Measurements of the impact of atmospheric aging on physical and optical properties of ambient black carbon particles in Los Angeles

Abstract

Understanding how physical and optical properties of atmospheric black carbon (BC) particles vary in time and space is critical for reducing uncertainty in climate forcing estimates from ambient BC. In this study, ambient BC was measured in Rubidoux, California, approximately 90 km (55 miles) downwind of downtown Los Angeles. Collocated NOx and NOy measurements were used to estimate the photochemical age of the sampled air. Sampling was conducted throughout entire days between February 3, 2015 and March 12, 2015 to capture diurnal and daily variations in ambient BC. Both ambient and thermally-denuded air was sampled in 15-min cycles to compare the physical and optical properties of coated versus uncoated BC particles. Physical properties of individual BC particles including mass and coating thickness were measured using a Single-Particle Soot Photometer (SP2), and BC optical properties were measured using a Photoacoustic Extinctiometer (PAX) at 870 nm. The mean BC mass concentration (±standard deviation) for the campaign was 0.12 ± 0.08 μg m−3. BC mass concentrations were higher on weekdays than weekends, though only differences between 11:00 and 17:00 h were statistically distinguishable. The fraction of total BC particles that were thickly-coated (f) was found to be relatively low, with a mean of 0.05 ± 0.02 over the campaign. Values for f peaked in the afternoon when photochemical pollutant concentrations are also generally at a maximum. Further, f at 15:00–16:00 h was found to be statistically higher on weekends than weekdays, potentially due to a higher relative amount of ambient SOA to BC on weekends versus weekdays, which would enhance SOA coating of primary BC particles as they age during transport from the western Los Angeles basin to our sampling site on weekends. Differences at other hours during the photochemically active period of the day (10:00–14:00 h) were not statistically different although the weekend values were systematically higher. Comparing f with the photochemical age (PCA) of sampled air showed increases in f as PCA increases; the mean value (±95% confidence interval) of f for PCA < 1 h was 0.037 ± 0.004, while that for PCA > 3 h was 0.12 ± 0.04. This suggests that even in winter, photochemistry in urban environments can lead to increased thickly coated BC particles. The mean (±95% confidence interval) enhancement in mass absorption cross-section (MAC) due to coatings on BC for our wintertime measurements in urban Los Angeles was found to be 1.03 ± 0.05. Comparisons to other studies that measure enhancement of MAC are presented.