Measurements addressing the initial stages of ozone recovery, the nucleation of, index of refraction of, and existence of large PSC particles

National Science Foundation, $821,131, September 2006 - September 2009, PI: T. Deshler, CoI: J. Mercer, Location: McMurdo Station, Antarctica  Submitted: 2 June 2005

In the past twenty years there has been a rather rapid decline in Antarctic stratosphere ozone in the spring. This decline is due directly to halogens released into the atmosphere since the 1930s. Coupling halogen releases with cold austral polar stratospheric temperatures, which lead to clouds and thus surfaces to convert inactive to active chlorine, sets the stage for rapid, chlorine induced, catalytic conversion of ozone to diatomic oxygen as sunlight returns to the polar winter stratosphere. This led to the "ozone hole" which reached unprecedented minimums in the late 1990s as stratospheric chlorine peaked. Profile measurements during this period indicated 4-6 km vertical regions in the mid to lower stratosphere which were devoid of ozone. The first warning of the danger of halogen releases was published in the 1970s, just about 20 years after atmospheric releases of halogens became common. These warnings were confirmed in the mid 1980s and the first controls on halogen release were established with the Montreal protocol in 1987. This protocol, with its amendments, has significantly reduced halogen releases, which has been measured. Our records now suggest that stratosphere chlorine peaked in the late 1990s.

We propose here to continue in situ balloonborne ozonesonde measurements through 2008. These ozone measurements, begun in 1986, documented the decline and minimum in ozone observed as chlorine increased and reached its maximum. Now the emphasis is shifting to observing the first signs of ozone recovery. Ozonesondes are uniquely capable of observing in the altitude range suffering the greatest chemical loss, and thus able to separate chemical and transport effects. Thus these instruments may be among the first to establish the ozone benefits resulting from declining chlorine. During 2007 we propose to collaborate with European colleagues in a second international Antarctic campaign to test 3D chemical transport modeling of ozone loss. This campaign will occur as part of the Intentional Polar Year, and will consist of a second year of winter/spring ozone measurements from McMurdo. Laboratory investigations of the chemistry within electrochemical cell ozonesondes will help establish a transfer function for data sets which include measurements with different ozonesondes and solution strengths. This supports our WMO involvement to establish recommendations for ozonesonde operating procedures.

Polar stratospheric cloud (PSC) observations will be continued to address questions related to the nucleation of nitric acid hydrates, the existence of large particles within Antarctic PSCs, and the index of refraction of PSC particles. Measurements to address questions related to nucleation of nitric acid hydrates within PSCs will include collaboration with Dr. Francois Vial, Laboratoire Meterologique Dynamique, France, to use data obtained in VORCORE in 2005 to assess the accuracy of analyzed temperature fields used in back trajectory calculations. Measurements of large particles require a new instrument presently funded for development under an NSF MRI grant. We will continue our development of a double angle particle counter to measure particle index of refraction. We will continue collaborating with Dr. Marcel Snels, Institute of Atmospheric Sciences and Climate (ISAC) of the National Research Council, Rome, on the lidar measurements from McMurdo, which help guide our PSC measurements.

The intellectual merit of this work lies in obtaining and analyzing measurements which may lead to the first signs of ozone recovery, to clarifying the electrochemistry occurring in ozonesondes, to a better understanding of the nucleation of nitric acid hydrate particles, to documenting the existence of large PSC particles in the Antarctic, and to continuing the development of an index of refraction spectrometer.

The broader impacts of this work have several aspects. Measurements establishing the first signs of ozone recovery are important reassurances to the world community in support of the commercial sacrifices which have been made to limit the release of chlorine into the atmosphere. Thus, in addition to the scientific interest, there are broad social implications dependent on maintaining ozone measurements through the first decade after maximum chlorine has been reached in the stratosphere. This research also contributes to the training and education of a post doctoral scholar, engineer, technician, and graduate students. Four of the nine members of my group are women thus increasing the contribution of underrepresented groups in science.