The effective density is an essential physical property of atmospheric aerosols. The study of the effective density helps characterize the aerodynamic behavior of aerosol particles, providing vital information on the chemical composition, mixing state, and aging process of the particles. Size-resolved effective density measurements based on the Aerodynamic Aerosol Classifier–Scanning Mobility Particle Sizer (AAC–SMPS) tandem system were conducted in Beijing from March 10 to 31, 2023. This study analyzed the relationship between the aerodynamic diameter and effective density and the characteristics of the effective density under different pollution conditions. During the entire observation period, the average effective densities for the 150-, 250-, 350-, 450-, and 550-nm particles were 1.48 ± 0.07, 1.59 ± 0.08, 1.69 ± 0.09, 1.73 ± 0.09, and 1.75 ± 0.13 g/cm3, respectively, i.e., the effective density was highly particle size-dependent, exhibiting higher values at larger aerodynamic diameters. The size-resolved effective density under different pollution conditions differed and was related to the air mass variations. Under dusty conditions, the effective density of particles was highest under the influence of dust air masses from the northern areas. The effective density of small particles (150–350 nm) of the northern clean air masses was lower than that of the southern polluted air masses; however, the opposite was observed for large particles (450–550 nm). The diurnal effective density variation was generally more pronounced during daytime. Under both clean and dusty conditions, there was a sudden effective density decrease at 150 nm during the evening peak traffic. Under the influence of new particle formation (NPF) events, a significant effective density decrease was observed when the accumulation mode began to grow. 

Size-resolved effective density of ambient aerosols measured by an AAC–SMPS tandem system in Beijing

Abstract Multiple charge correction is essential in charger-classifier systems, where aerosols acquire a distribution of charges. A key example is the unipolar charger–particle mass analyzer (PMA) system. Recently, the Interpolated Average Charge (INTAC) algorithm was introduced as a simplified method for multiple charge correction in such systems. This work experimentally validates the INTAC algorithm across various aerosol types. First, we compare experimentally measured average particle mass with INTAC predictions for spherical particles of known density, finding that the algorithm performs within an average absolute error of 8.09% ± 5.45% for particles smaller than 100 fg. Further validation is conducted on aerosols with diverse morphological properties, revealing that large errors can arise due to uncertainties in both the mass-mobility relationship and the charge acquired by aggregate particles. However, direct fitting to experimental charge and effective density data reduces the absolute average error to 5.64% ± 5.72%. Finally, we discuss the use of a unipolar charger–PMA system for selecting particles with a narrow mass distribution. Results indicate that such systems transmit a broad range of masses at smaller particle sizes, which should be considered when designing experiments. GRAPHICAL ABSTRACT

Multiple charge correction in particle mass analyzers: Experimental validation of the interpolated average charge (INTAC) algorithm

Abstract Accurate knowledge of particle density is essential for many aspects of aerosol science. Yet, density is often characterized poorly and incompletely for internally mixed particles, particularly for dry particles, with previous studies focused primarily on deliquescent (aqueous) droplets. Instead, densities for dry internally mixed particles are often inferred from mass composition measurements in combination with predictive models assuming ideal mixing, with the accuracy of such models not estimated. We determined particle densities from mobility diameter measurements (using a Scanning Mobility Particle Sizer, SMPS) for dried particles classified by their aerodynamic size (using an Aerosol Aerodynamic Classifier, AAC) for a range of two-component organic-inorganic particles containing known proportions of organic and inorganic species. We examined all permutations of mixing between four different organic (water soluble nigrosin dye, citric acid, polyethylene glycol-400, and ascorbic acid) and three different inorganic (sodium chloride, ammonium sulfate, and sodium nitrate) species. The accuracy and precision in our measured particle densities were ∼5% and ∼1%, respectively, for nonvolatile particles. Substantial deviations in particle density from ideal mixing (up to 20%) were observed. We tested descriptions of the non-ideal mixing for our systems by representing the volume change of mixing using Redlich-Kister (RK) polynomials in terms of mass fraction or in terms of mole fraction, with both approaches performing similarly. Graphical Abstract

https://www.tandfonline.com/doi/pdf/10.1080/02786826.2022.2062293?needAccess=true

Densities of internally mixed organic-inorganic particles from mobility diameter measurements of aerodynamically classified aerosols

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Densities of internally mixed organic-inorganic particles from mobility diameter measurements of aerodynamically classified aerosols