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

Sergio Rodríguez,Jessica López-Darias

doi:10.1016/j.atmosres.2021.105793

Abstract

We investigated the sources and processes affecting the vertical distribution of tropical PMx aerosols (particulate matter -PM- smaller than 10, 2.5 and 1 μm, PM10, PM2.5 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 PMx levels were predominantly low, except near to PMx sources, where distinctive PM1 / PM2.5 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 PMx 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, PMx 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 PMx transport from North Africa, both under dust and dust-free conditions, is associated with complex vertical stratifications, even within the dusty Saharan Air Layer.

Highlights

Atmospheric aerosols are a natural component of the Earth System (Carslaw et al, 2010)
In this study we performed high resolution measurements of PMx to investigate the sources and processes contributing to the variability of tropical PMx aerosols in two Cape Verde islands
By transecting PMx measurements at ground level we found that local sources had a rather low-scale impact and characteristic PM1/PM2.5 ratios linked to vehicle exhaust, biomass burning and Cape Verdean dust emissions

Summary

Introduction

Atmospheric aerosols are a natural component of the Earth System (Carslaw et al, 2010). Semi-directly, by absorbing solar radiation, promoting cloud evaporation and hindering rains (Rosenfeld, 2000; Seinfeld et al, 2016). Biomass burning, urban and industrial aerosols are of major concern due to their impacts. Pyrogenic emissions tend to produce tiny aerosols (Ito et al, 2019), whereas desert dust tends to be coarser (Raes et al, 2000). Tiny aerosols are formed by nucleation (Kulmala et al, 2016) of gaseous precursors and within clouds (Quan et al, 2021). Some key properties as particle size, chemical composition and minerally modulates aerosols impacts (Raes et al, 2000; Rodríguez et al, 2012)

Methods

Results

Conclusion