Abstract Relative humidity (RH) is one of the most important parameters in the study of fog-haze. This paper first estimates the contributions of the key quantities (temperature, water vapor, air pollutant) and their combinations to the relative change of RH, and then investigates the relationships of RH with the long-term variation of haze and fog days based on the meteorological data in the Yangtze River Delta over the period 1970–2010. The main conclusions are as follows. (1) Temperature is the foremost factor influencing RH, with the effect of specific humidity and direct contribution of air pollutant being second and third. (2) RH shows a prominent descending trend in the Yangtze River Delta, due to global warming and the ‘analogous heat island effect’ (AHIE). (3) Decreasing RH was responsible for the reduction of fog days. The AHIE can explain the phenomenon in which fog days in metropolises are less than in small cities. (4) Granger causality analysis of inter-annual variation further shows that increasing aerosol loading Ganger-causes the increasing haze days, rather than meteorological parameters such as RH. High RH would enhance hygroscopic growth of particle and then suppress the planet boundary layer height (PBLH) and lead to more haze days, lower PBLH further increases aerosols and RH, this positive feedback mechanism can be established under the condition of aerosol loading being relatively stable and beyond the threshold for haze formation. (5) If aerosol emissions maintain the status quo, climate cooling would result in serious fog-haze events occurring more frequently in Yangtze River Delta. The research provide a scientific basis for understanding the influence of meteorological factors on RH and the connection between the variation of long term fog-haze days and RH under the background of climate change.

中文翻译：

---

长三角地区相对湿度影响因素及其与雾霾事件长期变化的关系

摘要 相对湿度（RH）是雾霾研究中最重要的参数之一。本文首先估算了关键量（温度、水汽、空气污染物）及其组合对相对湿度相对变化的贡献，然后基于1970-2010年长三角地区气象数据. 主要结论如下。(1) 温度是影响相对湿度的首要因素，其次是比湿度的影响和空气污染物的直接贡献。(2) 由于全球变暖和“类似热岛效应”(AHIE)，RH 在长三角地区呈显着下降趋势。(3) 降低相对湿度是雾日减少的原因。AHIE 可以解释大都市雾天比小城市少的现象。(4) 年际变化的格兰杰因果关系分析进一步表明，气溶胶负荷的增加导致雾霾日数增加，而不是相对湿度等气象参数。高RH会增强粒子的吸湿生长，进而抑制行星边界层高度（PBLH）并导致更多雾日，低PBLH进一步增加气溶胶和RH，这种正反馈机制可以在气溶胶负载相对稳定的条件下建立并超过雾霾形成的阈值。(5) 如果气溶胶排放保持现状，气候变冷将导致长三角地区更频繁地发生严重的雾霾事件。