Drought and record wildfires during the 3-year La Niña: assessing air pollution impacts in Northeastern Mexico

The Monterrey Metropolitan Area (MMA) in Northeastern Mexico, already burdened by significant industrial pollution, experienced a severe drought crisis during the 2020-2023 triple-dip La Niña. This prolonged climate anomaly triggered three major fire episodes in the Sierra Madre Oriental (SMO), sharply increasing particulate matter (PM10 and PM2.5) concentrations. To assess the impact of wildfire emissions on urban pollution, this study integrates ground-based air quality and meteorological measurements, satellite-derived data (VIIRS fire radiative power and MODIS aerosol optical depth), drought indicators from the North American Drought Monitor (NADM), and dispersion modeling using Nonparametric Wind Regression (NWR). Fire-attributable contributions increased by up to 53.3 $\mu$ gm $_{}^{-3}$ for PM10 and 12.8 $\mu$ gm $_{}^{-3}$ for PM2.5, frequently exceeding both Mexican and WHO air quality standards. On average, the three wildfires accounted for relative increases of 110% in PM10 and 49% in PM2.5 compared to non-fire conditions. These increases were linked to wind-driven smoke transport from the SMO to the MMA, demonstrating that three megafires substantially degraded urban air quality during a period of extreme drought. As climate change is expected to increase the frequency of multi-year ENSO episodes, thereby prolonging droughts and intensifying wildfire occurrence, our findings underscore the urgent need to incorporate biomass burning aerosol emissions into air quality management strategies and health impact assessments in other regions experiencing similar conditions.