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The secondary aerosol formation mechanism in the presence of ammonia (NH 3), is poorly understood, especially under high relative humidity (RH) conditions. In this study, a total of seven experiments were conducted from toluene/NOx photo-oxidation in the presence/absence of NH 3 under dry (~7% RH) and wet (>60% RH) conditions in a ~3 m3 smog chamber. A series of instruments including gas analysers, scanning mobility particle sizer (SMPS), aerosol mass spectrometry (HR-ToF-AMS) etc. were applied to measure the NOx and O 3 concentrations, the mass concentration and chemical composition of secondary aerosol. It was found that NH 3 could enhance the mass loading of secondary aerosol, especially under wet condition. However, the presence of NH 3 or increasing RH did not have a significant influence on SOA yield. The organic aerosol mass spectrum from AMS showed that the most abundant fragment was at m / z = 44, which was mainly from the fragmentation of carboxylic acids. Compared to the absence of NH 3 , the fraction of fragment at m / z = 44 and O:C was higher in the presence of NH 3 , regardless of dry or wet conditions. The highest O:C value of 0.71–0.75 was observed in the presence of NH 3 under wet condition, suggesting there could be a synergetic effect between the high RH and the presence of NH 3 , which jointly contributed to the photochemical aging process of SOA. The N:C increased in the presence of NH 3 under both dry and wet conditions, which might be attributed to the carboxylates and organic nitrates formed from the reaction between NH 3 and carboxylic acids. The results implied that SOA modelling should consider the role of NH 3 and water vapour, which might fill the gap of O:C between laboratory studies and field measurements. Unlabelled Image • NH 3 significantly enhanced SOA formation from toluene/NOx photo-oxidation under high RH condition. • High RH and NH 3 could jointly accelerate the photochemical aging process of SOA. • Both the addition and replacement of carboxylic functional group contributed to the formation of carboxylic acids. • NH 3 could react with the carboxylic acids formed from toluene, generating carboxylates and other organic nitrates. [ABSTRACT FROM AUTHOR]