Organics at the Gas-Aerosol Interface

V. Faye McNeill, Joel Thornton

Surface active organics in ambient aerosol

Organic material represents 10 – 90% of atmospheric aerosol mass,

and comes from a diverse array of sources. Naturally occurring

surface active organics such as fatty acids are common in continental

and marine aerosols. In solution, surfactant molecules partition to the

gas-liquid interface, forming a surface film and lowering the surface

tension. Surfactant molecules present in aqueous atmospheric aerosol

are thought to partition to the surface in an “inverted micelle”

configuration, forming an organic surface layer. With increasing

surfactant content, micelles may form, ultimately resulting in gel-like films.

Multilayer films of organic material have been shown to inhibit gas-aerosol mass transfer, but

relatively little is known about the effects of submonolayer to monolayer organic films on gas-aerosol

reactive uptake kinetics. Furthermore, the lifetime of these films in the oxidizing environment of the

atmosphere is not known.

The effect of surface active organics on reactive uptake

Submonolayer coverages of

surfactant on submicron aqueous

aerosols suppress the reactive

uptake of N₂O_5.

A nonlinear dependence of g_N2O5

on surface coverage is observed

for SDS and sodium oleate (oleic

acid proxy). This is attributed to

the fact that SDS and oleic acid

are “expanded state” surfactants.

Reference: McNeill, VF, Patterson, J, Wolfe, GM, and Thornton, JA, “The Effect of Varying levels of

Surfactant on the Reactive Uptake of N₂O₅ to Submicron Aqueous Aerosol” Atmos. Chem. Phys. 6,

1635-1644 (2006) link to paper

Detection of particle phase organics

We use a heated inlet to monitor

gas and particle phase composition

simultaneously with our CIMS. This

method is sufficiently sensitive for the

direct study of monolayer interfacial

organic chemistry on submicron

aqueous aerosols.

Using CIMS, we can detect particle

phase organic species with fast time

response, high selectivity, and low

fragmentation.

This figure shows oleic acid and oleic acid

ozonolysis product signals as a function of inlet

temperature. The majority of the oleic acid

and ozonolysis product molecules were observed

to be in the particle phase at ambient

temperature. Note that we were not able to

monitor nonanal in this experiment.

Ozone oxidation of oleic acid monolayers

O₃ oxidation kinetics for a single monolayer of oleic

acid on submicron aqueous aerosols were obtained

using an aerosol kinetics flow tube. O₃ was

introduced via a moveable injector. Particle-phase

oleic acid signal is shown here on a log scale vs.

reaction time, which is a function of the injector

position in the flow tube. The pseudo-first-order

rate constant, k^I, is obtained from the slope of the

decay curve.

O₃ oxidation of an oleic acid monolayer on

submicron aqueous aerosols appears to follow

a Langmuir Hinshelwood-type mechanism, in

which the reactive surface becomes saturated at

high [O₃]. Our results suggest that the 1 e-fold lifetime

of unsaturated organic surfactants at the surface

of aqueous aerosols is ~ 10 minutes under

atmospheric conditions.