Air masses and aerosols chemical components in the free troposphere at the subtropical northeast atlantic region

Abstract

The dynamics and the aerosol chemistry of the air masses reaching the free troposphere of the subtropical Northeast Atlantic region during the period 1995–98 have been studied. Seven days backward trajectories were calculated daily with HYSPLIT-4 model for Izana Global Atmospheric Watch (GAW) Observatory (28.3°N 16.5°W, 2367 m a.s.l.). These back-trajectories were classified by means of a k-means clustering strategy. The daily air masses have been coded using 16 variables to detect the aerosol load of each one of them. Four clusters were found: Cluster 1, representative of Atlantic oceanic middle troposphere air masses, (OMT), has an average frequency of occurrence of 50.6%. Cluster 2, which includes air masses originated in the African continent (AfD), has been recorded in a 19.8% of time. Cluster 3 represents a mixture at least of two of the next sources: Europe, Africa and Ocean, (EAM), with a frequency of 12.7%. Finally, Cluster 4 includes air masses with a high load of maritime aerosols, (MaA), and it has been detected in a 16.9%. An analysis of four aerosol components: NO3−, NH4+, non-sea-salt-SO42−, and mineral dust and its relation with the origin and transport of the air masses have been done. The highest quantities of mineral dust and nss-SO42− are linked with African air masses with a mean value of 86.5 and 1.9 μg/m3 respectively. Whereas the highest levels of NO3−, 1.0 μg/m3, and NH4+, 0.4 μg/m3, were obtained for AfD and EAM. The lowest levels were associated with OMT and MaA air masses types: 12.7, 0.6, 0.2, and 0.5 μg/m3 for dust, NO3−, NH4+, and nss-SO42− in average for the four studied years. However, it is remarkable that the values of the median for dust are 2.2 and 3.5 μg/m3 in clusters MaA and OMT respectively. Using non-parametric statistical tests the distributions of concentrations in each cluster by year have been compared in order to detect similarities. The results show that the aerosol loads of OMT and MaA air masses are quite similar and the same occurs for AfD and EAM air masses. However, the correlation analysis between the levels of anions and ammonium evidenced important differences among the air mass types. In AfD air masses is clear a low correlation between levels of nss-SO42− and NH4+ (r2 = 0.08) suggesting that the sulfate speciation was dominated by sulfate species others than ammonium sulfate, such as calcium sulfate. CaSO4⋅2H2O (gypsum) is mainly present in the coarse mode, where the radiative effects of sulfate are less important that in the accumulative mode. For OMT air masses is noticeable an important increasing on the correlation between the levels of anions and those of NH4+ for the two last years of the study period (1997–1998, r2 = 0.61 –0.85%) with respect to the first ones (1995–1996, r2 = 0.25–0.49%), coinciding with the second strongest ENSO (El Nino Southern Oscillation) event recorded. This behavior indicates a change in the speciation of the aerosol component.