Relationship between the sub-micron fraction (SMF) and fine-mode fraction (FMF) in the context of AERONET retrievals
<p>The sub-micron (SM) aerosol optical depth (AOD) is an optical separation based on the fraction of particles below a specified cutoff radius of the particle size distribution (PSD) at a given particle radius. It is fundamentally different from spectrally separated FM (fine-mode) AOD. We pres...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-03-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://amt.copernicus.org/articles/16/1103/2023/amt-16-1103-2023.pdf |
| Summary: | <p>The sub-micron (SM) aerosol optical depth (AOD) is an
optical separation based on the fraction of particles below a specified cutoff 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.</p>
<p>SMF (AERinv-derived) vs. FMF (SDA-derived) behavior is primarily dependent on the relative truncated portion (<span class="inline-formula"><i>ε</i><sub>c</sub></span>) 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 (<span class="inline-formula"><i>ε</i><sub>f</sub></span>). 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 <span class="inline-formula">1−<i>ε</i><sub>c</sub></span> and <span class="inline-formula"><i>ε</i><sub>c</sub></span>) with increasing cutoff radius. SMF vs. FMF slopes and intercepts derived from AERinv and SDA retrievals confirmed the general domination of <span class="inline-formula"><i>ε</i><sub>c</sub></span> over <span class="inline-formula"><i>ε</i><sub>f</sub></span> 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 an SMF vs. FMF analysis.</p> |
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| ISSN: | 1867-1381 1867-8548 |