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Inorganic and organic coatings on black carbon (BC) particles can enhance light absorption and affect atmospheric lifetimes of BC-containing particles and thus have significant implications for climate. To study the physical and chemical characteristics of atmospheric BC and BC-associated coatings, a soot particle-aerosol mass spectrometer was deployed during the winter of 2014-2015 in Fresno, a city located in the San Joaquin Valley of California, to selectively analyze BC-containing particles. Comparing soot particle-aerosol mass spectrometer measurements to those from the collocated single-particle soot photometer (SP2) and high-resolution aerosol mass spectrometer, we found that 17% of total submicrometer aerosol mass was associated with BC-containing particles, suggesting that a majority of the fine particles in Fresno contained no BC. Most BC-containing particles appeared to be associated with residential wood burning and vehicular traffic. These particles typically had a bulk-average mass ratio of coating to BC (R-coat/rBC) less than 2. However, during periods of persistent fog larger R-coat/rBC values were observed, with the coatings primarily composed of secondary inorganic and organic components that likely resulted from aqueous-phase processing. Specifically, compared to periods with less fog, the BC coating increased in concentration and contained a larger fraction of nitrate and oxidized organic matter. The size distributions of BC and associated organic coating were generally centered around 300nm in vacuum aerodynamic diameter. However, during foggy periods BC had an additional peak at 400nm and organics and nitrate displayed a prominent mode in the accumulation size range.

Plain Language Summary Various combustion processes can emit soot, or black carbon (BC), particles with properties highly relevant to climate change and respiratory health. Often, BC particles are coated with other materials derived from the same combustion source or added in the atmosphere as these particles age. This coating material can have profound effects on the behavior, properties, and longevity of BC. In this study we deployed an instrument called a soot particle-aerosol mass spectrometer to selectively analyze particles containing BC and investigate their coating, concentration, and composition in real time in Fresno, CA, in wintertime. The properties of BC particles were found to be mainly controlled by emission sources but were also significantly influenced by humid meteorological conditions.