Single-particle analysis of atmospheric aerosols at Cape Gris-Nez, English Channel: Influence of steel works on iron apportionment

The chemical composition of atmospheric aerosols from an environment subjected to air masses of multiple origins was investigated at a northern French in-shore site away from any direct urban influence. During a continuous 1-year campaign, particles were sampled weekly onto polycarbonate membranes. Twelve 1-week samples of distinct air-mass origins, according to computed backward trajectories, were characterized by scanning electron microscopy combined with energy-dispersive X-ray spectrometry (SEM-EDX). Approximately, 1000 particles per sample were automatically analyzed. Semi-quantitative analysis was carried out by applying Castaingʹs first approximation to characteristic X-ray intensities of 16 elements (Z>8). A total of 11,842 particles were classified into five clusters, based upon similarities in composition, using a hierarchical clustering analysis. These clusters include: Na-rich (70%), referred to as marine particles, Ca–S-rich (17%), Al–Si-rich (8%), Fe-rich (4%) and Si-rich (1%), assigned to continental—including both natural and anthropogenic—sources. In this area, which is frequently subjected to meteorological depressions, the total particle number concentration is significantly correlated (r2=0.97) with the number concentration of marine particles regardless of the global origin of the air mass (i.e. oceanic and/or continental). In the case of air masses originating predominantly from the continent, marine particles presumably come from an external mixing due to the convergence of separate air masses before the sampling site. Due to their key role in marine primary production and in global climate by radiative forcing, Fe-bearing particles have been intensively studied for the past two decades. Fe-containing particles were interpreted here as iron (oxy(hydr))oxides (55%) and aluminosilicates (34%). A statistical reanalysis of all Fe-bearing particles (including particles with low Fe contents, not initially tabulated in the distribution statistics) clearly discloses their large abundance in air masses transported over steel works.