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Black carbon (BC) emitted from incomplete combustion can result in\udsignificant impacts on air quality and climate. Understanding the mixing\udstate of ambient BC and the chemical characteristics of its associated\udcoatings is particularly important to evaluate BC fate and environmental\udimpacts. In this study, we investigate the formation of organic coatings on\udBC particles in an urban environment (Fontana, California) under hot and dry\udconditions using a soot-particle aerosol mass spectrometer (SP-AMS). The\udSP-AMS was operated in a configuration that can exclusively detect refractory BC (rBC)\udparticles and their coatings. Using the −log(NOx ∕ NOy)\udratio as a proxy for photochemical age of air masses, substantial formation\udof secondary organic aerosol (SOA) coatings on rBC particles was observed\uddue to active photochemistry in the afternoon, whereas primary organic\udaerosol (POA) components were strongly associated with rBC from fresh\udvehicular emissions in the morning rush hours. There is also evidence that\udcooking-related organic aerosols were externally mixed from rBC. Positive\udmatrix factorization and elemental analysis illustrate that most of the\udobserved SOA coatings were freshly formed, providing an opportunity to\udexamine SOA coating formation on rBCs near vehicular emissions. Approximately\ud7–20 wt % of secondary organic and inorganic species were estimated to be\udinternally mixed with rBC on average, implying that rBC is unlikely the\udmajor condensation sink of SOA in this study. Comparison of our results to\uda co-located standard high-resolution time-of-flight aerosol mass\udspectrometer (HR-ToF-AMS) measurement suggests that at least a portion of\udSOA materials condensed on rBC surfaces were chemically different from\udoxygenated organic aerosol (OOA) particles that were externally mixed with\udrBC, although they could both be generated from local photochemistry

Formation of secondary organic aerosol coating on black carbon particles near vehicular emissions