Measurement of externally mixed sodium containing particles in ambient air by single particle mass spectrometry

The number distribution as a function of sodium mass has been determined for individual particles in ambient air at an urban location about 40 km inland from the Pacific ocean. The measurements were made in real-time by single particle mass spectrometry. The aerosol is ‘externally mixed’ with respect to sodium, that is, a small fraction of the total number of particles are highly enriched in Na. Size distributions calculated for Na mass equivalent spherical NaCl particles over the size range 0.1–0.4 μm show a consistent peak at 0.19 μm. Samples taken after foggy mornings had number densities for Na containing particles around 9 cm⁻³ compared with 0.9 cm⁻³ for samples taken after clear mornings. Comparison of the size distribution over this size range with marine particle size distributions indicates that sub-μm Na containing particles at the inland sampling site are of marine origin. Sodium mass density for particles smaller than 0.4 μm was found to be ∼ 0.02 μg m⁻³ and for particles of diameter ⩽2.5 μm, it was estimated to be ∼ 1.4 μg m⁻³.