Isotopic constraints on past ozone layer in polar ice (ISOL-ICE)

臭氧层保护所有陆地生物免受有害的紫外线辐射；并间接影响地球表面的气候，包括温度和风，特别是两极附近的气候。人造卤化碳，例如用于冰箱和喷雾罐的，自1970年代末以来被释放到大气中，造成臭氧层的严重破坏，特别是在春季南极洲上空。由于1989年的《蒙特利尔议定书》禁止使用许多卤代烃，预计本世纪内臭氧层将恢复到20世纪60年代和70年代初的状况。然而，臭氧层的厚度也受到自然原因的影响，而自然原因对此知之甚少，这使得对未来臭氧和气候的预测不太确定。自然原因包括太阳活动的变化、火山喷发、生物卤化碳的释放和大气环流。目前，关于臭氧层在历史时间尺度上的自然变化的信息很少，即在20世纪初开始直接观测之前。然而，了解臭氧层的自然变异性及其根本原因对于评估气候和臭氧政策选择的有效性是必要的。为了提高对地面紫外线辐射的预测，这也是必要的。紫外线辐射被认为是一种环境致癌物质，也是人类健康的主要担忧。超越现代测量时代的一种方法是使用在极地冰芯测量的代用品。除了最近提出的一个生物标志物外，还没有过去紫外线辐射的定量指标。在这里，我们建议测量极地冰中硝酸根离子中氮和氧的同位素，以重建过去的紫外线辐射，从而重建臭氧层。同位素是相同元素的原子，具有相同数量的质子，但不同数量的中子。在南极洲内陆非常干燥的地区，雪在被降雪掩埋之前会暴露在阳光下数月。在暴露期间，雪中的硝酸盐被太阳紫外线辐射分解；在这个过程中，观察到硝酸盐中较重的氮同位素优先留在雪中，而较轻的氮同位素逃逸到上面的大气中。这种分馏取决于紫外线辐射的波长和持续时间。我们假设，一旦雪中的硝酸盐被深埋，它就保留了向下涌出的紫外线辐射的同位素指纹，从而保留了臭氧层的厚度。我们建议从积累率较低的东南极高原收集浅冰芯，以开发和应用基于硝酸盐稳定同位素的新冰芯代理，以限制南极上空臭氧层在过去1年的趋势。为此，我们将利用自20世纪50年代以来对南极洲上空臭氧层的观测来校准冰核信号，然后将这种关系外推到更遥远的过去。我们将使用数值模式来研究基于冰核重建过去臭氧层变化的根本原因。我们将尝试回答的具体问题包括：平流层臭氧在过去是否发生了变化；太阳的可变性、卤化碳的自然排放或火山喷发对重建的趋势有何贡献？

The ozone layer shields all land-based life forms from harmful ultraviolet radiation; and indirectly influences the climate at the Earth's surface, including temperature and winds, particularly near the poles. Man-made halocarbons, used for example in refrigerators and spray cans, were released to the atmosphere and have caused significant destruction of the ozone layer since the late 1970s, especially above Antarctica during spring-time. Because the use of many halocarbons was banned by the 1989 Montreal Protocol, the ozone layer is expected to recover to the conditions of the 1960s and early 1970s within this century.However, the thickness of the ozone layer is also influenced by natural causes, which are less well understood and which make predictions of future ozone and climate less certain. Natural causes include variations in the sun's activity, volcanic eruptions, release of biogenic halocarbons and atmospheric circulation. Currently there is very little information on the natural variability of the ozone layer over historic time scales, i.e. before direct observations started in the early 20th century. However, understanding the natural variability of the ozone layer and the underlying causes is necessary to evaluate the effectiveness of climate and ozone policy options. It is also necessary in order to improve predictions of ground level UV radiation, which is recognized as an environmental carcinogen and a major concern for human health.One way to go back in time beyond the era of modern measurements is the use of proxies measured in polar ice cores. Apart from a recently proposed biomarker there are no quantitative proxies of past UV radiation. Here we propose to measure the isotopes of nitrogen and oxygen in the nitrate ion in polar ice to reconstruct past ultraviolet radiation and therefore the ozone layer. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. In the very dry regions of inner Antarctica snow is exposed to sunlight for many months before being buried by snowfall. During that exposure the nitrate in the snow is decomposed by solar UV radiation; during that process the heavier nitrogen isotopes in nitrate are observed to stay preferentially in the snow, whereas the lighter ones escape to the atmosphere above. That fractionation depends on the wavelength and duration of the UV radiation. We hypothesize that once the nitrate in snow is buried at depth, it preserves an isotopic fingerprint of down-welling UV radiation and therefore of the thickness of the ozone layer.We propose to collect a shallow ice core from East Antarctic Plateau, where low accumulation rates prevail, to develop and apply a new ice core proxy based on the stable isotopes of nitrate, to constrain trends in the ozone layer above Antarctica over the last 1kyr. To do this, we will calibrate the ice core signal with observations of the ozone layer above Antarctica since the 1950s, and then extrapolate that relationship to the more distant past.Using numerical models we will investigate the underlying causes of the ice core based reconstruction of past variability in the ozone layer. Particular questions we will attempt to answer include: has stratospheric ozone changed in the past; and how did solar variability, natural emissions of halocarbons, or volcanic eruptions contribute to the reconstructed trends?