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
<p>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 a...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/11991/2018/acp-18-11991-2018.pdf |
| Summary: | <p>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 <q>road-to-ambient</q> 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 (<i>f</i>). 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 <i>f</i> 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, <i>f</i>
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 <i>f</i> (and
O<sub>3</sub>) were found to be systematically higher on weekends than weekdays,
suggesting that the <q>weekend effect</q> can create more thickly coated rBC
particles presumably due to enhanced secondary organic aerosol (SOA) and
reduced available rBC as condensation sites.</p> |
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| ISSN: | 1680-7316 1680-7324 |