Investigation of the Reaction of HCl and Ice- Introduction

(More information about the image at the right can be found at Chester's home page)
The atmospheric ozone layer has long protected humanity from the damaging effects of solar radiation by absorbing harmful ultraviolet radiation. However, atmospheric scientists have recently noticed a dramatic decrease in ozone levels in the stratosphere. In January of 1993, researchers discovered that ozone levels above the northern midlatitudes were 13 to 14 percent below normal (Zurer, Pamela. (May, 1993). Chemical and Engineering News, 8-18).. The National Aeronautics and Space Administration (NASA) documented that in 1993 the levels of ozone were the lowest in fourteen years (Zurer 1993). Many people blame the eruption of Mount Pinatubo in the Philippines for the decrease, yet atmospheric scientists claim that chlorofluorocarbons (CFC's) are the real culprits.

Since chlorofluorocarbons are used in industry as refrigerants and are sold to the public in products such as aerosol cans, the theories that have been proposed have come under considerable scrutiny. There is a need to investigate these theories in complete detail, so that a solution that is scientifically valid may be formulated.

Theories advanced by atmospheric scientists rely on many reactions taking place in order for the chlorofluorocarbons to destroy the ozone layer. This work investigated the role of heterogeneous reactions in the destruction of ozone in the stratosphere. Specifically, we investigate the reaction of hydrogen chloride (HCl) and solid water (ice).

Normally ozone is continually created and destroyed via the following reactions:
O₂ + hv --> O + O
O + O₂ --> O₃
O₃ + hv --> O₂ + O
These reactions take place about 40 kilometers above the earth in the stratosphere.

It is theorized that the CFC's rise into the upper atmosphere where they are photolyzed to produce another CFC and a chlorine free radical (Zurer 1993). One such reaction is:
CCl₂F₂ (CFC-12) + hv --> CClF₂ + Cl
Free chlorine radicals act as catalysts which deplete the population of O₃ molecules available for the reaction 1.3 by the process:
Cl + O₃ --> ClO + O₂
ClO + O --> Cl + O₂
O₃ + O --> 2O₂ (net)
Because the chlorine radical is regenerated in this process, it is available to destroy many other O₃ molecules by the same process.

One cycle that atmospheric scientists are particularly interested in understanding is the seasonal ozone depletion above the Antarctic. The Antarctic is an interesting system to study because it has suffered extensive damage, i.e. the well publicized ozone ``hole''. Additionally, in the winter, it becomes a closed system. During this season, the winds begin to circle above the polar region, creating what is called a polar vortex. The winds of the polar vortex region do not mix with the rest of the atmosphere until late spring.

The mechanism described in equation 1.5 does not explain the destruction of the ozone layer above the Antarctic for two reasons. First, it does not provide for a seasonal depletion of ozone as is observed above the Antarctic. Secondly, there are not enough O atoms present in the altitudes in which the most prominent destruction has occurred. In fact, these reactions have been shown to be insignificant at 40 km, stratospheric altitudes (Zurer 1993). Therefore another theory has to be considered.

The theory that accounts for seasonal depletion of ozone takes into account the reservoir species. These species are nodes in holding reactions. These reactions compete with ozone destroying mechanisms and have the effect of removing destructive species from the ozone cycle Wayne, Chemistry of Atmospheres 1991).The most prominent of these reservoir species is hydrogen chloride, HCl, which accounts for about 70 percent of atmospheric Cl.

To introduce a seasonal cycle, a mechanism is needed to explain how these reservoir species are converted into species active in for ozone destruction. It is proposed that HCl and ClONO₂ react on the surface of polar stratospheric ice particles in the clouds and produce molecular chlorine (Cl₂) and hypochlorous acid (HOCl). Tolbert, Science 240, 1988).

The clouds in the polar vortex region, called polar stratospheric clouds (PSC's), carry ice crystals which provide surfaces on which the above reactions can take place. There are two types of cloud particles present: PSC Type (I), smaller particles (1 $\mu$m) consisting of nitric acid trihydride (NAT), and PSC Type (II), larger particles (10 $\mu$m) constisting of mostly frozen water condensed on a NAT nucleus Fahey, J. Geo. Res., 94 1989). The reservoir species react on the surfaces of these PSC's all winter long, creating Cl₂ and HOCl. When the sun returns in the spring, the Cl₂ and HOCl are photolyzed, creating a deluge of ozone destroying radicals. Thus, the ozone hole is created over the Antarctic in the spring.

Experiment

In this photo the top hat piece which holds the Silicon surface is being heated to 1000C to clean the Silicon. Once the surface is clean it will be cooled down with liquid nitrogen and dosed with water to form an ice surface.

Recent results on measuring the sticking coefficient of HCl on Ice.