Undergraduate Research and Creativity

URECA

2008-2009

The effect of long chain fatty acid/fatty alcohol monolayers
on ice nucleation from aqueous salt droplets

Seanna Forrester and Daniel Knopf
School of Marine and Atmospheric Sciences

Future global climate change can have a significant impact on society and the biosphere. Therefore, it is important to assess the factors influencing climate. Atmospheric constituents such as CO₂ and aerosol particles can significantly affect the global radiative budget and thus climate. Whereas the effects of greenhouse gases are well known, the effects of aerosols on the radiative budget remain highly uncertain. Actual modeling suggests that aerosol particles may cool the atmosphere by directly scattering incoming ultraviolet radiation and absorbing terrestrial radiation, and indirectly by acting as cloud condensation nuclei and ice nuclei, changing the radiative properties of clouds. In the upper troposphere, temperatures promote the formation of ice leading to the formation of cirrus clouds that can reflect solar radiation and absorb terrestrial radiation.

The oceans are a major source of aerosol particles. It has been shown that many of these natural aerosols are comprised of sea salt and surface active amphiphilic organic compounds. These organic compounds can act as ice nuclei, altering the freezing temperature of the aerosol particle. Only a few studies have explored the effects of such monolayers acting as heterogeneous ice nuclei, and have shown that the monolayers induce freezing on pure water droplets at higher temperatures.

Here, we examine the effect of several different long chain organic molecules on ice nucleation from aqueous salt droplets. First we determine the monolayer properties by measuring pressure-area isotherms and the equilibrium spreading pressure (ESP) of four different organic surfactants on 5 wt% and 20 wt% aqueous NaCl solutions. This is done using a 612-D model Langmuir-Blodgett trough. Depending on the chain length, and amount of dissolved salt in the droplets, the molecules will pack more tightly or are structured further apart. This may greatly influence the freezing point of an aerosol. Freezing experiments are conducted on aqueous NaCl droplets coated with the different organics at ESP. The freezing experiments are carried out using a cryo-cooling stage coupled to an optical microscope. The isotherms and ESP values provide a detailed molecular understanding of the effect of surface active organic compounds on the freezing of aqueous salt particles.

This work was supported by the NOAA Climate Program Office, Atmospheric Composition & Climate Program, Grant NA08OAR4310545.