scholarly journals Retrieval of Aerosol Optical Depth above Clouds from OMI Observations: Sensitivity Analysis and Case Studies

2012 ◽  
Vol 69 (3) ◽  
pp. 1037-1053 ◽  
Author(s):  
Omar Torres ◽  
Hiren Jethva ◽  
P. K. Bhartia
Keyword(s):  
Sensitivity Analysis ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Forest Fires ◽  
Radiative Forcing ◽  
Large Fraction ◽  
Single Scattering ◽  
Aerosol Layer ◽  
Carbonaceous Particles ◽  
True Value

Abstract A large fraction of the atmospheric aerosol load reaching the free troposphere is frequently located above low clouds. Most commonly observed aerosols above clouds are carbonaceous particles generally associated with biomass burning and boreal forest fires, and mineral aerosols originating in arid and semiarid regions and transported across large distances, often above clouds. Because these aerosols absorb solar radiation, their role in the radiative transfer balance of the earth–atmosphere system is especially important. The generally negative (cooling) top-of-the-atmosphere direct effect of absorbing aerosols may turn into warming when the light-absorbing particles are located above clouds. The actual effect depends on the aerosol load and the single scattering albedo, and on the geometric cloud fraction. In spite of its potential significance, the role of aerosols above clouds is not adequately accounted for in the assessment of aerosol radiative forcing effects due to the lack of measurements. This paper discusses the basis of a simple technique that uses near-UV observations to simultaneously derive the optical depth of both the aerosol layer and the underlying cloud for overcast conditions. The two-parameter retrieval method described here makes use of the UV aerosol index and reflectance measurements at 388 nm. A detailed sensitivity analysis indicates that the measured radiances depend mainly on the aerosol absorption exponent and aerosol–cloud separation. The technique was applied to above-cloud aerosol events over the southern Atlantic Ocean, yielding realistic results as indicated by indirect evaluation methods. An error analysis indicates that for typical overcast cloudy conditions and aerosol loads, the aerosol optical depth can be retrieved with an accuracy of approximately 54% whereas the cloud optical depth can be derived within 17% of the true value.

scholarly journals Sunphotometry of the 2006–2007 aerosol optical/radiative properties at the Himalayan Nepal Climate Observatory – Pyramid (5079 m a.s.l.)

2010 ◽  
Vol 10 (1) ◽  
pp. 1193-1220 ◽  
Author(s):  
G. P. Gobbi ◽  
F. Angelini ◽  
P. Bonasoni ◽  
G. P. Verza ◽  
A. Marinoni ◽  
...  
Keyword(s):  
High Altitude ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Radiative Forcing ◽  
Monsoon Season ◽  
Good Reason ◽  
Single Scattering ◽  
Radiative Properties ◽  
Mountain Range ◽  
Monsoon Period

Abstract. In spite of being located at the heart of the highest mountain range in the world, the Himalayan Nepal Climate Observatory (5079 m a.s.l.) at the Ev-K2-CNR Pyramid is shown to be affected by the advection of pollution aerosols from the populated regions of southern Nepal and the Indo-Gangetic plains. Such an impact is observed along most of the period April 2006–March 2007 addressed here, with a minimum in the monsoon season. Backtrajectory-analysis indicates long-range transport episodes occurring in this period to originate mainly in the West Asian deserts. At this high altitude site, the measured aerosol optical depth is observed to be: 1) about one order of magnitude lower than the one measured at Gandhi College (60 m a.s.l.), in the Indo-Gangetic basin, and 2) maximum during the monsoon period, due to the presence of elevated (cirrus-like) particle layers. Assessment of the aerosol radiative forcing results to be hampered by the persistent presence of these high altitude particle layers, which impede a continuous measurement of both the aerosol optical depth and its radiative properties from sky radiance inversions. Even though the retrieved absorption coefficients of pollution aerosols was rather large (single scattering albedo of the order of 0.6–0.9 were observed in the month of April 2006), the corresponding low optical depths (~0.03 at 500 nm) are expected to limit the relevant radiative forcings. Still, the high specific forcing of this aerosol and its capability of altering snow surface albedo provide good reason for continuous monitoring.

scholarly journals Daytime aerosol optical depth above low-level clouds is similar to that in adjacent clear skies at the same heights: airborne observation above the southeast Atlantic

2020 ◽  
Author(s):  
Yohei Shinozuka ◽  
Meloë S. Kacenelenbogen ◽  
Sharon P. Burton ◽  
Steven G. Howell ◽  
Paquita Zuidema ◽  
...  
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Wavelength Dependence ◽  
Single Scattering ◽  
Airborne Lidar ◽  
Aerosol Layer ◽  
Low Level ◽  
Satellite Retrieval ◽  
Biomass Burning Aerosol ◽  
The Difference

Abstract. To help satellite retrieval of aerosols and studies of their radiative effects, we demonstrate that daytime 532 nm aerosol optical depth over low-level clouds is similar to that in neighboring clear skies at the same heights in recent airborne lidar and sunphotometer observations above the southeast Atlantic. The mean AOD difference is between 0 and −0.01, when comparing the two sides, each up to 20 km wide, of cloud edges. The difference is not statistically significant according to a paired t-test. Systematic differences in the wavelength dependence of AOD and in situ single scattering albedo are also minute. These results hold regardless of the vertical distance between cloud top and aerosol layer bottom. AOD aggregated over ~ 2° grid boxes for each of September 2016, August 2017 and October 2018 also shows little correlation with the presence of low-level clouds. We posit that a satellite retrieval artifact is entirely responsible for a previous finding of generally smaller AOD over clouds (Chung et al., 2016), at least for the region and season of our study. Our results also suggest that the same values can be assumed for the intensive properties of free-tropospheric biomass-burning aerosol regardless of whether clouds exist below.

scholarly journals Estimate of surface direct radiative forcing of desert dust from atmospheric modulation of the aerosol optical depth

2013 ◽  
Vol 13 (11) ◽  
pp. 5647-5654 ◽  
Author(s):  
A. di Sarra ◽  
D. Fuà ◽  
D. Meloni
Keyword(s):  
Water Vapour ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Spectral Range ◽  
Radiative Forcing ◽  
Oscillation Amplitude ◽  
Aerosol Layer ◽  
Surface Irradiance ◽  
Perturbation Versus ◽  
Direct Radiative Forcing

Abstract. Measurements carried out on the island of Lampedusa, in the central Mediterranean, on 7 September 2005, show the occurrence of a quasi-periodic oscillation of aerosol optical depth, column water vapour, and surface irradiance in different spectral bands. The oscillation has a period of about 13 min and is attributed to the propagation of a gravity wave able to modify the vertical structure of the planetary boundary layer, as also confirmed by satellite images. The wave occurred during a Saharan dust event. The oscillation amplitude is about 0.1 for the aerosol optical depth, and about 0.4 cm for the column water vapour. The modulation of the downward surface irradiances is in opposition of phase with respect to aerosol optical depth and water vapour column variations. The perturbation of the downward irradiance produced by the aerosols is determined by comparing the measured irradiances with estimated irradiances at a fixed value of the aerosol optical depth, and by correcting for the effect of the water vapour in the shortwave spectral range. The direct radiative forcing efficiency, i.e., the radiative perturbation of the net surface irradiance produced by a unit of optical depth aerosol layer, is determined at different solar zenith angles as the slope of the irradiance perturbation versus the aerosol optical depth. The estimated direct surface forcing efficiency at about 60° solar zenith angle is −(181 ± 17) W m−2 in the shortwave, and −(83 ± 7) W m−2 in the photosynthetic spectral range. The estimated daily average forcing efficiencies are of about −79 and −46 W m−2 for the shortwave and photosynthetic spectral range, respectively.

scholarly journals Sunphotometry of the 2006–2007 aerosol optical/radiative properties at the Himalayan Nepal Climate Observatory-Pyramid (5079 m a.s.l.)

2010 ◽  
Vol 10 (22) ◽  
pp. 11209-11221 ◽  
Author(s):  
G. P. Gobbi ◽  
F. Angelini ◽  
P. Bonasoni ◽  
G. P. Verza ◽  
A. Marinoni ◽  
...  
Keyword(s):  
High Altitude ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Radiative Forcing ◽  
Monsoon Season ◽  
Single Scattering ◽  
Radiative Properties ◽  
Mountain Range ◽  
Anthropogenic Aerosol ◽  
Aerosol Radiative

Abstract. In spite of being located at the heart of the highest mountain range in the world, the Himalayan Nepal Climate Observatory (5079 m a.s.l.) at the Ev-K2-CNR Pyramid is shown to be affected by the advection of pollution aerosols from the populated regions of southern Nepal and the Indo-Gangetic plains. Such an impact is observed along most of the period April 2006–March 2007 addressed here, with a minimum in the monsoon season. Backtrajectory-analysis indicates long-range transport episodes occurring in this year to originate mainly in the west Asian deserts. At this high altitude site, the measured aerosol optical depth is observed to be about one order of magnitude lower than the one measured at Ghandi College (60 m a.s.l.), in the Indo-Gangetic basin. As for Ghandi College, and in agreement with the in situ ground observations at the Pyramid, the fine mode aerosol optical depth maximizes during winter and minimizes in the monsoon season. Conversely, total optical depth maximizes during the monsoon due to the occurrence of elevated, coarse particle layers. Possible origins of these particles are wind erosion from the surrounding peaks and hydrated/cloud-processed aerosols. Assessment of the aerosol radiative forcing is then expected to be hampered by the presence of these high altitude particle layers, which impede an effective, continuous measurement of anthropogenic aerosol radiative properties from sky radiance inversions and/or ground measurements alone. Even though the retrieved absorption coefficients of pollution aerosols were rather large (single scattering albedo of the order of 0.6–0.9 were observed in the month of April 2006), the corresponding low optical depths (~0.03 at 500 nm) are expected to limit the relevant radiative forcing. Still, the high specific forcing of this aerosol and its capability of altering snow surface albedo provide good reasons for continuous monitoring.

scholarly journals Dust optical properties over North Africa and Arabian Peninsula derived from the AERONET dataset

2011 ◽  
Vol 11 (7) ◽  
pp. 20181-20201 ◽  
Author(s):  
D. Kim ◽  
M. Chin ◽  
H. Yu ◽  
T. F. Eck ◽  
A. Sinyuk ◽  
...  
Keyword(s):  
Refractive Index ◽  
Optical Properties ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
North Africa ◽  
Radiative Forcing ◽  
Arabian Peninsula ◽  
Single Scattering ◽  
Major Fraction ◽  
Remote Sensing Techniques

Abstract. Dust optical properties over North Africa and the Arabian Peninsula are extracted from the quality assured multi-year datasets obtained at 14 sites of the Aerosol Robotic Network (AERONET). We select the data with (a) large aerosol optical depth (AOD ≥ 0.4 at 440 nm) and (b) small Ångström exponent (Åext ≤ 0.2) for retaining high accuracy and reducing interference of non-dust aerosols. The result indicates that the major fraction of high aerosol optical depth days are dominated by dust over these sites even though it varies depending on location and time. We have found that the annual mean and standard deviation of single scattering albedo, asymmetry parameter, real refractive index, and imaginary refractive index for Saharan and Arabian desert dust is 0.946 ± 0.005, 0.752 ± 0.014, 1.498 ± 0.032, and 0.0025 ± 0.0036 at 550 nm wavelength, respectively. Dust aerosol selected by this method is less absorbing than the previously reported values over these sites. The weaker absorption of dust from this study is consistent with the previously studies using remote sensing techniques. These results can help to constrain uncertainties in estimating global dust shortwave radiative forcing.

scholarly journals Dust optical properties over North Africa and Arabian Peninsula derived from the AERONET dataset

2011 ◽  
Vol 11 (20) ◽  
pp. 10733-10741 ◽  
Author(s):  
D. Kim ◽  
M. Chin ◽  
H. Yu ◽  
T. F. Eck ◽  
A. Sinyuk ◽  
...  
Keyword(s):  
Refractive Index ◽  
Optical Properties ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
North Africa ◽  
Radiative Forcing ◽  
Arabian Peninsula ◽  
Single Scattering ◽  
Major Fraction ◽  
Remote Sensing Techniques

Abstract. Dust optical properties over North Africa and the Arabian Peninsula are extracted from the quality assured multi-year datasets obtained at 14 sites of the Aerosol Robotic Network (AERONET). We select the data with (a) large aerosol optical depth (AOD ≥ 0.4 at 440 nm) and (b) small Ångström exponent (Åext ≤ 0.2) for retaining high accuracy and reducing interference of non-dust aerosols. The result indicates that the major fraction of high aerosol optical depth days are dominated by dust over these sites even though it varies depending on location and time. We have found that the annual mean and standard deviation of single scattering albedo, asymmetry parameter, real refractive index, and imaginary refractive index for Saharan and Arabian desert dust is 0.944 ± 0.005, 0.752 ± 0.014, 1.498 ± 0.032, and 0.0024 ± 0.0034 at 550 nm wavelength, respectively. Dust aerosol selected by this method is less absorbing than the previously reported values over these sites. The weaker absorption of dust from this study is consistent with the studies using remote sensing techniques from satellite. These results can help to constrain uncertainties in estimating global dust shortwave radiative forcing.

scholarly journals Daytime aerosol optical depth above low-level clouds is similar to that in adjacent clear skies at the same heights: airborne observation above the southeast Atlantic

2020 ◽  
Vol 20 (19) ◽  
pp. 11275-11285 ◽  
Author(s):  
Yohei Shinozuka ◽  
Meloë S. Kacenelenbogen ◽  
Sharon P. Burton ◽  
Steven G. Howell ◽  
Paquita Zuidema ◽  
...  
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Wavelength Dependence ◽  
Single Scattering ◽  
Airborne Lidar ◽  
Aerosol Layer ◽  
Low Level ◽  
Satellite Retrieval ◽  
Biomass Burning Aerosol ◽  
The Difference

Abstract. To help satellite retrieval of aerosols and studies of their radiative effects, we demonstrate that daytime aerosol optical depth over low-level clouds is similar to that in neighboring clear skies at the same heights. Based on recent airborne lidar and sun photometer observations above the southeast Atlantic, the mean aerosol optical depth (AOD) difference at 532 nm is between 0 and −0.01, when comparing the cloudy and clear sides, each up to 20 km wide, of cloud edges. The difference is not statistically significant according to a paired t test. Systematic differences in the wavelength dependence of AOD and in situ single scattering albedo are also minuscule. These results hold regardless of the vertical distance between cloud top and aerosol layer bottom. AOD aggregated over ∼2∘ grid boxes for each of September 2016, August 2017 and October 2018 also shows little correlation with the presence of low-level clouds. We posit that a satellite retrieval artifact is entirely responsible for a previous finding of generally smaller AOD over clouds (Chung et al., 2016), at least for the region and time of our study. Our results also suggest that the same values can be assumed for the intensive properties of free-tropospheric biomass-burning aerosol regardless of whether clouds are present below.

Retrieval of aerosol single scattering albedo using joint satellite and surface visibility measurements

2020 ◽  
Author(s):  
Jing Li ◽  
Yueming Dong
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Radiative Forcing ◽  
Optical Parameter ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Retrieval Algorithm ◽  
Extensive Coverage ◽  
Satellite Sensors ◽  
Aerosol Radiative

<p>Aerosol single scattering albedo is a critical optical parameter that determines aerosol radiative effect. However, most existing passive satellite sensors such as MODIS and VIIRS only measures the intensity of reflected solar radiation and can only retrieve aerosol optical depth, while aerosol single scattering albedo needs to be assumed in the retrieval algorithm. On the other hand, if aerosol optical depth is known, it would be possible to retrieve aerosol single scattering albedo using satellite sensors.  In this study, we develop a machine learning based algorithm that retrieves aerosol single scattering albedo using joint visibility and satellite measurements. Combined with meteorology variables including relative humidity and boundary layer height, surface visibility can be converted to column aerosol optical depth. Then combining this converted aerosol optical depth with VIIRS measured TOA apparent reflectance, we retrieve aerosol single scattering albedo at over 2000 stations worldwide. The results compare well with AERONET retrieved SSA. However, compared with AERONET, visibility is recorded at every WMO meteorology station and has much more extensive coverage. We also applied our method to surface PM2.5 measurements obtained satisfactory results. Our work provides an aerosol single scattering albedo dataset with extensive coverage over land, which can be used for aerosol radiative forcing calculations and model validation.</p>

scholarly journals Estimate of surface direct radiative forcing of desert dust from atmospheric modulation of the aerosol optical depth

2013 ◽  
Vol 13 (1) ◽  
pp. 527-548
Author(s):  
A. di Sarra ◽  
D. Fuà ◽  
D. Meloni
Keyword(s):  
Water Vapour ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Spectral Range ◽  
Radiative Forcing ◽  
Oscillation Amplitude ◽  
Aerosol Layer ◽  
Surface Irradiance ◽  
Perturbation Versus ◽  
Direct Radiative Forcing

Abstract. Measurements carried out on the island of Lampedusa, in the central Mediterranean, on 7 September 2005, show the occurrence of a quasi periodic oscillation of aerosol optical depth, column water vapour, and surface irradiance in different spectral bands. The oscillation has a period of about 13 min and is attributed to the propagation of a gravity wave able to modify the vertical structure of the planetary boundary layer. The wave occurred during an event of Saharan dust at Lampedusa. The oscillation amplitude is about 0.1 for the aerosol optical depth, and about 0.4 cm for the column water vapour. The modulation of the downward surface irradiances is in opposition of phase with respect to aerosol optical depth and water vapour column variations. The perturbation to the downward irradiance produced by the aerosols is determined by comparing the measured irradiances with estimated irradiances at a fixed value of the aerosol optical depth, and by correcting for the effect of the water vapour in the shortwave spectral range. The direct radiative forcing efficiency, i.e. the radiative perturbation to the net surface irradiance produced by a unit optical depth aerosol layer, is determined at different solar zenith angles as the slope of the irradiance perturbation versus the aerosol optical depth. The estimated direct surface forcing efficiency at 60° solar zenith angle is −(181 ± 17) W m−2 in the shortwave, and −(83 ± 7) W m−2 in the photosynthetic spectral range. The estimated daily average forcing efficiencies are of about −79 and −46 W m−2 for the shortwave and photosynthetic spectral range, respectively.

scholarly journals Aerosol optical, microphysical and radiative properties at regional background insular sites in the western Mediterranean

2016 ◽  
Vol 16 (18) ◽  
pp. 12177-12203 ◽  
Author(s):  
Michaël Sicard ◽  
Rubén Barragan ◽  
François Dulac ◽  
Lucas Alados-Arboledas ◽  
Marc Mallet
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Forest Fires ◽  
Urban Areas ◽  
Radiative Forcing ◽  
Mediterranean Basin ◽  
Western Mediterranean ◽  
Radiative Properties ◽  
Regional Background ◽  
Western Mediterranean Basin

Abstract. In the framework of the ChArMEx (the Chemistry-Aerosol Mediterranean Experiment; http://charmex.lsce.ipsl.fr/) program, the seasonal variability of the aerosol optical, microphysical and radiative properties derived from AERONET (Aerosol Robotic Network; http://aeronet.gsfc.nasa.gov/) is examined in two regional background insular sites in the western Mediterranean Basin: Ersa (Corsica Island, France) and Palma de Mallorca (Mallorca Island, Spain). A third site, Alborán (Alborán Island, Spain), with only a few months of data is considered for examining possible northeast–southwest (NE–SW) gradients of the aforementioned aerosol properties. The AERONET dataset is exclusively composed of level 2.0 inversion products available during the 5-year period 2011–2015. AERONET solar radiative fluxes are compared with ground- and satellite-based flux measurements. To the best of our knowledge this is the first time that AERONET fluxes are compared with measurements at the top of the atmosphere. Strong events (with an aerosol optical depth at 440 nm greater than 0.4) of long-range transport aerosols, one of the main drivers of the observed annual cycles and NE–SW gradients, are (1) mineral dust outbreaks predominant in spring and summer in the north and in summer in the south and (2) European pollution episodes predominant in autumn. A NE–SW gradient exists in the western Mediterranean Basin for the aerosol optical depth and especially its coarse-mode fraction, which all together produces a similar gradient for the aerosol direct radiative forcing. The aerosol fine mode is rather homogeneously distributed. Absorption properties are quite variable because of the many and different sources of anthropogenic particles in and around the western Mediterranean Basin: North African and European urban areas, the Iberian and Italian peninsulas, most forest fires and ship emissions. As a result, the aerosol direct forcing efficiency, more dependent to absorption than the absolute forcing, has no marked gradient.

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