Formation Mechanism And Aerosol Radiative Effect Of A Severe Haze Episode In The Yangtze River Delta, China

The Yangtze River Delta (YRD) is one of the heaviest haze regions in China, and the haze events that happened in recent years greatly affected the daily life and health of the residents, which drew much attention of the society. In this study, WRF/Chem was used to study a severe haze episode that occurred over YRD in November 2013, and the formation mechanism as well as the aerosol radiation feedbacks were analyzed.(1) Weather pattern was stable in this episode. This episode was characterized by a high PM2.5 concentration (> 400 μg/m3), high relative humidity(> 80%) and low visibility (< 900 m). Weak northwestern or southwestern wind (<3 m/s) prevailed in YRD, which prevented clean air mass from diluting and dissipating pollutants in this region. Regional average results showed that PM2.5 concentration peaks correspondedclosely with a low wind speed and a low planetary boundary layer (PBL) height, and the maximal PM2.5/PM10 ratio of 0.89indicated fine particle dominance.(2) Horizontal dispersion analysis showed that the ventilation coefficient (VC) droppedfrom above 3000 m2/s (clean days) to below 1500 m2/s (polluted days), and the average VC for December for theperiod of 2008-2012 was 2119 m2/s; horizontal transport flux showed central and northwest YRD mainly outputtedpollutants in this episode. Vertically, because of the influence of the PBL and nocturnal inversion, the region of high PM2.5concentration (> 125 μg/m3) 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 abilityplayed a dominant role in the haze formation, and weak vertical dispersion ability, together with high relative humidity,aggravated the pollution.(3)Chemical analysis showed that, compared with PM2.5 in the northwest part of the YRD, PM2.5 inthe central YRD contained a higher proportion of nitrate and a lower proportion of black carbon and organic carbon. Theobserved NO3-/SO42- ratio was 1.54 for this episode, and the monthly average dropped to 1.40 for December 2013. Weconcluded that mobile sources contributed considerably to the episode. Moreover, in winter, higher NO3-/SO42- and nitratebeing the main component (29%) in PM2.5 made central YRD different from the Beijing-Tianjin-Hebei region.(4) During the episode, the downward short wave flux at the ground surface (SWDOWN) of northwestern and central YRD reduced by 18-30W/m2 and 24-30W/m2. The reduction of daytime surface temperature was larger than that of the nighttime surface temperature. PBL height for the whole YRD declined, and the wind speed dropped slightly (<0.15 m/s). Surface relative humidity increased by 1-4%. For chemistry, PM2.5 concentration of the central and northwestern YRD increased by 6-18μg/m3(<15%).The regional average results showed that the maximal reduction of SWDOWN, latent heat and sensible heat due to aerosol radiation feedback were 88 W/m2,12W/m2 and 37 W/m2, respectively. The PBL height, daytime temperature and wind speed dropped by 276 m,1 ℃ and 0.33 m/s, respectively.From the analysis of VC, that value reduced in almost the whole YRD; VC dropped by 8-24% for central and northwestern YRD. The vertical profile of average difference of pollutants showed that PM2.5 and PM10 concentration increased below 950 hPa, withthe maximalincreasement of 7 μg/m3 and 8 μg/m3, and reduced between 950-800 hPa, with the maximal reduction of 3.5 μg/m3 and 4.5 μg/m3, which may be due to the reduction of PBL height. That was opposite for O3, which declined by 1.1 μg/m3 below 960 hPa and increased by 1.7 μg/m3 between 960-770 hPa. This phenomenon of O3 may be due to the weakened downward shortwave radiation caused by heavy loads of aerosol concentration below 950 hPa, which reduced the O3 generation rate, and vice versa for 960-770 hPa.To sum up, the aerosol radiation feedback weakened the dispersion ability of the atmosphere and aggravated the pollution, but the effect was limited (<15%). We conclude that this severe haze event was caused by the large-scale stagnant weather patterns.