大气/植被界面氨气交换通量及其对氮沉降总量的贡献

Rapid growth of reactive nitrogen (Nr) emissions in China has caused serious air pollution and led to excess Nr deposition, with growing concern regarding the effects on receiving natural ecosystems. Quantifying the Nr deposition flux, including spatial and temporal patterns, is important to assess its environmental impacts and regulation strategies. Most of current measurements in China are based on the amount of Nr sampled by open rain collectors that appear to have large underestimation of total Nr deposition, since the dry deposition of Nr is often omitted from observation-based flux studies. This project proposes to use field observations from current monitoring network (10 sites) operated by ourselves, combined with chemistry analysis and a chemical transport model to better quantify the ammonia (NH3) exchange between atmosphere and vegetation in Northern China. The work will apply a vacuum infiltration technique to extract the apoplastic solution of the leaves. Then H+ and NH4+ concentrations in the apoplast will be determined to obtain gaseous NH3 concentration inside the leaves, i.e. the so-called stomatal compensation point. This parameter is used to understand the regulation of plant-atmosphere NH3 exchange, which will be considered in the dry deposition calculation. Observations of NH3 fluxes will be used to constrain the calculated gaseous dry deposition amount. It will complement the previous study by the additional estimation of the atmospheric deposition flux of Nr via gases and particles (dry deposition). The project will also examine the magnitude and the importance of dry deposition, compared with precipitation (wet deposition). The project is expected to significantly advance our current understanding of Nr deposition impacts in China, through methodology innovations.

我国快速增长的活性氮排放不仅影响空气质量还造成了过量氮沉降，严重威胁生态系统健康。准确认识大气氮沉降通量及其时空变化对于评估其环境危害并控制排放具有重要的科学意义和迫切现实需求。当前我国氮沉降观测以降水为主，缺乏对干沉降通量的系统性研究，因而显著低估了氮沉降总量。本申请将以京津冀地区大气干湿沉降研究网络为基础，综合运用野外观测、实验室研究和数值模拟等手段，研究氨气在大气/植被（森林和农作物）界面的交换通量。本项目拟应用真空渗滤技术分离植物叶片质外体溶液，通过测定其中H+和NH4+含量计算植物叶子中NH3浓度，确定叶孔NH3饱和点。该参数将应用于大气NH3干沉降通量模拟，并使用实测大气NH3浓度和通量数据进行验证。本项目旨在完善大气氮以颗粒物和气体干沉降途径输入到地表环境的通量测算体系，进而揭示干沉降在大气氮沉降总量中的贡献。本项目的实施将为深入理解我国氮沉降的影响提供理论和方法参考。

作为非约束性污染物且来源复杂，氨气排放一直未得到有效控制，是氮沉降的主要成分。我国气体和颗粒物干沉降观测资料较少，尤其缺乏大气/植被界面氨气交换过程研究。氨气交换过程的方向取决于叶片中氨气饱和点，本项目发现农田（玉米）和森林（杨树）叶片饱和点具有明显日变化。为了精细化表征这一关键参数，建立了真空渗滤技术分离植物叶片质外体溶液-通过测定其中氢离子和铵盐含量计算叶片氨气浓度-进而确定叶孔氨气饱和点的实验方法。通过2年的外场观测，研究了大气/植被界面氨气交换过程，发现传统的阻力模式高估了农田生长季氨气干沉降通量81.4%。考虑氨气双向交换后重新估算的华北农田氮沉降总量为44.3 kg N ha-1 yr-1，比传统估算方法减少了27%，说明氮沉降的精细化评估需要考虑这一重要过程。研究还发现了氮沉降的一些新现象，包括：1）颗粒物铵盐随着霾污染的加重，存在明显的粒径转移现象，峰值浓度从清洁天0.43-0.65 μm转移到轻度污染的0.65-1.1 μm和重度污染的1.1-2.1 μm，颗粒物铵盐由细颗粒物向粗颗粒物的转移必然增加铵盐的干沉降；2）2016年7月19-21日华北发生了历史罕见的极端暴雨，期间观测到的铵盐和硝酸盐湿沉降通量分别占年总量的21.8%和3.8%，极端暴雨降水事件对铵盐湿沉降的贡献显著高于硝酸盐，需要在氮沉降效应评估中予以重视；3）传统的氮沉降被动采样在收集降水同时仅能捕获部分气溶胶和气态污染物（如NH3、HNO3）干沉降，将显著低估霾污染背景下的氮沉降总量约40%。基于上述新认识，以华北大气干湿沉降研究网络为基础，综合运用野外观测、化学分析和数值模拟等手段，本项目进一步完善了大气氮以降水、颗粒物和气体干沉降途径输入到地表环境的通量测算体系。集成的研究方法从华北地区推广到全国尺度，量化了氨气干沉降的时空分布格局，发现华北是我国氨气浓度和沉降量最高的区域，而城市地区氨气来源与非农业源有直接联系（基于氮稳定同位素测量结果）。本项目的实施将为深入理解我国氮沉降的变化规律和生态环境影响提供理论和方法参考。