01 Nov 2022
01 Nov 2022
Status: this preprint is currently under review for the journal AMT.

Relationship between the sub-micron fraction (SMF) and fine mode fraction (FMF) in the context of AERONET retrievals

Norman T. O'Neill1, Keyvan Ranjbar2, Liviu Ivănescu1, Thomas F. Eck3, Jeffrey S. Reid4, David M. Giles3, Daniel Pérez-Ramírez5, and Jai Prakash Chaubey6 Norman T. O'Neill et al.
  • 1Centre d'applications et de recherches en télédétection (CARTEL), Université de Sherbrooke, Sherbrooke, J1K 2R1, Canada
  • 2Flight Research Laboratory, National Research Council, Ottawa, ON, K1V 1J8, Canada
  • 3NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
  • 4US Naval Research Laboratory, Monterey, CA 93943, USA
  • 5Applied Physics Department, University of Granada, Granada 18071, Spain
  • 6Department of Physics and Atmospheric Sciences, Dalhousie University, Halifax, NS, B3H 4R2, Canada

Abstract. The sub-micron (SM) aerosol optical depth (AOD) is an optical separation based on the fraction of particles below a specified cut off radius of the particle size distribution (PSD) at a given particle radius. It is fundamentally different from spectrally separated FM (fine mode) AOD. We present a simple (AOD-normalized) SM fraction versus FM fraction (SMF vs FMF) linear equation that explains the well-recognized empirical result of SMF generally being greater than the FMF. The AERONET inversion (AERinv) products (combined inputs of spectral AOD and sky radiance) and the Spectral Deconvolution Algorithm (SDA) products (input of AOD spectra) enable, respectively, an empirical SMF vs FMF comparison at similar (columnar) remote sensing scales across a variety of aerosol types.

SMF (AERinv derived) vs FMF (SDA derived) behavior is primarily dependent on the relative truncated portion (εc) of the coarse mode (CM) AOD associated with the cutoff portion of the CM PSD and, to a second order, the cutoff FM PSD and FM AOD (εf). The SMF vs FMF equation largely explains the SMF vs FMF behavior of the AERinv vs SDA products as a function of PSD cutoff radius ("inflection point") across an ensemble of AERONET sites and aerosol types (urban industrial, biomass burning, dust, maritime and a mixed class of Arctic aerosols). The overarching dynamic was that the linear SMF vs FMF relation pivots clockwise about the approximate (SMF, FMF) singularity of (1, 1) in a "linearly inverse" fashion (slope and intercept of approximately 1 − εc and εc) with increasing cutoff radius. SMF vs FMF slopes and intercepts derived from AERinv and SDA retrievals confirmed the general domination of εc over εf in controlling that dynamic. A more general conclusion is the apparent confirmation that the optical impact of truncating modal (whole) PSD features can be detected by a SMF vs FMF analysis.

Norman T. O'Neill et al.

Status: open (until 07 Dec 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2022-284', Anonymous Referee #1, 16 Nov 2022 reply
  • RC2: 'Comment on amt-2022-284', Anonymous Referee #2, 24 Nov 2022 reply

Norman T. O'Neill et al.

Norman T. O'Neill et al.


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Short summary
Aerosols are atmospheric particles that vary in size (radius) from a fraction of a micrometer (µm) to around 20 µm. They tend to be either smaller than 1 µm (like smoke or pollution) or larger than 1 µm (like dust or sea-salt). Their optical effect (scattering and absorbing sunlight) can be divided into FM (fine mode) and CM (coarse mode) parts using a cutoff radius around 1 µm or a spectral (color) technique. We present and validate a theoretical link between the 2 types of FM and CM divisions.