Reduction of atmospheric BC levels in the Chilean Central Andes: An unexpected impact of COVID-19

Abstract:

The COVID-19 pandemic, declared in March 2020 by the WHO, has forced governments to take closure and lockdown measures. In this context, on March 18, 2020, in a bilateral decision, the land border between Chile and Argentina, located in the highest altitude sector of the Andes, was closed, allowing only truck traffic, essential for exchanging goods between these countries. This action has presented a unique opportunity to verify a hypothesis previously raised by other researches: Local vehicle emissions have a considerable impact on the cryosphere of the Central Andes of Chile, which plays a fundamental role in the water supply and the ecological balance of the area. Here, vehicular activity is the main sources of Black Carbon (BC) emission to the atmosphere. This compound, due to its great capacity to absorb solar energy, can cause an acceleration in the melting of snow/ice, reason why is considered of special concern in this area.

Measurements of atmospheric BC were carried out in Portillo, Chilean Central Andes, in the "Nunatak" laboratory-refuge (32.84°S, 70.13° W, 3000 m.a.s.l). This site has a strategic location: it is next to the road and close to the border between Chile and Argentina. BC concentrations were monitored in PM2.5 by a Multi- Angle Absorption Photometer (MAAP, Model 5012, Thermo), based on the optical attenuation at a wavelength of 637 nm. Levels of BC from March to June were compared for the years 2019 and 2020. The measured atmospheric mass concentration of BC in the period March-June 2020 were shown to be 30 ± 13% lower than those from the same period of 2019, with a mean value of 0.25 ± 0.08 µg m^-3 in 2020, in comparison with a mean of 0.35 ± 0.07 µg m^-3 in 2019. This agrees with a decrease in vehicular traffic of 46 ± 10%, given that, during the study period, in 2019 a total of 191,259 vehicles circulated in the area, while in 2020 there were 102,903. The magnitude of the decrease in BC atmospheric levels has apparently been lower than that of the reduction in vehicular traffic. This could be due to the traffic restriction applying only to private vehicles and buses, but truck traffic, which has diesel engines with BC emissions significantly higher than those of gasoline engines, did not show significant variations.

Acknowledgments: Fondecyt-ANID Postdoc. Proj 3190513, Fondef-ANID IDeA I+D ID19I10359