Laboratory Studies of Atmospheric Heterogeneous and

Multiphase Chemistry


Research Staff: Laura T. Iraci and Samantha Ashbourn

Interactions between gases and particles occur throughout the Earth's atmosphere and have consequences such as the formation of acid rain and the destruction of stratospheric ozone. To understand how these processes occur in our highly complex atmosphere, our research group studies heterogeneous (gas/surface) interactions in a controlled laboratory setting. By carefully varying parameters such as temperature, relative humidity, and particle composition, we can isolate the response due to changes in each of these conditions in the real atmosphere. Our results can then be used in an integrated analysis with field measurements and modeling studies to produce a more complete understanding of our environment and to predict how future changes in temperature or particulates may, in turn, affect the chemistry of the atmosphere.

We have recently completed a study of the interaction of methanol with aqueous sulfuric acid solutions which mimic the particles found in the lower stratosphere. The motivation for this work was two-fold: The conclusion of co-workers at Ames that a significant methanol sink is needed to balance the global budget, and their observation of unexpectedly high levels of methanol in the remote marine atmosphere. In the laboratory we measured the uptake of methanol in aqueous 45 - 70 wt% H2SO4 solutions at temperatures between 197 and 231 K. The solubility increases with decreasing temperature and increasing acidity, with an effective Henry's law coefficient ranging from 105 - 108 moles liter-1 atm-1. This equilibrium uptake of methanol into sulfuric acid aerosol particles in the upper troposphere and lower stratosphere will not appreciably alter gas-phase concentrations of methanol and does not provide the "missing sink" in the atmosphere.

In addition to simple dissolution and other equilibrium processes, we also observed a reaction between methanol and sulfuric acid at room temperature. Unfortunately, this reaction is too slow to provide a sink for gaseous methanol at the temperatures of the upper troposphere and lower stratosphere. It is also too slow to produce sufficient quantities of soluble reaction products to explain the large amount of unidentified organic material seen in particles of the upper troposphere. These results are in contradiction to recent rate coefficients reported by workers at JPL.

Another current project investigates the behavior of the halogen species hypobromous acid (HOBr), hydrobromic acid (HBr) and hydrochloric acid (HCl) at temperatures warmer than those which lead to polar stratospheric cloud formation. The behavior of HOBr under these conditions is important, as it may enable chlorine activation and subsequent ozone destruction in "warm" regions of the atmosphere. A new laboratory facility is under construction, and upon its completion we will continue our studies of the uptake and reactivity of halogenated and small organic molecules on sulfuric acid solutions which model the global sulfate aerosol layer. Other areas of interest include the fate of ionic and nonionic solutes upon freezing of cloud droplets, and studies of aqueous phase transformations of oxygenated compounds of biogenic origin.

Point of Contact: Laura T. Iraci, 650/604-,