大气氮氧化物-硝酸-硝酸盐转化体系中稳定氮同位素分馏机制研究

Quantitative analysis of NOx sources is one of the most urgent problems in air pollution control in China. Nitrate-based stable nitrogen isotope technique (δ15N-NO3−) is an effective method to solve this problem. However, due to the effect of isotope fractionation in the processes of atmospheric NOx conversion into NO3−, there is some uncertainty in the analytical results. In view of this problem, the stable nitrogen isotope fractionation mechanism in the atmospheric NOx-HNO3-NO3− conversion system will be studied scientifically in the urban area with serious NOx pollution (taking Yantai City as an example) in this project. Specific elements include measuring the concentrations and δ15N of NOx, HNO3 and NO3− in different atmospheric conditions, such as high temperature, low temperature, and haze, and analyzing the influence factors; constructing the atmospheric NOx-HNO3-NO3− fractionation system and quantifying the fractionation effects of different conversion processes in the system; in addition, establishing the fractionation characterization system based on air quality data O3, SO2 and meteorological parameters T, RH to further improve the practicability of building fractionation system. The purpose of this project is to scientifically quantify isotope fractionation effects in the atmospheric NOx-HNO3-NO3− conversion systems and to provide theoretical and technical support for accurate tracing of atmospheric NOx.

定量解析氮氧化物来源是目前我国空气污染治理亟需解决的难题之一。基于硝酸盐的稳定氮同位素技术是解决该难题的有效手段，然而受大气氮氧化物转化硝酸盐过程中同位素分馏效应的影响，解析结果存在一定的不确定性。本项目针对此问题，以氮氧化物污染严重的城市区域为靶区（烟台市为例），对大气氮氧化物-硝酸-硝酸盐转化体系中稳定氮同位素分馏机制进行科学研究。具体内容包括：实测高温、低温、灰霾等不同大气状态下氮氧化物、硝酸、硝酸盐浓度及氮同位素值，摸清其影响因素；基于此，构建大气氮氧化物-硝酸-硝酸盐分馏体系，量化体系中不同转化过程的分馏效应；此外，为进一步增强构建分馏体系的实用性，建立以空气质量数据臭氧、二氧化硫与气象参数温度、湿度为环境因子的分馏表征体系。本项目旨在科学量化大气氮氧化物-硝酸-硝酸盐转化体系中同位素分馏效应，为同位素手段在大气氮氧化物的精准溯源提供理论和实用性技术支撑。

稳定氮同位素技术是解析硝酸盐来源的强有力工具，可以有效助力于解决我国日益严重的大气硝酸盐污染问题。大气氮氧化物转化为硝酸盐过程中同位素分馏效应对利用稳定氮同位素技术解析硝酸盐结果的不确定性至关重要。为此，本研究在典型城市区域对不同大气状态下氮氧化物-硝酸-硝酸盐转化体系展开实测研究。研究结果表明大气氮氧化物-硝酸-硝酸盐转化中确实存在氮同位素分馏，且随着转化的进行，氮同位素分馏程度不断增大；在该体系中各组分摩尔占比与氮同位素分馏效应共同受环境因子影响，特别是在冬季，存在显著的相关性。本研究基于大气硝酸盐形成过程及同位素分馏经验公式，建立了大气氮氧化物-硝酸-硝酸盐分馏体系方程；基于实测的各组分摩尔占比与氮同位素特征对分馏体系进行不断验证、优化，结果显示优化后的体系可有效估算大气硝酸盐形成过程中的氮同位素分馏效应。此外，本研究对冬季各组分摩尔占比与环境因子进行多元线性回归分析，建立了基于环境因子的各组分摩尔占比表达方程，最终实现了利用环境因子量化同位素分馏效应的目的。总之，本研究对大气氮氧化物-硝酸-硝酸盐转化体系中的氮同位素分馏机制进行了深入研究，相关认识可进一步降低利用氮同位素技术对大气硝酸盐溯源时的不确定性。