scholarly journals Synergy between CALIOP and MODIS instruments for aerosol monitoring: application to the Po Valley

2010 ◽  
Vol 3 (4) ◽  
pp. 893-907 ◽  
Author(s):  
P. Royer ◽  
J.-C. Raut ◽  
G. Ajello ◽  
S. Berthier ◽  
P. Chazette
Keyword(s):  
Signal To Noise Ratio ◽  
Orthogonal Polarization ◽  
Large Area ◽  
Total Uncertainty ◽  
Aerosol Model ◽  
Po Valley ◽  
Modis Aod ◽  
The Mean ◽  
Level 1 ◽  
Level 2

Abstract. In this study aerosol optical properties are studied over the Po Valley from June 2006 to February 2009 using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations/Cloud-Aerosol LIdar with Orthogonal Polarization (CALIPSO/CALIOP) and Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua and Terra. The choice of the Po valley has been driven by the numerous occurrences of pollutant events leading to a mean MODIS-derived aerosol optical depth (AOD) of 0.27 (±0.17) at 550 nm over a large area of ~120 000 km2. AOD derived from MODIS, AERONET and CALIOP have been compared. The comparison with AERONET sun-photometers has highlighted an overestimation of AOD from MODIS radiometers of 0.047 for Aqua and 0.088 for Terra. A systematic underestimation of AOD derived from CALIOP Level-2 products has been observed in comparison to Aqua (0.060) and Terra (0.075) MODIS values. Considering those discrepancies a synergistic approach combining CALIOP level-1 data and MODIS AOD has been developed for the first time over land to retrieve the equivalent extinction-to-backscatter ratio at 532 nm (LR). MODIS-derived AOD were indeed used to constrain CALIOP profiles inversion. A significant number of CALIOP level-1 vertical profiles have been averaged (~200 individual laser shots) in the Po Valley, leading to a signal-to-noise ratio (SNR) higher than 10 in the planetary boundary layer (PBL), which is sufficient to invert the mean lidar profiles. The mean LR (together with the associated variabilities) over the Po Valley retrieved from the coupling between CALIOP/MODIS-Aqua and CALIOP/MODIS-Terra are ~78±22 sr and ~86±27 sr, respectively. The total uncertainty on LR retrieval has been assessed to be ~12 sr using a Monte Carlo approach. The mean LR determined from a look-up table through a selection algorithm in CALIOP level 2 operational products (~63±8 sr) show a good agreement for daytime inversion (70±11 sr for Aqua and 74±14 sr for Terra). These values appear close to what is expected for pollution aerosols in an urban area. Contrarily large differences are observed when considering nighttime CALIOP profiles inverted with daytime AOD from MODIS (63±7 sr for CALIOP level-2 compared with 89±28 sr for CALIOP/Aqua and 103±32 sr for CALIOP/Terra synergies). They can be explained by a significant evolution of AOD between lidar and radiometer passing times. In most of cases, the mean aerosol extinction coefficient in the PBL significantly differs between the level-2 operational products and the result CALIPSO/MODIS synergy results. Mean differences of 0.10 km−1 (~50%) and 0.13 km−1 (~60%) have indeed been calculated using MODIS-Aqua/CALIOP and MODIS-Terra/CALIOP coupling studies, respectively. Such differences may be due to the identification of the aerosol model by the operational algorithm and thus to the choice of the LR.

scholarly journals Synergy between CALIOP and MODIS instruments for aerosol monitoring: application to the Po Valley

2010 ◽  
Vol 3 (2) ◽  
pp. 1323-1359 ◽  
Author(s):  
P. Royer ◽  
J.-C. Raut ◽  
G. Ajello ◽  
S. Berthier ◽  
P. Chazette
Keyword(s):  
Signal To Noise Ratio ◽  
Extinction Ratio ◽  
Orthogonal Polarization ◽  
Aerosol Extinction ◽  
Large Area ◽  
Total Uncertainty ◽  
Aerosol Model ◽  
Po Valley ◽  
The Mean ◽  
Level 2

Abstract. We propose here a synergy between Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations/Cloud-Aerosol LIdar with Orthogonal Polarization (CALIPSO/CALIOP) and Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua and Terra in order to retrieve aerosol optical properties over the Po Valley from June 2006 to February 2009. Such an approach gives simultaneously access to the aerosol extinction vertical profile and to the equivalent backscatter-to-extinction ratio at 532 nm (BER, inverse of the lidar ratio). The choice of the Po valley has been driven by the great occurrences of pollutant events leading to a mean MODIS-derived aerosol optical thickness of 0.27(±0.17) at 550 nm over a large area of ~120 000 km2. In such area, a significant number of CALIOP level-1 vertical profiles can be averaged (~200 individual laser shots) leading to a signal-to-noise ratio greater than 10 in the planetary boundary layer (PBL) sufficient to perform a homemade inversion of the mean lidar profiles. The mean BER (together with the associated variabilities) over the Po Valley retrieved from the coupling between CALIOP/MODIS-Aqua and CALIOP/MODIS-Terra are ~0.014(±0.003) sr−1 and ~0.013(±0.004) sr−1, respectively. The total uncertainty on BER retrieval has been assessed to be ~0.003 sr−1 using a Monte Carlo approach. These mean BER values retrieved have been compared with those given by the level-2 operational products of CALIOP ~0.016(±0.003) sr−1. The values we assessed appear close to what is expected above urban area. A seasonal cycle has been observed with higher BER values in spring, summer and fall, which can be associated to dust event occurring during this period. In most of cases, the mean aerosol extinction coefficient in the PBL diverges significantly between the level-2 operational products and the result of our own inversion procedure. Indeed, mean differences of 0.10 km−1 (~50%) and 0.13 km−1 (~60%) have been calculated using MODIS-Aqua/CALIOP and MODIS-Terra/CALIOP synergies, respectively. Such differences may be due to the identification of the aerosol model by the operational algorithm and thus to the choice of the BER.

Dust lidar ratios retrieved from the space-borne CALIOP measurements using the MODIS AOD as a constraint

2020 ◽  
Author(s):  
Sang-Woo Kim ◽  
Man-Hae Kim ◽  
Ali Omar
Keyword(s):  
Mean Values ◽  
Deep Blue ◽  
Source Regions ◽  
Modis Aod ◽  
Lidar Ratio ◽  
The Mean ◽  
Level 1 ◽  
Log Normal ◽  
Log Normal Distribution ◽  
Level 2

<p class="MDPI31text"><span lang="EN-US">Dust lidar ratios are retrieved by a synergetic use of CALIOP and MODIS products for 5 years from 2007 to 2011. The CALIOP level 1 total attenuated backscatter data is used for the retrieval and the CALIOP level 2 aerosol profile product is used to determine dust layers. Quality assured (QA > 1 for dark target ocean, QA = 3 for deep blue land) aerosol optical depth (AOD) data from the MODIS level 2 aerosol product is used as constraint. MODIS AOD retrievals and CALIOP attenuated backscatter profiles closer than 10 km from the center of MODIS pixel are defined as collocated measurements. Clouds are screened out for both CALIOP and MODIS. The retrieval is performed for the whole column of the atmosphere from 30 km to the surface adopting a constant lidar ratio of 30 sr for aerosols of clear air above the detected layers. The retrieved dust lidar ratios show a log-normal distribution with mean (median) values of 39.5 ± 16.8 (38.1) sr and 46.6 ± 36.3 (39.2) sr for ocean and land, respectively. The mean values are comparable to the value of 44 sr currently used in the CALIOP level 2 aerosol algorithm but the median values are relatively lower. There is a distinct regional variation in the retrieved dust lidar ratios. Dust lidar ratio is larger for the Saharan Desert (49.5 ± 36.8 sr) than the Arabian Desert (42.5 ± 26.2 sr), which is consistent with many previous studies. Dust aerosols transported to the Mediterranean Sea (44.4 ± 15.9 sr), Mid Atlantic (40.3 ± 12.4 sr) and Arabian Sea (37.5 ± 12.1 sr) show lower values compared with their source regions. An aging process of the long-range transported dust to remote ocean may be responsible for low lidar ratios. Dust lidar ratio over ocean in East Asia is 41.8 ± 27.6 sr is comparable with previous studies. Over Taklamakan and Gobi Deserts region the retrieved dust lidar ratios (35.5 ± 31.1 sr) show low values but still comparable with previous studies. Dust lidar ratios for Australia (35.4 ± 34.4 sr) are also relatively low compared with other regions. Although the mean AOD difference between CALIOP and MODIS is small (close to zero), the distribution of the AOD difference shows that the CALIOP AOD is biased low. However, when including clear air AOD for CALIOP, AODs from the two sensors become more comparable. A conclusion that can be drawn from this is that retrieving only for the detected layers in the CALIOP algorithm is one of the major reasons for lower AODs for CALIOP than MODIS. Lidar ratios retrieved in this study are strongly affected by MODIS AOD, because it is used as a constraint for the retrieval. </span></p>

scholarly journals Daytime aerosol extinction profiles from the combination of CALIOP profiles and AERONET products

2013 ◽  
Vol 6 (2) ◽  
pp. 3983-4038 ◽  
Author(s):  
C. Marcos ◽  
R. Pedrós ◽  
J. L. Gómez-Amo ◽  
M. Sicard ◽  
M. P. Utrillas ◽  
...  
Keyword(s):  
Signal To Noise Ratio ◽  
Mediterranean Coast ◽  
Saharan Dust ◽  
Orthogonal Polarization ◽  
Aerosol Extinction ◽  
Lidar Measurements ◽  
Level 1 ◽  
The Vertical Distribution ◽  
Level 2 ◽  
Good Agreement

Abstract. The solar background illumination has a strong effect on CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) measurements, leading to a decrease in the signal-to-noise ratio of the lidar signal. Because of this, CALIOP level 2 data algorithms might be limited in the retrieval of the properties of the aerosols in the atmosphere. In this work, we present a methodology that combines CALIOP level 1 data with AERONET (Aerosol RObotic NETwork) measurements to retrieve aerosol extinction profiles and lidar ratios in daytime conditions. In this way, we fulfill a two-fold objective: first, we obtain more accurate daytime aerosol information; second, we supplement column integrated measurements from AERONET sun photometers with information about the vertical distribution of aerosols. The methodology has been applied to Burjassot (39.30° N, 0.25° W) and Barcelona (41.39° N, 2.11° E) AERONET stations in the Mediterranean coast of Spain in the period from June 2006 to September 2011. We have found good agreement for the extinction profiles in several study cases of ground lidar measurements in Barcelona, coincident with CALIOP overpasses. Finally, the methodology has proved to be useful for the study of special episodes such as Saharan dust outbreaks.

scholarly journals Assessment of the Level-3 MODIS daily aerosol optical depth in the context of surface solar radiation and numerical weather modeling

2013 ◽  
Vol 13 (2) ◽  
pp. 675-692 ◽  
Author(s):  
J. A. Ruiz-Arias ◽  
J. Dudhia ◽  
C. A. Gueymard ◽  
D. Pozo-Vázquez
Keyword(s):  
Solar Radiation ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Surface Solar Radiation ◽  
Numerical Weather ◽  
Modis Aod ◽  
Level 3 ◽  
Weather Modeling ◽  
The Mean ◽  
Level 2

Abstract. The daily Level-3 MODIS aerosol optical depth (AOD) product is a global daily spatial aggregation of the Level-2 MODIS AOD (10-km spatial resolution) into a regular grid with a resolution of 1° × 1°. It offers interesting characteristics for surface solar radiation and numerical weather modeling applications. However, most of the validation efforts so far have focused on Level-2 products and only rarely on Level 3. In this contribution, we compare the Level-3 Collection 5.1 MODIS AOD dataset from the Terra satellite available since 2000 against observed daily AOD values at 550 nm from more than 500 AERONET ground stations around the globe. Overall, the mean error of the dataset is 0.03 (17%, relative to the mean ground-observed AOD), with a root mean square error of 0.14 (73%, relative to the same), but these errors are also found highly dependent on geographical region. We propose new functions for the expected error of the Level-3 AOD, as well as for both its mean error and its standard deviation. Additionally, we investigate the role of pixel count vis-à-vis the reliability of the AOD estimates, and also explore to what extent the spatial aggregation from Level 2 to Level 3 influences the total uncertainty in the Level-3 AOD. Finally, we use a radiative transfer model to investigate how the Level-3 AOD uncertainty propagates into the calculated direct normal and global horizontal irradiances.

scholarly journals Self-reported waist circumference compared with the ‘Waist Watcher’ tape-measure to identify individuals at increased health risk through intra-abdominal fat accumulation

1998 ◽  
Vol 80 (1) ◽  
pp. 81-88 ◽  
Author(s):  
T.S. Han ◽  
M.E.J. Lean
Keyword(s):  
Waist Circumference ◽  
Screening Tool ◽  
Reference Method ◽  
Abdominal Fat ◽  
Fat Accumulation ◽  
Tape Measure ◽  
The Mean ◽  
Level 1 ◽  
Level 2 ◽  
Action Levels

We evaluated the accuracy of self-reported home-assessed and self-measured waist circumference in 101 men and eighty-three women aged 28–67 years. The main outcome measures were subjects' self-reported and self-measured waist circumference, and self-classification according to the previously defined waist action level 1 (940 mm in men, 800 mm in women) and action level 2 (1020 mm in men, 880 mm in women), and waist circumference measured by the investigator using the ‘Waist Watcher’ tape-measure, as the reference method. The mean errors (95% CI limits of agreement) for subjects' self-reported waist circumference (self-reported minus reference; mm) were −67 (95% CI −210, 77) in men and −43 (95% CI −211, 123) in women, and for self-measured waist circumference (mm) using the ‘Waist Watcher’ (self-measured minus reference) were −5 (95% CI −62, 52) in men and −4 (95% CI −50, 42) in women. The proportions of subjects classified into waist action level 1 or action level 2 by the investigator were used as the reference method. Self-reported waist circumference of men and women respectively would be classified correctly in different categories based on action level 1 with sensitivities of 58·3 and 78·7%, and specificities of 98·5 and 98·7%, and action level 2 with sensitivities of 38·3 and 48·9%, and specificities of 98·5 and 98·7%. Using the ‘Waist Watcher’ with different colour bands based on the action levels, male and female subjects respectively classified themselves into correct categories according to action level 1 with sensitivities of 100 and 98·7%, and specificities of 98·1 and 98·2%, and according to action level 2 with sensitivities of 98·1 and 100%, and specificities of 100% for both sexes. Only 2% of the sample misclassified themselves into the wrong categories according to waist circumference action levels. In conclusion, people tend to underestimate their waist circumference, but the ‘Waist Watcher’ tape-measure offers advantages over self-reported home-assessed measurement, and may be used as a screening tool for self-classifying the risk of ill health through intra-abdominal fat accumulation.

scholarly journals Megha-Tropiques/SAPHIR measurements of humidity profiles: validation with AIRS and global radiosonde network

2013 ◽  
Vol 6 (6) ◽  
pp. 11405-11437 ◽  
Author(s):  
K. V. Subrahmanyam ◽  
K. K. Kumar
Keyword(s):  
Regression Analysis ◽  
North America ◽  
East Asia ◽  
Correlation Coefficient ◽  
Correlation Coefficients ◽  
The Mean ◽  
Level 1 ◽  
Level 2 ◽  
Humidity Profiles ◽  
Dry Bias

Abstract. The vertical profiles of humidity measured by SAPHIR (Sondeur Atmospherique du Profil d' Humidité Intropicale par Radiométrie) on-board Megha-Tropiques satellite are validated using Atmosphere Infrared Sounder (AIRS) and ground based radiosonde observations during July–September 2012. SAPHIR provides humidity profiles at six pressure layers viz., 1000–850 (level 1), 850–700 (level 2), 700–550 (level 3), 550–400 (level 4) 400–250 (level 5) and 250–100(level 6) hPa. Segregated AIRS observations over land and oceanic regions are used to assess the performance of SAPHIR quantitatively. The regression analysis over oceanic region (125° W–180° W; 30° S–30° N) reveal that the SAPHIR measurements agrees very well with the AIRS measurements at levels 3, 4, 5 and 6 with correlation coefficients 0.79, 0.88, 0.87 and 0.78 respectively. However, at level 6 SAPHIR seems to be systematically underestimating the AIRS measurements. At level 2, the agreement is reasonably good with correlation coefficient of 0.52 and at level 1 the agreement is very poor with correlation coefficient 0.17. The regression analysis over land region (10° W–30° E; 8° N–30° N) revealed an excellent correlation between AIRS and SAPHIR at all the six levels with 0.80, 0.78, 0.84, 0.84, 0.86 and 0.65 respectively. However, again at levels 5 and 6, SAPHIR seems to be underestimating the AIRS measurements. After carrying out the quantitative comparison between SAPHIR and AIRS separately over land and ocean, the ground based global radiosonde network observations of humidity profiles over three distinct geographical locations (East Asia, tropical belt of South and North America and South Pacific) are then used to further validate the SAPHIR observations as AIRS has its own limitations. The SAPHIR observations within a radius of 50 km around the radiosonde stations are averaged and then the regression analysis is carried out at the first five levels of SAPHIR. The comparison is not carried out at sixth level due to inaccuracies of radiosonde measurements of humidity at this level. From the regression analysis, it is found that the SAPHIR observations agree very well with the radiosonde observations at all the five levels with correlation coefficients 0.65, 0.72, 0.84, 0.88 and 0.78 respectively. Among the three regions considered for the present study, the correlation was poor at the first level over East Asia. Further, statistical analysis showed that at first level the SAPHIR observations have wet bias at low humidity magnitudes and dry bias at high humidity magnitudes. The humidity magnitude at which wet bias changes to dry bias varied from one level to the other. The mean bias between the radiosonde and the SAPHIR observations are also estimated separately for the three regions. The mean bias profiles showed that SAPHIR has wet bias at all the five levels over South/North America and South Pacific regions. However, the results showed dry bias at all the levels except 2nd and 3rd levels, where it showed wet bias, over East Asia. In a nutshell, the results indicated that SAPHIR has wet bias over dry regions and dry bias over wet regions. The important outcome of the present study is the quantitative validation of the SAPHIR humidity observations using both space and ground based measurements. The present results are very encouraging and envisage the great potential of SAPHIR observations for meteorological applications especially in understanding the hydrological cycle at shorter temporal and spatial scales in the Tropics.

Improvement of Image Steganography Using Discrete Wavelet Transform

2020 ◽  
Vol 38 (1A) ◽  
pp. 83-87
Author(s):  
Manal K. Oudah ◽  
Rula S. Khudhair ◽  
Saad M. Kaleefah ◽  
Aqeela N. Abed
Keyword(s):  
Wavelet Transform ◽  
Discrete Wavelet Transform ◽  
Signal To Noise Ratio ◽  
Discrete Wavelet ◽  
Signal To Noise ◽  
Band Frequency ◽  
Message Length ◽  
Frequency Components ◽  
Level 1 ◽  
Level 2

Recently the Discrete-Wavelet-Transform (DWT) has been represented as signal processing powerful tool to separate the signal into its band frequency components. In this paper, improvement of the steganography techniques by hiding the required message into the suitable frequency band is presented. The results show that the increase of the message length will reduce the Peak Signal to Noise Ratio (PSNR), while the PSNR increases with the increasing the DWT levels. It should be noted that the PSNR reduction was from -13.8278 to -17.77208 when increasing the message length from 161 to 505 characters. In this context, the PSNR is increased from -13.8278 to 7.0554 and from -17.7208 to 1.7901 when the DWT increased from level (1) to level (2).

scholarly journals Comparison of Two Different Cloud Climatologies Derived from CALIOP-Attenuated Backscattered Measurements (Level 1): The CALIPSO-ST and the CALIPSO-GOCCP

2013 ◽  
Vol 30 (4) ◽  
pp. 725-744 ◽  
Author(s):  
H. Chepfer ◽  
G. Cesana ◽  
D. Winker ◽  
B. Getzewich ◽  
M. Vaughan ◽  
...  
Keyword(s):  
Signal To Noise Ratio ◽  
Coupled Model ◽  
Orthogonal Polarization ◽  
Cloud Detection ◽  
Model Intercomparison ◽  
Spatiotemporal Resolution ◽  
Wide Range ◽  
Intercomparison Project ◽  
Level 1 ◽  
High Level

Abstract Two different cloud climatologies have been derived from the same NASA–Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)-measured attenuated backscattered profile (level 1, version 3 dataset). The first climatology, named Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations–Science Team (CALIPSO-ST), is based on the standard CALIOP cloud mask (level 2 product, version 3), with the aim to document clouds with the highest possible spatiotemporal resolution, taking full advantage of the CALIOP capabilities and sensitivity for a wide range of cloud scientific studies. The second climatology, named GCM-Oriented CALIPSO Cloud Product (CALIPSO-GOCCP), is aimed at a single goal: evaluating GCM prediction of cloudiness. For this specific purpose, it has been designed to be fully consistent with the CALIPSO simulator included in the Cloud Feedback Model Intercomparison Project (CFMIP) Observation Simulator Package (COSP) used within version 2 of the CFMIP (CFMIP-2) experiment and phase 5 of the Coupled Model Intercomparison Project (CMIP5). The differences between the two datasets in the global cloud cover maps—total, low level (P > 680 hPa), midlevel (680 < P < 440 hPa), and high level (P < 440 hPa)—are frequently larger than 10% and vary with region. The two climatologies show significant differences in the zonal cloud fraction profile (which differ by a factor of almost 2 in some regions), which are due to the differences in the horizontal and vertical averaging of the measured attenuated backscattered profile CALIOP profile before the cloud detection and to the threshold used to detect clouds (this threshold depends on the resolution and the signal-to-noise ratio).

scholarly journals Evaluation of acceptability of the use of internal and external condoms in couples: a cross-over study

2020 ◽  
Vol 9 (9) ◽  
pp. e846997942
Author(s):  
Tatiana Moreira Afonso ◽  
Taciana Silveira Passos ◽  
Marcos Antonio Almeida-Santos
Keyword(s):  
Repeated Measures ◽  
Analogue Scale ◽  
Ease Of Use ◽  
Wave Type ◽  
Four Dimensions ◽  
Level 3 ◽  
The Mean ◽  
Level 1 ◽  
Level 2 ◽  
Better Than

Objective: To evaluate the acceptability (comfort, ease of use, lubrication, and pleasure) of internal and external condom use in stable couples. Metodology: This randomized cross-over trial included 54 couples (108 adult individuals). Participants were asked to evaluate acceptability measures of both types of condoms using the Visual Analogue Scale.  We developed a three-level hierarchical model [level 1= three repeated measures (wave, type of condom); level 2= individual (gender, age); level 3= couple (sequence of presentation of female versus external condom)]. Results: The "ease of use" dimension represented the biggest difference between the types of condoms for men (Hedges 'g = 0.96; p <0.0001) and women (Hedges' g = 1.62; p <0.0001). Regardless of gender, external condoms performed better (1.61; 95% HPD = 1.41 – 1.81). Men gave lower scores in general (-0.62; 95% HPD = -1.16 − -0.10). Conclusion: The external condom scored better than the internal condom for women and their male partners. The scores tended to improve with the repetitive use of the condom. There is no evidence of the influence of the age of the individuals or the randomly selected sequence of use of condoms with regards to the mean score of the four dimensions.

scholarly journals The role of cloud contamination, aerosol layer height and aerosol model in the assessment of the OMI near-UV retrievals over the ocean

2016 ◽  
Vol 9 (7) ◽  
pp. 3031-3052 ◽  
Author(s):  
Santiago Gassó ◽  
Omar Torres
Keyword(s):  
Particle Shape ◽  
Spherical Shape ◽  
Detailed Examination ◽  
Index Of Refraction ◽  
Retrieval Algorithm ◽  
Orthogonal Polarization ◽  
Aerosol Layer ◽  
Ozone Monitoring Instrument ◽  
Aerosol Model ◽  
Modis Aod

Abstract. Retrievals of aerosol optical depth (AOD) at 388 nm over the ocean from the Ozone Monitoring Instrument (OMI) two-channel near-UV algorithm (OMAERUV) have been compared with independent AOD measurements. The analysis was carried out over the open ocean (OMI and MODerate-resolution Imaging Spectrometer (MODIS) AOD comparisons) and over coastal and island sites (OMI and AERONET, the AErosol RObotic NETwork). Additionally, a research version of the retrieval algorithm (using MODIS and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) information as constraints) was utilized to evaluate the sensitivity of the retrieval to different assumed aerosol properties. Overall, the comparison resulted in differences (OMI minus independent measurements) within the expected levels of uncertainty for the OMI AOD retrievals (0.1 for AOD < 0.3, 30 % for AOD > 0.3). Using examples from case studies with outliers, the reasons that led to the observed differences were examined with specific purpose to determine whether they are related to instrument limitations (i.e., pixel size, calibration) or algorithm assumptions (such as aerosol shape, aerosol height). The analysis confirms that OMAERUV does an adequate job at rejecting cloudy scenes within the instrument's capabilities. There is a residual cloud contamination in OMI pixels with quality flag 0 (the best conditions for aerosol retrieval according to the algorithm), resulting in a bias towards high AODs in OMAERUV. This bias is more pronounced at low concentrations of absorbing aerosols (AOD 388 nm  ∼  < 0.5). For higher aerosol loadings, the bias remains within OMI's AOD uncertainties. In pixels where OMAERUV assigned a dust aerosol model, a fraction of them (< 20 %) had retrieved AODs significantly lower than AERONET and MODIS AODs. In a case study, a detailed examination of the aerosol height from CALIOP and the AODs from MODIS, along with sensitivity tests, was carried out by varying the different assumed parameters in the retrieval (imaginary index of refraction, size distribution, aerosol height, particle shape). It was found that the spherical shape assumption for dust in the current retrieval is the main cause of the underestimate. In addition, it is demonstrated in an example how an incorrect assumption of the aerosol height can lead to an underestimate. Nevertheless, this is not as significant as the effect of particle shape. These findings will be incorporated in a future version of the retrieval algorithm.

Sign in / Sign up

Export Citation Format

Share Document