Simultaneous measurements of urban and rural particles in Beijing - Part 2: Case studies of haze events and regional transport

Two parallel field studies were conducted simultaneously at both urban and rural sites in Beijing from 1 to 29 November 2016. An online single-particle chemical composition analysis was used as a tracer system to investigate the impact of heating activities and the formation of haze events. Central heating elevated EC-Nit (elemental carbon-nitrate), EC-Nit-Sul (EC-nitrate-sulfate), and ECOC-Nit (ECOC: internal-mixed elemental carbon and organic carbon) levels by 1.5-2.0 times due to the increased use of coal in the urban areas. However, in the rural areas, residential heating, which mainly consumes low-quality coal, and biomass burning elevated ECOC-Nit-Sul, NaK-Nit, and OC-Sul levels by 1.2-1.5 times. Four severe haze events (hourly PM2.5 > 200 mu g m(-3)) occurred at both sites during the studies. In each event, a pattern of transport and accumulation was found. In the first stage of the pattern, particles were regionally transported from the south and southwest and accumulated under air stagnation, creating significant secondary formation, then PM2.5 was elevated to 300 mu g m(-3). At both sites, the severe haze occurred due to different patterns of local emission, transport, and secondary processes. At Pinggu (PG), the sulfate-rich residential coal burning particles were dominant. The regional transport between PG and Peking University (PKU) was simulated using the Weather Research and Forecasting HYbrid Single-Particle Lagrangian Integrated Trajectory (WRF-HYSPLIT) model, confirming that the transport from PG to PKU was significant, but PKU to PG occurred occasionally. These cases can explain the serious air pollution in the urban areas of Beijing and the interaction between urban and rural areas. This study can provide references for enhancing our understanding of haze formation in Beijing.

2016年11月1日至29日，在北京的城市和农村地区同时进行了两项平行的实地研究。利用在线单颗粒化学成分分析作为示踪系统，研究供暖活动的影响以及霾事件的形成。由于城市地区煤炭使用量增加，集中供暖使EC - Nit（元素碳 - 硝酸盐）、EC - Nit - Sul（元素碳 - 硝酸盐 - 硫酸盐）和ECOC - Nit（ECOC：内部混合的元素碳和有机碳）水平升高了1.5 - 2.0倍。然而，在农村地区，主要消耗劣质煤的居民供暖以及生物质燃烧使ECOC - Nit - Sul、NaK - Nit和OC - Sul水平升高了1.2 - 1.5倍。在研究期间，两个地点都发生了四次严重的霾事件（每小时PM2.5 > 200μg/m³）。在每次事件中，都发现了一种传输和累积模式。在该模式的第一阶段，颗粒从南部和西南部区域传输，并在空气停滞的情况下累积，产生显著的二次生成，然后PM2.5升高到300μg/m³。在两个地点，严重的霾是由于本地排放、传输和二次过程的不同模式而产生的。在平谷（PG），富含硫酸盐的居民燃煤颗粒占主导地位。利用天气研究与预报混合单粒子拉格朗日综合轨迹（WRF - HYSPLIT）模型模拟了PG和北京大学（PKU）之间的区域传输，证实了从PG到PKU的传输是显著的，但从PKU到PG的传输偶尔发生。这些案例可以解释北京城市地区严重的空气污染以及城乡之间的相互作用。这项研究可为增进我们对北京霾形成的理解提供参考。