Validation of aerosol backscatter profiles from Raman lidar and ceilometer using balloon-borne measurements
<p>Remote-sensing measurements by light detection and ranging (lidar) instruments are fundamental for the monitoring of altitude-resolved aerosol optical properties. Here we validate vertical profiles of aerosol backscatter coefficient (<span class="inline-formula"><i>β&l...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/21/2267/2021/acp-21-2267-2021.pdf |
| Summary: | <p>Remote-sensing measurements by light detection and ranging (lidar)
instruments are fundamental for the monitoring of altitude-resolved aerosol
optical properties. Here we validate vertical profiles of aerosol
backscatter coefficient (<span class="inline-formula"><i>β</i><sub>aer</sub></span>) measured by two independent lidar
systems using co-located balloon-borne measurements performed by Compact
Optical Backscatter Aerosol Detector (COBALD) sondes. COBALD provides
high-precision in situ measurements of <span class="inline-formula"><i>β</i><sub>aer</sub></span> at two wavelengths
(455 and 940 nm). The two analyzed lidar systems are the research Raman
Lidar for Meteorological Observations (RALMO) and the commercial CHM15K
ceilometer (Lufft, Germany). We consider in total 17 RALMO and 31 CHM15K
profiles, co-located with simultaneous COBALD soundings performed throughout
the years 2014–2019 at the MeteoSwiss observatory of Payerne (Switzerland).
The RALMO (355 nm) and CHM15K (1064 nm) measurements are converted to 455 and 940 nm, respectively, using the Ångström exponent profiles
retrieved from COBALD data. To account for the different receiver field-of-view (FOV) angles between the two lidars (0.01–0.02<span class="inline-formula"><sup>∘</sup></span>) and COBALD
(6<span class="inline-formula"><sup>∘</sup></span>), we derive a custom-made correction using Mie-theory
scattering simulations. Our analysis shows that both lidar instruments
achieve on average a good agreement with COBALD measurements in the boundary
layer and free troposphere, up to 6 km altitude. For medium-high-aerosol-content measurements at altitudes below 3 km, the mean <span class="inline-formula">±</span> standard
deviation difference in <span class="inline-formula"><i>β</i><sub>aer</sub></span> calculated from all considered
soundings is <span class="inline-formula">−2</span> % <span class="inline-formula">±</span> 37 % (<span class="inline-formula">−0.018</span> <span class="inline-formula">±</span> 0.237 Mm<span class="inline-formula"><sup>−1</sup></span> sr<span class="inline-formula"><sup>−1</sup></span> at 455 nm) for <span class="inline-formula">RALMO−COBALD</span> and <span class="inline-formula">+</span>5 % <span class="inline-formula">±</span> 43 %
(<span class="inline-formula">+</span>0.009 <span class="inline-formula">±</span> 0.185 Mm<span class="inline-formula"><sup>−1</sup></span> sr<span class="inline-formula"><sup>−1</sup></span> at 940 mm) for <span class="inline-formula">CHM15K−COBALD</span>. Above 3 km altitude, absolute deviations generally decrease, while
relative deviations increase due to the prevalence of air masses with low
aerosol content. Uncertainties related to the FOV correction and spatial- and
temporal-variability effects (associated with the balloon's drift with
altitude and different integration times) contribute to the large standard
deviations observed at low altitudes. The lack of information on the aerosol
size distribution and the high atmospheric variability prevent an accurate
quantification of these effects. Nevertheless, the excellent agreement
observed in individual profiles, including fine and complex structures in
the <span class="inline-formula"><i>β</i><sub>aer</sub></span> vertical distribution, shows that under optimal
conditions, the discrepancies with the in situ measurements are typically
comparable to the estimated statistical uncertainties in the remote-sensing
measurements. Therefore, we conclude that <span class="inline-formula"><i>β</i><sub>aer</sub></span> profiles measured
by the RALMO and CHM15K lidar systems are in good agreement with in situ
measurements by COBALD sondes up to 6 km altitude.</p> |
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| ISSN: | 1680-7316 1680-7324 |