A haze episode occurred on 12 September 2009 in Shanghai, when the mass ratio of PM1.0/PM2.5 (PM: particulate matter) reached 0.8. A similar variation of hygroscopic growth factor distribution was observed for Aitken mode particles and accumulation mode particles, implying that the enhancement of fine particles was caused by local atmospheric processing. The hygroscopicity measurements in combination with chemical analysis provided strong support for the significant contribution of (NH4)2SO4 and NH4NO3 to the haze episode. The molar ratio of [NH4 + ]/([NO3 − ] + 2[SO42 − ]) rose up to 0.96, coincident with the large increase in NH3 concentration, suggesting that the available NH3 played a vital role in the enhancement of particulate sulfate and nitrate during the haze episode.