Effect of relative humidity on light scattering by mineral dust aerosol asmeasured in the marine boundary layer over the tropical Atlantic Ocean

Using an integrating nephelometer, we measured aerosol light scattering dur ing a month-long experiment conducted in April 1994 on Barbados, West Indie s(13.17 degrees N, 59.43 degrees W). Concurrent measurements of aerosol com position showed that the trade wind aerosol mass was alternatively dominate d by sea salt and by mineral dust transported from North Africa. Dust conce ntrations ranged from 0.9 to 257 mu g m(-3), and sea salt ranged from 9.4 t o 29 mu g m(-3). Nonsea-salt sulfate (nss SO4=) was generally a minor compo nent with concentrations ranging from 0.5 to 4.2 mu g m(-3). We obtained hy groscopic growth factors (HGF) by calculating the ratio of the aerosol scat tering coefficient (ASC) measured at ambient relative humidity (RH), which ranged from 67% to 83%, to the ASC measured with the sample air stream heat ed so as to reduce the RH to under 40%. Over the course of the experiment, HGF ranged from 1.13 to 1.69, with lower values corresponding to high dust concentration periods. By a variety of procedures, including the chemical a pportioning of ASC, we derived the HGF for mineral dust; depending on the t echnique used, values ranged from 1.0 to 1.1, In contrast, we obtain for se a salt an HGF of 1.8 +/- 0.2, a value consistent with previously reported m easurements. The low HGF values obtained for dusty periods suggest that onl y minor amounts of hygroscopic materials are associated with dust. Conseque ntly, the radiative properties of North African dust in the atmosphere will be relatively insensitive to RH changes.