Effective lidar ratios of dense dust layers over North Africa derived from the CALIOP measurements

Liu, Zhaoyan; Winker, David M.; Omar, Ali; Vaughan, Mark; Trepte, Charles R.; Hu, Y.; Powell, Kathleen A.; Sun, Wenbo; Lin, Bing

doi:10.1016/j.jqsrt.2010.05.006

Cited by 48 publications

(37 citation statements)

References 36 publications

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“…This increase in quartz fraction and decrease in illite fraction results in an overall increase of the dust refractive index during dust events (recall that quartz has a relatively high refractive index and illite has a low refractive index, per Table 5). We suggest that the increase in refractive index and particle size during dust storms can have a significant effect on the lidar ratio, and that this is a possible cause of the discrepancy between the Liu et al (2011) paper and our study. Our results also differ from some other studies, though.…”

Section: Final Comments About Dustcontrasting

confidence: 91%

“…For instance, 97 % of the "all" dust AERONET retrievals occur at τ A (440) < 1.5, and the optical properties of these retrievals do not necessarily apply to dust events with higher optical depths. Indeed, Liu et al (2011) obtained a median effective lidar ratio (ELR) of 36 (532 nm wavelength) for opaque dust layers over Northern Africa (Sahara and Sahel), and deduced a lidar ratio of 40 sr for thin to moderately dense African dust layers after multiple scattering effects are considered (see also Wandinger et al, 2010). Additionally, the high dust loads of τ A (532) ≥ 3 in Liu et al (2011) correspond to very low fine volume fractions, and probably different mineralogical compositions than the AERONET retrievals of our study.…”

Section: Final Comments About Dustmentioning

confidence: 99%

“…Indeed, Liu et al (2011) obtained a median effective lidar ratio (ELR) of 36 (532 nm wavelength) for opaque dust layers over Northern Africa (Sahara and Sahel), and deduced a lidar ratio of 40 sr for thin to moderately dense African dust layers after multiple scattering effects are considered (see also Wandinger et al, 2010). Additionally, the high dust loads of τ A (532) ≥ 3 in Liu et al (2011) correspond to very low fine volume fractions, and probably different mineralogical compositions than the AERONET retrievals of our study. Dust events tend to lift silt-and sand-sized particles into the atmosphere (i.e., particles greater than 2 µm diameter), which have a different composition than clay particles ), but the median coarse mode effective radius at all of the AERONET sites in this study is only 1.6-2 µm (see Table 4).…”

Section: Final Comments About Dustmentioning

confidence: 99%

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Comparison of CALIPSO aerosol optical depth retrievals to AERONET measurements, and a climatology for the lidar ratio of dust

et al. 2012

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Abstract. We compared CALIPSO column aerosol optical depths at 0.532 µm to measurements at 147 AERONET sites, synchronized to within 30 min of satellite overpass times during a 3-yr period. We found 677 suitable overpasses, and a CALIPSO bias of −13 % relative to AERONET for the entire data set; the corresponding absolute bias is −0.029, and the standard deviation of the mean (SDOM) is 0.014. Consequently, the null hypothesis is rejected at the 97 % confidence level, indicating a statistically significant difference between the datasets. However, if we omit CALIPSO columns that contain dust from our analysis, the relative and absolute biases are reduced to −3 % and −0.005 with a standard error of 0.016 for 449 overpasses, and the statistical confidence level for the null hypothesis rejection is reduced to 27 %. We also analyzed the results according to the six CALIPSO aerosol subtypes and found relative and absolute biases of −29 % and −0.1 for atmospheric columns that contain the dust subtype exclusively, but with a relatively high correlation coefficient of R = 0.58; this indicates the possibility that the assumed lidar ratio (40 sr) for the CALIPSO dust retrievals is too low. Hence, we used the AERONET size distributions, refractive indices, percent spheres, and forward optics code for spheres and spheroids to compute a lidar ratio climatology for AERONET sites located in the dust belt. The highest lidar ratios of our analysis occur in the non-Sahel regions of Northern Africa, where the median lidar ratio at 0.532 µm is 55.4 sr for 229 retrievals. Lidar ratios are somewhat lower in the African Sahel (49.7 sr for 929 retrievals), the Middle East (42.6 sr for 489 retrievals), and Kanpur, India (43.8 sr for 67 retrievals). We attribute this regional variability in the lidar ratio to the regional variability of the real refractive index of dust, as these two parameters are highly anti-correlated (correlation coefficients range from −0.51 to −0.85 for the various regions). The AERONET refractive index variability is consistent with the variability of illite concentration in dust across the dust belt.

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Section: Final Comments About Dustcontrasting

confidence: 91%

Section: Final Comments About Dustmentioning

confidence: 99%

Section: Final Comments About Dustmentioning

confidence: 99%

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Comparison of CALIPSO aerosol optical depth retrievals to AERONET measurements, and a climatology for the lidar ratio of dust

et al. 2012

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“…Accordingly, as a second screening metric, profiles are removed if aerosol particle scattering is not detected to within 250 m of the surface in the 5-km along-track average. Like clouds, dense aerosol layers can exhibit visible optical depths approaching 3.0, thus, limiting CALIOP profiling to the surface Vaughan et al, 2009;Liu et al, 2011). This metric screens profiles limited by simple undersampling, in addition to cases where optically-thin aerosol particle layers scatter at magnitudes below layer-detection thresholds.…”

Section: Caliop and Naaps Aod Datasetsmentioning

confidence: 99%

Evaluating nighttime CALIOP 0.532 μm aerosol optical depth and extinction coefficient retrievals

Campbell¹,

Tackett

Reid³

et al. 2012

Atmos. Meas. Tech.

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Abstract. NASA Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) Version 3.01 5-km nighttime 0.532 µm aerosol optical depth (AOD) datasets from 2007 are screened, averaged and evaluated at 1 • × 1 • resolution versus corresponding/co-incident 0.550 µm AOD derived using the US Navy Aerosol Analysis and Prediction System (NAAPS), featuring two-dimensional variational assimilation of quality-assured NASA Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR) AOD. In the absence of sunlight, since passive radiometric AOD retrievals rely overwhelmingly on scattered radiances, the model represents one of the few practical global estimates available from which to attempt such a validation. Daytime comparisons, though, provide useful context. Regional-mean CALIOP vertical profiles of night/day 0.532 µm extinction coefficient are compared with 0.523/0.532 µm ground-based lidar measurements to investigate representativeness and diurnal variability. In this analysis, mean nighttime CALIOP AOD are mostly lower than daytime (0.121 vs. 0.126 for all aggregated data points, and 0.099 vs. 0.102 when averaged globally per normalised 1 • × 1 • bin), though the relationship is reversed over land and coastal regions when the data are averaged per normalised bin (0.134/0.108 vs. 0140/0.112, respectively). Offsets assessed within single bins alone approach ±20 %. CALIOP AOD, both day and night, are higher than NAAPS over land (0.137 vs. 0.124) and equal over water (0.082 vs. 0.083) when averaged globally per normalised bin. However, for all data points inclusive, NAAPS exceeds CALIOP over land, coast and ocean, both day and night. Again, differences assessed within single bins approach 50 % in extreme cases. Correlation between CALIOP and NAAPS AOD is comparable during both day and night. Higher correlation is found nearest the equator, both as a function of sample size and relative signal magnitudes inherent at these latitudes. Root mean square deviation between CALIOP and NAAPS varies between 0.1 and 0.3 globally during both day/night. Averaging of CALIOP along-track AOD data points within a single NAAPS grid bin improves correlation and RMSD, though day/night and land/ocean biases persist and are believed systematic. Vertical profiles of extinction coefficient derived in the Caribbean compare well with ground-based lidar observations, though potentially anomalous selection of a priori lidar ratios for CALIOP retrievals is likely inducing some discrepancies. Mean effective aerosol layer top heights are stable between day and night, indicating consistent layeridentification diurnally, which is noteworthy considering the potential limiting effects of ambient solar noise during day.

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“…For a vertically pointing Mie lidar, the unknown lidar ratio is a key parameter for the retrieval of the aerosol extinction coefficient. However, previous research has found that the lidar ratio of dust can vary between 20 and 100 sr [3] , depending on its originating sources and dust-pollutant mixing state. Therefore, without knowing the actual lidar ratio, it is difficult to accurately measure the dust extinction coefficient with a vertically pointing Mie ldar.…”

mentioning

confidence: 98%

Optical and hygroscopic properties of Asian dust particles based on a horizontal Mie lidar: case study at Hefei, China

et al. 2017

Chin. Opt. Lett.

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As an extension of the Mie lidar technique to measure the extinction coefficient of the surface particles, a horizontally pointing Mie lidar is used for determining the optical properties of Asian dust, which is an approach without knowing the actual lidar ratio. A long lasting dust event is observed based on this approach in May 2014. The "no dust," "pure dust," and "polluted dust" stage is observed during this event, and their optical and hygroscopic properties are discussed. Some new optical and hygroscopic features are observed, which benefit from the quantitative, multi-wavelength, and continuous measurement of the extinction related optical properties of dust particles.

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Effective lidar ratios of dense dust layers over North Africa derived from the CALIOP measurements

Cited by 48 publications

References 36 publications

Comparison of CALIPSO aerosol optical depth retrievals to AERONET measurements, and a climatology for the lidar ratio of dust

Comparison of CALIPSO aerosol optical depth retrievals to AERONET measurements, and a climatology for the lidar ratio of dust

Evaluating nighttime CALIOP 0.532 μm aerosol optical depth and extinction coefficient retrievals

Optical and hygroscopic properties of Asian dust particles based on a horizontal Mie lidar: case study at Hefei, China

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