Measurements of airborne particulate and gaseous sulphur and nitrogen species in the area of the Azores, Atlantic Ocean

Ground-level concentrations of particulate sulphate, nitrate, chloride, ammonium and iron, total particle count and gaseous nitric acid and sulphur dioxide have been measured in marine air on Santa Maria, an island in the Azores in the mid-North Atlantic Ocean. During one 10 d period of persistent anticyclonic conditions, the air sampled had not been in contact with continental regions for many days. The mean concentrations over this period of non-sea-salt sulphate, 0.40 mu g m(-3); nitrate, 0.24 mu g m(-3); ammonium, 0.08 mu g m(-3) and nitric acid, 0.09 mu g m(-3) were comparable with measurements reported from more remote marine locations. In another part of the campaign, air which had crossed the British Isles three days earlier was sampled; this contained substantially increased concentrations of sulphate, nitrate and nitric acid. Average concentrations for the entire campaign are comparable with other reported measurements from the mid-North Atlantic region. Concentrations of the sulphur containing species sulphur dioxide and non-sea-salt sulphate in the ''clean air'' period are of a magnitude which is consistent with production from the oxidation of dimethyl sulphide (DMS) based on measurements taken during this campaign and from previous studies in the literature. Concentrations of nitric acid and nitrate during the ''clean air'' period appear to imply a source other than continental emissions, which could result from release of precursors from the sea, and/or downward transfer from the free troposphere.