The National science Foundation (NSF) Environmental Chemical Sciences (ECS) program of the Division of Chemistry and the Atmospheric Chemistry (ATM) program of the Division of Atmospheric and Geospace Sciences, and the Deutsche Forschungsgemeinschaft(DFG)(German research Foundation will fund the international collaboration in chemistry (ICC) project of Prof. Elisabeth Moyer of the University of Chicago and a collaborative team of German investigators which includes Volker Ebert, Ottmar Mohler and Steven Wagner of the University of Heidelberg and Harald Saathoff of the University of Gottingen. Prof. Moyer and her collaborators will study the mechanism of ice nucleation and growth via measurements of the isotopic composition of water under atmospherically-relevant conditions at the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) experimental chamber in Karlsruhe, Germany, a uniquely suitable facility for this study. The study will add new measurement capability to AIDA that will support future research in the area of ice nucleation and growth. The project will provide excellent training opportunities to students at the University of Chicago and the University of Heidelberg. It will also provide a valuable educational experience to high school students from the Woodlawn School, a University of Chicago-operated charter school serving the low-income African-American community surrounding the university.
The project stands to greatly increase understanding the mechanism of ice nucleation and growth and the role of water in the stratosphere. It will improve stratospheric ozone and climate predictions, which are of major importance to science policy and economics.
Recognizing the unique international training opportunities to US students, the Office of International Science and Engineering (OISE) contributed funds towards this international collaboration in chemistry project.
This project centered on studies of the microphysics of cirrus (ice) clouds, especially those at the coldest regions where they form, the tropical tropopause region at ~ 16 km altitude, where temperatures regularly drop below 200 K. Measurements from high-altitude aircraft had suggested that water vapor concentrations these ultracold cirrus were higher than would be expected, i.e. either that a different type of ice was forming, with a different saturation vapor pressure, or that ice growth was somehow inhibited. Since these ultracold cirrus are the last stages in the dehydration of air as it rises into the dry stratosphere, where the Earth's protective ozone layer lies, understanding any complication with that dehydration is important for understanding climate and atmospheric chemistry: stratospheric water vapor plays a direct role in ozone destruction. This project sought to study basic microphysics in a laboratory environment, and to explore a range of temperatures across the 200 K threshold to understand whether some aspect of ice formation and growth changed at lower temperatures. We sought to use a novel tracer for understanding cirrus microphysics, monitoring the evolving isotopic comoposition of water vapor (the ratio of the naturally occuring isotopically substituted water, HDO, to ordinary H2O) as cirrus formed. Many proposed hypotheses explaining the anomalous supersaturation would produce different isotopic signatures. The project therefore involved the design and construction of a new instrument to measure HDO and H2O by tunable diode laser absorption spectroscopy. The instrument was designed for use not from aircraft but in a laboratory facility, the AIDA aerosol and cloud chamber at the Karlsruhe Institute of Technology, Germany, which is capable of simulating the temperatures and pressures of the parts of the atmosphere where cirrus clouds form. The entire chamber can be cooled as low as 185 K. The new instrument, the Chicago Water Isotope Spectrometer (Chi-WIS), was deployed at AIDA during four campaigns in 2012-2013. During that time we were able to measure isotopic evolution during formation of cirrus clouds at temperatures from 235 to 185 K. These measurements allowed us to measure fundamental physical properties of ice formation that had not previously been directly measured. We made the first measurements of the "fractionation factor" for the HDO/H2O system at these temperatures: that is, the degree of preferential deposition of HDO rather than H2O as ice. That preference means that surrounding water vapor becomes more depleted in HDO as cirrus clouds grow. All previous modeling of isotopic evolution at these temperatures in Earth system models had use extrapolations of measurements from warmer temperatures made in the 1960s. Our measurements suggest that those extrapolations are in fact reasonable. Our measurements are consistent with the early studies from the 1960s, and disagree with some recent work questioning those results. We also made the first experimental confirmation of the modification of that fractionation in conditions where the number of ice particles is small enough that their growth is limited by the ability of water molecules to diffuse to the ice crystals. These effects had been broadly assumed but never demonstrated experimentally. And lastly, we showed that cirrus clouds formed at temperatures below 200 K had no particular anomalies in growth rates, in surrounding water concentration, or in isotopic properties. The implication is that the previous aircraft measurements suggesting anomalies below 200 K may have been in error and that we need not hypothesize new and unknown processes that occur only at ultracold temperatures. These results are fundamental contributions to the microphysics of ice clouds. They do not suggest new scientific dilemmas, but instead confirm existing ideas and theories, suggesting that scientific knowledge of the microphysics of cold ice clouds is more sound than had been assumed.
