AbstractNitrogen (N) is a constituent element of ambient organic aerosol; however, we have limited knowledge of the total quantity of organic N (ON) and its relative importance to aerosol N due to a lack of effective analytical methods for ON. Here, we report first online measurements of ON in particulate matter (PM_2.5) in urban Shanghai in summer of 2021 using a newly developed aerosol nitrogen analyzer. ON accounted for an average of 25% of total aerosol N and varied in the range of 5%–64% on the hourly scale. Hourly variations identified four distinct types of ON episodes that occurred in different hours of a day. Analysis aided by concurrently measured source indicative pollutants revealed that vehicular emissions, nocturnal chemistry, daytime photochemistry or combined primary sources, and secondary chemistry could drive up ON under different day‐to‐day atmospheric conditions. Our results help bridge the knowledge gap in the emissions and formation of organic aerosol.