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. Black carbon (BC) particles can have deleterious human health consequencesand impact regional and global climate. Uncertainties remain in part due toincomplete knowledge on the evolution of physical properties and mixing stateof black carbon from sources to the remote atmosphere. We aim to understandhow “road-to-ambient” processing and longer timescale aging in an urbanplume 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 downtownLos Angeles. The near-road measurements were made at distances ranging from30 to 114 m downwind of Interstate 405 in Los Angeles. These results werecompared with measurements performed 100 km east of Los Angeles in Redlands,California. Coatings on rBC particles were quantified using two methods. Asdistance from the highway increased at the near-road site, we observeddecreases in rBC mass and number concentrations and increases in the numberfraction of rBC particles with thick coatings (f). The latter likelyoccurred due to rapid processing of the highway plume and entrainment ofurban background particles. Most rBC-containing particles measured near thehighway were either uncoated or thinly coated. In Redlands, we found that rBCmass concentrations on weekdays were similar to those observed at thefurthest measured distance from the highway (114 m). However, rBC numberconcentrations for the smallest measured sizes were an order of magnitudelower in Redlands than all measured distances from the highway. Observationsof f indicate that values in Redlands during periods when estimatedphotochemical age (PCA) was highest (6–8 h) were similar to correspondingvalues at the furthest measured distance from the highway. This suggests thatthe residence time of air in the Los Angeles Basin under typical summertimeconditions measured during this campaign may not be sufficient for rBC toacquire thick coatings. However, under certain meteorological conditions, fwas observed to be ∼0.20 in Redlands, with coating thickness histogramsshowing a larger contribution of rBC particles with coating thickness&gt; 80 nm. This occurred during a weekend day when local emissions fromdiesel vehicles were lower (compared to weekdays) and winds brought air fromthe desert regions to Redlands, both of which would increase the relativecontribution of remote sources of rBC. Afternoon values of f (andO3) were found to be systematically higher on weekends than weekdays,suggesting that the “weekend effect” can create more thickly coated rBCparticles presumably due to enhanced secondary organic aerosol (SOA) andreduced available rBC as condensation sites.

atmospheric chemistry and physics

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