Abstract. The planar differential mobility analyzer (DMA), functioning as a particle sizer, exhibits superior transmission and selection accuracy at ambient pressure relative to its cylindrical counterparts. It also presents integration potential with atmospheric pressure interface mass spectrometry (API-MS) for enhanced cluster detection with an additional ion mobility dimension. In this study, the performance of a commercially available planar DMA (DMA P5) was evaluated. The device is capable of sizing particles below 3.9 nm, with larger sizes measurable through a sheath gas flow restrictor. The resolving power was appraised under various recirculation arrangements, including suction and counterflow modes along with different sheath flow rates, using electrosprayed tetra-alkyl ammonium salts. The peak resolving powers for tetrahexylammonium (THA+) achieved in suction and counterflow modes were 61.6 and 84.6, respectively. The DMA P5 offers a sizing resolution that is 5 to 16 times greater than that of cylindrical DMAs. Resolving power displayed a near-linear relationship with the square root of the applied voltage (VDMA) in counterflow mode. Conversely, the resolving power for THA+ ceased its linear enhancement with VDMA beyond a VDMA of 3554.3 V, entering a plateau which is ascribed to the perturbations in sample flow impacting the laminar nature of sheath flow. The DMA P5 transmission efficiency reaches 54.3 %, markedly surpassing that of conventional DMAs by nearly 1 order of magnitude. Moreover, the mobility spectrum of various electrosprayed tetra-alkyl ammonium salts and the mass-to-charge versus mobility 2D spectrum of sulfuric acid clusters were characterized using the DMA P5 MS system.
Abstract. Phase state and morphology of aerosol particles play a critical role in determining their effect on climate. While aerosol acidity has been identified as a key factor affecting the multiphase chemistry and phase transitions, the impact of acidity on phase transition of multicomponent aerosol particles has not been extensively studied in situ. In this work, we employ an aerosol optical tweezer (AOT) to probe the impact of acidity on the phase transition behavior of levitated aerosol particles. Our results reveal that higher acidity decreases the separation relative humidity (SRH) of aerosol droplets mixed with ammonium sulfate (AS) and secondary organic aerosol (SOA) proxy, such as 3-methylglutaric acid (3-MGA), 1,2,6-hexanetriol (HEXT) and 2,5-hexanediol (HEXD) across aerosol pH in atmospheric condition. Phase separation of organic acids was more sensitive to acidity compared to organic alcohols. We found the mixing relative humidity (MRH) was consistently higher than the SRH in several systems. Phase-separating systems, including 3-MGA/AS, HEXT/AS, and HEXD/AS, exhibited oxygen-to-carbon ratios (O:C) of 0.67, 0.50, and 0.33, respectively. In contrast, liquid-liquid phase separation (LLPS) did not occur in the high O:C system of glycerol/AS, which had an O:C of 1.00. Additionally, the morphology of 38 out of the 40 aerosol particles that underwent LLPS was observed to be a core-shell. Our findings provide a comprehensive understanding of the pH-dependent LLPS in individual suspended aerosol droplets and pave the way for future research on phase separation of atmospheric aerosol particles.
Abstract. Single particle analysis is essential for a better understanding of the particle transformation process and to predict its environmental impact. In this study, we developed an aerosol optical tweezer (AOT) Raman spectroscopy system to investigate the phase state and morphology of suspended aerosol droplets in real time. The system comprises four modules: optical trapping, reaction, illumination and imaging, and detection. The optical trapping module utilizes a 532 nm laser and a 100 × oil immersion objective to stably trap aerosol droplets within 30 s. The reaction module allows us to adjust relative humidity (RH) and introduce reaction gases into the droplet levitation chamber, facilitating experiments to study liquid–liquid phase transitions. The illumination and imaging module employs a high-speed camera to monitor the trapped droplets, while the detector module records Raman scattering light. We trapped sodium chloride (NaCl) and 3-methyl glutaric acid (3-MGA) mixed droplets to examine RH-dependent morphology changes. Liquid–liquid phase separation (LLPS) occurred when RH was decreased. Additionally, we introduced ozone and limonene/pinene to generate secondary organic aerosol (SOA) particles in situ, which collided with the trapped droplet and dissolved in it. To determine the trapped droplet's characteristics, we utilized an open-source program based on Mie theory to retrieve diameter and refractive index from the observed whispering gallery modes (WGMs) in Raman spectra. It is found that mixed droplets formed core–shell morphology when RH was decreased, and the RH dependence of the droplets' phase transitions generated by different SOA precursors varied. Our AOT system serves as an essential experimental platform for in situ assessment of morphology and phase state during dynamic atmospheric processes.
Reproducibility of sootFor high quality data, reproducibility of the flame in all experiments was ensured.The mass and number concentrations were measured four times during the entire measurement to guarantee that the same soot particles were generated.Figure S1 and Figure S2 show the particle mass and number concentration, respectively, associated with four sizes of fresh soot.Figure S1.Particle mass of fresh soot with mobility size of 75, 100, 150, and 200 nm during the measurement.
Diffractive optical element (DOE) array promises compact shape for full-parallax integral imaging three-dimensional (3D) display. However, DOEs suffer from large chromatic aberration due to the strong wavelength-dependent nature of diffraction phenomena that degrade the quality of reconstructed 3D images. An end-to-end DOE optimization approach is proposed to reduce chromatic aberration for integral imaging. The end-to-end optimization framework includes RGB pre-processing convolutional neural networks and achromatic optics optimization design of rotationally symmetric DOE. An optical display model based on diffractive optics is proposed to simulate the integral imaging 3D display process for achromatic optical optimization design. The pre-processed elemental image arrays are modulated by an optimized DOE array to reconstruct the achromatic 3D images. A 3D artifacts scene without chromatic aberration is reconstructed in different views with the proposed method, and both peak signal to noise ratio (PSNR) and structural similarity (SSIM) are improved compared to the conventional Fresnel DOE.
Continuous measurement of 98 volatile organic compounds (VOCs) was conducted during 2017-2019 at a regional background site (Shanxi) located at northeast of Zhejiang Province, YRD region, China. The average concentration of total VOCs (TVOCs) was 25.4±18.4 ppbv, and an increasing trend (+12.2%) was observed. Alkanes were the most abundant VOC group among all seasons, accounting for 43.5% of TVOCs. Oxygenated VOCs (OVOCs), aromatics, halides and alkenes contributed 15.9%, 15.7%, 11.7% and 10.3% of TVOCs concentration, respectively. Biogenic VOCs (BVOCs) and OVOCs showed distinguished diurnal cycle from primary anthropogenic VOCs. Photochemical reactivity analysis based on ozone formation potential (OFP) and OH loss rate (L OH ) indicated that aromatics and alkenes were the most significant contributor, respectively. Toluene, xylene (m/p- and o-), ethene and propene were the largest contributor of annual OFP, with the mean OFP being 33.8±44.3 μg/m 3 , 31.9±32.1 μg/m 3 , 9.29±11.4 μg/m 3 , 22.1±21.3 μg/m 3 and 12.8±19.5μg/m 3 , respectively. Seven sources were identified with positive matrix factorization (PMF): petrochemical industry (13.8%), biogenic emission (1.0%), solvent usage-toluene (16.9%), vehicular exhaust (43.8%), Integrated circuits industry (3.8%), solvent usage-C8 aromatics (10.9%), and gasoline evaporation (9.8%). Vehicular exhaust was the most significant source (43.8%) during the whole measurement period. Solvent usage, petrochemical industry, and gasoline evaporation showed high temperature dependency. The integrated contribution of solvent usage and industrial processes were higher than vehicular exhaust during hot months. These sources also have higher chemical reactivities and can contribute more on O 3 formation. Our results are helpful on determining the control strategies aiming at alleviating O 3 pollution.
Abstract. The morphological transformation of soot particles via condensation of low-volatility materials constitutes a dominant atmospheric process with serious implications for the optical and hygroscopic properties, as well as atmospheric lifetime of the soot. We consider the morphological transformation of soot aggregates under the influence of condensation of vapors of sulfuric acid, and/or limonene ozonolysis products. This influence was systematically investigated using a Differential Mobility Analyzer coupled with an Aerosol Particle Mass Analyzer (DMA–APM) and the Tandem DMA techniques integrated with a laminar flow-tube system. We hypothesize that the morphology transformation of soot results (in general) from a two-step process, i.e., (i) filling of void space within the aggregate and (ii) growth of the particle diameter. Initially, the transformation was dominated by the filling process followed by growth, which led to the accumulation of sufficient material that exerted surface forces, which eventually facilitated further filling. The filling of void space was constrained by the initial morphology of the fresh soot as well as the nature and the amount of condensed material. This process continued in several sequential steps until all void space within the soot aggregate was filled. And then “growth” of a spherical particle continued as long as vapors condensed on it. We developed a framework for quantifying the microphysical transformation of soot upon the condensation of various materials. This framework used experimental data and the hypothesis of “ideal sphere growth” and void filling to quantify the distribution of condensed materials in the complementary filling and growth processes. Using this framework, we quantified the percentage of material consumed by these processes at each step of the transformation. For the largest coating experiments, 6, 10, 24, and 58 % of condensed material went to filling process, while 94, 90, 76, and 42 % of condensed material went to growth process for 75, 100, 150, and 200 nm soot particles, respectively. We also used the framework to estimate the fraction of internal voids and open voids. This information was then used to estimate the volume-equivalent diameter of the soot aggregate containing internal voids and to calculate the dynamic shape factor, accounting for internal voids. The dynamic shape factor estimated based on the traditional assumption (of no internal voids) differed significantly from the value obtained in this study. Internal voids are accounted for in the experimentally derived dynamic shape factor determined in the present study. In fact, the dynamic shape factor adjusted for internal voids was close to 1 for the fresh soot particles considered in this study, indicating the particles were largely spherical. The effective density was strongly correlated with the morphological transformation responses to the condensed material on the soot particle, and the resultant effective density was determined by the (i) nature of the condensed material and (ii) morphology and size of the fresh soot. In this work we quantitatively tracked in situ microphysical changes in soot morphology, providing details of both fresh and coated soot particles at each step of the transformation. This framework can be applied to model development with significant implications for quantifying the morphological transformation (from the viewpoint of hygroscopic and optical properties) of soot in the atmosphere.
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