On-line alkali analysis of individual aerosol particles in urban air

On-line measurements of individual alkali-containing submicron particles in ambient air have been performed at two urban sites in Göteborg, on the west coast of Sweden. Results obtained with a recently developed particle beam mass spectrometer are presented. This instrument combines an optimized particle inlet with mass spectrometric techniques and detection of particle-bound alkali metal is based on surface ionization technique. The concentrations of Na and K in submicron particles were followed over periods of a few weeks. Mass concentrations of alkali in ambient air varied in the range 0.02–100 ng m−3 during the measurement periods depending on air mass history, wind direction, season, and contributions from local sources. The number of alkali-containing particles varied between 0.1 and 100 cm−3. Most detected individual particles contained a relatively small amount of Na, and few particles had sufficient Na content to be identified as pure sea-salt particles. The detected aerosol was concluded to be dominated by emissions from combustion of biomass and fossil fuels, with a significant contribution from sea-salt particles only during intrusion of marine air. This conclusion was supported by backward air mass trajectories and calculated K/Na weight ratios that generally agreed well with earlier measurements in urban environments.