Vertical variations of sea-salt modification in the boundary layer of spring Arctic during the ASTAR 2000 campaign

Airborne and ground-based aerosol/gas measurements were carried out in Svalbard between mid-March and mid-April 2000. From the viewpoint of vertical features of sea-salt modification in the lower troposphere (1500 m) of the Arctic spring, more than 1000 individual sea-salt particles were analyzed with scanning electron microscopy energy dispersive X-ray spectrometry (SEM-EDX) in the present study. Individual particle analysis suggested a vertical gradient of sea-salt modification in fine sea-salt particles with an altitude of 59–1485 m above sea level (asl), e.g. chlorine liberation rates of 33.0% (212 m asl) and 81.0% (1266 m asl) on 15 March, and 72.7% (100 m asl), 83.8% (495 m asl) and 95.8% (1411 m asl) on 26 March. Sea salts may be dominantly modified with SO₂ and SO₄ ²⁻ under Arctic haze conditions with higher SO₂ concentration (≥2 nmol m⁻³), whereas they are dominantly modified with NO₃ ⁻ and reactive nitrogen oxides under Arctic background conditions, with [HNO₃] of 0.15–1.3 nmol m⁻³ and [SO₂] of 0.04–2 nmol m⁻³. Vertical trends in sea-salt modification suggested that it makes a significant contribution to the formation of reactive halogen species in the upper boundary layer and the lower free troposphere of the spring Arctic.