The accurate and precise mass spectrometric measurement of organic compounds in atmospheric aerosol particles is a challenging task that requires analytical developments and adaptations of existing techniques for the atmospheric application. Here we describe the development and characterization of an atmospheric pressure chemical ionization Orbitrap mass spectrometer (APCI-Orbitrap-MS) for the measurement of organic aerosol in real time. APCI is a well-known ionization technique, featuring minimal fragmentation and matrix dependencies, and allows rapid alternation between the positive and negative ionization mode. As a proof of principle, we report ambient organic aerosol composition in real-time, with alternating ionization, high mass resolution ( R = 140 000) and accuracy (<2 ppm). The instrument was calibrated in the negative ion mode using 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) model aerosol. We obtain a detection limit of 1.3 ng/m3. Based on the performed calibration using MBTCA particles, the ambient concentration of MBTCA in the particle phase measured in an urban area in Mainz, Germany, ranged between 10 and 80 ng/m3. For the first time, we apply a nontarget screening approach on real-time data, showing molecular variability between ambient day- and nighttime aerosol composition. The detected compounds were grouped in the night- and daytime and analyzed by ultrahigh-resolution MS (UHRMS) visualization methods. Among several prevalent biogenic secondary organic aerosol (BSOA) markers, 24 organic mononitrates and one organic dinitrate were detected. We further estimate that, on average, organic nitrates contribute to 5% and 14% of the measured particulate organic aerosol at day and night, respectively.