Composition and Evolution of Reactive Organic Carbon in Urban Air

Reactive organic carbon (ROC) is the sum of both gas- and particle-phase organic compounds excluding methane, serving as the fuel for atmospheric oxidation processes. Comprehensive characterization of organic mixtures, however, has been a long-standing challenge. Here, we investigate the speciation, properties, and evolution of ROC in a holistic view based on comprehensive field observations using four advanced mass spectrometers at an urban site in Guangzhou, a megacity in southern China. The summed concentration of over 1000 organic species detected in the gas phase averaged 124.7 μgC/m³, with oxygenated organics accounting for the largest fraction (44% by carbon mass, the same below), whereas the concentration of organic aerosol was 8.1 μgC/m³ on average. The observed ROC was dominated by volatile species (84%), while semi- and intermediate-volatile species that were not routinely measured contributed 10%. C₁-C₈ compounds constituted the major fraction of ROC (85%), most of which were long-lived oxidation products, along with anthropogenic alkanes and aromatics. Over 16 h of photochemical aging, the observed ROC mass decreased by 14%, which was much lower than in previous attempts in urban air, highlighting a better carbon closure with the advancement of mass spectrometry techniques. Our work provides insights into the evolution of the speciation and properties of ROC during oxidation processing.