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

Abstract

Abstract. Black carbon particles can have deleterious human health consequences and impact regional and global climate. 10Uncertainties 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 114 m 15 downwind of Interstate 405 in Los Angeles. These results were compared with measurements performed 100 km 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 20 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 (114 m). 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 (6-8 hours) were similar to corresponding values at the furthest measured distance from the highway. This suggests that the residence time of air in the 25 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 > 80 nm. 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 30 sources of rBC. Afternoon values of f (and O 3 ) 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.Atmos. Chem. Phys. Discuss., https://doi

Constructing Shapes and Mixing Structures of Black Carbon Particles With Applications to Optical Calculations

“…A Soot Particle Aerosol Mass Spectrometer (SP‐AMS) can measure the average coating thickness (coating‐to‐core mass ratio) of BC particle population (Cappa et al., 2012; Wang et al., 2019), while a Single Particle Soot Photometer (SP2) can obtain the coating thickness (coating‐to‐core diameter ratio) of individual BC particles (Krasowsky et al., 2018; S. Liu et al., 2015; Schwarz et al., 2006). Previous studies have used the SP‐AMS and SP2 to investigate the optical effects of core size and coating thickness of BC particles (D. Liu et al., 2017; H. Liu et al., 2020; Unga et al., 2018; Wang et al., 2018; Xie et al., 2020; Zanatta et al., 2018).…”

Mixing state of black carbon at different atmospheres in north and southwest China

“…We compared the number fraction of the thickly coated BC at different measurement sites from literature (Ueda et al, 2016;Schwarz et al, 2008a, b;Wu et al, 2017;Wang et al, 2014Huang et al, 2012;Metcalf et al, 2012;Shiraiwa et al, 2007;McMeeking et al, 2012;Subramanian et al, 2010;Saha et al, 2018;Krasowsky et al, 2018;Holder et al, 2014) and the results are shown in Fig. 3.…”