Mountain-Facilitated Lee-Slope Transport and Daytime Boundary Layer Mixing of Volcano Plumes Exacerbates Air Pollution Over Arequipa, Peru

Severe air pollution plagues Arequipa, Peru, due to anthropogenic and natural emissions. Persistent volcano emission in the vicinity of Arequipa makes it among the largest SO2 sources in the world. Because volcano plumes mostly exist in the free troposphere and stratosphere where horizontal transport acts rather quickly, previous studies mostly focused on their global-scale impacts. Whether these plumes can affect near-surface air quality has not attracted much research attention. This study uses WRF-Chem simulations to reveal that in the presence of northerly/northwesterly winds and favorable mountain meteorology, the plume from volcano Sabancaya (elevation 5,960 m, similar to 80 km north of Arequipa) can be brought down to near the surface of Arequipa through two steps of transport and dispersion processes: (a) With northerly/northwesterly winds, the free troposphere plume from Sabancaya is transported southward and intercepted by Mountain Chachani located between Sabancaya and Arequipa and subsequently transported downward to Arequipa by nighttime downslope winds linked to large-amplitude lee-side mountain gravity waves. Often the plume reaches down to be close to the boundary layer over Arequipa. (b) In the following day, convective boundary layer growth brings the above boundary-layer plume to near the surface through vertical mixing processes, thus exacerbating ambient air pollution in Arequipa. A mechanism on how volcano plumes above 6-km height cause air pollution over the lower-lying Arequipa city is therefore revealed for the first time. The mountain dynamic effect in inducing the large-amplitude mountain lee waves is further illustrated by an idealized simulation excluding mountain's thermal effect.