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Field measurements of the chemical composition of boundary-layer clouds that formed in clean, marine air are presented as a background reference point for comparison to cloud water composition in more polluted regions. An impaction-based sampler was used to simultaneously collect cloud water on two stages, where the ratio of droplet diameter was ∼1.1 for the two droplet size fractions. Analysis revealed that large droplets were more concentrated than smaller cloud droplets by a factor of 1.5 for sea-salt-derived species. Cloud water concentrations of ionic species were generally five times greater the concentrations of the same ions in rain water. Aqueous-phase solute concentrations in cloud varied over two orders of magnitude but generally were quite low, correlated to each other and to aerosol (CN) concentration, but negatively correlated to LWC. Air-equivalent solute concentrations were calculated, allowing the detection of the influence of air-mass trajectory on cloud-water composition. A multivariate statistical analysis of the cloud water data suggested sea salt, biogenic, crustal, and anthropogenic emission source contributions; the last two sources existed only for continental air-mass trajectories. Coastal and oceanic trajectories were selected for the purpose of estimating a northern hemisphere, mid-latitude, marine, background cloud water composition of 8 neq m −3 non-sea salt SO 4 2− and 4 neq m −3 NO 3 − . |
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