成都次降水稳定氢氧同位素特征及水汽来源分析

Analysis of Stable Hydrogen and Oxygen Isotope Characteristics and Vapor Sources of Event-based Precipitation in Chengdu

基于对成都地区2016年9月至2017年10月采集的113场次降水样品氢氧同位素的分析,发现大气次降水中δD、δ^(18)O、^(17)O、d-excess和^(17)O-excess有显著的季节性变化,旱季高雨季低,反映了该地区旱、雨两季水汽来源不同;地区大气降水线斜率和截距都偏小,表明成都降水来源于具有不同稳定同位素比率的源地,且雨滴在降落过程中发生了二次蒸发;三氧同位素大气降水线(δ'^(17)O=0. 528 9δ'^(18)O+0. 007 5)斜率介于海洋气团(0. 529)与干空气(0. 518)之间,表明成都地区处于海洋气团向内陆迁移的路径上; d值接近全球平均值,而^(17)O-excess值远较海水大,表明成都的水汽来源由海洋气团主导,且到达该地区的过程中同位素经历了严重的富集; d-excess在旱季出现的极低值可能是受到了人工降雨的影响,^(17)O-excess除了与水汽源地的相对湿度有关外,还会受到上游气团对流作用的影响,此外,成都当地的气象因素对不同季节次降水的^(17)O-excess值有不同程度的影响.

Based on analysis of hydrogen and oxygen isotopes in 113 rainfall samples collected from September 2016 to October 2017 in Chengdu,which is a typical representative of humid areas affected by multiple moisture sources,the compositional characteristics of hydrogen and oxygen isotopes(2 H,18O,and17O)and the water vapor sources of precipitation were analyzed.It was found thatδD,δ18O,δ17O,d-excess,and17O-excess in atmospheric event-based precipitation have significant seasonal variation.In the dry season they are high and in the wet season are low,reflecting the different moisture sources during two seasons(dry and wet).The slope and intercept of the Local Meteoric Water Line were small,indicating that the precipitation originated from sources with various stable isotope ratios and that raindrops were subject to secondary evaporation during their landing process.The Local Meteoric Water Line slope for the triple oxygen isotopes(δ’17O=0.528 9δ’18O+0.007 5)ranged between the slopes for seawater vapor and dry air,and the value of17O-excess was far larger than that of seawater.This indicates that the Chengdu area lies in the path of marine air masses moving toward inland regions.The atmospheric precipitation mainly came from these marine air masses and the isotope had undergone serious enrichment in the process of reaching the area.The d values were close to the global average,and the extremely low value of dexcess in the dry season may be affected by artificial rainfall operations.In addition to the relative humidity of the water vapor source,17O-excess is also affected by the upstream air mass convection;moreover,the17O-excess of the precipitation was not affected by the meteorological factors over the whole study period,so the17O-excess could be considered tracers of evaporative conditions at the vapor source in Chengdu.The precipitation17O-excess in different seasons provides additional information to better understand the precipitation formation processes in Chengdu.