Secondary organic aerosol (SOA) formation in coastal regions, where a substantial world population resides, remains poorly understood, hindering estimation of its effects on air quality and human health. Here, SOA formation from coastal air oxidation was investigated using a field-deployed oxidation flow reactor at Tanxu Island in the Yangtze River Delta, China. High-humidity conditions with relative humidity (RH) >90% resulted in maximum SOA production of 3.7 μg m^-3, more than three times that under RH <90% (1.2 μg m^-3). This discrepancy could be attributed to an enhanced contribution of aqueous-phase processing, together with the supply of anthropogenic precursors from continental outflow. In turn, when humid coastal air was transported to nearby cities such as Shanghai, SOA production could increase, as indicated by the organic aerosol loading effect. Closure analysis indicated that the gas-phase OH oxidation of measured gaseous organics explained 57% of the measured SOA production at equivalent photochemical ages <3 days and RH <80%, with the overlooked contributions from oxygenated aromatics (18%) and furans (6%) highlighted. By isolating air mass origins, we found that SOA production with continental air masses was more than three times that in marine air masses at RH <90%; the unexplained fraction for SOA production in continental air masses (49%) was also higher than that in marine air masses (35%), suggesting unidentified anthropogenic precursors such as volatile chemical products. These results provide quantitative constraints on SOA formation in coastal air, with implications for understanding SOA pollution in downwind coastal cities.