Sulfate is a significant chemical component of atmospheric aerosols; however, the chemistry governing its formation over a large portion of the globe, particularly the high latitudes and southern hemisphere remains unconstrained. At present there are sparse measurements of sulfur dioxide fluxes from anthropogenic and natural sources, particularly in the southern hemisphere. Sulfate aerosols interact strongly with incoming solar and outgoing terrestrial radiations by scattering and absorption, therefore size distribution is of high significance to understand the mode of interaction of sulfate aerosols with radiation. The primary objective of this study is to develop an isotopic probe of atmospheric sulfate chemistry in a pristine marine environment at Cape Grim Baseline Air Pollution Station, Australia to identify the role of sea salt alkalinity on the oxidation of sulfur. Oxygen and sulfur isotope measurements will be used to better constrain the sources and chemistry controlling the formation and transport of sulfate aerosol in the atmosphere. These results will be used to interpret paleo-sulfate aerosol data obtained from Antarctic ice cores and to identify the contribution of anthropogenic sulfur to the natural sulfur cycle.
This research will help to elucidate the impact of sulfur chemistry on climate in a clean oceanic background at Cape Grim Station, Australia. This will establish a baseline for evaluating temporal and spatial variations in sulfate aerosols and will provide a basis for the future interpretation of ice core data. In view of the inherent problems associated with global chemistry models used to reproduce the sulfate levels, this new analytical tool has the potential to improve global chemistry models and in turn this may enable a better diagnosis of potential changes that result from anthropogenic sulfate sources. This project also has the broader goal of furthering the development of human resources in the field by training a postdoctoral scientist and graduate and undergraduate students.
According to the International Panel on Climate Change the greatest uncertainties in climate change is the role of particulates (aerosols) in the atmosphere. Aerosols effect the solar radiation coming in and IR out of the atmosphere in a fashion that is inadequately understood. A major weakness in quantifying the role of aerosols is that their chemical composition determines these properties and is difficult to measure appropriately. The chemical properties of their surfaces will determine how particles transform during their transport which may be hemispheric. These chemical alterations are extraordinarily difficult to determine by conventional concentration measurements. Furthermore, resolving the source of aerosols is another facet that is difficult to assign. In this project, we used a new tracer of these processes, multi stable isotope ratio measurements. The composition and alteration of isotope ratios coupled with knowledge of the quantum chemical control of them allows chemical alterations to be determined in a completely unique way. In addition, source recognition may be achieved. Two of the most abundant and important atmospheric aerosols are nitrate and sulfate. They intersect with climate, human health, agriculture, and materials degradation, hence understanding these features has a very significant societal component. Their transport is also important because the issues become international and mediation complex. Any enhancement in understanding these properties may significantly impact the ability to make political and social decisions, as well as advance basic understanding of global atmospheric chemical science. In a previously funded NSF grant, this lab excavated a snow pit at the South Pole. This type of sampling as compared to conventional coring provides a high time resolution (6 month vs. years) and we were able to examine a 20 year record of aerosol sulfate for all oxygen and sulfur isotopes that was transported to the South Pole. This inclusion of all isotopes had never been done anywhere, and the time evolution was a key component. The basis for the proposal was to determine how the sulfate aerosols were created via oxidation and, to determine if we may evaluate global properties of oxidation processes as they are an important component of the Earths totals chemical system. The observations far exceeded our expectations. We were able to establish our chronology quite precisely as we were able to recognize the Pinatubo and Cerro Hudson volcanic debris and their associated perturbation of the upper atmospheric (stratospheric) perturbation. This gave us a benchmark for both time for hemispheric transport and magnitude because the output from these volcanic events is well characterized. What we also found and had not been noted before, was that during El Nino years, there exists a conveyor belt arising in the equatorial region of sulfur species that impinge on the ozone chemistry of the equatorial region and are then transported to the South Pole (Fig. 1a and b). This correlated well with the ozone measurements made be various satellites and we now have a means to measure sources at the equatorial region, define their chemical change over distance, and the time scale. A greater surprise was that in the sulfur record, following the largest El Nino in 200 years (1998/1999) we found a massive stratospheric transport of sulfur that exceeded the size of effect produced by the Pinatubo eruption. We have resolved that this derives from the massive wildfires that occurred in Indonesia and South America. The wildfires injected massive quantities of material to the stratosphere where the photochemical reaction of sulfur dioxide and subsequent reaction with ozone occurred along its transit to the South Pole. This process had not been recognized and it fully exploited the new isotope approach. These findings will have applications in the future because with climate change, the extent of wildfires is predicted to increase and this provides a monitor to determine what the global impact will be. This process had not been recognized before, nor would it have been by conventional techniques. It might be better understood by examination of the ice record during times of know dryness and cold where more particulate and natural wildfires are expected. This would provide a more direct route to understanding what may be expected in the future. The results of much of the work involving climate, global processes and a polar expedition is of public interest and as reported, we did many presentations, including outreach to schools and the public. We engaged a wide range of undergraduate students who benefitted from a research experience, did public presentations, wrote results, and in many occasions, won awards. The students also were derived from a highly diverse background and the project was highly successful from both an ability to communicate exterior to the University and include a component of career development.
