Using spectral methods to obtain particle size information from optical data: applications to measurements from CARES 2010

Abstract

Abstract. Multi-wavelength in situ aerosol extinction, absorption and scattering measurements made at two ground sites during the 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES) are analyzed using a spectral deconvolution method that allows extraction of particle-sizerelated information, including the fraction of extinction produced by the fine-mode particles and the effective radius of the fine mode. The spectral deconvolution method is typically applied to analysis of remote sensing measurements. Here, its application to in situ measurements allows for comparison with more direct measurement methods and validation of the retrieval approach. Overall, the retrieved finemode fraction and effective radius compare well with other in situ measurements, including size distribution measurements and scattering and absorption measurements made separately for PM 1 and PM 10 , although there were some periods during which the different methods yielded different results. One key contributor to differences between the results obtained is the alternative, spectrally based definitions of "fine" and "coarse" modes from the optical methods, relative to instruments that use a physically defined cut point. These results indicate that for campaigns where size, composition and multi-wavelength optical property measurements are made, comparison of the results can result in closure or can identify unusual circumstances. The comparison here also demonstrates that in situ multi-wavelength optical property measurements can be used to determine information about particle size distributions in situations where direct size distribution measurements are not available.

Spectral Derivatives of Optical Depth for Partitioning Aerosol Type and Loading

“…Based on the in-situ measurements, the multispectral optical properties can not only provide information on particle size distribution [41] but also can calculate aerosol fine and coarse mode scattering and extinction coefficients by means of the extended spectral deconvolution algorithm (DSA+) methodology provided by Kaku et al [42]. To enhance subtle spectral features, derivative spectroscopy methods (see [43] for a recent review) on the basis of nth-order derivatives have been applied to the investigation of pharmacology, medical biology and satellite oceanography [44][45][46][47][48][49][50].…”

Aerosol Total Volume Estimation From Wavelength‐ and Size‐Resolved Scattering Coefficient Data: A New Method

“…Although the corresponding Ångström exponent can also be applied for describing this wavelength dependence (e.g., Delene & Ogren, 2002), such an alternative is not considered in our initial analysis. It should be noted that the SCA_ratio represents a quite narrow spectral range (0.45–0.7 μm); thus, it is mostly sensitive to the submicron particles (e.g., Atkinson et al, 2018). In other words, the SCA_ratio alone is not well suited for describing a link between the light scattering and PM 10,v .…”

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

“…In the most controlled experiments, volume-sampled SMF estimates are compared with FMF retrievals derived from applying the SDA to volume-sampled, three-channel CRD (cavity ring-down) or three-channel nephelometer "spectra" (Atkinson et al, 2010;Kaku et al, 2014 (Ka); Atkinson et al, 2018). In general we find, as expected, the SMF to be FMF in Atkinson et al (2010) and Ka: this is notably true at the near-IR (700 nm) wavelength in Ka (ACE-Asia data of their Fig.…”