资源环境科技发展态势分析平台

We investigated the sources and processes affecting the vertical distribution of tropical PM_x aerosols (particulate matter -PM- smaller than 10, 2.5 and 1 μm, PM₁₀, PM_2.5 and PM₁, respectively) in the low troposphere of Santo Antão and São Vicente islands, in Cape Verde archipelago, a region where a better understanding of aerosols is needed due to their involvement in tropical meteorology and their impact on air quality, ocean and climate. We found that local sources had a low-scale impact. From transect measurements at ground level, we found that PM_x levels were predominantly low, except near to PM_x sources, where distinctive PM₁ / PM_2.5 ratios were measured, linked to vehicle exhaust (0.96), biomass burning (0.67) and Cape Verdean dust (0.36) emissions. The depth of the marine boundary layer (MBL) and the vertical distributions of PM_x showed wide variability prompted by meteorological conditions. The trade winds prevailed in the MBL, whereas other airflows were situated above it: North-Atlantic, African easterly airflow and Saharan Air Layer. Under North-Atlantic airflow conditions, the MBL extended to 1400 m above sea level (m.a.s.l.). Above this altitude, PM_x concentrations decreased quickly (< 3 μg/m³) due to the free troposphere conditions. Under Saharan dust conditions, the MBL was confined to just 70 m.a.s.l., whereas a complex dust stratification was observed above, characterized by alternating dry air layers with high dust loads (PM₁₀ ~ 100 μg/m³) and more humid air layers with lower aerosol loads (PM₁₀ ~ 40 μg/m³). Within the dry easterly African airflow occurring above the marine stratocumulus typical of the MBL top (placed at 500 m.a.s.l.), we detected layers enriched in hydrophilic aerosols (PM₁₀: ~ 8 μg/m³). These were imbedded in relatively humid air (RH ~48%), probably linked to secondary aerosol formation by in-cloud processes in the marine stratocumulus situated below. We found that PM_x transport from North Africa, both under dust and dust-free conditions, is associated with complex vertical stratifications, even within the dusty Saharan Air Layer.