氮同位素示踪灰霾期氨的气粒转化及细颗粒铵根演化过程

Severe and persistent haze episodes, which are characterized by extremely high concentrations of fine particulate matter, are one of the most concerned atmospheric environmental pollution. Gas ammonia is considered to be catalyst for the formation of PM2.5, the conversion of gas ammonia to fine particle NH4+ is one of the major contributors to the generation of secondary inorganic aerosol and even to PM2.5. 15N is more likely to be enriched in fine particle caused by the equilibrium fractionation effect during ammonia gas-to-particle conversion process in haze episodes(as a closed system), δ15N values of fine particle ammonium and gas ammonia deplete with the generation of ammonium. Based on our existing research, the δ15N of fine particle ammonium nitrogen was more and more negative if the gas ammonia consistently transformed to ammonium during haze events, otherwise the δ15N of ammonium will be increased. The project is intended to take Nanchang city as a case study, the characteristics of gas ammonia and ammonium nitrogen isotope of size-resolved particles during different stages of haze period will be extracted to better understand the impact of ammonia/ammonium conversion on the formation and explosive of fine particle. The concentrations and nitrogen isotopic compositions of gas ammonia , and the composition of water soluble inorganic ions of size-resolved particles，and acid gases as well as the meteorological conditions such as temperature and relatively humidity will also be disclosed to deeply characterize the conversion of ammonia gas-to-particle and temporal evolution of particle ammonium based on Rayleigh fractionation, and to have a better understand on the mechanisms and impact factors during its evolution in ambient atmosphere. In general, the objective of this project is to enhance the understanding of the atmospheric chemical process of gas ammonia and fine particle ammonium and the role on the formation and growth process of fine particle. Therefore, our study will provide scientific information and reasonable suggestions for the control of air pollution and haze episodes.

以高浓度细颗粒物为特征的灰霾污染是我国面临的严重大气环境问题，其中气态氨向颗粒铵的转化是促进二次无机气溶胶甚至是PM2.5生成的主要因素之一。灰霾期（近似密闭系统），氨的气粒转化过程引起的同位素平衡分馏效应使15N更容易富集在细颗粒中，细颗粒铵及气态氨的δ15N值随铵根的生成而降低。申请人前期研究发现：灰霾期气态氨如果以消耗为主，细颗粒铵的δ15N值将越来越负（瑞利分馏引起），否则其δ15N值会升高。因此，本研究拟以南昌市为研究区域，分析灰霾期气态氨及分级粒径颗粒铵氮同位素特征，揭示灰霾期气态氨及不同粒径颗粒铵演化过程；同时提取酸性气体浓度、温度和湿度等气象因素、二次无机组分粒径分布特征等信息，利用瑞利分馏原理表征氨的气粒转化过程，阐释其转化机理和影响因素。本项目的研究结果有助于理解我国灰霾污染下氨的气粒转化过程及其在细颗粒气溶胶形成和增长中的作用，为大气环境污染控制提供科学依据。

近二十年来，随着我国人口以及随之匹配的工农业资源需求的迅速增长，氮素被广泛应用于工农业生产的重要环节；生产生活中各个环节的氨释放使得我国成为全球最大的氨排放国家。含氮化合物（氨、氮氧化合物及有机氮等）在大气环境、气候变化、圈层系统互作中扮演着重要角色。由于气态氨可显著促进细颗粒物二次无机组分的增长，氨被认为是我国突出环境问题-雾霾暴发的重要推手。厘清大气氨排放来源及其致霾机制，确定主要源的排放特征并建立氨减排技术，是减缓我国大气复合污染的关键基础。为此，本项目重点研究了（I）我国主要省会城市及4个背景站点日尺度上的细颗粒物铵根污染特征;(II)京津冀城市群和西南城市气态氨来源；（III）南昌市冬季气态氨小时尺度及日尺度的变化特征。研究发现：（I）主要省会城市冬季细颗粒物铵根浓度时空差异显著，具体表现为京津冀>西北城市>长三角>华中城市>成渝>西南城市；铵根浓度及其百分比（在二次无机水溶性离子中的比例）随污染暴发显著增长，然而我国主要省会城市冬季平均的铵根百分比相似，为17%-22%；(II)灰霾发生时期，北京/天津/石家庄城市区的气象条件、细颗粒物组成以及NH4+氮同位素特征较为相似且同步变化；相比于清洁天，灰霾期NH4+氮同位素显著偏正，可能表明灰霾期城市大气氨来源发生变化。同位素混合模型的源解析结果表明，灰霾期城市近地表NH3主要来自于城市内部排放（氨逃逸，排泄物等），农业源对近地表NH3贡献比例极低。灰霾期的静稳条件可能限制了NH3的区域传输，这使得氨逃逸部分的贡献显著提高；（III）南昌市冬季（12月份）大气氨平均浓度为3.8±0.3ppbv，低于北京/上海等城市观测数据；日尺度上，气态氨在11点左右达到高值且与风速和气温相关性良好。此外，由于硝酸根成为城市细颗粒物重要组分，研究也初步探讨了硝酸根的生成机制及氮氧化物来源。本项目的初步研究结果有助于理解我国灰霾污染下无机氮组分的来源及其在细颗粒气溶胶形成和增长中的作用，可为大气环境污染控制提供科学依据。