RESUMO
Black carbon transport from the Santiago Metropolitan Area, Chile, up to the adjacent Andes Cordillera and its glaciers is of major concern. Its deposition accelerates the melting of the snowpack, which could lead to stress on water supply in addition to climate feedback. A proposed pathway for this transport is the channelling through the network of canyons that connect the urban basin to the elevated summits, as suggested by modelling studies, although no observations have validated this hypothesis so far. In this work, atmospheric measurements from a dedicated field campaign conducted in winter 2015, under severe urban pollution conditions, in Santiago and the Maipo canyon, southeast of Santiago, are analysed. Wind (speed and direction) and particulate matter concentrations measured at the surface and along vertical profiles, demonstrate intrusions of thick layers (up to 600 m above ground) of urban black carbon deep into the canyon on several occasions. Transport of PM down-valley occurs mostly through shallow layers at the surface except in connection with deep valley intrusions, when a secondary layer in altitude with return flow (down-valley) at night is observed. The transported particulate matter is mostly from the vicinity of the entrance to the canyon and uncorrelated to concentrations observed in downtown Santiago. Reanalyses data show that for 10% of the wintertime days, deep intrusions into the Maipo canyon are prevented by easterly winds advecting air pollutants away from the Andes. Also, in 23% of the cases, intrusions proceed towards a secondary north-eastward branch of the Maipo canyon, leaving 67% of the cases with favourable conditions for deep penetrations into the main Maipo canyon. Reanalyses show that the wind directions associated to the 33% anomalous cases are related to thick cloud cover and/or the development of coastal lows.
Assuntos
Poluentes Atmosféricos , Camada de Gelo , Poluentes Atmosféricos/análise , Carbono , Chile , Monitoramento Ambiental , Material Particulado/análise , VentoRESUMO
Ice and snow in the Central Andes contain significant amounts of light-absorbing particles such as black carbon. The consequent accelerated melting of the cryosphere is not only a threat from a climate perspective but also for water resources and snow-dependent species and activities, worsened by the mega-drought affecting the region since the last decade. Given its proximity to the Andes, emissions from the Metropolitan Area of Santiago, Chile, are believed to be among the main contributors to deposition on glaciers. However, no evidence backs such an assertion, especially given the usually subsident and stable conditions in wintertime, when the snowpack is at its maximum extent. Based on high-resolution chemistry-transport modeling with WRF-CHIMERE, the present work shows that, for the month of July 2015, up to 40% of black carbon dry deposition on snow or ice covered areas in the Central Andes downwind from the Metropolitan area can be attributed to emissions from Santiago. Through the analysis of aerosol tracers we determine (i) that the areas of the Metropolitan Area where emissions matter most when it comes to export towards glaciers are located in Eastern Santiago near the foothills of the Andes, (ii) the crucial role of the network of Andean valleys that channels pollutants up to remote locations near glaciers, following gentle slopes. A direct corollary is that severe urban pollution, and deposition of impurities on the Andes, are anti-correlated phenomena. Finally, a two-variable meteorological index is developed that accounts for the dynamics of aerosol export towards the Andes, based on the zonal wind speed over the urban area, and the vertical diffusion coefficient in the valleys close to ice and snow covered terrain. Numerous large urban areas are found along the Andes so that the processes studied here can shed light on similar investigations for other glaciers-dependent Andean regions.