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Discussion papers | Copyright
https://doi.org/10.5194/acp-2017-1020
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Nov 2017

Research article | 09 Nov 2017

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This discussion paper is a preprint. A revision of the manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP).

Characterizing the evolution of physical properties and mixing state of black carbon particles: from near a major highway to the broader urban plume in Los Angeles

Trevor S. Krasowsky1, Gavin R. McMeeking2, Constantinos Sioutas1, and George Ban-Weiss1 Trevor S. Krasowsky et al.
  • 1Sonny Astani Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, 90089, U.S.A
  • 2Handix Scientific, Boulder, 80301, U.S.A

Abstract. Black carbon particles can have deleterious human health consequences and impact regional and global climate. Uncertainties remain in part due to incomplete knowledge on the evolution of physical properties and mixing state of black carbon from sources to the remote atmosphere. We aim to understand how road-to-ambient processing and longer timescale aging in an urban plume affect black carbon physical properties. Refractory black carbon (rBC) was measured during summer 2016 using a Single Particle Soot Photometer (SP2) in two distinct environments: near a major freeway and downwind of downtown Los Angeles. The near-road measurements were made at distances ranging from 30 to 114m downwind of Interstate 405 in Los Angeles. These results were compared with measurements performed 100km east of Los Angeles in Redlands, California. Coatings on rBC particles were quantified using two methods. As distance from the highway increased at the near-road site, we observed decreases in rBC mass and number concentrations and increases in the number fraction of rBC particles with thick coatings (f). The latter likely occurred due to rapid processing of the highway plume and entrainment of urban background particles. Most rBC-containing particles measured near the highway were either uncoated or thinly-coated. In Redlands, we found that rBC mass concentrations on weekdays were similar to those observed at the furthest measured distance from the highway (114m). However, rBC number concentrations for the smallest measured sizes were an order of magnitude lower in Redlands than all measured distances from the highway. Observations of f indicate that values in Redlands during periods when estimated photochemical age was highest (6–8hours) were similar to corresponding values at the furthest measured distance from the highway. This suggests that the residence time of air in the Los Angeles basin under typical summertime conditions measured during this campaign may not be sufficient for rBC to acquire thick coatings. However, under certain meteorological conditions, f was observed to be ~0.20 in Redlands, with coating thickness histograms showing a larger contribution of rBC particles with coating thickness >80nm. This occurred during a weekend day when local emissions from diesel vehicles were lower (compared to weekdays) and winds brought air from the desert regions to the Northeast of Los Angeles, both of which would increase the relative contribution of remote sources of rBC. Afternoon values of f (and O3) were found to be systematically higher on weekends than weekdays, suggesting that the “weekend effect” can create more thickly-coated rBC particles presumably due to enhanced secondary organic aerosol and reduced available rBC as condensation sites.

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Short summary
Black carbon (BC) particles can have deleterious health consequences and impact regional and global climate. We aim to understand how processing of BC near highways, and longer timescale aging in an urban plume, affect its physical properties. Measurements in distinct regions of the Los Angeles basin indicate that health-relevant physical properties can change within 100 m of the freeway. Climate-relevant properties downwind of Los Angeles depend on day of the week and overall meteorology.
Black carbon (BC) particles can have deleterious health consequences and impact regional and...
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