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Abstract
Aerosol columnar size distributions (SDs) are commonly provided by aerosol inversions based on measurements of both spectral extinction and sky radiance. These inversions developed for a fully clear sky offer few SDs for areas with abundant clouds. Here, we estimate SDs from spectral extinction data alone for cloudy coastal and maritime regions using aerosol refractive index (RI) obtained from chemical composition data. Our estimation involves finding volume and mean radius of lognormally distributed modes of an assumed bimodal size distribution through fitting of the spectral extinction data. We demonstrate that vertically integrated SDs obtained from aircraft measurements over a coastal site have distinct seasonal changes, and these changes are captured reasonably well by the estimated columnar SDs. We also demonstrate that similar seasonal changes occur at a maritime site, and columnar SDs retrieved from the combined extinction and sky radiance measurements are approximated quite well by their extinction only counterparts (correlation exceeds 0.9) during a 7-year period (2013–2019). The level of agreement between the estimated and retrieved SDs depends weakly on wavelength selection within a given spectral interval (roughly 0.4–1 µm). Since the extinction-based estimations can be performed frequently for partly cloudy skies, the number of periods where SDs can be found is greatly increased.
Highlights
Studies with focus on regional and global climate change are primarily concerned with size spectra of aerosol particles in a vertical atmospheric column, the so-called columnar size distributions (SDs) [1,2]
These distributions are commonly acquired from spectrally resolved aerosol optical depth (AOD) and sky radiance measured by Cimel sunphotometers (CSPHOTs) as part of the National Aeronautics and Space Administration (NASA)
To evaluate the model SDs, we consider (1) SDs obtained from the integrated airborne measurements (TCAP) and (2) SDs offered by the conventional AERONET inversions
Summary
As sunlight streaks through the atmosphere, it is attenuated and scattered by aerosol particles. There is accumulating evidence that such information can rarely be collected mostly because of the frequent presence of clouds located above a ground-based instrument. These clouds can contribute strongly to the measured sunlight scattering, making inferences of aerosol properties impossible. These clouds have no impact on sunlight attenuation if they do not block a straight line between the Sun and the observing instrument. It is demonstrated that the aerosol size distributions obtained from the combined (attenuation and scattering) and individual (attenuation only) measurements in the coastal and maritime environment are, on average, in a good agreement. Since the sunlight attenuation can be measured much more frequently than attenuation and scattering together, the attenuation-only based size distributions will provide a more complete picture of how particles behave in time and space and interact with sunlight and clouds
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