Size-Resolved Surface-Active Substances of Atmospheric Aerosol: Reconsideration of the Impact on Cloud Droplet Formation

Abstract

Our current understanding of the importance of surface-active substances (SAS) on atmospheric aerosol cloud-forming efficiency is limited, as explicit data on the content of size-resolved ambient aerosol SAS, which are responsible for lowering the surface tension (σ) of activating droplets, are not available. We report on the first data comprising seasonal variability of size-segregated SAS concentrations in ambient aerosol particulate matter (PM). To assess the impact of SAS distribution within PM on cloud droplet activation and growth, a concept of surfactant activity was adopted and a parametrization developed; i.e., surfactant activity factor (SAF) was defined, which allowed translation of experimental data for use in cloud parcel modeling. The results show that SAS-induced σ depression during cloud activation may affect droplet number ( N) as much as a 2-fold increase in particle number, whereas by considering also the size distribution of particulate SAS, N may increase for another 10%. This study underscores the importance of size-resolved SAS perspective on cloud activation, as data typically obtained from aqueous extracts of PM and PM may result in misleading conclusions about droplet growth due to large mass fractions of supermicron particles with SAS deficit and little or no influence on CCN and N.

The surface tension of surfactant-containing, finite volume droplets

“…Gérard et al (7,29) recently reported total surfactant concentrations in collected ambient fine aerosol reaching several tens to hundreds millimolar across Europe. Moreover, surfactant concentrations are enhanced in smaller (<200-nm diameter) atmospheric particles (46). Such atmospheric observations combined with our results suggest surfactants in some cases will substantially impact the activation of atmospheric particles into cloud droplets.…”

“…The surface monolayer model predictions suggest surfactant concentrations on the order of several tens to 100 mM, consistent with reported atmospheric concentrations, are required to substantially affect N d (i.e., modify SS c by >20%). These predictions highlight the necessity of a more comprehensive understanding of the types and concentrations of surfactants in aerosol, as to date only a few studies in a few environments have been reported (7,8,10,29,46,47). Global simulations of N d have only rarely incorporated such surface partitioning models (5,6), in part because partitioning effects had not been experimentally confirmed until this work.…”

“…We also demonstrate experimentally that surfactant partitioning in finitesized droplets is substantially affected by the presence of a cosolute (glutaric acid, NaCl). Considering atmospheric aerosols are complex chemical mixtures containing inorganic salts and organic molecules, of which surfactants are an important component (7,46), the surfactant-solute interactions and resulting size-dependent surface-bulk partitioning effects must be further explored.…”

Electrochemical Evidence of non-Volatile Reduced Sulfur Species in Water-Soluble Fraction of Fine Marine Aerosols

“…In the previous paper of Frka et al [36], a broad voltammetric peak that was recorded around a potential of −0.70 V in continental, coastal, and urban aerosol samples as well as in the isolated atmospheric HULIS, could be possibly associated with the presence of RSS. These RSS contribute to the surfactant activity, since HULIS material, i.e., HULIS concentration can be regarded as a rough upper-limit estimate of aerosol water-soluble SAS [53]. Moreover, RSS are an important part of the total S pool in aerosol samples.…”

Supplementary material to &quot;Marine organic matter in the remote environment of the Cape Verde Islands – An introduction and overview to the MarParCloud campaign&quot;

“…Phase sensitive alternating current (AC) voltammetry was applied for SAS analysis, being already used as a successful tool for the determination of SAS in a plethora of environmental aquatic samples (Frka, et al 2009;Frka, et al 2012;Kroflič, et al 2018). Measurements (outof-phase mode, frequency 77 Hz, and amplitude 10 mV) were performed with μAutolab-type II (Eco Chemie B. V., The Netherlands), GPES 4.6 software (Eco Chemie B. V., The Netherlands), followed the method of Ćosović and Vojvodić (1998) Gašparović, et al 2017).…”