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Goals and Objectives Abstract Water Evaporation From Tropospheric Aerosols Patrick Louden and Dr. Christopher Lawrence Department of Chemistry,Grand Valley State University, Allendale, MI 49401 References Background Future Work Acknowledgements Dr. Christopher P. Lawrence Alex Gilde With the recent discovery of the ubiquity of organic material in tropospheric aerosols, it has been postulated that the rates of water evaporation and condensation into the aerosols could be affected by thin surfactant films, which could ultimately affect cloud formation. Nathanson et al. have begun to study the effect of water evaporation from sulfuric acid solutions through the short-chain surfactant, butanol. They have found that a nearly full monolayer of butanol fails to reduce water evaporation from the acid. This unexpected result raises many questions about the mechanism of water evaporation. We used molecular modeling to help answer some of these questions as it allowed us to examine the trajectory by which a molecule leaves the liquid at the molecular level. We also are able to study this problem under conditions closer to that of the troposphere because we are free of certain experimental limitations and we intend to do so in the future. •To understand water evaporation from a surfactant covered aerosol using simulations •To understand why there is no change in the rate of evaporation in the sulfuric acid system An aerosol is a small droplet of liquid suspended in air. Aerosols will often have impurities located within, or on them. Our research focuses on water aerosols, located within the troposphere. The troposphere is the lowest layer of the atmosphere, this is where most of the weather occurs. LaMer et al. studied water evaporation through long-chain surfactants (12 to 18 carbon atoms in length). They found that the rate of evaporation decreases by a factor of 10,000 in the presence of the surfactant. As the organic material in the troposphere is expected to consist of short-chain molecules, Nathanson et al. studied evaporation of sulfuric acid at -60 o C with butanol. They found that even with a full monolayer of butanol there was negligible change in the rate of evaporation of the acid. •Finish running and analyzing the sulfuric acid simulations •Determine the source of the increase in condensation rate in going from water to sulfuric acid. •Work toward a system that is more consistent with tropospheric aerosols. •Explore the inability of the model to quantitatively reproduce the experimental observations. • LaMer, V. K. L.; Healy, T. W.; Aylmore, L. A. G. J. Colloid Sci. 1964, 19, 673. •Lawrence, J. R.; Glass, S. V.; Nathanson, G. M. J. Phys. Chem. A 2005, 109, 7449. Simulation Model CH 3 CH 2 CH 2 CH 2 H O ( + ) ( - ) ( + ) Butanol Water/Butanol Interface To being our simulation we introduced a box of water molecules. We then allowed the simulation to run for about 200 picoseconds to allow the water molecules to reach a more favorable state. After this we introduced butanol molecules on the top and bottom of the box of water. Simulation Problem A typical simulation run takes about one day to complete. In that time, we see no evaporation occurrences. To get a statistically meaningful value for the rate, we need to see about one hundred evaporations occur. Since this is impossible due to the time required, we instead measure the rate of condensation which is equal to the rate of evaporation at equilibrium. The rate of condensation can be calculated using far less computer time. Again we ran the simulation for 200 picoseconds to allow the system to reach a more favorable state. Lastly we introduce a water molecule in the gas phase with a random position and velocity and allow it to hit the interface. We then are able to count the fraction of molecules that condense into the interface. 0 8 18 32 50 72 98 100 91 83 60 33 37 52 Butanol Condense % After 250 runs of our simulation, we obtain the following results Sulfuric Acid/Butanol Interface We are unable to compare our results to Nathanson’s because of the differences in our systems. To attempt to model his experiment, we simulated sulfuric acid coated in butanol Our initial results show us that at the 50 butanol coverage, the percent that condense is about 50%. This is an improvement from the water-butanol simulation, however not the 100% that Nathanson obtained through experiment. We ran the simulation at 27 o C to ensure that it was not only a temperature dependency. Our data shows us that it is not temperature dependant. O H H -2q +q +q Water Fixed charges to allow for hydrogen bonding We treated the bonds and angles of the molecules as springs Surfactant Water 27 o C Acid 27 o C Acid 27 o C Acid 60wt% 64wt% 68wt% 8 91% 93% 96% 95% 18 83% 86% 86% 89% 32 59% 64% 77% 72% 50 28% 51% 56% 31% 72 18% 21% 27% 13% Surfactant Water -60 o C Acid -60 o C Acid -60 o C Acid 60wt.% 64wt.% 68wt.% 8 91% 97% 97% 96% 18 83% 93% 93% 90% 32 59% 77% 74% 73% 50 28% 47% 56% 50% 72 18% 14% 24% 18% A repulsive potential prevents the oxygen atoms from getting too close to one another A torsion potential is used to model bond rotation To explain why there is an increase in the number of condensations at butanol coverage above 50 butanol molecules, we looked at how the butanol molecules are laying on the surface of the interface. The butanol can lay on the surface in different ways. When there are only a few butanol molecules on the surface they tend to lie flat, and they grow increasingly in height the more that are packed onto the interface. The graph above shows the probability of the height of the butanol molecules at different coverage. At very low coverage, the peak in the distribution lies about halfway between standing straight up and laying down. As the coverage increases, the butanol molecules have a greater tendency to stand straight up. However, as the coverage increases above 50 butanol molecules, a large number of the butanol molecules stand in the opposite direction as noted by the negative heights. Tail Head The image to the left shows a bilayer of butanol. With the head of the butanol molecules exposed to the surface, the incoming water molecule may become bound to the butanol and remain there indefinitely. To account for this, we considered an alternate definition of condensation in which the incoming water must make contact with other water molecules in order to be counted. This definition will lead to smaller condensation percentages and is used throughout the remainder of our work. A weak attraction is included between CHx groups
