成都典型霾污染中高浓度PM2.5硝酸盐的成因

Regional haze pollution events occurred frequently over China in recent years. It is urgent to gain in-depth knowledge about the key mechanisms of their formation and evolution in order to adopt effective measures to control air pollution, especially for certain important regions. Among these regions is the Sichuan Basin, due to its complex terrain, special meteorological conditions, and shifting major emission sources and urbanization processes. Chengdu, one of the two mega cities located on the western edge of the basin, thus probably has special mechanisms on its haze characterization (e.g. rapid-growth and high-concentration nitrate in PM2.5). This proposal will focus on the characterization and formation mechanism of typical hazes in Chengdu, based on long-term monitoring and short-term observation campaign conducted at its urban and rural sites simultaneously. There are two aims to achieve for this study: 1) to sum up typical types of haze pollutions and to extract their outbreak characteristics and key chemical speciation of PM2.5, and to form typical haze case base with high-concentration nitrate in PM2.5, by combining continuous monitoring of regular air pollutants in its surveillance net and a super station in downtown Chengdu, and enhanced short-term campaign. 2) To set up the response relationship between secondary inorganic aerosol (such as sulfate, nitrate) and their precursors during the formation and disappearing processes of regional haze events, and extract the key parameters for chemical speciation of PM2.5 indicating the typical pollution process; to further investigate the characteristics and mechanism of nitrate accumulation under high humidity and stagnant condition by combining a mesoscale weather and chemistry transport numerical model, i.e. WRF-CHEM; and thus to clarify the effect of meteorology and the underlying mechanism involved in typical haze outbreak and development.

在近年来我国区域霾污染频发的背景下，针对特定区域典型霾污染的形成、发展的气象影响及关键机理研究严重不足。由于四川盆地污染源排放与地形地势复杂且局地气象与区域气候独特，其特大城市成都的典型霾污染类型特征（如高浓度PM2.5硝酸盐）与成因可能具有独特性和复杂性。有鉴于此，本项目拟结合在成都城区和远郊的长期连续和短期加强观测、模型模拟等对典型霾污染成因进行解析：1）结合站网和超级站长期监测和化学物种采样/分析的强化观测，归纳典型霾污染类型、爆发特征及关键物种，形成高浓度硝酸盐霾污染案例库。2）通过对典型霾污染发生、发展的综合分析，建立PM2.5中二次无机物种硫酸盐、硝酸盐与气态前体物之间的响应关系、演变规律并提取特征物种指标；借助中尺度气象-化学传输数值模式（如WRF-CHEM）模拟等，明晰静稳、高湿条件下PM2.5硝酸盐快速增长、高浓度保持的特征及气象影响，并进一步探究其生成、积累的内在机制。

2013年“大气十条”实施以来，成都市空气质量持续改善，但PM2.5和O3依然超标，并与NO2在全国重点城市中排名靠后，显著落后于四川盆地另一特大城市重庆。本项目基于在成都城区和郊区的PM2.5化学物种采样和全组分分析并结合高分辨率观测等，明晰了PM2.5的化学组成特征；利用改进的消光算法解析了PM2.5的化学消光贡献，明确了霾污染关键物种；通过对典型霾污染生消过程的分析，探究了硝酸盐快速增长、高浓度保持的特征及其生成机制。2018年夏-2019年春，成都城、郊PM2.5的主要成分均为有机物（OM）和硝酸盐，其中硝酸盐在春季复合污染时期的消光贡献更大；城区的二次污染比郊区严重，SO2在城区更容易转化为硫酸盐，NOx则在郊区更容易转化为硝酸盐。无论城区或郊区，PM2.5的主要来源均为二次源、燃烧源和机动车排放源。.基于成都城区冬季高分辨率PM2.5化学组分在线监测数据，发现PM2.5中NO3-的占比和NOR值均随着其浓度的增加而增加，突出了硝酸盐的生成在冬季霾污染中的重要作用。与四川盆地内另一特大城市重庆相比，硝酸盐对成都PM2.5重污染过程的促发和加剧作用更为突出。多相反应途径和富铵环境加速了NOx转化为硝酸盐。在夜间的高相对湿度条件下，N2O5的非均相水解以及硫酸盐水溶液对气相HNO3和NH3的吸收作用均显著增强，成为硝酸盐的主要生成途径；气相氧化（如白天的NO2+∙OH或夜间的NO3∙+碳氢化合物）是低相对湿度条件下主要的生成途径；此外，可能存在其他非均相路径导致硝酸盐的生成，如NO2的直接摄取和水解可产生HNO3和HONO；非均相反应对白天的硝酸盐生成也有重要的作用。鉴于SO42-、NO3-和NH4+之间互相影响，并与其前体物以及大气中的水汽构成一个复杂的二次无机颗粒物体系，协同减排NOx、SO2和NH3可有效改善成都市空气质量和能见度水平，并可能是消除重污染天的关键。