SO measurements in a clean coastal environment of the southwestern Europe: Sources, transport and influence in the formation of secondary aerosols.

Two years of SO measurements at El Arenosillo observatory located in the Gulf of Cadiz (Atlantic Ocean) were investigated. Annual hourly averages of 1.9 ± 1.5 μg m and p95 between 3 and 4.4 μg m were recorded, showing clean and background environments. Monthly means vary between 1.5 and 2.4 μg m, a monthly evolution was not found. SO fields from the MERRA2 model were used to identify SO sources and its transport, which could be affecting the studied region. Although SO records were low, major conductive for SO increases were observed in specific periods. A selection methodology was applied to extract these events, which showed a mean of ~11 μg m. Surface meteorological observations and ERA5 meteorological fields from the ECMWF model were used to assess the weather conditions. SO increases, in cold months occurred under conditions governed by synoptic-scale. Two types of transport scenarios were identified: SO transport defined as direct impact, which is the sum of the plumes from Portugal and the Huelva area; and indirect impact, where SO and sulphate particle emissions from Portugal were transported and accumulated in the Gulf of Cadiz and then carried inland, where new particle formation were observed. Episodes with high SO concentrations were also reported in warm periods associated with pure sea-land breezes. The SO peaks under sea-land breezes were associated with the transport of SO from the south of Portugal to the Gulf of Cadiz, whereas SO from the east of the Iberian Peninsula and north of Africa reached the Mediterranean Sea and were then transported to the Atlantic Ocean following the Strait of Gibraltar. Blocking of the airflows from the Mediterranean Sea to the Atlantic Ocean turns the Gulf of Cadiz into a chemical reservoir, where chemical species such as SO can accumulate, triggering new particle formation processes.