Abstract:The absorption of water by ambient aerosols and the influence of ambient relative humidity on the extinction coefficient are investigated under the assumptions that the atmosphere is in static stability during low visibility process caused by radiative cooling using the continuous observations data (visibility, relative humidity, temperature, wind etc.) from Shanghai Pudong automatic weather stations in 2009. The subtle features of meteorological elements are also studied with such high temporal resolution (1 min) data. The results show that the hygroscopic growth factor f(RH) of the extinction coefficient increased continuously with RH and featured a first slow then quick uptrend. The curves obtained from different seasons show that f(RH) was relatively higher in summer and autumn than that in winter and spring. Overall, the average f(RH) in 7 cases could reach to 6.6 when RH increased from less than 40% to 95%. We also present comparisons with other researches in the world, and the result shows that the behavior of the average of hygroscopic growth factor of the extinction coefficient in the cases is similar to the calculated growth factor of the scattering coefficient for ammonium sulfate. Further study also shows that the decrease of relative humidity is lagging about 1-2 hours behind the increase of temperature and visibility, which is conjectured that the humidity sensor fails to change quickly enough to measure the variation of relative humidity of the local air mass.