Abstract
Abstract. Understanding the impact of sea spray aerosol (SSA) on the climate and atmosphere requires quantitative knowledge of their chemical composition and mixing states. Furthermore, single-particle measurements are needed to accurately represent large particle-to-particle variability. To quantify the mixing state, the organic volume fraction (OVF), defined as the relative organic volume with respect to the total particle volume, is measured after generating and collecting aerosol particles, often using deposition impactors. In this process, the aerosol streams are either dried or kept wet prior to impacting on solid substrates. However, the atmospheric community has yet to establish how dry versus wet aerosol deposition influences the impacted particle morphologies and mixing states. Here, we apply complementary offline single-particle atomic force microscopy (AFM) and bulk ensemble high-performance liquid chromatography (HPLC) techniques to assess the effects of dry and wet deposition modes on the substrate-deposited aerosol particles' mixing states. Glucose and NaCl binary mixtures that form core–shell particle morphologies were studied as model systems, and the mixing states were quantified by measuring the OVF of individual particles using AFM and compared to the ensemble measured by HPLC. Dry-deposited single-particle OVF data positively deviated from the bulk HPLC data by up to 60 %, which was attributed to significant spreading of the NaCl core upon impaction with the solid substrate. This led to underestimation of the core volume. This problem was circumvented by (a) performing wet deposition and thus bypassing the effects of the solid core spreading upon impaction and (b) performing a hydration–dehydration cycle on dry-deposited particles to restructure the deformed NaCl core. Both approaches produced single-particle OVF values that converge well with the bulk and expected OVF values, validating the methodology. These findings illustrate the importance of awareness in how conventional particle deposition methods may significantly alter the impacted particle morphologies and their mixing states.
Highlights
The chemical composition of the ocean and sea surface microlayer (SSML) directly affects the mixing states of sea spray aerosol (SSA), which is generated by film and jet drops (Prather et al, 2013; Jacobson, 2001; Vignati et al, 2010; de Leeuw et al, 2011; Wang et al, 2017)
Phase bleeding describes an instance in which the viscosity of the two phaseseparated materials is too similar at a given relative humidity (RH), and the phase contrast between organic and inorganic components is relatively weak in the atomic force microscopy (AFM) images
Since phase imaging inherently relies on measuring differences in tip–sample interactions originating from different viscoelastic properties, lowering the RH will further converge the two different viscosities of organic and inorganic components together, lessening the accuracy of the core and shell phase boundary determination
Summary
The chemical composition of the ocean and sea surface microlayer (SSML) directly affects the mixing states of sea spray aerosol (SSA), which is generated by film and jet drops (Prather et al, 2013; Jacobson, 2001; Vignati et al, 2010; de Leeuw et al, 2011; Wang et al, 2017). AFM measurements can be performed under controlled relative humidity (RH), while producing high-resolution 3-D height and phase images, and directly measuring viscoelastic properties of materials (Lee et al, 2017a, b) In this regard, simultaneous acquisition of the 3-D height and phase images over individual particles can be used to quantify their mixing states or organic volume fraction (OVF). Unlike liquid–liquid phase separation, to the best of our knowledge, parameterization to predict a solid–semisolid or solid–liquid phase separation does not yet exist (Bertram et al, 2011; You et al, 2013, 2014; Krieger et al, 2012; Song et al, 2012) Both chemical systems are highly relevant to SSA, with glucose contributing up to 5.2 % and 14.4 % of the total organic mass of PM2.5 and PM10–2.5 SSA, respectively (Jayarathne et al, 2016). An experimental approach to restructure the dry-deposited particles by performing a hydration and dehydration cycle is introduced and validated
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