Factors controlling sea salt abundances in the urban atmosphere of a coastal South American megacity

Abstract

The South Atlantic oceanic influence in the ambient air of Buenos Aires was studied on the basis of the measured concentrations of Cl-, Mg2+ and Na+, as chemical markers of marine aerosols. A total of 113 fine (PM2.5) and 113 coarse (PM2.5-10) samples were collected over a one-year period in an inland sampling site located ∼250 km from the open sea and ∼7.5 km from the shore of the La Plata River, which flows into the Atlantic Ocean. The ratio rion-PM between the added concentrations of the three ions and the corresponding aerosol mass concentration was also used as a sea salt indicator. The behavior of these indicators under various meteorological conditions was used to identify and characterize the presence of sea salt in the urban aerosol. The influence of regional meteorological conditions was assessed by means of the Potential Source Contribution Function (PSCF) while that of local conditions was assessed by categorized percentile distributions analysis. The pattern of the PSCF for different ranges of the four sea salt indicators, exhibiting a transition from lowest values under continental influence to highest values under oceanic influence, provided robust evidence that the marine aerosol from the South Atlantic Ocean reaches the city of Buenos Aires. The rion-PM ratio, which combines the opposite effects of wind speed on the aerosol mass and ion concentrations, was identified as the most sensitive indicator of sea salt aerosol variations. Percentile distributions of the rion-PM ratio, disaggregated according to onshore (NE, E, SE, S) and offshore (N, NW, W, SW) winds and speeds above and below the median (4.3 m s-1), clearly indicated that the highest levels of marine aerosol occurred under onshore winds and wind speeds > 4.3 m s-1. In addition to characterizing the oceanic influence in Buenos Aires, we reported the expected sea salt levels under different conditions and estimated the magnitude of chloride depletion. This is the first study on sea salt levels in the urban atmosphere of this coastal megacity that reports and makes available a set of consistent concentrations of marine aerosol markers measured over a one-year period.