Abstract: Microdroplets have many unique features derived from the large surface-volume ratio and the ultrahigh electric field at the air-water interface, therefore they can remarkably accelerate various slow reactions and trigger reactions that won’t occur in bulk solutions. The size of atmospheric aerosols is similar to that of the microdroplets, so reactions performed in microdroplets and ambient conditions can act as good simulations of important atmospheric processes, which will provide a better understanding of the role of aerosols. Aldehydes are important oxidized species in the atmosphere derived from releases of human activities, such as vehicle emissions and biomass burning, which can generate volatile organic compounds (VOCs) and are highly related to saltrich aerosol formation through further oxidation. Studying of aldehydes’ hydration and oxidation reactions is very helpful for understanding the complex conversion process from VOCs to secondary organic aerosols (SOA). In this work, several aromatic aldehydes with different electron donating or withdrawing groups were studied by spraying their solutions into microdroplets, then directly characterizing them by online mass spectrometry. It showed that these aromatic aldehydes could be oxidized into carboxylic acids under the space-restricting condition of microdroplets, and the hydration reactions of aromatic aldehydes could be induced by carboxylate under the extreme pH environment and abundant reactive oxygen species provided by the air-water interface of microdroplets. Together, the products of oxidation and hydration reactions would combine to yield relatively unstable “carboxylate-aldehyde hydrate” complex ions. The lifetime of the ions was roughly estimated by conducting a collision-induced disassociation (CID) experiment and comparing it to the instrument running procedure, and the structure was checked using CID and a cross-forming reaction between two different aldehydes. This work revealed that the complex ions were potential intermediates of the atmospheric aldehyde oxidation reaction, providing a previously unknown pathway of atmospheric carboxylic acids and SOA. This result supported the view that atmospheric oxidation might take place in aerosols, such as fog and clouds, which was the missing part of SOA sources and matched the observations. However, unlike other liquid phase or gaseous simulation research of the oxidation process of aldehydes, the oxidation reaction would occur even without the participation of UVs or hydrogen peroxide addition in microdroplets, because the air-water interface could generate a large number of reactive oxygen species (ROS) like hydroxyl radicals and hydrogen peroxide, implicating that the air-water interface of aerosols might be an important place to perform many atmospheric reactions.