Dust and tropical PMx aerosols in Cape Verde: Sources, vertical distributions and stratified transport from North Africa.

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, PM 10 , PM 2.5 and PM 1 , 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 PM 1 / 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 (PM 10 ~ 100 μg/m3) and more humid air layers with lower aerosol loads (PM 10 ~ 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 (PM 10 : ~ 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. [Display omitted] • Local PM x sources have distinct PM 1 /PM 2.5 ratios: 0.96 vehicle exhaust, 0.67 biomass burning and 0.36 Cape Verdean dust • Under dust free conditions background levels of PM x are rather low • High PM x concentrations are observed under Saharan and Cape Verdean dust conditions • The dusty Saharan Air Layer have a stratified vertical structure • Under dust free conditions, layers of hydrophilic aerosols are observed above the marine stratocumulus [ABSTRACT FROM AUTHOR]