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

Abstract

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 the impacts of BC at different temporal and spatial scales. These uncertainties are largely due to the heterogeneous nature of BC in terms of its spatiotemporal distribution, mixing state, and coating composition. Here, we seek to further understand the size and mixing state of BC emitted from various sources and aged over different timescales using field measurements in the Los Angeles region. We measured refractory black carbon (rBC) with a single-particle 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 were dominant and measured air masses were representative of well-aged background over the Pacific Ocean. In the second and third campaigns (December 2017 and November 2018, respectively), atypical Santa Ana wind conditions allowed us to measure biomass burning rBC (BCbb) from air masses dominated by large biomass burning events in California and fossil fuel rBC (BCff) from the Los Angeles Basin. We observed that the emissions source type heavily influenced both the size distribution of the rBC cores and the rBC mixing state. BCbb had thicker coatings and larger core diameters than BBff. We observed a mean coating thickness (CTBC) of ∼40–70 nm and a count mean diameter (CMD) of ∼120 nm for BCbb. For BCff, we observed a CTBC of ∼5–15 nm and a CMD of ∼100 nm. Our observations also provided evidence that aging led to an increased CTBC for both BCbb and BCff. Aging timescales < ∼1 d were insufficient to thickly coat freshly emitted BCff. However, CTBC for aged BCff within aged background plumes was ∼35 nm thicker than CTBC for fresh BCff. Likewise, we found that CTBC for aged BCbb was ∼18 nm thicker than CTBC for fresh BCbb. The results presented in this study highlight the wide variability in the BC mixing state and provide additional evidence that the emissions source type and aging influence rBC microphysical properties.

Mixing state of refractory black carbon aerosol in the South Asian outflow over the northern Indian Ocean during winter

“…S1d in the Supplement), show significant emissions over the continental areas upwind, a part of which would be transported to the oceanic regions during the ICARB-2018. Earlier, based on the observations using the optical attenuation technique (aethalometer) over the upwind locations (Kharagpur, Bhubaneswar, Vizag, Trivandrum), Kompalli et al (2013Kompalli et al ( , 2014b reported that the highest equivalent black carbon (EBC; optically measured BC) mass concentrations throughout the year are seen during the winter period (mean values ranging from ∼ 5389 ± 1245 ng m −3 over Trivandrum to 11 691±4457 ng m −3 over Kharagpur), which highlighted the source strength during this season. Also, the east coast of India is more industrialized compared to the west coast and/or peninsular India (Fig.…”

“…Also, the east coast of India is more industrialized compared to the west coast and/or peninsular India (Fig. S1c and d in the Supplement; Moorthy et al, 2005;Kompalli et al, 2013).…”

“…As the impact of continental outflow increases in the NIO-W, the coating on rBC increased once again (median RCT ∼ 1.92 and ACT ∼ 86 nm). Interestingly, highly coated BC particles were found less frequently over the NIO-W (with west coast air masses) compared to its eastern counterpart, the NIO-E region, which experienced east coast and/or Bay of Bengal air masses originating from more industrialized upwind locations, e.g., Moorthy et al (2005), Kompalli et al (2013). Thus, a clear contrast in the mixing state parameters is evident, which is due to differences in respective coastal sources (Moorthy et al, 2008;Peng et al, 2016;Gong et al, 2016) and possible transit times over these two regions.…”

“…Lawrence and Lelieveld (2010) have highlighted many field experiments that attempted to assess the impact of continental outflow of anthropogenic emissions from South Asia to the surrounding oceanic regions and its climate implications. Past field campaigns, such as the Indian Ocean Experiment (INDOEX) during 1998-1999 (Ramanathan et al, 2001) and the Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB) during March-May 2006 (phase 1) and December-January 2008/09 (phase 2) (Moorthy et al, 2008;Babu et al, 2012;Kompalli et al, 2013), have characterized regional aerosols over the northern Indian Ocean during different seasons.…”

Investigating Carbonaceous Aerosol and Its Absorption Properties From Fires in the Western United States (WE‐CAN) and Southern Africa (ORACLES and CLARIFY)

“…Figure 7 shows that the model underestimates the MAC BC at 660 nm downwind of southern Africa. Model values are similar for ORACLES‐2016 and CLARIFY (8.91 and 8.72 m 2 g −1 , respectively), as our current model scheme uses a fixed AE of 1.5 globally, which may not be a good representation of real world conditions downwind of Africa (Ko et al., 2020). Given higher BC:OA ratios for ORACLES and CLARIFY compared to WE‐CAN, OA contributes less to the simulated MAC BC at 660 nm.…”