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Quantifying the Hygroscopic Growth of Individual Submicrometer Particles with Atomic Force Microscopy Holly S. Morris, Armando D. Estillore, Olga Laskina, Vicki H Grassian, and Alexei V Tivanski Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b04349 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on March 8, 2016
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Analytical Chemistry
Quantifying the Hygroscopic Growth of Individual Submicrometer Particles with Atomic Force Microscopy Holly S. Morris, Armando D. Estillore, Olga Laskina, Vicki H. Grassian#,*, and Alexei V. Tivanski* Department of Chemistry, University of Iowa, Iowa City, IA 52242, United States ABSTRACT: The water uptake behavior of atmospheric aerosol dictates their climate effects. In many studies, aerosol particles are deposited onto solid substrates to measure water uptake, however, the effects of the substrate are not well understood. Furthermore, in some cases, methods used to analyze and quantify water uptake of substrate deposited particles use a two-dimensional (2D) picture of particle to monitor growth by following changes in the particle diameter with relative humidity (RH). However, this 2D analysis assumes that the droplet grows equally in all directions. If particle growth is not isotropic in height and diameter, this assumption can cause inaccuracies when quantifying hygroscopic growth factors (GFs), where GF for a for a spherical particle is defined as the ratio of the particle diameter at a particular relative humidity divided by the dry particle diameter (typically about 5% RH). However, as shown here, anisotropic growth can occur in some cases. In these cases, a three-dimensional (3D) analysis of the growth is needed. This study introduces a way to quantify the hygroscopic growth of substrate deposited particles composed of model systems relevant to atmospheric aerosols using atomic force microscopy (AFM) which gives information on both the particle height and area and thus a three-dimensional view of each particle. In this study, we compare GFs of submicrometer sized particles composed of single component sodium chloride (NaCl) and malonic acid (MA), as well as binary mixtures of NaCl and MA, and NaCl and nonanoic acid (NA) determined by AFM using area (2D) equivalent diameters, similar to conventional microscopy methods, and compared to GFs determined using volume (3D) equivalent diameter. We also compare these values to GFs determined by a hygroscopic tandem differential mobility analyzer (HTDMA) (substrate free, 3D method). It was found that utilizing volume equivalent diameter for quantifying GFs with AFM agreed well with those determined by substrate-free HTDMA method, regardless of particle composition but area equivalent derived GFs varied for different chemical systems. Furthermore, the NaCl and MA mixture was substrate-deposited both wet and dry, revealing that the hydration state of the particle at the time of impaction influences how the particle grows on the substrate upon water uptake. Most importantly, for the binary mixtures different populations of particles can be distinguished with AFM, an individual particle method, whereas HTDMA sees the ensemble average. Overall, this study establishes the methodology of using AFM to accurately quantify the water uptake of individual submicrometer particles at ambient conditions and over a wide range of RH values. Furthermore, the importance of single particle AFM analysis was demonstrated.
Understanding water uptake of atmospheric particles is important in order to determine the climate effects of aerosols.1 Hygroscopic growth refers to the change in size due to the uptake of water upon changing relative humidity (RH) and is dictated by the chemical composition of the aerosol. Hygroscopicity affects particle morphology,2 cloud condensation nuclei activity,3 and heterogeneous reactions.4 In addition, particle size changes upon absorbing or releasing water and therefore, alters how the particle scatters solar radiation.2 The quantification of change in the size of a particle due to water absorption or release is called a growth factor (GF) and is defined by Equation 1, where D(RH) is the diameter of the particle at a particular RH and D0 is the diameter of the dry particle () =
( )
(1)
A common method for studying hygroscopic growth of particles is by using a hygroscopic-tandem differential mobility analyzer (HTDMA).1,5-7 HTDMA quantifies water uptake by measuring aerosol size distribution at varying RH. While this method has proven to be reliable, it is limited by the fact that, for a heterogeneous population of particles, the measurement yields an ensemble average of thousands of particle measurements, and doesn’t allow careful examination of the extremes of the distribution that are not accurately represented by the average. Thus, it is important to understand particle composition and hygroscopic properties at the single particle level.12-16 Furthermore, HTDMA is generally restricted to relatively small particle size (
