The optical properties of mineral particles suspended in seawater were calculated from the Mie scattering theory for different size distributions and complex refractive indices of the particles. The ratio of the spectral backscattering coefficient to the sum of the spectral absorption and backscattering coefficients of seawater, b _b(λ)/[a(λ) + b _b(λ)], was analyzed as a proxy for ocean reflectance for varying properties and concentrations of mineral particles. Given the plausible range of variability in the particle size distribution and the refractive index, the general parameterizations of the absorption and scattering properties of mineral particles and their effects on ocean reflectance in terms of particle mass concentration alone are inadequate. The variations in the particle size distribution and the refractive index must be taken into account. The errors in chlorophyll estimation obtained from the remote sensing algorithms that are due to the presence of mineral particles can be very large. For example, when the mineral concentration is 1 g m^-3 and the chlorophyll a concentration is low (0.05 mg m^-3), current global algorithms based on a blue-to-green reflectance ratio can produce a chlorophyll overestimation ranging from ∼50% to as much as 20-fold.

From laboratory measurements, we determined the spectral mass‐specific absorption, a_p^*(λ), and scattering, b_p^*(λ, coefficients for terrigenous mineral‐rich particulate assemblages suspended in seawater. The samples were derived mostly from surface soils in different locations and consisted of small particles (,10 mm). Both a_p^*(λ and b_p^*(λ showed large variability associated with variations in particle size distribution (PSD) and origin of samples. Variations in a_p^*(λ produced by changes in PSD are consistent with the package effect, in that samples with a higher percentage of small‐sized particles have higher a_p^*(λ. The variability among the samples is also associated with composition of particulate matter. For example, a_p^*(λ at blue wavelengths varied from ~0.05 m² g⁻¹ for organic‐dominated soil dust to ~0.1‐0.5 m² g⁻¹ for mineral‐dominated samples. The effects of particulate composition are reflected in a broad range of imaginary refractive index of particles, which in the blue can exceed 0.2‐0.3 for mineral‐dominated samples rich in iron oxides. The patterns of the variability in the scattering coefficient among the samples are quite intricate because of the effects of PSD and composition. In general, b_p^*(λ ranged from about 0.5 to 1.5 m² g⁻¹, and the spectral behavior varied from nearly flat spectra to the spectral dependency ~l2 λ with a slope λ as high as ~1.3 for the sample with the largest contribution of small particles.

The spectral optical properties of Asian mineral dust suspended in seawater exhibit significant variability associated with the origin (and, hence, the chemistry and mineralogy) and particle size distribution of the samples. The measurements of dust samples from different locations show that the mass‐specific absorption coefficient of particles, a^*_p, at a wavelength of light λ = 440 nm, varies from about 0.028 m² g⁻¹ for the soil dust from Chinese desert near Dunhuang to 0.15 m² g⁻¹ for the soil dust of volcanic origin in Cheju Island (South Korea). At l5 400 nm, this range is 0.05–0.23 m² g⁻¹. The aerosol sample collected in the Sea of Japan during a massive dust storm in East Asia shows a^*_p(λ) > 0.1 m² g⁻¹ for λ < 425 nm. The mass‐specific scattering coefficient, b^*_p(λ, ranges from about 0.8 to 1.5 m² g⁻¹ at blue and green wavelengths for the samples examined. The single scattering albedo, ω_o, increases with wavelength. For λ > 400 nm, ω_o was >0.78 for the sample from Cheju Island and >0.9 for other samples. In the near‐infrared region (750–850 nm), where absorption by dust particles is small or undetectable, ω_o was close to 1.