Abstract
WRF-Chem was used to study a severe haze episode that occurred over the Yangtze River Delta (YRD), China, in November 2013. This episode was characterized by a high PM2.5 concentration (＞ 400 μg m^(-3)), high relative humidity (＞ 80%) and low visibility (＜ 900 m). Regional average results showed that PM_(2.5) concentration peaks corresponded closely with a low wind speed and a low planetary boundary layer (PBL) height, and the maximal PM_(2.5)/PM_(10) ratio of 0.89 indicated fine particle dominance. Horizontal dispersion analysis showed that the ventilation coefficient (VC) dropped from above 3000 m^2 s^(-1) (clean days) to below 1500 m^2 s^(-1) (polluted days), and the average VC for December for the period of 2008-2012 was 2119 m^2 s^(-1); horizontal transport flux showed central and northwest YRD mainly outputted pollutants in this episode. Vertically, because of the influence of the PBL and nocturnal inversion, the region of high PM_(2.5) concentration (＞ 125 μg m^(-3)) extended to 1 km height during daytime, but was confined to below 200 m at night. However, near-surface inversion was observed even on clean days. Therefore, we concluded that poor horizontal dispersion ability played a dominant role in the haze formation, and weak vertical dispersion ability, together with high relative humidity, aggravated the pollution. Chemical analysis showed that, compared with PM_(2.5) in the northwest part of the YRD, PM_(2.5) in the central YRD contained a higher proportion of nitrate and a lower proportion of black carbon and organic carbon. The observed NO_3^-/SO_4^(2-) ratio was 1.54 for this episode, and the monthly average dropped to 1.40 for December 2013. We concluded that mobile sources contributed considerably to the episode. Moreover, in winter, higher NO_3^-/SO_4^(2-) and nitrate being the main component (29%) in PM_(2.5) made central YRD different from the Beijing-Tianjin-Hebei region.
Highlights
The Yangtze River Delta (YRD), located on the eastern coast of China, has an area of 99,600 km2 and a population of 108.6 million
An onlinecoupled meteorology and chemistry model—the Weather Research and Forecasting (WRF) model with chemistry (WRF-Chem)—was used to simulate a heavy haze episode that occurred in the YRD from November 28 to December 11, 2013
The wind speed directly affects the horizontal dispersion ability of the atmosphere, and it influences the evolution of the planetary boundary layer height (PBLH), thereby indirectly influencing the vertical aspect
Summary
The Yangtze River Delta (YRD), located on the eastern coast of China, has an area of 99,600 km and a population of 108.6 million. It includes the Shanghai municipality, eight cities of Jiangsu Province, and seven cities of Zhejiang Province and has become one of the fastest growing economic regions in China in recent decades. YRD has become one of the four heaviest haze regions in China, with the other three being the Pearl River Delta (PRD), Beijing–Tianjin–Hebei (BTH) region, and Chongqing. Among the studies on haze in the YRD, the study of Fu et al (2008) analyzed a haze episode of 2007 based on observation and found that stagnant dispersion conditions played a predominant role in the haze formation.
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