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 (BC) 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 114 m 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 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 (PCA) was highest (6–8 h) 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 > 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 Redlands, 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 (SOA) and reduced available rBC as condensation sites.

Characterising mass-resolved mixing state of black carbon in Beijing using a morphology-independent measurement method

“…Though a number of different proposed model treatments give different predictions of BC mixing state (Jacobson, 2001;Murphy et al, 2006;Péré et al, 2009), none of these give perfect agreement with atmospheric observations, either in situ or remote (Fierce et al, 2016). As the coated BC particles may act as cloud condensation nuclei (CCN) in the atmosphere, the coating thickness information of BCcontaining particles can also influence the wet removal rate and thus its atmospheric lifetime and overall climate forcing potential (Kuwata et al, 2007;Schroder et al, 2015;Motos et al, 2019). Taylor et al (2014) shows that the size distribution of BC-containing particles is important for the simulation of BC lifetime, vertical profile and transportation.…”

Characterising Mass-resolved Mixing State of Black Carbon in Beijing Using a Morphology-Independent Measurement Method

“…In summer, the primary emissions are less compared to winter, and SOA contributes more in Beijing (Sun et al, 2018;Hu et al, 2016). Previous studies show that an internal mixture would be expected for BC associated with a larger secondary fraction (Krasowsky et al, 2018;Bondy et al, 2018). In addition, the rBC-containing particles experience more intense photochemical processing in summer, which may promote the formation of secondary species internally mixed with BC (Xu et al, 2018).…”

Measurements to determine the mixing state of black carbon emitted from the 2017–2018 California wildfires and urban Los Angeles

“…The campaign-wide f lag,neg was 0.017 for the first campaign (September 2017), 0.018 for the second campaign (December 2017), and 0.026 for the third campaign (November 2018). Comparatively, Dahlkötter et al (2014) observed an f lag,neg of ∼ 0.046 during an airborne field campaign measuring an aged biomass burning plume, and additionally calculated a higher fragmentation rate of ∼ 0.4 to 0.5, based on their aforementioned alternative method (Laborde et al, 2012). Sedlacek et al (2012) reported an f lag,neg > 0.6 for ground-based measurements of a biomass burning plume in Long Island, New York, originating from Lake Winnipeg, Canada.…”

“…The incandescence signal was calibrated using Aquadag, and the scattering signal was calibrated using polystyrene latex spheres. Further details regarding the governing principles and operation of the SP2 can be found in numerous publications (Stephens et al, 2003;Schwarz et al, 2006;Gao et al, 2007;Moteki and Kondo, 2007;Laborde et al, 2012;Dahlkötter et al, 2014;Krasowsky et al, 2016).…”

“…Dahlkötter et al (2014) used a method examining the tail end of the time-dependent scattering cross section in order to determine if a rBC-containing particle was fragmenting, thereby calculating a higher fragmentation rate relative to f lag,neg . Details of the time-dependent scattering cross-section method can be found in Laborde et al (2012) and Dahlkötter et al (2014). This method to calculate a refined fragmentation rate was not used in Sedlacek et al (2012) nor in this study.…”