Collaborative Research: The impact of bromine chemistry on the isotopic composition of nitrate at Summit, Greenland

硝酸盐是高山和极地雪中发现的主要离子之一，但它仍然是冰芯记录中最不被了解的化学成分之一。最近的研究表明，硝酸盐同位素比率是研究冰雪岩心中硝酸盐的有力工具。硝酸盐的同位素组成已被证明包含有关硝酸盐(氮氧化物)来源和沉积前大气中将氮氧化物转化为硝酸盐的氧化过程的信息。由于羟基和过氧基的同位素组成与臭氧非常不同，现在人们可以区分在大气中产生硝酸盐的不同氧化过程的影响。在格陵兰岛顶峰，对雪中硝酸盐氧同位素组成的季节性观测，不能用当地的标准光化学来理解。造成模型和观测结果差异的最有可能的原因是箱式模型没有从顶峰以外的区域输送硝酸盐，以及卤素化学(即Bro)对硝酸盐的影响。最近的测量表明，在顶峰上方的边界层中出现了比预期更高的BRO。该项目的目的是量化溴化学对春季和夏季硝酸盐生产的影响。该方法包括现场测量和实验室测量以及建模。在春季和夏季野外季节，将在现场进行BRO和多个气相测量，而雪和大气样品的同位素分析将在实验室完成。雪和冰川冰中硝酸盐的氧同位素组成有可能定量恢复古大气氧化剂的浓度，但重要的是限制这种示踪剂在多大程度上反映局部与区域或半球尺度的化学。此外，由于Summit的BRO很可能来自自然来源，而这一来源受到气候变化的影响，因此卤素化学对氮氧化物的影响对解释近代和深层冰芯的硝酸盐记录具有重要意义。

Nitrate is one of the major ions found in alpine and polar snow, yet it continues to be one of the least understood chemical components in ice core records. Recent work ahs shown nitrate isotope ratios to be a powerful tool for the study of nitrate in snow and ice cores. The isotopic composition of nitrate has been shown to contain information about the source of the nitrate (nitrogen oxides) and the oxidation processes that convert nitrogen oxides to nitrate in the atmosphere prior to deposition. Because hydroxyl and peroxy radicals have very different isotopic compositions than ozone, one can now distinguish the impact of the different oxidation processes that produce nitrate in the atmosphere. Seasonal observations of the oxygen isotopic composition of nitrate in snow at Summit, Greenland, cannot be understood in terms of standard, local photochemistry. The most likely causes of the model and observation discrepancy are that the box model lacks transport of nitrate from regions outside of Summit and the influence of halogen chemistry (i.e., BrO) on nitrate. Recent measurements suggest a higher than expected presence of BrO in the boundary layer above Summit. The aim of this project is to quantify the influence of bromine chemistry on nitrate production in the spring and summer. The approach includes field and laboratory measurements as well as modeling. During spring and summer field seasons, BrO and multiple gas-phase measurements will be conducted on-site, while isotopic analyses of snow and atmospheric samples will be completed in the laboratory. The oxygen isotopic composition of nitrate in snow and glacial ice holds potential for quantitatively reconstructing paleoatmospheric oxidant concentrations, but it is important to constrain how much this tracer reflects local versus regional or hemispheric scale chemistry. Furthermore, since BrO at Summit most likely originates from a natural source, and this source is affected by changes in climate, the influence of halogen chemistry on nitrogen oxides has important implications for the interpretation of recent and deep ice core records of nitrate.