Spatio-temporal trends and source apportionment of fossil fuel and biomass burning black carbon (BC) in the Los Angeles Basin Amirhosein Mousavi a , Mohammad H. Sowlat a , Sina Hasheminassab b , Andrea Polidori b , Constantinos Sioutas a, ⁎ a University of Southern California, Department of Civil and Environmental Engineering, Los Angeles, CA, USA b South Coast Air Quality Management District, Diamond Bar, CA, USA HIGHLIGHTS • We observed a decrease in bc and bc ff levels from 2012 to 2016 at all sites. • Fossil fuel combustion BC (BC ff ) is the dominant source of BC in the Los Angeles air basin. • Biomass burning also contributes to am- bient BC levels, especially in the colder periods. • BC ff peaked during traffic rush hours, but biomass burning BC (BC bb ) peaked at nighttime. • Tracer and sensitivity analyses con- firmed the outputs of the EBC source ap- portionment model. GRAPHICAL ABSTRACT abstract article info Article history: Received 28 March 2018 Received in revised form 31 May 2018 Accepted 2 June 2018 Available online xxxx Editor: P. Kassomenos In this study, we evaluated the spatial and temporal trends of black carbon (BC) in the Los Angeles Basin between 2012–2013 and 2016–2017. BC concentrations were measured in seven wavelengths using Aethalometers (AE33) at four sites, including central Los Angeles (CELA), Anaheim, Fontana, and Riverside. Sources of BC were quantified using the equivalent black carbon (EBC) model. Results indicate that total BC concentrations nearly doubled in colder period compared to the warm period. Source apportionment results revealed that fossil fuel combustion has higher an- nual contributions (ranging from 82% in Riverside to 91% in CELA) than biomass burning (ranging from 9.3% in CELA to 18.7% in Riverside) to the total BC concentrations at all sites. This trend was more clearly observed at the sites closer to major freeways, such as CELA and Anaheim. The relative contribution of fossil fuel to total BC concentrations was higher in the warm period, whereas biomass burning had higher contributions in the colder period. The diurnal variation of fossil-fuel-originated BC (BC ff ) to the total BC concentrations revealed major rises during the traffic rush hours, espe- cially in the warm period. In contrast, the fraction of BC originating from biomass burning (BC bb ) peaked at nighttime, particularly in the cold period, reaching values as high as 25–30% of total BC concentration. Moreover, we observed a clear decrease in both absolute BC concentrations as well as relative contributions of BC ff to total BC concentrations from 2012–2013 to 2016–2017, which can be attributed to the implementation of strict regulations in California to re- duce transportation-related PM emissions. Results from the present study suggest that as these regulations become in- creasingly stricter, the relative contributions of traf fic sources to BC also decrease, thereby making the impact of non- fossil fuel combustion sources, such as biomass burning, to the overall BC levels more significant. © 2018 Elsevier B.V. All rights reserved. Keywords: Black carbon Fossil fuel Biomass burning Source apportionment Los Angeles Science of the Total Environment 640–641 (2018) 1231–1240 ⁎ Corresponding author at: 216C Kaprielian Hall, 3620 South Vermont Ave., Los Angeles, CA 90089, USA. E-mail addresses: amousavi@usc.edu, (A. Mousavi), sowlat@usc.edu, (M.H. Sowlat), SHasheminassab@aqmd.gov, (S. Hasheminassab), apolidori@aqmd.gov, (A. Polidori), sioutas@usc.edu (C. Sioutas). https://doi.org/10.1016/j.scitotenv.2018.06.022 0048-9697/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv