10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Influences of Primary Emission and Secondary Coating Formation on the Mixing State of Black Carbon-containing Particles
ALEX LEE, Chia-Li Chen, Jun Liu, Derek Price, Raghu Betha, Lynn Russell, Xiaolu Zhang, Christopher Cappa, National University of Singapore
Abstract Number: 284 Working Group: Carbonaceous Aerosol
Abstract Black carbon (BC) emitted from incomplete combustion can result in significant impacts on air quality and climate. Understanding the mixing state of ambient BC and the chemical characteristics of its associated coatings is particularly important to evaluate BC fate and environmental impacts. In this study, we investigate formation of coating on BC particles and aerosol mixing state near traffic emissions under hot and dry conditions in Fontana, California using an Aerodyne soot-particle aerosol mass spectrometer (SP-AMS). The SP-AMS was operated in a configuration that can exclusively detect refractory BC (rBC) particles and their non-refractory coating materials, including organic, nitrate, sulfate, ammonium and chloride.
Substantial formation of secondary organic aerosol (SOA) coatings on rBC particles was observed due to active photochemistry in the afternoon, whereas primary organic aerosol (POA) components were strongly associated with rBC from fresh vehicular emissions in the morning rush hours. Nitrate peaks were observed in the late morning due to formation of nitric acid via OH radical oxidation of NO2 in traffic emissions. Using the −log(NOx ∕ NOy) ratio as a proxy for photochemical age of air masses, most of the observed SOA coatings were freshly formed, providing an opportunity to examine SOA coating formation on rBCs near vehicular emissions. Comparison of our results to a co-located standard high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) measurement suggests that approximately 7–20 wt % of secondary organic and inorganic species were estimated to be internally mixed with rBC on average, implying that rBC is unlikely the major condensation sink of secondary species.
Event Trigger mode of SP-AMS was used to measure chemical composition of individual rBC-containing particle. Preliminary results of cluster analysis of single-particle measurements indicate the presence of various types of primary and secondary coatings based on their mass spectral characteristics. The mixing state index, χ, was calculated for quantifying the degree of mixing of rBC and coating materials (i.e., χ = 0 and 1 means that all particles are fully externally and internally mixed, respectively). The mixing state index varied between 0.2 and 0.6 with the average of 0.38 (± 0.06) in this study, and was not sensitive to the photochemical age of air masses. Nevertheless, the average particle species diversity (Dα) and the bulk population species diversity (Dγ), which represent the average effective number of species in each particle and the effective number of species in the population, respectively, slightly increased with primary traffic emissions and/or nitrate formation but gradually decreased with SOA formation during the afternoon. These observations indicate that substantial SOA formation observed in the afternoon led to more homogeneous particle compositions.