Optical properties of coated black carbon aggregates: numerical simulations, radiative forcing estimates, and size-resolved parameterization scheme

The formation of black carbon fractal aggregates (BCFAs) from combustion and subsequent ageing involves several stages resulting in modifications of particle size, morphology, and composition over time. To understand and quantify how each of these modifications influences the BC radiative forcing, the optical properties of BCFAs are modelled. Owing to the high computational time involved in numerical modelling, there are some gaps in terms of data coverage and knowledge regarding how optical properties of coated BCFAs vary over the range of different factors (size, shape, and composition). This investigation bridged those gaps by following a state-of-the-art description scheme of BCFAs based on morphology, composition, and wavelength. The BCFA optical properties were investigated as a function of the radius of the primary particle (ao), fractal dimension (Df), fraction of organics (forganics), wavelength (λ), and mobility diameter (Dmob). The optical properties are calculated using the multiple-sphere T-matrix (MSTM) method. For the first time, the modelled optical properties of BC are expressed in terms of mobility diameter (Dmob), making the results more relevant and relatable for ambient and laboratory BC studies. Amongst size, morphology, and composition, all the optical properties showed the highest variability with changing size. The cross sections varied from 0.0001 to 0.1 µm2 for BCFA Dmob ranging from 24 to 810 nm. It has been shown that MACBC and single-scattering albedo (SSA) are sensitive to morphology, especially for larger particles with Dmob > 100 nm. Therefore, while using the simplified core–shell representation of BC in global models, the influence of morphology on radiative forcing estimations might not be adequately considered. The Ångström absorption exponent (AAE) varied from 1.06 up to 3.6 and increased with the fraction of organics (forganics). Measurement results of AAE ≫ 1 are often misinterpreted as biomass burning aerosol, it was observed that the AAE of purely black carbon particles can be ≫ 1 in the case of larger BC particles. The values of the absorption enhancement factor (Eλ) via coating were found to be between 1.01 and 3.28 in the visible spectrum. The Eλ was derived from Mie calculations for coated volume equivalent spheres and from MSTM for coated BCFAs. Mie-calculated enhancement factors were found to be larger by a factor of 1.1 to 1.5 than their corresponding values calculated from the MSTM method. It is shown that radiative forcings are highly sensitive to modifications in morphology and composition. The black carbon radiative forcing ΔFTOA (W m−2) decreases up to 61 % as the BCFA becomes more compact, indicating that global model calculations should account for changes in morphology. A decrease of more than 50 % in ΔFTOA was observed as the organic content of the particle increased up to 90 %. The changes in the ageing factors (composition and morphology) in tandem result in an overall decrease in the ΔFTOA. A parameterization scheme for optical properties of BC fractal aggregates was developed, which is applicable for modelling, ambient, and laboratory-based BC studies. The parameterization scheme for the cross sections (extinction, absorption, and scattering), single-scattering albedo (SSA), and asymmetry parameter (g) of pure and coated BCFAs as a function of Dmob were derived from tabulated results of the MSTM method. Spanning an extensive parameter space, the developed parameterization scheme showed promisingly high accuracy up to 98 % for the cross sections, 97 % for single-scattering albedos (SSAs), and 82 % for the asymmetry parameter (g).

Fuel molecular structure effect on soot mobility size in premixed C6 hydrocarbon flames

Soot formation in premixed laminar flames is examined for a canonical set of flames burning C6 hydrocarbon fuels. Particle mobility size and flame temperature measurements are complemented by flame structure calculations using detailed flame chemistry. Specifically, the evolution of the detailed soot particle size distribution (PSDF) is compared for n-hexane, n-hexene, 2-methylpentane, cyclohexane and benzene at a carbon-to-oxygen ratio of 0.69 and maximum flame temperature of 1800 K. Under this constraint, the overall sooting process is comparable as evidenced by similar time resolved bimodal PSDF. However, the first inception of particles and the persistence of nucleation-sized particles with time are depend upon the structure of the parent fuel. For the given conditions, the fastest onset of soot is observed in cyclohexane and benzene flames and the observed evolution of the PSDF also shows that nucleation-sized particles disappear sooner in cyclohexane and benzene flames. Flame structure computations incorporating detailed chemistry show a clear connection between the early onset of soot particles as fuel specific routes to PAH formation are predicted in the pre-flame region of the cyclohexane and benzene flames. These observations illustrate the impact of alkane, alkene, cycloalkane and aromatic fuel structure on soot formation in premixed flames. Analysis of soot particle morphology by atomic force microscopy indicates that most of size distribution is composed of aggregates. Simple aggregate mobility diameter analysis shows the spherical assumption taken to interpret the mobility diameter does not impact the PSDF number density result but the inferred volume fraction for aggregates deviates by up to an order of magnitude depending on the morphology assumptions adopted.

Atmospheric fungal nanoparticle bursts

Aerosol nanoparticles play an important role in the climate system by affecting cloud formation and properties, as well as in human health because of their deep reach into lungs and the circulatory system. Determining nanoparticle sources and composition is a major challenge in assessing their impacts in these areas. The sudden appearance of large numbers of atmospheric nanoparticles is commonly attributed to secondary formation from gas-phase precursors, but in many cases, the evidence for this is equivocal. We report the detection of a mode of fungal fragments with a mobility diameter of roughly 30 nm released in episodic bursts in ambient air over an agricultural area in northern Oklahoma. These events reached concentrations orders of magnitude higher than other reports of biological particles and show similarities to unclarified events reported previously in the Amazon. These particles potentially represent a large source of both cloud-forming ice nuclei and respirable allergens in a variety of ecosystems.

A novel study of the morphology and effective density of externally mixed black carbon aerosols in ambient air using a size-resolved single-particle soot photometer (SP2)

The morphology of externally mixed black carbon (extBC) aerosols, an important factor affecting radiative forcing, was studied by a tandem method coupling a differential mobility diameter (DMA) with a single-particle soot photometer (SP2). Ambient particles were selected by the DMA, and the size-resolved extBC particles were distinguished from those of a thick coating (internally mixed) and quantified by the SP2. Time differences between the DMA size selection and the SP2 measurement were processed previously, as well as the effects of multicharged particles. Based on the mass-mobility relationship, the fractal dimension of the extBC particles was obtained, with a value of 2.36 ± 0.04. This value is comparable with those of diesel exhaust particles, implying a predominant contribution of vehicle emissions to the ambient extBC in urban Beijing. The effective densities (ρeff) of the extBC in the mobility diameter range of 140–750 nm were also derived, with values gradually decreasing from 0.34 g cm−3 at 140–160 nm to 0.12 g cm−3 at 700 nm. The ρeff values were generally lower than those measured using the DMA-aerosol particle mass analyzer (APM) system. This was most likely due to the lower BC masses determined by the SP2 compared to those from the APM, since the SP2 measured the mass of pure refractory BC instead of the entire BC aggregate consisting of both refractory BC and nonrefractory components measured by the APM.

Experimental study of H₂SO₄ aerosol nucleation at high ionization levels

One hundred and ten direct measurements of aerosol nucleation rate at high ionization levels were performed in an 8 m3 reaction chamber. Neutral and ion-induced particle formation from sulfuric acid (H2SO4) was studied as a function of ionization and H2SO4 concentration. Other species that could have participated in the nucleation, such as NH3 or organic compounds, were not measured but assumed constant, and the concentration was estimated based on the parameterization by Gordon et al. (2017). Our parameter space is thus [H2SO4] =4×106-3×107 cm−3, [NH3+ org] = 2.2 ppb, T=295 K, RH = 38 %, and ion concentrations of 1700–19 000 cm−3. The ion concentrations, which correspond to levels caused by a nearby supernova, were achieved with gamma ray sources. Nucleation rates were directly measured with a particle size magnifier (PSM Airmodus A10) at a size close to critical cluster size (mobility diameter of ∼ 1.4 nm) and formation rates at a mobility diameter of ∼ 4 nm were measured with a CPC (TSI model 3775). The measurements show that nucleation increases by around an order of magnitude when the ionization increases from background to supernova levels under fixed gas conditions. The results expand the parameterization presented in Dunne et al. (2016) and Gordon et al. (2017) (for [NH3+org] = 2.2 ppb and T=295 K) to lower sulfuric acid concentrations and higher ion concentrations. The results make it possible to expand the parameterization presented in Dunne et al. (2016) and Gordon et al. (2017) to higher ionization levels.

Experimental study of H₂SO₄ aerosol nucleation at high ionization levels

One hundred and ten direct measurements of aerosol nucleation rate at high ionization levels were performed in an 8 m3 reaction chamber. Neutral and ion-induced particle formation from sulphuric acid (H2SO4) as a function of ionization and H2SO4 concentration was studied. Other species that could participate in the nucleation were not measured. The measurements extend the parameter space of measurements described by (Dunne, 2016) (at T = 295 K and RH = 38 %) by expanding to lower H2SO4 concentrations (4x106–3x107 cm−3) and higher ion concentrations (1700–19000 cm−3). The ion concentrations, which correspond to levels caused by a nearby supernova, were achieved with gamma ray sources. Nucleation rates were directly measured with a particle size magnifier (PSM Airmodus A10) at a size close to critical cluster size (mobility diameter of ~ 1.4 nm) and formation rates at mobility diameter of ~ 4 nm were measured with a CPC (TSI model 3775). The measurements show that nucleation increases by around a factor of five when the ionization increases from background to supernova levels under fixed gas conditions. The results expand the parametrization from (Dunne, 2016) to lower sulphuric acid concentrations and higher ion concentrations.