Effects of atmospheric particles from Southern California on the optical properties of seawater

Abstract

The role of particles deposited from the atmosphere to the ocean is poorly understood in ocean optics. We examined the light absorption and scattering properties of atmospheric particulate matter, which was collected by dry deposition near the Pacific coastline in La Jolla, Southern California, and then suspended in particle‐free seawater for subsequent laboratory measurements. Within these suspensions, we measured the spectral absorption and beam attenuation coefficients, particle size distribution, and particle mass concentration of atmospheric particles. We also determined the 4‐week average aerosol deposition rates at the experimental site over a period of 9 months. The mass‐specific particle absorption ap*(λ) and scattering bp*(λ) coefficients showed considerable variability among the samples because of the variations in particle size distribution and composition. For example, ap*(440) was on average 0.042 m2 g−1 but exhibited over fivefold range among the samples. At near‐infrared wavelengths (750–800 nm), the magnitude of ap* was significant (>0.02 m2 g−1 for a number of samples) and is likely attributable to the presence of black carbon. The bp*(λ) values were virtually wavelength‐independent because of the significant presence of relatively large particles (>10 μm) in our samples. The magnitude of bp*(λ) varied within a 3.5‐fold range, with an average value of ∼0.27 m2 g−1. Our determinations of absorption and scattering by atmospheric particles coupled with radiative transfer simulations suggest that aerosol deposition may have sizable effect on ocean optical properties, including remote sensing reflectance under certain scenarios of deposition events, residence time of deposited particles within the water column, and particle properties.