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In this study, the positive matrix factorization (PMF) receptor model\ud(version 5.0) was used to identify and quantify major sources contributing\udto particulate matter (PM) number concentrations, using PM number size\uddistributions in the range of 13 nm to 10 µm combined with several\udauxiliary variables, including black carbon (BC), elemental and organic\udcarbon (EC/OC), PM mass concentrations, gaseous pollutants, meteorological,\udand traffic counts data, collected for about 9 months between August 2014\udand 2015 in central Los Angeles, CA. Several parameters, including particle\udnumber and volume size distribution profiles, profiles of auxiliary\udvariables, contributions of different factors in different seasons to the\udtotal number concentrations, diurnal variations of each of the resolved\udfactors in the cold and warm phases, weekday/weekend analysis for each of\udthe resolved factors, and correlation between auxiliary variables and the\udrelative contribution of each of the resolved factors, were used to identify\udPM sources. A six-factor solution was identified as the optimum for the\udaforementioned input data. The resolved factors comprised nucleation,\udtraffic 1, traffic 2 (with a larger mode diameter than traffic 1 factor),\udurban background aerosol, secondary aerosol, and soil/road dust. Traffic\udsources (1 and 2) were the major contributor to PM number concentrations,\udcollectively making up to above 60 % (60.8–68.4 %) of the total number\udconcentrations during the study period. Their contribution was also\udsignificantly higher in the cold phase compared to the warm phase.\udNucleation was another major factor significantly contributing to the total\udnumber concentrations (an overall contribution of 17 %, ranging from\ud11.7 to 24 %), with a larger contribution during the warm phase than\udin the cold phase. The other identified factors were urban background\udaerosol, secondary aerosol, and soil/road dust, with relative contributions\udof approximately 12 % (7.4–17.1), 2.1 % (1.5–2.5 %), and 1.1 %\ud(0.2–6.3 %), respectively, overall accounting for about 15 %\ud(15.2–19.8 %) of PM number concentrations. As expected, PM number\udconcentrations were dominated by factors with smaller mode diameters, such\udas traffic and nucleation. On the other hand, PM volume and mass\udconcentrations in the study area were mostly affected by sources with\udlarger mode diameters, including secondary aerosols and soil/road dust.\udResults from the present study can be used as input parameters in future\udepidemiological studies to link PM sources to adverse health effects as well\udas by policymakers to set targeted and more protective emission standards\udfor PM

Source apportionment of ambient particle number concentrations in central Los Angeles using positive matrix factorization (PMF)