Challenges of measuring nascent soot in flames as evidenced by high-resolution differential mobility analysis

<p>Nascent soot particles with mobility diameters ≤10 nm were measured in an ethylene/air premixed flame to shed light on the challenges and potential artifacts affecting studies on soot inception by differential mobility analysis (DMA) techniques. The size distribution functions (SDFs) of particles with charge acquired either naturally or diffusively upon ion seeding were measured at several positions in the flame using rapid-dilution probing and a high-resolution DMA for different values of the ratio of dilution ratio to residence time (DR/Δ<i>t</i>). The SDFs are roughly bimodal with a sub-3 nm mode and a larger one that appears either downstream in the flame or for low DR/Δ<i>t</i>s. Soot nuclei smaller than 3 nm preferentially acquire positive charge, which brings into question the assumption of steady-state charging probability of flame sampled soot nuclei in the bipolar diffusion neutralizer. The approximately polarity-symmetric lognormal SDF of larger particles is attributed to nuclei coagulation. Naturally charged particles increase in number when lowering DR/Δ<i>t</i>, suggesting either their collisional charging by flame chemi-ions or particle nucleation by condensation of neutral molecules on ions or both. The critical conditions for suppressing particle coagulation and charge redistribution in the sampling system were not achieved under most conditions, despite the fact that values of DR/Δ<i>t</i>s were more favorable to such a suppression in the present experiment as compared to other studies in the literature. As a result, the identification of this “asymptotic” regime, which is critical to determine the parent SDFs and the charge state of nascent soot in the flame, is still elusive.</p> <p>© 2016 American Association for Aerosol Research</p

Factors Influencing Ultrafine Particulate Matter (PM_0.1) Formation under Pulverized Coal Combustion and Oxyfiring Conditions

This paper explores the early processes of coal ultrafine ash (<i>D</i> < 100 nm) formation under both conventional air-blown and oxyfiring conditions. An innovative flow reactor and a high-resolution differential mobility analysis technique have been coupled to measure the particle size distribution functions (PSDFs) in a size range extending down to 1 nm. Information on the formed fly ash chemical nature has been obtained by ultraviolet−visible (UV−vis) light absorption and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM−EDXS). Five coals of different rank, covering a broad range of ash compositions, have been tested under three oxygen concentration levels. A multimodal behavior of coal ultrafine ash PSDFs has always been observed. The first mode at 1−5 nm has been attributed to carbonaceous particles based on the UV−vis light absorption measurements and the results obtained burning a carbon black powder under the same operating conditions. The volume fractions of larger mode particles have been correlated to coal components. SEM−EDXS analyses have mostly supported the correlation indications. Results suggest that the particle size modes derive from size-selective nucleation of refractory oxides and metal nanoparticles and their subsequent growth. The oxygen concentration influences the size of nucleating particles and the preferential vaporization of some compounds with respect to others through both char-burning temperatures and the local reducing properties of the gas environment. Nevertheless, an enhanced oxygen concentration promotes ultrafine particle formation

Observation and Modeling of the Solar Wind Turbulence Evolution in the Sub-Mercury Inner Heliosphere

This letter exploits the radial alignment between the Parker Solar Probe and BepiColombo in late 2022 February, when both spacecraft were within Mercury’s orbit. This allows the study of the turbulent evolution, namely, the change in spectral and intermittency properties, of the same plasma parcel during its expansion from 0.11 to 0.33 au, a still unexplored region. The observational analysis of the solar wind turbulent features at the two different evolution stages is complemented by a theoretical description based on the turbulence transport model equations for nearly incompressible magnetohydrodynamics. The results provide strong evidence that the solar wind turbulence already undergoes significant evolution at distances less than 0.3 au from the Sun, which can be satisfactorily explained as due to evolving slab fluctuations. This work represents a step forward in understanding the processes that control the transition from weak to strong turbulence in the solar wind and in properly modeling the heliosphere.</p

Linking Small-scale Solar Wind Properties with Large-scale Coronal Source Regions through Joint Parker Solar Probe-Metis/Solar Orbiter Observations

The solar wind measured in situ by Parker Solar Probe in the very inner heliosphere is studied in combination with the remote-sensing observation of the coronal source region provided by the METIS coronagraph aboard Solar Orbiter. The coronal outflows observed near the ecliptic by Metis on 2021 January 17 at 16:30 UT, between 3.5 and 6.3 R ⊙ above the eastern solar limb, can be associated with the streams sampled by PSP at 0.11 and 0.26 au from the Sun, in two time intervals almost 5 days apart. The two plasma flows come from two distinct source regions, characterized by different magnetic field polarity and intensity at the coronal base. It follows that both the global and local properties of the two streams are different. Specifically, the solar wind emanating from the stronger magnetic field region has a lower bulk flux density, as expected, and is in a state of well-developed Alfvénic turbulence, with low intermittency. This is interpreted in terms of slab turbulence in the context of nearly incompressible magnetohydrodynamics. Conversely, the highly intermittent and poorly developed turbulent behavior of the solar wind from the weaker magnetic field region is presumably due to large magnetic deflections most likely attributed to the presence of switchbacks of interchange reconnection origin.</p