Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv Diurnal and seasonal trends and source apportionment of redox-active metals in Los Angeles using a novel online metal monitor and Positive Matrix Factorization (PMF) Amirhosein Mousavi, Mohammad H. Sowlat, Constantinos Sioutas * University of Southern California, Department of Civil and Environmental Engineering, Los Angeles, CA, USA ARTICLE INFO Keywords: Source apportionment PMF Los Angeles Redox-active metals ABSTRACT In the present study, we identified the sources of four redox-active metals, including Iron (Fe), Chromium (Cr), Cupper (Cu), and Manganese (Mn) and quantified the contribution of these sources to PM 2.5 concentrations in central Los Angeles, California, by employing time-resolved measurements (i.e., a time resolution of 2 h) with a recently developed online metal monitor and Positive Matrix Factorization (PMF). Size distribution of ambient PM (14 nm–10 μm) was measured using the Scanning Mobility Particle Sizer (SMPS) and Optical Particle Sizer (OPS). Auxiliary variables were also collected, including elemental (EC) and organic carbon (OC), gaseous pollutants (NO 2 and O 3 ), meteorological parameters (including relative humidity (RH) and temperature), and traffic data (for heavy- (HDVs) and light-duty vehicles (LDVs)). A 4-factor solution was found to be optimum for the chemically-speciated dataset, whereas a 5-factor solution appeared to be most plausible for the size dis- tribution data. The factors included fresh traffic, soil/road dust, urban background aerosol, secondary aerosol, and nucleation (only resolved for the size distribution data). Fresh traffic was the major contributor to Fe and Cu concentrations, whereas Cr was mostly found in the urban background aerosol (reflecting a mixture of small local sources as well as aged traffic emissions), and Mn mostly came from both soil/road dust and was to a lesser degree found in urban background aerosol. Secondary aerosol did not contribute to the concentrations of any of these metals, but was associated with very high loading of OC, as expected. Even though the urban background aerosol and secondary aerosol appeared to be characterized by “aged” particles and have a rather homogeneous spatial distribution, the reactions and processes involved in their formation are entirely different. Our results provide insights into the sources of redox-active metals in central Los Angeles. They also underscore the benefits of novel measurement techniques for PM-bound metals, which could enhance our understanding of the sources of atmospheric aerosols by providing us with measurements with finer time resolutions that otherwise would not have been possible using traditional filter-based measurement techniques. 1. Introduction Air pollution is becoming an increasingly severe problem in urban areas around the globe and can cause human health impacts (Naddafi et al., 2012). Particularly, exposure to ambient particulate matter (PM) has been known to cause human health impacts ranging from re- spiratory and cardiovascular diseases to neurodegenerative effects (Davis et al., 2013; Delfino et al., 2010; Gauderman et al., 2015). His- torically, the focus of earlier epidemiological studies has been on total mass of PM, while the findings from more recent studies suggest that, other than the total PM mass, physical characteristics (including number concentration, particle size, and surface area) as well as che- mical composition of PM are also critical in driving the health end- points (Davis et al., 2013; Delfino et al., 2010). Iron (Fe), Chromium (Cr), Cupper (Cu), and Manganese (Mn) are among the most important redox-active transition metals that have been linked to human health effects mainly because of their tox- icological properties. This is believed to be due to the ability of these metals to induce oxidative stress through generating reactive oxygen species (ROS), which eventually leads to inflammation of target cells and organs (Li et al., 2009; Tao et al., 2003). Therefore, the study of sources, transport, and spatio-temporal characteristics of these redox- active metals becomes of paramount importance, as this information will help policy makers to more effectively regulate and mitigate their sources and to bring down exposure to these toxic species. Positive matrix factorization (PMF) has been globally used as a https://doi.org/10.1016/j.atmosenv.2017.11.034 Received 24 July 2017; Received in revised form 12 November 2017; Accepted 18 November 2017 * Corresponding author. 3620 S. Vermont Ave. KAP210, Los Angeles, CA 90089, USA. E-mail address: sioutas@usc.edu (C. Sioutas). Atmospheric Environment 174 (2018) 15–24 Available online 21 November 2017 1352-2310/ © 2017 Elsevier Ltd. All rights reserved. T