Soot Aerosol Research Articles

Simulated contrail-processed aviation soot aerosols are poor ice-nucleating particles at cirrus temperatures

Abstract. Aviation soot surrogates processed in contrails are believed to become potent ice nuclei at cirrus temperatures. This is not verified for real aviation soot, which can have vastly different physico-chemical properties. Here, we sampled soot particles from in-use commercial aircraft engines and quantified the effect of contrail processing on their ice nucleation ability at T< 228 K. We show that aviation soot becomes compacted upon contrail processing, but that does not change their ice nucleation ability in contrast to other soot types. The presence of H2SO4 condensed in soot pores, the highly fused nature of the soot primary particles and their arrangement are what limit the volume of pores generated upon contrail processing, in turn limiting sites for ice nucleation. Furthermore, we hypothesized that contrail-processed aviation soot particles emitted from alternative jet fuel would also be poor ice-nucleating particles if their emission sizes remain small (< 150 nm).

Sedimentation and photophoretic levitation of aerosol clusters in the free molecular regime

Sedimentation of fractal aerosol clusters in rarefied gas medium in the dark and under external illumination similar to sunlight is studied in a numerical experiment taking into account orientation effects, depending on fractal dimension varying within a broad range. The calculation of forces and their moments, including friction forces and photophoretic forces, was carried out on the basis of approximation of free molecular gas kinetic regime and previously developed Monte-Carlo algorithms. Calculation involved 10000 clusters, each of them containing 160 primary spherical particles and efficiently absorbing sunlight and IR radiation, similarly to the particles of soot aggregates.The velocity of multi-particle cluster settling in the absence of illumination, when particle temperatures are equal to the temperature of the gaseous environment, is a distinct function of fractal dimension and is rather close to the velocity of settling of single spherical particles. The settling velocity approximation depending on fractal dimension has been obtained.Illumination brings dramatic changes into sedimentation pattern. The range of cluster sedimentation rate variations broadens substantially, which is a manifestation of gravito-photophoretic effect, so that some clusters even start to levitate. The proportion of levitating clusters is essentially dependent on fractal dimension. For soot-like clusters under irradiation similar to sunlight and pressure equal to atmospheric at the altitude of 30 km, the proportion of levitating aggregates is approximately 7%, which is in good agreement with the experimental data. In the dark and under irradiation, the aggregates rotate, and their trajectories are shaped as spirals.In the context of photophoretic phenomena, a result of principle has been obtained: under irradiation with sunlight, gravito-photophoretic levitation of aerosol soot-like aggregates composed of identical primary particles is possible at a pressure corresponding to the stratospheric altitudes. This result points to the possibility of substantial influence of gravito-photophoretic effect on transport and localization of soot aerosol in the Earth’s stratosphere and mesosphere.

Influence of soot aerosol properties on the counting efficiency of instruments used for the periodic technical inspection of diesel vehicles

Abstract. In this work, we investigated the influence of different types of soot aerosol on the counting efficiency (CE) of instruments employed for the periodic technical inspection (PTI) of diesel vehicles. Such instruments report particle number (PN) concentration. Combustion aerosols were generated by a prototype bigCAST, a miniCAST 5201 BC, a miniCAST 6204 C, and a miniature inverted soot generator (MISG). For comparison purposes, diesel soot was generated by a Euro 5b diesel test vehicle with by-passed diesel particulate filter (DPF). The size-dependent counting efficiency profile of six PN–PTI instruments was determined with each one of the aforementioned test aerosols. The results showed that the type of soot aerosol affected the response of the PN–PTI sensors in an individualised manner. Consequently, it was difficult to identify trends and draw conclusive results about which laboratory-generated soot is the best proxy for diesel soot. Deviations in the counting efficiency remained typically within 0.25 units when using laboratory-generated soot compared to Euro 5b diesel soot of similar mobility diameter (∼ 50–60 nm). Soot with a mobility diameter of ∼ 100 nm generated by the MISG, the lowest size we could achieve, resulted in most cases in similar counting efficiencies as those generated by the different CAST generators at the same particle size, showing that MISG may be a satisfactory – and affordable – option for PN–PTI verification; however, further optimisation will be needed for low-cost soot generators to comply with European PN–PTI verification requirements.

Differences and similarities in optical properties of coated fractal soot and its surrogates

Atmospheric soot (or black carbon, BC) affects climate through solar light absorption and scattering, which depend strongly on the particle morphology and composition. Initially, soot particles are fractal aggregates of spherules made of elemental carbon (EC), but condensation of atmospheric trace vapors adds non-EC materials and often results in particle compaction. The optical properties of such processed soot differ from those of fractal soot, and the changes are caused both by particle volume increase from coating addition and by restructuring of the EC backbone. In laboratory studies of soot optics, surrogates such as carbon black (CB) and nigrosin are often used in place of flame-generated soot. Our goal was to investigate if compositional and morphological differences between these surrogates and soot may produce different processing rates and optical responses. In our experiments, we generated fractal soot, compact CB, agglomerated CB (via coagulation of compact CB), and spherical nigrosin aerosol particles, subjected them to supersaturated vapor of dioctyl sebacate (DOS) to form a coating layer, and investigated the morphological response of these four particle types to coating addition and removal. Using coated and coated-denuded aerosol particles with known composition and morphology, we quantified the contributions of volume increase and restructuring to light scattering and absorption enhancements. By comparing experimental measurements against different particle optics models we show that it is crucial to account for larger, multiply charged particles present in the mobility-classified aerosol. Producing a disproportionately high contribution to absolute values of optical cross sections, such larger particles also result in lesser optical enhancements due to slower growth by vapor condensation. Scattering increases for all particle types due to the addition of a coating layer, and also due to restructuring for fractal soot (strongly) and agglomerated CB (weakly). Absorption increases only due to coating addition caused by the coating layer for all particle types. We find that simple optical models, such as Mie, are often sufficient to provide reasonable closure with experimental results for bare and coated aerosols, but only after accounting for the contributions from multiply charged particles, both in terms of their stronger optical cross sections and slower condensational growth. We conclude that CB is an appropriate surrogate for soot in aerosol aging studies where the effects of restructuring do not need to be considered and that nigrosin can be used as a general model for light-absorbing aerosols but is not representative of optical properties of soot.

Soot aerosols from commercial aviation engines are poor ice-nucleating particles at cirrus cloud temperatures

Abstract. Ice-nucleating particles catalyze ice formation in clouds, affecting climate through radiative forcing from aerosol–cloud interactions. Aviation directly emits particles into the upper troposphere where ice formation conditions are favorable. Previous studies have used proxies of aviation soot to estimate their ice nucleation activity; however, investigations with commercial aircraft soot from modern in-use aircraft engines have not been quantified. In this work, we sample aviation soot particles at ground level from different commercial aircraft engines to test their ice nucleation ability at temperatures ≤228 K as a function of engine thrust and soot particle size. Additionally, soot particles were catalytically stripped to reveal the impact of mixing state on their ice nucleation ability. Particle physical and chemical properties were further characterized and related to the ice nucleation properties. The results show that aviation soot nucleates ice at or above relative humidity conditions required for homogeneous freezing of solution droplets (RHhom). We attribute this to a mesopore paucity inhibiting pore condensation and the sulfur content which suppresses freezing. Only large soot aggregates (400 nm) emitted under 30 %–100 % thrust conditions for a subset of engines (2 out of 10) nucleate ice via pore condensation and freezing. For those specific engines, the presence of hydrophilic chemical groups facilitates the nucleation. Aviation soot emitted at thrust ≥ 100 % (sea level thrust) nucleates ice at or above RHhom. Overall, our results suggest that aviation soot will not contribute to natural cirrus formation and can be used in models to update impacts of soot–cirrus clouds.

Chemical properties and single-particle mixing state of soot aerosol in Houston during the TRACER campaign

Abstract. A high-resolution soot particle aerosol mass spectrometer (SP-AMS) was used to selectively measure refractory black carbon (rBC) and its associated coating material using both the ensemble size-resolved mass spectral mode and the event trigger single particle (ETSP) mode in Houston, Texas, in summer 2022. This study was conducted as part of the Department of Energy Atmospheric Radiation Measurement (ARM) program's TRacking Aerosol Convection interactions ExpeRiment (TRACER) field campaign. The study revealed an average (±1σ) rBC concentration of 103 ± 176 ng m−3. Additionally, the coatings on the BC particles were primarily composed of organics (59 %; 219 ± 260 ng m−3) and sulfate (26 %; 94 ± 55 ng m−3). Positive matrix factorization (PMF) analysis of the ensemble mass spectra of BC-containing particles resolved four distinct types of soot aerosol, including an oxidized organic aerosol (OOABC,PMF) factor associated with processed primary organic aerosol, an inorganic sulfate factor (SO4,BC,PMF), an oxidized rBC factor (O-BCPMF), and a mixed mineral dust–biomass burning aerosol factor with significant contribution from potassium (K-BBBC,PMF). Additionally, K-means clustering analysis of the single-particle mass spectra identified eight different clusters, including soot particles enriched in hydrocarbon-like organic aerosol (HOABC,ETSP), sulfate (SO4,BC,ETSP), two types of rBC, OOA (OOABC,ETSP), chloride (ClBC,ETSP), and nitrate (NO3,BC,ETSP). The single-particle measurements demonstrate substantial variation in BC coating thickness with coating-to-rBC mass ratios ranging from 0.1 to 100. The mixing state index (χ), which denotes the degree of homogeneity of the soot aerosol, varied from 4 % to 94 % with a median of 40 %, indicating that the aerosol population lies in between internal and external mixing but has large temporal and source type variability. In addition, a significant fraction of BC-containing particles, a majority enriched with oxidized organics and sulfate, exhibit sufficiently high κ values and diameters conducive to activation as cloud nuclei under atmospherically relevant supersaturation conditions. This finding bears significance in comprehending the aging processes of rBC-containing particles and their activation into cloud droplets. Our analysis highlights the complex nature of soot aerosol and underscores the need to comprehend its variability across different environments for accurate assessment of climate change.

An optical soot sizing method without prior morphology parameters

Great concern has been focused on the particle size distribution (PSD) of soot as a crucial factor of the physical and chemical properties. Optical methods are widely used to analyze the PSD of soot due to their non-invasive and real-time measurement, where the morphology parameters are essential for retrieving the PSD from optical signals. However, these parameters are difficult to obtain in field measurement. In this work, we propose a new PSD sensing method for soot without prior knowledge of the morphology parameters based on the light-scattering angular spectrum using a hybrid inverse algorithm. The light scattering angular spectrum is the intensity distribution of scattering light at different observation angles for simultaneously characterizing the PSD and the morphological parameters, which are decoupled by a hybrid inverse algorithm consisting of Tikhonov regularization and adaptive weighted particle swarm optimization (AW-PSO). A compacted and lens-free prototype sensor is designed to verify our method. The calibration experiments by spherical aerosols show that our prototype sensor provides high-precision light scattering angular spectrum measurement with maximum relative error (RE) of 14.78 %. In the tests of soot, the maximum Kullback-Leibler Divergence (DKL) of PSD is 0.28 in comparison with the reference volume equivalent PSD. The sensing method proposed in this paper provides volume equivalent PSD measurement for soot without requirement of the morphology parameters, and our prototype sensor shows great potential for soot and other non-spherical aerosol analysis in routine field measurements outside the laboratory.

Field study on the diffusion and advection effects of vehicle-emitted soot aerosols on snow albedo in the Chilean Andes

Emissions from road traffic are one of the most important sources of soot aerosols and they can affect the surfaces in the area. In the case of snow surfaces, this effect may lead to changes in the radiative forcing and snow melt, also influenced by particle transport such as particle diffusion and advection. An experimental campaign in The Andes, Chile, was carried out measuring the radiance and irradiance of the snow perpendicularly to a road with high traffic load, together with the meteorological conditions (to identify the diffusion and advection phenomena), the aerosol size distribution, the concentration of particles (PM1, PM2.5 and PM10), and the concentration of black carbon (BC) in both, the atmosphere and the snowpack. The aim of the study was to quantify the contribution of four factors affecting the albedo (black carbon -BC- concentration in snow, grain size, cloudiness, and roughness) by comparing the measured albedo of the contaminated snow surface with that of the same surface prior to contamination, considered as a reference. Results showed a trade-off between diffusion and advection. Close to the road, diffusion was predominant, leading to an increase in BC concentration and a reduction in snow albedo. On the contrary, far from the road, where winds are channeled along the centre of the valley, advection of particles became dominant, leading to another increase in particle concentration and another reduction in snow albedo. Among the factors contributing to reduce the snow albedo, BC concentration dominates at all distances from the road, although the effect of the grain size becomes as significant as that of BC in the centre of the valley, with the effects of surface roughness and cloudiness remaining minor. This information can be used in snow models to get a better knowledge of the effect of particle deposition on albedo reductions.

Investigating an indirect aviation effect on mid-latitude cirrus clouds – linking lidar-derived optical properties to in situ measurements

Abstract. Aviation has a large impact on the Earth's atmosphere and climate by various processes. Line-shaped contrails and contrail cirrus clouds lead to changes in the natural cirrus cloud cover and have a major contribution to the effective radiative forcing from aviation. In addition, aviation-induced aerosols might also change the microphysical properties and optical properties of naturally formed cirrus clouds. Latter aerosol–cloud interactions show large differences in the resulting effective radiative forcing, and our understanding on how aviation-induced aerosols affect cirrus cloud properties is still poor. Up to now, observations of this aviation-induced aerosol effect have been rare. In this study, we use combined airborne lidar and in situ ice cloud measurements to investigate differences in the microphysical and optical properties of naturally formed cirrus clouds, which formed in regions that are highly affected by aviation-induced aerosol emissions and, of those, which formed in regions rather unaffected by aviation. Urbanek et al. (2018) showed that those cirrus clouds, which are more affected by aviation-induced soot emission, are characterized by larger values of the particle linear depolarization ratio (PLDR). In this follow-on study we relate collocated lidar measurements performed aboard HALO during the ML-CIRRUS mission of the particle linear depolarization ratio with in situ cloud probe measurements of the number concentration and effective diameter of the ice particles. In situ measurements for both cloud types (high-PLDR-mode – aviation-affected – and low-PLDR-mode – pristine – cirrus) can be reliably compared in a temperature range between 210 and 215 K. Within this temperature range we find that high-PLDR-mode cirrus clouds tend to show larger effective ice particle diameters with a median value of 61.4 compared to 50.7 µm for low-PLDR-mode pristine cirrus clouds. Larger effective ice particles in aviation-influenced (high-PLDR-mode) cirrus are connected to lower ice particle number concentration with a median value of 0.05 compared to 0.11 cm−3 (low-PLDR-mode), which evolved in more pristine regions with only little impact from aviation. We suspect that a suppression of homogeneous ice formation by the heterogeneously freezing soot aerosol particles included in the areas affected by air traffic is the cause of the reduced ice crystal concentrations.

Effect of pressure and CO2 dilution on soot formation in laminar inverse coflow flame at conditions close to autothermal reforming

Autothermal reforming (ATR) is an important technology for hydrogen production from natural gas, where soot formation deserves particular attention as it may cause catalyst poisoning. The typical configuration of ATR reactors is that of an inverse diffusion flame (IDF). In this study, soot formation near ATR conditions is investigated experimentally and numerically, focusing on the effects of pressure and CO2 dilution in IDFs with pressure reaching 5 bar. The fuel stream consists of methane diluted with CO2 or N2. The mole fraction of O2 in the oxidant stream is 70%. Polycyclic aromatic hydrocarbons (PAHs) and soot volume fraction are measured by the planar laser-induced fluorescence (PLIF) and planar laser-induced incandesce (PLII) methods. High-fidelity simulations with a detailed soot aerosol model are also performed, and an empirical reactive soot inception model is proposed. A comparison of the results shows that the soot behavior is well predicted by the empirical reactive inception model but not fully captured by the physical inception model based on irreversible dimerization, demonstrating the importance of radical involvement in the soot inception process. Both measurements and predictions show a linear relationship between peak soot volume fraction and pressure, which can be explained by the linear increase of PAH molar concentration with pressure. Simulation analysis indicates that the dimer adsorption and HACA mechanism have a similar quantitative contribution to the soot growth. The CO2 shows stronger suppression effects in soot formation at the investigated pressures compared to N2. The contributions of chemical and thermal effects to soot suppression are numerically analyzed and the results indicate that the chemical effect of CO2 is mainly due to the removal of H radicals through the reaction CO2 + H→CO + OH, providing a soot suppression contribution of two times larger compared to the thermal one.

Soot Aerosols from Wheat Stubble Burning Lead to Ice Nucleation and Heavy Rainfall Over Arid Rajasthan, India

Soot Aerosols from Wheat Stubble Burning Lead to Ice Nucleation and Heavy Rainfall Over Arid Rajasthan, India

Atmospheric nanoparticles hygroscopic growth measurement by a combined surface plasmon resonance microscope and hygroscopic tandem differential mobility analyzer

Abstract. The hygroscopic growth of atmospheric aerosols plays an important role in regional radiation, cloud formation, and hence climate. Aerosol hygroscopic growth is often characterized by hygroscopic tandem differential mobility analyzers (HTDMAs), and Xie et al. (2020) recently demonstrated that hygroscopic growth measurements of a single particle are possible using a surface plasmon resonance microscope-azimuthal rotation illumination (SPRM-ARI). The hygroscopic properties of ambient aerosols are not uniform and often exhibit large relative humidity (RH) and size variabilities due to different chemical compositions and mixing states. To better understand the contribution of different aerosol components and establish a link between the apparent hygroscopic properties of single particles and bulk aerosols, we conduct combined hygroscopic growth measurements using a SPRM-ARI and an HTDMA as a case study to prove the concept (experimental information: 100–200 nm, during noontime on 28 September 2021 and 22 March 2022 in Hefei, China). According to the distinct hygroscopic growth behavior from single-particle probing using a SPRM-ARI, the individual particles can be classified into three categories defined as non-hygroscopic (NH), less hygroscopic (LH), and more hygroscopic (MH). The mean growth factor (GF) of the three categories can be utilized to reproduce the GF distribution obtained from the HTDMA measurement. The chemical compositions of individual particles from the three categories are identified to be organic carbon (OC), soot (mainly elemental carbon), fly ash, and secondary aerosols (mainly OC and sulfate), using scanning electron microscopy (SEM) with an energy-dispersive spectrometer (EDS). The coupled SPRM–HTDMA measurement suggests a size-dependent variation of aerosol chemical components, i.e., an increase of OC fraction with increasing particle sizes, which agrees reasonably well with the chemical compositions from collected aerosol samples. This likely links the hygroscopic properties of individual particles to their bulk hygroscopic growth and chemical composition.

The importance of accurately modelling soot and radiation coupling in laminar and laboratory-scale turbulent diffusion flames

Ultrafine and highly light absorbing soot particles have been known to be harmful to human health and a major climate forcer. Understanding the mechanisms of soot formation and developing reliable and robust soot models are essential to design efficient and clean combustion systems. Thermal radiation is often the dominant heat transfer mode in both turbulent and laminar flames and affects soot formation through its influence on temperature. Significant progress has been made in modelling soot formation, thermal radiation transfer, and turbulence radiation interaction in hydrocarbon flames over the last three decades. However, the optically thin approximation has still been used to simplify thermal radiation transfer modelling and turbulence radiation interaction has often been neglected in soot formation modelling without proper justification. In this paper, soot models developed for laminar and laboratory scale turbulent jet diffusion flames, non-gray radiative heat transfer models, and turbulence radiation interaction models are reviewed. The aim of this review is to highlight the importance of thermal radiation heat transfer in both laminar and turbulent diffusion flames and turbulence radiation interaction in turbulent jet diffusion flames. Use of the optically thin approximation is often not justified even in laminar diffusion flames. This is particularly true in diffusion flames at elevated pressures or microgravity. Although the prediction of soot in both laminar and turbulent flames is primarily influenced by soot kinetic model, it is also significantly affected by aerosol dynamic model, thermal radiation heat transfer model, and turbulence radiation interaction model. It is important to accurately model soot aerosol dynamics and non-gray thermal radiation transfer in laminar diffusion flames and additionally to adequately model turbulence radiation interaction in turbulent flames for the purpose of validating reaction mechanisms and soot kinetic models.

Impacts of Cloud‐Processing on Ice Nucleation of Soot Particles Internally Mixed With Sulfate and Organics

AbstractSoot particles are one of the major aerosol components (in particle number concentration) in the troposphere and can impact cirrus formation. Depending on their physicochemical properties, soot particles can nucleate ice in the cirrus regime via pore condensation and freezing (PCF) at a lower ice saturation threshold than that of the homogeneous freezing of solution droplets. Soot aerosol undergoing multiple cloud cycles can change its porosity and surface wettability, thus modulating its PCF ability. In this study, organic‐rich and ‐lean size‐selected propane flame soot particles were coated with different thicknesses of sulfuric acid (H2SO4) and exposed to thermodynamic conditions under 228 K and relative humidity with respect to water 104% to simulate contrail formation. Ice‐activated soot particles from the mimicked contrail were sublimated before being tested for ice nucleation (IN) at T ≤ 233 K. Additionally, soot particle size and mass distribution as well as morphology were characterized. The results demonstrate that the increase in mesopore availability induced by cloud‐processing plays a key role in regulating soot IN. All cloud‐processed soot become more compacted, however, only if more PCF relevant mesopores are generated, can the compacted soot exhibit promoted IN. If a H2SO4 coating and/or enriched organics occupy the pore volume, inhibition of PCF is observed for the cloud‐processed soot. Notably, a thick H2SO4 coating enhances the PCF of 400 nm organic‐lean soot particles because the partially engulfed but collapsed soot‐core may have new mesopores produced by compaction. Therefore, the mesopore abundance plays the key role in evaluating aged soot IN ability.

Numerical investigation on the accuracy of size information of fractal soot aerosols retrieved by lidar: Optical property, morphology effect, and parameterization scheme

Lidar is effective in retrieving size distribution of atmospheric particles such as soot aerosols in real-time based on Mie scattering theory. However, the fundamental spherical shape assumption of Mie theory is obviously discrepant with the actual situations of atmospheric soot particles which are fractal and mixed with coating materials. In this study, the effects of fractal shapes and morphological parameters such as fractal dimension (Df), volume fraction (Vf), and particle radius (r) of soot aerosols on the size information retrieved by lidar based on Mie theory were investigated from the numerical aspect, closed-cell and coated-aggregate models were employed to represent soot aerosols thinly and heavily coated by non-absorbing materials, respectively. Optical properties of fractal soot particles were calculated at first, then backscattering and extinction properties were employed to retrieve backscattering equivalent radius (rbk) and extinction equivalent radius (rext) based on Mie theory, just like the lidar retrieval. Results revealed that optical properties of spherical and fractal models differ significantly with the variation of volume equivalent radius (rvol). Radius retrieval errors based on extinction cross-section are much smaller than that based on backscattering cross-section. The retrieval accuracy of coated-aggregate is higher than bare fractal aggregate and closed-cell models. Retrieved size distribution parameters mean radius are generally underestimated while standard deviation are overestimated for both models. Furthermore, parameterization schemes connecting rext and rvol of soot were constructed, retrieval errors are effectively reduced from about 25% to 10%, which is meaningful for the improvement of size retrieval accuracy of soot using lidar observations.

The effects of CO2/CH4 ratio on soot formation for autothermal reforming of methane at elevated pressure

Autothermal reforming of methane (ATR) technique is a promising technology for H2 production. Soot formed in the combustion region of ATR may deactivate the downstream catalyst used to enhance the reforming reactions. In this work, the effects of CO2/CH4 ratio on soot production and main gas-phase components involved in reforming reactions were investigated by experiment and numerical simulations in laminar inverse coflow diffusion flames, at conditions close to ATR. The CO2/CH4 ratio ranges from 0.27 to 4.90, and the pressure ranges from 1 atm to 10 atm. The oxidant consists 70% O2 and 30% N2 to mimic the oxy-combustion in the ATR process. The soot, polycyclic aromatic hydrocarbon (PAHs), OH* distribution, and flame structure were measured using planar laser-induced incandesce, planar laser-induced fluorescence, OH* luminescence, and flame luminescence, respectively. High fidelity simulations with a reduced gas-phase kinetic mechanism and soot aerosol models were also undertaken to supplement the findings. The results show that soot formation is suppressed by a higher CO2/CH4 ratio, while the level of soot reduction is attenuated as the pressure increases. The peak soot volume fraction increases with the flame height, and then levels off. PAHs concentration monotonically increases with flame height. The spatial distributions of PAH and soot along flame height were not well captured by numerical simulations, and the used physical based soot inception model is expected to account for the discrepancy. The peak soot volume fraction correlates with CO2/CH4 ratio by an exponentially decaying function, with the exponent decreasing with increasing pressure. A similar trend of PAHs concentration along with CO2/CH4 ratio was also observed. Non-sooting flame is achievable at elevated pressure if the CO2/CH4 ratio is higher than 4.0, at the expense of lowering the syngas yield in the combustion region. The simulations also predicted that the flame temperature and concentration of H2 and CO decrease with CO2/CH4 ratio.

Impact of water uptake and mixing state on submicron particle deposition in the human respiratory tract (HRT) based on explicit hygroscopicity measurements at HRT-like conditions

Abstract. Particle hygroscopicity plays a key role in determining the particle deposition in the human respiratory tract (HRT). In this study, the effects of hygroscopicity and mixing state on regional and total deposition doses on the basis of the particle number concentration for children, adults, and the elderly were quantified using the Multiple-Path Particle Dosimetry model, based on the size-resolved particle hygroscopicity measurements at HRT-like conditions (relative humidity = 98 %) performed in the North China Plain. The measured particle population with an external mixing state was dominated by hygroscopic particles (number fraction = (91.5 ± 5.7) %, mean ± standard deviation (SD); the same below). Particle hygroscopic growth in the HRT led to a reduction by around 24 % in the total doses of submicron particles for all age groups. Such a reduction was mainly caused by the growth of hygroscopic particles and was more pronounced in the pulmonary and tracheobronchial regions. Regardless of hygroscopicity, the elderly group of people had the highest total dose among three age groups, while children received the maximum total deposition rate. With 270 nm in diameter as the boundary, the total deposition doses of particles smaller than this diameter were overestimated, and those of larger particles were underestimated, assuming no particle hygroscopic growth in the HRT. From the perspective of the daily variation, the deposition rates of hygroscopic particles with an average of (2.88 ± 0.81) × 109 particles h−1 during the daytime were larger than those at night ((2.32 ± 0.24) × 109 particles h−1). On the contrary, hydrophobic particles interpreted as freshly emitted soot and primary organic aerosols exhibited higher deposition rates at nighttime ((3.39 ± 1.34) × 108 particles h−1) than those in the day ((2.58 ± 0.76) × 108 particles h−1). The traffic emissions during the rush hours enhanced the deposition rate of hydrophobic particles. This work provides a more explicit assessment of the impact of hygroscopicity and mixing state on the deposition pattern of submicron particles in the HRT.

Albedo reduction for snow surfaces contaminated with soot aerosols: Comparison of experimental results and models

Highly reflective snow surfaces are sensitive to contamination by light-absorbing particles. When such particles are deposited onto a snow surface, they reduce snow albedo, which leads to snow surface warming and accelerates snowmelt. To better understand and characterize the effects of light-absorbing particles on snow albedo, an experimental campaign was conducted in Cotos Port, Madrid Spain. A spectroradiometric system consisting of six spectroradiometers simultaneously measuring sun irradiance and snow radiance spectra over the range from 300 to 2500 nm was used. Measured snow albedo spectra were compared to modeled snow albedo spectra obtained with a novel snow radiative transfer model (OptiPar) developed by our research group. Comparison of these experimental and OptiPar-modeled results with results from open modeling software, such as SNICAR and snowTARTES, has demonstrated that the detailed consideration of soot aerosol shape and size is helpful for a good spectral albedo prediction and that OptiPar provides enough flexibility for optimal fitting.

Soot biodegradation by psychrotolerant bacterial consortia.

To probe the bioavailability of soot released into the atmosphere is pivotal to understanding their environmental impacts. Soot aerosol absorbs organic matter, creating a hot spot for biogeochemical transformation and the global carbon cycle. Soot primarily contains condensed aromatics chemically recalcitrant; however, oligotrophic microorganisms might use it as a nutritional source. This study investigated the influence of psychrotolerant bacterial consortia on soot. Significant increase in the bacterial biomass, reduction in water-insoluble organic carbon (OC) and elemental carbon (EC) in soot residues and increase in water-soluble OC in the filtrate signifies the use of soot as a carbon and nutritional source. The influence on morphology and composition of soot was reported using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy, and Energy Dispersive X-Ray analysis (EDX). The FTIR analysis showed significant variations in the pattern of soot spectra, suggesting degradation. Elemental analysis and EDX showed a reduction in carbon percentage. Besides, the reduction of optical density with incubation time signifies the OC and EC consumption. This study shows that soot can be a substrate and pivotal factor in the microbial food web. Nowadays, soot emission to the environment is growing; therefore, soot involvement in microbe-mediated processes should be closely focused.

The Effect of Varying Aerosol Concentrations on Visibility and Particle Size Distribution in Urban Atmosphere: Validating OPAC Results using MERRA Satellite Aerosol Data

Well validated simulation software and models are meant to add value through a more precise representation of different atmospheric features and climate characteristics that are physically out of reach. In this work, comparison and validation was carried out using the Modern Era Retrospective Analysis for Research and Applications (MERRA) averaged Angstrom Exponents (α) and averaged visibilities for ten urban countries against simulated Optical Properties of Aerosols and Clouds (OPAC) urban components. The simulations of OPAC were performed at 0%, 50%, 70%, 80%, 90%, 95%, 98% and 99% relative humidities (RH) while the MERRA satellite data were extracted at an average of 78% relative humidity. results based on α values showed that OPAC was only able to simulate well the insoluble and soot aerosol particle size distributions of India’s urban atmosphere such that it had approximately the same insoluble and soot aerosol particle size distributions as India. For the rest of the nine countries (USA, Brazil, Indonesia, China, Japan, Mexico, Nigeria, Pakistan and Ethiopia), OPAC was seen to either overestimate or underestimate water soluble, insoluble and soot aerosol particle size distributions. For visibility and water soluble or insoluble aerosol concentrations, OPAC showed good simulated values that were approximately the same as those found within Ethiopia’s and Japan’s urban atmosphere. OPAC also had visibility value and insoluble aerosol concentration that were approximately the same as those found within China’s atmosphere. For all other countries (India, USA, Brazil, Indonesia, Mexico, Nigeria and Pakistan) OPAC either overestimated their visibilities and underestimated their water soluble, insoluble and soot aerosol concentrations or vice versa. With both OPAC and MERRA, the presence of fine mode aerosol particles in an urban atmosphere was established with α values > 1. But MERRA also showed otherwise for three countries (Nigeria, Ethiopia and Pakistan) that had α values < 1 which implied the presence of coarse mode aerosol particles. The relationship between visibility and α satisfied the direct power law for OPAC, for MERRA the relationship approximately satisfied the power law for most of the countries.