From lowland plains to the Altiplano: The impacts of regional transport of wildfire smoke on the air quality of Bolivian cities

Abstract

Wildfires emerged as a global concern in recent decades, because they deteriorate the air quality in urban centres situated downwind, impact human health, ecosystems and regional climate. In this study, we assessed the air quality of Bolivia's three largest metropolitan areas during the period 2017–2019 by analysing in situ criteria air pollutant data (PM10, O3 and NOx) coupled with satellite (aerosol optical depth AOD and total CO column), fire foci data, and airmass backward trajectories. The Bolivian lowland plains were highly impacted by smoke transported from the Amazon and Cerrado biomes. Due to its proximity to the fire foci, Santa Cruz was the most affected city, where PM10, CO and AOD increased between 1.3- and 3.9-fold in September (maximum fire activity) compared to April (lowest fire activity). In Cochabamba and La Paz-El Alto, the impacts of wildfires on AOD and CO were lower (15–16% increase). A case study of regional smoke in September 2019 using Cochabamba as an example revealed that: i) airmass trajectories passed over regions with high fire radiative power density in southern Bolivia, Paraguay and northern Argentina; ii) the advection of wildfire smoke caused unusual episodes of enhanced nocturnal O3 concentrations (20–45 μg/m3) over a four-day pollution event. The results indicated that local sources (e.g., motorised traffic, brick kilns and dust resuspension) largely contributed to deteriorate the air quality, particularly in the dry months, surpassing the World Health Organization recommendations for PM10, in La Paz-El Alto and Cochabamba. We highlight the need of international cooperation among South American countries to combat fire practices in the region, given the transboundary nature of the biomass burning plumes. Moreover, it is crucial to implement local measures aimed at tackling local air pollution sources, especially in a changing climate in which drier conditions can favour the onset and propagation of wildfires and accumulation of air pollutants.