Comprehensive chemical profile and source apportionment of PM2.5 in Buenos Aires: Insights from the southernmost megalopolis

Understanding aerosol composition is essential for identifying sources and assessing impacts. We analyzed the chemical profile of 100 24-hour PM_2.5 samples and used this data for mass reconstruction and source apportionment. Samples were collected between April 2019 and March 2020 at a site located in Buenos Aires, Argentina. PM_2.5 concentrations ranged from 4.2 $\mu$g m⁻³ to 51.4 $\mu$g m⁻³, with a mean of 17.5 $\mu$g m⁻³, and maxima during biomass burning (BB) events. Samples were classified according to the presence or absence of BB events affecting the area, and mass reconstruction was performed. Optimal OM/OC ratios were determined to be 2.5 (BB-samples) and 1.9 (non-BB samples), being OM $\sim$  65% and $\sim$  54%, respectively. On average, other components accounted for $\sim$  14% geological minerals $>$ $\sim$  10% inorganic ions $>$ $\sim$  6% elemental carbon $>$ $\sim$  3% sea salt $>$ $\sim$  2% non crustal K. Source contributions were further studied using Positive Matrix Factorization. Open biomass burning was the main contributor to PM_2.5 (28.4%) and total carbon (25.7%), highlighting the significance of long-range pollutant transport. The temporal variability of this factor aligns with fire events identified using fire location, back-trajectory analysis, and aerosol classification schemes. The remaining factors found were: SOA + soil + road dust (17.7%), mobile sources powered by low sulfur (15.8%) and high sulfur fuels (11.1%), construction + grills (12.1%), agriculture (9.3%) and thermal power plants + industry (5.6%). This study provides relevant information for air quality management, highlighting knowledge gaps on primary and secondary sources affecting the site.