size distribution is investigated, for the first time to our knowledge, and compared to that on soot, which has been widely studied. Understanding the impact of humidity on such characteristics is essential for storage, handling, processing, and eventual performance of nanomaterials. More specifically, broadly used agglomerates of flame-made silica nanoparticles are humidified at various saturation ratios, S = 0.2-1.5, and dried before characterization with a differential mobility analyzer (DMA), an aerosol particle mass (APM) analyzer, and transmission electron microscopy. At high humidity, the constituent single and/or aggregated (chemically bonded) primary particles (PPs) rearrange to balance the capillary forces induced by condensation-evaporation of liquid bridges between PPs. Larger agglomerates restructure more than smaller ones, narrowing their mobility size distribution. After humidification at S = 1.5 and drying, agglomerates collapse into compact structures that follow a fractal scaling law with mass-mobility exponent D_fm = 3.02 ± 0.11 and prefactor k_m = 0.27 ± 0.07. This critical S = 1.5 for silica agglomerates is larger than the 1.26 obtained for soot because of the hydrophilic surface of silica that delays water evaporation. The relative effective density, ρ_eff/ρ, of collapsed agglomerates becomes invariant of mobility diameter, d_m, similar to that of fluidized and spray-dried granules. The average silica ρ_eff/ρ = 0.28 ± 0.02 is smaller than the 0.36 ± 0.04 measured for the humidified-dried soot because of the larger size of silica aggregates, d_m/ d_p, and number of constituent primary particles, n_p, of diameter d_p. This is verified by tandem-DMA (TDMA) measurements, yielding maximum d_m = 3 d_p or 5 d_p and n_p = 13 or 36 for the soot or silica aggregates studied here, in good agreement with those reported from microscopy and high-pressure agglomerate dispersion. A scaling law relating the initial d_m,o to d_m, D_fm, and k_m after condensation-drying is developed. The mass-mobility relationship of collapsed silica and soot agglomerates obtained by combining this law with fast TDMA measurements is in excellent agreement with that measured by the direct, but tedious, DMA-APM analysis.

Molecular Adsorption Mechanism of Elemental Carbon Particles on Leaf Surface

Plant leaves can effectively capture and retain particulate matter (PM), improving air quality and human health. However, little is known about the adsorption mechanism of PM on leaf surface. Black carbon (BC) has great adverse impact on climate and environment. Four types of elemental carbon (EC) particles, carbon black as a simple model for BC, graphite, reduced graphene oxide, and graphene oxide, and C₃₆H₇₄/C₄₄H₈₈O₂ as model compounds for epicuticular wax were chosen to study their interaction and its impact at the molecular level using powder X-ray diffraction and vibrational spectroscopy (infrared and Raman). The results indicate that EC particles and wax can form C-H···π type hydrogen bonding with charge transfer from carbon to wax; therefore, strong attraction is expected between them due to the cooperativity of hydrogen bonding and London dispersion from instantaneous dipoles. In reality, once settled on the leaf surface, especially without wax ultrastructures, BC with extremely large surface-to-volume ratio will likely stick and stay. On the other hand, BC particles can lead to phase transition of epicuticular wax from crystalline to amorphous structures by creating packing disorder and end- gauche defects of wax molecular chain, potentially causing water loss and thereby damage of plants.