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Ammonia (NH 3) is an important base gas in the atmosphere and has received extensive research interest due to its role on secondary aerosol formation. However, the effects of NH 3 on the photochemical smog oxidants formation including ozone and secondary aerosol did not attract much attention. In this study, a set of smog chamber experiments was conducted under toluene/NOx photo-oxidation base condition and adding NH 3 in the following photochemical regimes referred as hydrocarbon, NOx-limited and transitional regimes. Dedicated instruments including gas analysers, scanning mobility particle sizer (SMPS), aerosol mass spectrometry (HR-ToF-AMS) were used to measure the gaseous compounds, aerosol mass concentration and chemical composition. It was found that the presence of NH 3 did not affect the O 3 formation rate. However, the particle number concentration increased dramatically after NH 3 was injected in any photochemical regimes. This effect has been mainly attributed to the rapid formation of organic ammonium and ammonium nitrate. The mass spectra of organic aerosols from AMS showed that the most abundant fragments were at m/z 28, 29, 43 and 44, which became higher after the addition of NH 3 in the transition and NOx-limited regimes, indicating that NH 3 could enhance the formation of the compounds containing carbonyl and carboxylic acid functional groups. Compared to hydrocarbon-limited regime, higher atomic nitrogen-to-carbon (N:C) ratios were observed when NH 3 were injected into transition and NOx-limited regimes, and this is mainly due to more organic nitrate formation. Four factors, which were assigned to inorganic nitrate aerosols, carboxylic acid compounds, carbonyl compounds and N-containing organic compounds (NOC), were identified and interpreted by applying the positive matrix factorization (PMF) to the AMS dataset. These results provide new insights into the complex chemistry of toluene/NOx/NH 3 during different O 3 formation regimes, and imply the benefit of controlling NH 3 to mitigate PM 2.5 when O 3 formation is under VOC-limited regime. [Display omitted] • How NH 3 affects particle formation in different photochemical regimes was investigated. • NH 3 does not affect O 3 formation but affect particle formation and composition. • Higher O:C and N:C were observed after NH 3 injection in NOx-limited regime. • Four factors contributing to the secondary aerosols were identified by PMF analysis. [ABSTRACT FROM AUTHOR]