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。

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.