Air quality assessment of a wildfires event affecting Mexico City’s atmosphere using PIXE

An event involving multiple biomass burning wildfires that occurred in southern and central Mexico during May 2019 was investigated using Particle Induced X-ray Emission (PIXE). The event severely affected the Mexico City Metropolitan Area (MCMA), resulting in elevated concentrations of various pollutants, including fine particulate matter (PM2.5), which accumulated in the region over several days. PM_2.5 mass concentration reached a maximum value of 63 µg/m³, exceeding the Mexican Air Quality Standard of 45 µg/m³ in 24 h, in effect in Mexico in 2019. PM_2.5 samples were collected during the wildfires episode and analyzed by PIXE to determine their elemental concentrations. Additionally, a high-volume PM_2.5 sampler operating in parallel provided samples to determine organic carbon (OC) and elemental carbon (EC). Time series of PM_2.5, OC, and EC, as well as those elements considered respectively good markers of biomass burning (K) and fossil fuel combustion (S and V) were generated. The results revealed trends in accordance with their origin, verifying the presence of particles proceeding from common sources, either from biomass burning or fossil fuel combustion. These results were cross compared with aerosol transport trajectories generated using the HYSPLIT model for computing simple air parcel trajectories, showing to be in agreement. Non-soil (smoke) potassium was estimated using its corresponding enrichment factor before, during, and after the most critical period of the event (May 9–18, 2019), indicating that wildfires were indeed the main contributors to the local atmospheric pollution around the peak of the event. Tracers time series proved to be a very valuable asset to link the pollutants to their sources.