Aqueous-phase secondary organic aerosols (aqSOA) constitute a large fraction of SOA, thereby exerting significant influence on air quality, climate, and human health. However, its formation mechanisms remain unclear due to limited observational evidence. We conducted field measurements of particulate matter (PM) composition by deploying high-resolution aerosol mass spectrometry in a suburban environment during autumn in Nanjing, China. The characteristics and formation pathways of aqSOA are comprehensively investigated by using Positive Matrix Factorization (PMF) method. Our results show that aqSOA accounted for 27.6 % of oxidized organic aerosols, exhibiting elevated O:C ratios (0.78) and strong correlations with nitrate and aerosol liquid water (ALW). The important role of acid-catalyzed reactions is also revealed by the enhanced production of aqSOA at lower aerosol pH conditions. Under elevated nitrate and ALW levels, a pronounced morning aqSOA peak was frequently observed; whereas a noon-time aqSOA peak was also observed on several days, likely governed by photochemistry and aqueous-phase reactions. These findings highlight the critical roles of nitrate, ALW, acidity, and photochemistry in driving aqSOA production in polluted urban environments. This study advances the mechanistic understanding of aqSOA formation and provides insights into the mitigation of SOA in Eastern China.