Determination of an Asian dust radiative signature over the North Pacific Ocean and Hawaii from surface and satellite observations in UV and visible wavelengths

Predicting the diverse radiative impact of mineral dust requires an understanding of peculiar dust properties at various geographical regions. We combine direct modeling, satellite observations, and surface measurements of atmospheric radiation, optics, and aerosol chemical composition to identify a spectral radiative signature of aged Asian dust reaching the North Pacific Ocean and Hawaii. Focusing on springs of 1997-2000, we make use of routine ground-based measurements conducted at Mauna Loa Observatory (MLO), Hawaii, in conjunction with satellite data of TOMS and AVHRR remote sensors.

MLO surface measurements of aerosol optics (light scattering coefficient at three wavelengths and single-wavelength light absorption coefficient) and aerosol composition were utilized to calculate spectral optical properties of Asian dust over a range of wavelengths during identified dusty days. Intrinsic measurement limitations and underlying assumptions required for such a modeling do not permit the developing of a unique optical model. Therefore a set of models have been calculated and used in a discrete-ordinate radiative transfer code to model the downwelling irradiances at the surface and radiances at the top of the atmosphere to simulate collocated surface radiation measurements and satellite observations, respectively. We consider several types of the aerosol vertical distribution, supported by the backward air-mass trajectories indicating the origin and altitude of the dust layers. The complementarity between surface downwelling irradiances and TOA radiances provides an additional useful constrain on dust optical properties. We demonstrate that this approach, merging different types of aerosol and radiation measurements, allows the quantitative characterization the Asian dust optical properties needed for climate modeling and remote sensing applications.

Acknowledgements. We gladly thank Drs. McPeters, Johnson and Torres for providing TOMS data and Drs. Stowe and Knapp for assistance with AVHRR data.