Continental-scale transport of sea salt aerosol

Compared to soil dust, for which there is an extensive literature documenting transport across and between continents, little attention has been given to the possible occurrence of sea salt far from its marine home. This paper examines an incursion of sea salt aerosol into the North American interior, more than 1000 km from any ocean water. On 14 December 2008, high fine-particle (< 2.5 um) chloride concentrations were recorded by three independent air-quality monitoring programs across several states and provinces of the North American Great Plains (Figure 1). Na, Mg, Cl, K, Ca, and Sr were all present in the proportions of sea water and at fine-particle concentrations normally found only at exposed coastal sites. This detailed marine chemical signature, together with the observed paucity of coarse particles, allows regional saline lakes and road de-icing chemicals to be ruled out as possible sources (Figure 2). The 14 December 2008 sea salt came to the Great Plains from the north, and its chemical and particle-size signatures resemble those found above 60 deg N, at Yellowknife and Point Barrow. Forward and backward trajectories support such a sub-arctic or arctic origin, but are inconclusive as to mechanisms of mobilization and exact transport pathways. Figure 1. Fine-particle chloride on 14 December 2008 in three national air monitoring networks: IMPROVE (Interagency Monitoring of PROtected Visual Environments, rural US); CSN (Chemical Speciation Network, urban US); and NAPS (National Air Pollution Surveillance, Canada). Bubble areas are proportional to 24h concentrations.

Figure 2. Sodium and chloride concentrations in coarse (Dp > 2.5 um) and fine (Dp < 2.5 um) particles at Winnipeg, MB. Winnipeg salt is normally coarse road salt; the 14 December 2008 sample was fine sea salt.