Measurements to Determine Mixing State of Black Carbon Emitted From the 2017/2018 California Wildfires and Urban Los Angeles

Abstract

The effects of atmospheric black carbon (BC) on climate and public health have been well established, but large uncertainties remain regarding the extent of BC’s impacts at different temporal and spatial scales. These uncertainties are largely due to BC’s heterogeneous nature in terms of its spatiotemporal distribution, mixing state, and coating properties. Here, we seek to further understand the mixing state evolution of BC emitted from various sources and aged over different 10 timescales using field measurements in the Los Angeles region. We measured refractory black carbon (rBC) with a singleparticle soot photometer (SP2) on Catalina Island, California (~70 km southwest of downtown Los Angeles) during three different time periods. During the first campaign (September 2017), westerly winds dominated and thus the sampling location was upwind of the dominant regional sources of BC (i.e., urban emissions from the Los Angeles basin). In the second and third campaigns (December 2017, November 2018), atypical wind conditions caused measured rBC to include 15 important contributions from large wildfires in California and urban emission from the Los Angeles basin. We observed a larger number fraction of thickly coated particles (fBC) and increased coating thickness (CTBC) during the first campaign (~0.27 and ~36 nm, respectively), and during portions of the third campaign when we suspect that rBC was transported longrange from the Camp Fire in Northern California (~0.35 and ~52 nm, respectively), compared to other time periods. In contrast, during periods when we suspect that measured rBC was dominated by Southern California fires or urban emissions, 20 both fBC and CTBC were significantly lower, with a mean fBC of ~0.03 and median CTBC ranging from ~0 to 10 nm. From our rBC measurements and meteorological analyses, we conclude that an aging timescale on the order of ~hours is not long enough for rBC to become thickly coated under the range of sources sampled and atmospheric conditions during this campaign. On average, we found that measured rBC had to age more than a day to become thickly-coated. Aging timescales for developing thick coatings were found to be longer in this study relative to a number of previous observational studies 25 conducted with an SP2, suggesting that rBC aging is heavily impacted by regional atmospheric context. https://doi.org/10.5194/acp-2019-769 Preprint. Discussion started: 7 October 2019 c © Author(s) 2019. CC BY 4.0 License.