Water-soluble organic compounds (WSOCs) readily uptake water and form atmospheric droplets. Understanding the water-uptake ability of these WSOCs can improve our understanding of their radiative effects and thus can improve current climate models. In this study, we measure the subsaturated and supersaturated droplet growth of four WSOCs: levoglucosan, sucrose, raffinose, and trehalose. Specifically, we use three distinct nanoscale droplet growth methods: cavity ring-down spectroscopy (CRDS), hygroscopic tandem differential mobility analysis (H-TDMA), and cloud condensation nuclei (CCN) counting and report optical growth factor (f RH), growth factor (G f ), refractive indices, and critical activation diameters (d p50 ) for each aerosol system. Koḧler theory (assuming a droplet surface tension equal to that of pure water) is applied to all droplet growth measurements, and the single-parameter hygroscopicity, κ, is reported for the four WSOCs measured on the different platforms. The three experimental methods measure decreased hygroscopicity with an increase in WSOC molecular weight. When assuming the droplets are dilute, κ values from measurements are consistent with known values. The three experimental methods have comparable precision with systematic deviations in the average κ for each method. Few studies have employed f RH data to obtain κ values. The current two methods to estimate κ from f RH data produce similar results, are dry aerosol size-dependent, and agree with κ values that assume dilute solution behavior. Thus, the presented measured data set and analysis contribute to the comparison of subsaturated and supersaturated data from optical and geometric methods for organic aerosol.

Abstract. Organosulfates (OSs) are important constituents of secondary organic aerosols, but their hygroscopic properties and cloud condensation nucleation (CCN) activities have not been well understood. In this work we employed three complementary techniques to characterize interactions of several OSs with water vapor under sub- and supersaturated conditions. A vapor sorption analyzer was used to measure mass changes in OS samples with relative humidity (RH, 0 %–90 %); among the 11 organosulfates examined, only sodium methyl sulfate (methyl-OS), sodium ethyl sulfate (ethyl-OS), sodium octyl sulfate (octyl-OS) and potassium hydroxyacetone sulfate were found to deliquesce as RH increased, and their mass growth factors at 90 % RH were determined to be 3.65 ± 0.06, 3.58 ± 0.02, 1.59 ± 0.01 and 2.20 ± 0.03. Hygroscopic growth of methyl-, ethyl- and octyl-OS aerosols was also studied using a humidity tandem differential mobility analyzer (H-TDMA); continuous hygroscopic growth was observed, and their growth factors at 90 % RH were determined to be 1.83 ± 0.03, 1.79 ± 0.02 and 1.21 ± 0.02. We further investigated CCN activities of methyl-, ethyl- and octyl-OS aerosols, and their single hygroscopicity parameters (κccn) were determined to be 0.459 ± 0.021, 0.397 ± 0.010 and 0.206 ± 0.008. For methyl- and ethyl-OS aerosols, κccn values agree reasonably well with those derived from H-TDMA measurements (κgf) with relative differences being < 25 %, whereas κccn was found to be ∼ 2.4 times larger than κgf for octyl-OS, likely due to both the solubility limit and surface tension reduction.

Volatile organic matter that is suspended in the atmosphere such as α-Pinene and β-caryophyllene undergoes aging processes, as well as chemical and photooxidation reactions to create secondary organic aerosol (SOA),...

Understanding the Martian climate requires a detailed characterization of the optical properties of Martian dust as it is a ubiquitous component of the atmosphere. The continued improvement of Martian atmosphere observations motivates measurements of terrestrial simulants under controlled conditions to support field studies and computational modeling. This investigation demonstrates an in situ method to entrain and directly measure the absorption spectrum of six Martian dust simulants using photoacoustic spectroscopy. Measured size distributions and absorption cross sections (C abs) are used in Mie theory calculations to retrieve the effective imaginary component of the refractive index (k eff) of each sample while constraining the real component to n = 1.5, the refractive index of silica. The weakly absorbing simulants have k eff values ranging between 0.002 and 0.03 at a wavelength of 500 nm that are correlated with the mass fraction of iron oxides.

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