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Environmental Processes

Size-resolved surface active substances of atmospheric aerosol: reconsideration of the impact on cloud droplet formation Ana Krofli#, Sanja Frka, Martin Simmel, Heike Wex, and Irena Grgi# Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02381 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Environmental Science & Technology

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Size-resolved surface active substances of atmospheric aerosol: reconsideration of the

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impact on cloud droplet formation

3 Ana Kroflič†┴║, Sanja Frka†‡║*, Martin Simmel§, Heike Wex± and Irena Grgić†

4 5 †

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Department of Analytical Chemistry, National Institute of Chemistry, Ljubljana, 1000,

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Slovenia; ┴

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Atmospheric Chemistry, Leibniz Institute for Tropospheric Research, Leipzig, 04318,

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Germany; ‡

Division for Marine and Environmental Research, Ruđer Bošković Institute, Zagreb, 10000,

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Croatia; §

Modelling of Atmospheric Processes, Leibniz Institute for Tropospheric Research, Leipzig,

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04318, Germany; ±

Experimental Aerosol and Cloud Microphysics, Leibniz Institute for Tropospheric Research,

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Leipzig, 04318, Germany

16 17

║

Co-first authors contributed equally to this work

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*Corresponding author

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Tel: +385 1 4561 185

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Fax: +385 1 4680 242

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Email address: [email protected]

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Abstract

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Our current understanding of the importance of surface active substances (SAS) on

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atmospheric aerosol cloud-forming efficiency is limited, as explicit data on the content of

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size-resolved ambient aerosol SAS, which are responsible for lowering the surface tension (σ)

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of activating droplets, are not available. We report on the first data comprising seasonal

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variability of size-segregated SAS concentrations in ambient aerosol particulate matter (PM).

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To assess the impact of SAS distribution within PM on cloud droplet activation and growth, a

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concept of surfactant activity was adopted and a parametrization developed, i.e. surfactant

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activity factor (SAF) was defined, which allowed translation of experimental data for use in

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cloud parcel modeling. Results show that SAS-induced σ depression during cloud activation

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may affect droplet number (Nd) as much as a two-fold increase in particle number, whereas by

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considering also the size distribution of particulate SAS, Nd may increase for another 10%.

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This study underscores the importance of size-resolved SAS perspective on cloud activation,

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as data typically obtained from aqueous extracts of PM2.5 and PM10 may result in misleading

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conclusions about droplet growth due to large mass fractions of supermicron particles with

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SAS deficit and little or no influence on CCN and Nd.

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Introduction

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One of the major uncertainties in the current understanding of the Earth’s climate is the effect

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that aerosol particles have on cloud formation processes and the resulting impact on the

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radiative balance.1,2 The ability of an aerosol particle to act as a cloud condensation nucleus

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(CCN) and to become a cloud droplet (i.e., its cloud-forming efficiency) is described by the

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Köhler theory.3 The theory accounts for a competition between the Kelvin term, which

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considers also the surface tension of particulate aqueous phase (σ), and the Raoult term,

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which links the surrounding water vapor pressure to the solute content. The interplay between 2 ACS Paragon Plus Environment

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both effects results in a size- and composition-dependent growth of hydrated particles/droplets

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in humid air and only a fraction of hygroscopic particles competing for the available water

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vapor in the surrounding can grow beyond their critical sizes for spontaneous growth at a

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given supersaturation. As larger particles nucleate easier than small ones, the effect of

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chemical composition on the cloud-forming efficiency is most significant in the submicron

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size-range particles. Coupling with inorganic components,4–6 both model and natural organic

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surface active substances (SAS) or surfactants have been shown to lower σ of atmospheric

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cloud, rain, and fog waters and aqueous extracts of atmospheric aerosol from various

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sources.7–13 By influencing the Kelvin effect, σ depression caused by SAS could promote

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cloud droplet formation and importantly affect cloud properties such as albedo.14 On the other

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hand, it is believed that displacement of SAS from the droplet bulk to the particle/droplet

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surface simultaneously attributes to a change in the Raoult term (opposing effect as from the

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σ depression) which also has to be accounted for.15,16 In a recent publication, however, a

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closure between chemical composition, growth factor measurements and CCN activity could

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not be achieved under particular conditions which was explained by a lack in accounting for

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the σ reduction of aerosol aqueous phase.17 Although diverse modeling efforts have been

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made to assess the influence of the observed σ depression on cloud droplet nucleation and

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growth, the magnitude and even the sign of the cumulative effect of SAS on cloud droplet

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number (Nd) are still subjects of controversy.16,18–23 The direct experimental evidence for σ

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depression in microscopic droplets remains elusive. Nozière et al. (2014) examined the

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dynamic development of surface tension in droplets containing SAS, and demonstrated that

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the effect of SAS on σ of growing droplets cannot be directly measured due to limitations of

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state-of-the-art instrumentation for studying cloud droplet formation which is unable to detect

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kinetically governed σ effects of aerosol SAS on CCN activity.24 By applying an alternative

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approach, Ruehl et al. (2016) measured the size of haze droplets up to the point of cloud 3 ACS Paragon Plus Environment


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droplet activation and beyond, for particles containing SAS.25 The observed droplet sizes at

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activation being larger than expected by the Köhler theory are explained by organics

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accumulated on the surface of haze droplets, forming a compressed film which reduces σ and

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allows for larger droplets to form prior to their activation. Gérard et al. (2016) further

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measured σ of isolated SAS from atmospheric aerosol samples at a range of dilutions,

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discussing that atmospheric haze droplets prior to activation still have the σ clearly lower than

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that of pure water.26 Nevertheless, a widely used parameterization in atmospheric models

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known as the κ-Köhler theory27 describing CCN activation ignores the influence of SAS by

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assuming that the σ of growing droplets is that of pure water, such that the experimentally

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derived hygroscopicity parameter κ correctly describes particle activation when used together

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with this assumption. Therefore, a more rigorous exploration of the content of ambient SAS

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and the conditions under which they exert an effect on droplet activation are urgently needed

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to elucidate their true impact on cloud-formation processes and global indirect aerosol

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forcing. Here, the particular examination of SAS distribution in particles of submicron size

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ranges, considered as critical for CCN activation, is explicitly addressed,17,26 because SAS

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might occur enriched at these sizes, compared to the bulk samples.

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Until now no explicit data on the size-resolved SAS in ambient particulate matter (PM) have

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been published. Former efforts were limited to measuring SAS in various bulk aerosol water

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extracts,28–36 including those that applied pre-concentrating solid-phase extraction (SPE)

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protocols26,37,38 used at the same time for the isolation and concentration of aerosol HUmic-

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LIke Substances (HULIS)e.g.,13. This is mainly due to the sensitivity limitations of classical

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methods used for low-level SAS quantification in atmospheric samples. Notwithstanding, the

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herein applied electrochemical method (phase sensitive alternating current (ac) voltammetry;

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out of phase mode) has been proven to be sensitive and directly applicable for the qualitative

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and quantitative characterization of SAS in various natural aqueous systems,e.g.,39–42 including 4 ACS Paragon Plus Environment

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bulk precipitation43,44 and aqueous aerosol extracts,34 serving as a platform for low-level SAS

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determination in aqueous extracts of size-segregated aerosols.

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We report the first data on size-segregated SAS concentrations in ambient aerosol particles

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with aerodynamic diameters between 0.04 and 15.6 µm, divided into nine size bins. For cloud

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modeling purposes, novel parametrization was developed which allowed us to account for σ

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effects on droplet formation in a size-resolved manner. For this purpose, size-resolved

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surfactant activity factors (SAF) were derived from the electrochemically determined size-

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segregated SAS, which were originally considered dissolved within the adsorbed water layer

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on the surface of the corresponding size-segregated particle population (i.e., within the water

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shell of hydrated particles/droplets in the particular size bin). SAS distribution within PM was

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obtained by combining size-resolved SAF with the measured PM mass size distribution. Size-

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segregated proxy data for σ were estimated according to Gérard et al.,26 the only report on the

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σ curves for the natural SAS of ambient PM2.5 samples. Finally, a cloud parcel model was

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applied to inspect size-resolved σ effects of ambient SAS on the kinetics of droplet growth

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and activation.

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EXPERIMENTAL SECTION

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Aerosol sampling

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Seasonal size-segregated and PM10 aerosol samples were collected in parallel at an urban

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background site of Ljubljana, Slovenia (approx. 279,000 inhabitants, 298 m a.s.l.). Size-

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segregated aerosols were collected on pre-baked aluminum foils (500 °C for 24 h) with a 10-

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stage low-pressure Berner cascade impactor (HAUKE, LPI 25/0,015/2) operated at ambient

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temperature at a flow rate of 25.8 L min−1. 2–3 consecutive 48–72 h samples were combined

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into each investigated sample set to ensure at least 90 µg of PM deposit for each size fraction 5 ACS Paragon Plus Environment


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as described elsewhere.45 In addition to ensure the sufficient amount of material for chemical

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analyses, stages 1 and 2 of each sample set were processed together (called stage 2a herein).

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In total, we examined four impactor sample sets (one for each season: winter (14/02–

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20/02/2015), spring (15/05–25/05/2015), summer (10/08–21/08/2015) and autumn (05/11–

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11/11/2015), each comprising nine size-segregated stages, representing the average of 6 to 12

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days. In parallel, PM10 samples were collected onto prebaked glass fiber filters (0.47 mm;

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Whatman) for 24 h at 2.3 m3 h−1 using a Leckel sampler. In each season (winter 02/2015;

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spring 05/2015; summer 08/2015; autumn 11/2015), 6 daily PM10 samples were processed

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together. The collected samples and the foil/filter blanks (no air drawn through the

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impactor/sampler) were stored at −18 °C until analysis.

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PM size distribution

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The impactor comprises 10 stages of nominal aerosol size ranges expressed in aerodynamic

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equivalent diameter, d; however, as already explained, stages 1 and 2 were combined (stage

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2a: 0.038–0.104 µm; stage 3: 0.104–0.16 µm, stage 4: 0.16–0.305 µm, stage 5: 0.305–0.56

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µm, stage 6: 0.56–1.01 µm, stage 7: 1.01–2.1 µm, stage 8: 2.1–3.99 µm; stage 9: 3.99–8.06

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µm, stage 10: 8.06–15.6 µm). Aerosol mass of each stage of size-segregated aerosol was

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determined by weighing the sample before and after the sampling; prior to weighing, all

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samples were conditioned for at least 24 h at a relative humidity of 50±5% and a temperature

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of 20±1 °C. For numerical modeling purposes (see below), discrete size-bin PM mass

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distributions (Figure 1a) were inverted to continuous size distributions and the model was

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initialized with the continuous PM size distributions of the respective seasons shown in

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Figure 1b. For details about the applied numerical treatment see Supporting Information (SI),

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Appendix 1.

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Aerosol pretreatment

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SAS of size-segregated aerosol and PM10 samples were concentrated by adopting a standard

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solid phase extraction (SPE) protocol for the isolation of aerosol HULIS.26,46,47 Aqueous PM

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extracts were first obtained by extraction of joined samples in 15–24 mL of high purity

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reagent Milli-Q water (Millipore, Bedford, MA, USA) for 24 h at 4 ºC, followed by filtration

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through a 0.22 µm pore size filter (Supelco, USA). Aqueous extracts were further treated

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according to the SPE procedure steps on C18 columns (SEP-PAK VAC, 3 mL, 500 mg,

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Waters; see SI, Appendix 3) to obtain MeOH isolates. Aqueous aerosol extracts and HULIS

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extracts (obtained after evaporation of MeOH extracts to dryness and redissolution in Milli-Q

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water) were further analyzed for their water-soluble organic carbon (WSOC and HULIS-C,

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respectively) and SAS content. Blank filters were also subjected to the same processing steps

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to ensure appropriate extraction procedure and control the purity of final extracts.

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SAS extraction efficiency. SAS extraction yield was determined as a ratio between SAS

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concentrations (in equiv. of model surfactant T-X-100) in aerosol aqueous extracts after and

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before the C18 SPE procedure. The applied procedure enabled high SAS yields (nearly

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100%), indicating their complete isolation from the aerosol water-soluble organic matter (SI,

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Appendix 3).

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Water-soluble organic carbon analysis

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WSOC and HULIS-C were analyzed by sensitive Combustion TOC Analyzer (Teledyne,

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Apollo 9000 HS) with a non-dispersive infrared gas detector (NDIR). In order to eliminate the

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inorganic carbonates, each sample was acidified with 2 M HCl to pH 2–3 before analysis.

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WSOC or HULIS-C of each sample was calculated as an average of three to five replicate

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measurements.



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SAS analysis

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Electrochemical measurements. The phase sensitive ac voltammetry was applied for SAS

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analysis, being already used as a successful tool for the determination of SAS in a plethora of

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environmental aquatic samples.39 Measurements (out-of-phase mode, frequency 77 Hz, and

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amplitude 10 mV) were performed with µAutolab-type II (Eco Chemie B. V., The

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Netherlands), GPES 4.6 software (Eco Chemie B. V., The Netherlands). A standard

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polarographic Metrohm cell of 50 cm3 with a three-electrode system was used: working

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electrode – hanging mercury drop electrode (HMDE; Metrohm, Switzerland; A = 0.01245

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cm2), reference electrode – Ag/AgCl/3 mol L−1 KCl, auxiliary electrode – platinum coil.

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Before each measurement, previously purified (450 °C for 5 h; charcoal organic residue

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removal) saturated NaCl (Kemika, Croatia) solution was added to the sample to adjust the

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electrolyte concentration to 0.55 mol L−1. In brief, because of their amphiphilic nature, SAS

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have the capability of spontaneous adsorption at different phase boundaries. Due to the

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hydrophobic character of the mercury electrode, its interface with aqueous electrolyte solution

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serves as a good model for the study of SAS.48 SAS adsorption at the surface of the mercury

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electrode changes its double layer capacitance, which is directly proportional to the measured

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capacity current (IC) at the applied potential. At the optimal potential for the adsorption of

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amphiphilic SAS (-0.6 V versus Ag/AgCl), IC decrease (compared to the capacity current of

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the supporting electrolyte alone (IC0)) is linearly dependent on the adsorbate concentration in

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aqueous solution, allowing for the precise quantification of low SAS concentrations.

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Method sensitivity. Interfacial tensiometers and drop shape analyzers are commonly in use for

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measurements of σ decrease due to the presence of SAS in aqueous solutions. However, due

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to the insufficient sensitivity, their usefulness for the investigation of ambient PM samples

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with low-level SAS is limited.e.g.,9,12,13 Taking tetraoctylphenolethoxylate (T-X-100) as typical

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model SAS, we compared sensitivities of ac voltammetry and tensiometry obtained with a 8 ACS Paragon Plus Environment

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digital tensiometer (Kruss, K10T, GmbH, Hamburg). Ac voltammetry is shown to be two

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orders of magnitude more sensitive than the conventional tensiometry (Figure S1b, for details

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refer to SI, Appendix 2).

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Calibration. Many scientists strive for the quantification of ambient aerosol surfactants,

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although without knowing their chemical nature or strength (i.e., how surface active they

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are).49 Traditional model surfactants are often used as surrogate substances for these versatile

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species;8,50,51 however, particular model surfactants have already been shown to fail to

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reproduce the overall surface-active properties of aerosol water-soluble fraction.e.g.,52 Some

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authors have already reported on the potential of more realistic representation of aerosol SAS

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deduced from real atmospheric samples.13,53 As we show that aerosol SAS fraction is

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completely isolated within the HULIS material with joint water soluble and hydrophobic

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properties, HULIS concentration can be regarded as a rough upper-limit estimate of aerosol

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SAS. Thus, instead of using model surfactants as commonly applied today, calibration with

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HULIS material isolated from the ambient PM10 samples collected in parallel to Berner

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impactor samples was performed for accurate quantification of SAS in size-segregated

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aerosols (details within SI, Appendices 3 and 4). Calibration plots ∆IC vs. concentration of

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HULIS-C were constructed and are shown in Figure S2. Limits of SAS detection and

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quantification are 0.1 and 0.16 mgC L−1 (equiv. of HULIS-C), respectively. Relative standard

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deviation obtained from three to five repetitive analyses of the same solution containing 1

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mgC L−1 is less than 5%.

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Numerical modeling

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The spectral microphysics model SPECS describes standard Köhler theory-based aerosol

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particle growth as well as droplet nucleation and growth with respect to water content using a

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2-dimensional grid.54 Thus, both, water and particulate mass were divided into discrete bins



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(360 for particulate mass with mass doubling every 8 bins, 264 for water mass with mass

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doubling every 4 bins). Dry aerosol was assumed to be an internal mixture where 50% of the

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particulate mass consisted of ammonium sulfate and 50% were insoluble substances, the latter

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not contributing to the Raoult term at all. The number of moles of solute was kept constant

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during the modeling. Surface tension (σ) was iteratively calculated for every time step from

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the respective dry PM size distribution and the condensed water load (giving us time- and

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size-resolved PM/water concentrations, c, see details below). Note that SAS bulk-surface

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partitioning was not accounted for, which could lead to an overestimation of the effect of σ on

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the modeled droplet growth.15,16 Nevertheless, the main focus of this work is to demonstrate

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the influence of size-resolved SAS on droplet growth, as compared to assuming a size

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independent σ. We do not want to elaborate on the absolute effect of σ, as that is still

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controversial. Recent literature has reported contradictory results, ranging from a possibly

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large effect of σ on atmospheric CCN concentrations (increase by a factor of 10)17 to cases for

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which a range of results, including a negligible impact on CCN activation55, has been

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reported.

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The model was initialized with the continuous dry PM size distributions of the respective

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seasons shown in Figure 1b. For the modeling purposes, d was converted into geometric

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diameters, geo, via the formula: geo = water/AP with particle density AP = 1.5 g cm3

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and water density water = 1.0 g cm3. Total aerosol number concentrations were 3.9×109,

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5.4×109, 1.4×1010 and 1.5×1010 m-3 for spring, summer, autumn and winter, respectively.

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Initially, equilibrium was assumed at a relative humidity (RH) of 99%, together with

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accounting for a possible σ reduction due to the presence of SAS in the distinct aerosol size

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fractions. The air parcel was lifted to a height of 250 m, starting at the 99% RH level, with a

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constant updraft velocity of 0.5 m s−1 as a standard case. The only microphysical process

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considered was water vapor condensation, i.e. droplet nucleation and further droplet growth 10 ACS Paragon Plus Environment

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by condensation. For the latter, a water accommodation coefficient of 0.5 was taken. The

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growth calculation was interactively coupled to the vapor phase, meaning that e.g.,

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supersaturation was explicitly calculated at each time step. Again, we stress that at each time

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step, σ was also calculated from the condensed water load (see size-dependent σ evolution

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during the modeling in SI, Appendix 11, Figure S11). Due to the 2D model configuration, the

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underlying dry aerosol particle distribution remained perfectly conserved throughout the

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model run while the spectrum referring to water mass evolved.

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As the determination of the influence of SAS on the σ of aerosol water layer is beyond the

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scope of this study, experimentally-obtained σ (mN m−1) dependence on PM/water

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concentration (c in µg µL−1 aqueous solution) was taken from Gerard et al.26 and a model

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function was fitted to the data (details in SI, Appendix 5, Eq. S2) to obtain the equation used

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in the aerosol model (Case 1):

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=

.

.

+ 36.638

(1)

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Note, cumulative SAS within PM2.5 was considered in the cited work; therefore, as we wanted

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to also explicitly account for SAS distribution within PM, further data treatment was needed.

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To account for a particular case of size-resolved SAS described below (Case 2), adapted PM

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size distributions with considering experimentally-derived surfactant activity factors (SAF)

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were initialized as well (Figure 1d; obtained according to SI Eq. S3, see Appendix 6) and used

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only for the modeling of σ in Case 2, i.e. for the calculation of c (the modeled PM size

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distribution remained unchanged).

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SAF$ =

%&%' %&% ().* +,' +, ().*

(2)

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where SASi and PMi are the experimentally determined SAS and PM concentrations of the

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corresponding size bin i, respectively. Namely, SAF is a weight that corrects σ dependence on

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c derived from Gerard et al.26 for the size-segregated SAS aspect brought-up in this work, 11 ACS Paragon Plus Environment


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describing the enrichment of SAS in the particular size bin: SAF values >1 represent the

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fractions enriched with SAS, while values 1 at stages 3 and 4 for all seasons (Figure

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1c), which resulted in more droplets formed in the case of SEG-SAS in comparison to EVEN-

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SAS (except in winter). With this we show that averaged SAS data over a broader PM size

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range (PM2.5 or PM10) are not necessarily representative for the cut-off size range and can

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result in misleading conclusions about the droplet growth. As an extreme case, consideration

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of the water-soluble size-segregated SAS distribution expressed on the surface area of particle

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population (FILM-SAS case; details in SI, Appendix 7) resulted in a relatively low Nd

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compared to EVEN-SAS and SEG-SAS (SI, Figure S6). Furthermore, a SAS-induced σ

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decrease during droplet growth affected Nd about as much or even more than the factor-of-two

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increase in CCN in WAT-2 (Figure 3a). Nenes et al.61 found numerous conditions where the

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effect of chemical composition on Nd was comparable to the effect of the CCN number. Under

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marine conditions, the influence of σ accounted for up to 50% of the Twomey effect, whereas

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under urban conditions it exceeded it. Moreover, for urban aerosols, inclusion of the σ effect

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exerted by water-soluble organic compounds may increase the CCN number concentration up

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to 110% relative to the case in which only the inorganic aerosol composition is considered.60

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Atmospheric relevance

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To date, the influence of SAS on cloud formation has mainly been exploratory and partially

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speculative, as explicit data on the abundance and size distribution of aerosol SAS,

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responsible for lowering σ of activating droplets, have been unknown. Electrochemical

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determination of low concentrations of SAS, together with the developed parametrization

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enabled us to model droplet activation and growth with considering the size-resolved σ effects

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on cloud formation. According to our results, SAS-induced σ depression during 18 ACS Paragon Plus Environment

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CCN activation (Case 1, without considering the influence of SAS partitioning between the

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droplet surface and the bulk which likely overestimates the σ effect) may influence Nd to a

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similar extent or even more than increasing particle number by a factor of two. The

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investigated water-soluble SAS unlikely exert the observed surface-film effect on droplet

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activation, as there is not enough SAS for lowering the σ of submicron particles substantially

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(SI, Appendix 11). It should rather be volatile organic compounds62 or organics adsorbed at

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the droplet surface from the gas phase,63 as proposed recently. Based on considering also the

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size distribution of particulate SAS, we showed that Nd may increase for another 10% (up to

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12% if moderate σ depression is considered), already when compared with a non-size

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resolved treatment of SAS. This effect is especially pronounced in spring and summer, when

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smaller particles (d ~ 100−250 nm) are enriched with SAS. The demonstrated influence of

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size-segregated SAS on cloud formation is comparable to a 26% increase in apparent particle

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hygroscopicity due to acetaldehyde adsorption to the aerosol-gas interface and the resulting σ

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reduction.62 Although the observed 10% droplet number increase is not likely to result in

431

significant changes with respect to cloud lifetime and precipitation formation in urban

432

regions, there could be a special situation where such CCN number increase triggers the

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precipitation. As the latter is most likely to happen under much cleaner than tested conditions

434

with low CCN numbers (where SAS effect on droplet number is expected to be larger),

435

studies should be repeated on aerosol samples from pristine environments (remote, maritime,

436

polar etc.). We believe that the abovementioned increase in Nd due to size-resolved SAS most

437

importantly influences the optical properties of clouds, which could have an indirect impact

438

on the climate forcing. If globally relevant, 10% increase in Nd changes the cloud albedo for

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up to ~1%, corresponding to a radiative forcing in the order of −1 Wm−2.14,50 To better assess

440

the magnitude of the size-resolved SAS effect on cloud droplet formation, further large-scale



441

field studies and climate predictions using the same approach are needed, especially in areas

442

that are critical for the climate response, such as oceanic regions.

443 444

Acknowledgments

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We thank the Faculty of Chemistry and Chemical Technology University of Ljubljana,

446

Slovenia (UL FCCT) for the permission to install the Berner impactor in the area of the

447

faculty. The authors acknowledge Dr. Bojan Šarac from UL FCCT for the access to the

448

tensiometer and Dr. Janja Turšič from the Environment Agency of the Republic of Slovenia,

449

Ljubljana for providing PM10 samples and performing WSOC analyses. We gratefully

450

acknowledge Dr. Hartmut Herrmann from TROPOS for enabling this international

451

collaboration and Dr. Frank Stratmann also from TROPOS for a fruitful discussion. The

452

authors acknowledge the financial support from European Union Seventh Framework

453

Programme (FP7 2007-2013) Marie Curie FP7-PEOPLE-2011-COFUND (GA no291823)

454

through NEWFELPRO project (Contract no. 47) and the Slovenian Research Agency

455

(research core funding No. P1-0034).

456 457

Supporting Information

458

Detailed procedures of data manipulation: continuous PM and SAS distributions, sensitivity

459

of electrochemical determination of SAS, SAS isolation and extraction efficiency,

460

quantification of SAS, surface tension dependence on PM concentration, different

461

representations of SAS within aqueous aerosols, updraft velocity and lower-limit surface

462

tension impacts, drop number and cut-off diameter; surface tension evolution during droplet

463

nucleation; 1 Table, 11 Figures.

464 465

References


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(1)

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Aerosol size

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ACS Paragon Plus Environment Surfactant activity factor = Surfactant enrichment

1 Size-resolved surface active substances of ... - ACS Publications

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