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1. What are the methods used to retrieve aerosol composition and investigate global distributions?

2. How are CALIOP-MODIS retrievals compared?

3. What technique is used for inversion in the retrieval procedure?

4. How was the surface reflection over the ocean calculated?

Accepted Answer

Several ground-based remote sensing methods have been developed to retrieve aerosol composition. These methods include using direct and diffuse solar radiation in the visible and near infrared wavelength regions measured by pyranometers and pyrheliometers (Kudo et al., 2010a), conventional Mie-scattering lidar (Nishizawa et al., 2007, 2008, 2011, 2017), high-spectral-resolution lidar or Raman lidar data from the Asian Dust and Aerosol 45 Lidar Observation Network (AD-Net; Sugimoto et al., 2015, 2016), and Aerosol Robotics Network (AERONET; Holben et al., 1998). Satellite remote sensing has also been used, such as retrieving the spatiotemporal distributions of sulfate, carbonaceous, DS, and SS aerosols from spectral information on radiances observed by satellite imagers (Higurashi and Nakajima, 2002; Kim et al., 2007). The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the Cloud Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) satellite has been utilized to classify aerosols at different altitudes (Winker et al., 2010). In the CALIOP Version 4 product, tropospheric aerosols are subdivided into seven types, and stratospheric aerosols into four types (Kim et al., 2018). These methods assume that aerosols consist of a few components with different sizes, light-absorbing features, and shapes. Synergistic remote sensing methods using active and passive sensors have been developed, combining passive sensors like spectral radiometers and polarimeters with active sensing by lidar to retrieve aerosol data obtained by a single instrument.

Accepted Answer

The CALIOP-MODIS retrievals in 2010 are compared with the CALIPSO and MODIS standard products and AERONET products. The CALIPSO standard product comprises the monthly means of AOD and the extinction coefficient (EC) in the cloud-free daytime data set of the CALIPSO Lidar Level 3 Tropospheric Aerosol product Version 4 (Tackett et al., 2018). The MODIS standard product comprises the monthly means of AOD in the MYD08_M3 Collection 6.1 Aqua Atmosphere Monthly Global Product (Platnick et al., 2015). The annual means were calculated from the monthly means. The AERONET products comprise AOD, SSA, AF, and fine and coarse mode radii in the level 2 data set of 120 the version 3 inversion (Giles et al., 2019; Sinyuk et al., 2020). DS are optimized to all CALIOP and MODIS measurements. DVC and DMR are defined as the volume concentration and median radius, respectively, at a relative humidity of 0%. Only the vertical layers discriminated as aerosols in the VFM data are targeted for retrieval, and the CALIOP-MODIS retrieval is conducted for only clear sky data in the daytime. If clouds 130 are detected in the VFM data, the CALIOP-MODIS retrieval is not conducted.

Accepted Answer

The optimal estimation technique developed by Kudo et al. (2016) is used for inversion in the retrieval procedure. This technique simultaneously optimizes the state vector with measurements and a priori constraints by minimizing an objective function. The minimization is conducted using an iterative algorithm with logarithmic transformation for stable and fast convergence. CALIOP measurements, which can have negative values due to signal noise, are transformed to ensure accurate results. This technique allows for efficient and accurate retrieval of state vectors in the presence of complex measurements and constraints.

Accepted Answer

The surface reflection over the ocean was calculated using the physical model of Nakajima and Tanaka (1983). This model utilized the surface wind speed as an input to determine the reflection. The model takes into account various factors such as wind speed, temperature, and humidity to accurately calculate the reflection over the ocean surface. By incorporating these parameters, the model provides a realistic representation of the ocean's surface reflection, which is crucial for accurate MODIS observations. The use of this model ensures that the calculated radiances from the PSTAR vector radiative transfer model are reliable and reflect the actual conditions of the ocean surface. This approach enhances the accuracy of the forward model and contributes to the overall quality of the MODIS observations.

Accepted Answer

Uncertainties of CALIOP-MODIS retrieval products were evaluated through simulations of CALIOP and MODIS observations. Simulations were conducted for 16 aerosol compositions, different AOD values, land and ocean surfaces, and solar zenith angles. The transport of WS, LA, and DS in the free atmosphere was considered for biomass-burning and dust cases. Vertical profiles for transported aerosols assumed a normal distribution. Boundary layer height was set at 2 km, and EC of aerosols decreased linearly with altitude. DMRs of 0.07, 0.1, and 0.15 mm were used for WS and LA, and 1.0, 2.0, and 4.0 mm for DS. AOD values at 532 nm were 0.05, 0.1, 0.3, 0.5, 0.7, and 1.0. Three land surface types were considered, with surface albedo values taken from the ECOSTRESS Spectral Library database. Surface wind speeds of 5, 15, and 25 m/s were used for the ocean surface. Solar zenith angles of 0, 20, 40, and 60 degrees were used. Random errors were added to simulated data, with CALIOP and MODIS observations having random errors less than +-15%, +-20%, and +-50% for ABC at 532 nm, +-5% for radiances at bands 1 and 2, and +-0.10 for surface albedo. Surface wind speed errors over the ocean were considered less than +-5 m/s. A total of 1152 simulations were conducted using forward models described in Sects.3.1.2.2 and 3.1.2.3.

Accepted Answer

The AODs at 532 and 1064 nm of CALIOP-MODIS retrievals agreed well with those of AERONET, with slopes of the relationships almost 1.0. The means and standard deviations of the relative differences between the CALIOP-MODIS retrievals and AERONET products were 0.04 +- 0.70 for AOD at 532 nm, and 0.14 +- 0.50 for AOD at 1064 nm. SSA retrievals were fewer and had high uncertainty when AOD was small, and the CALIOP-MODIS retrievals were underestimated. The mean +- standard deviation of the absolute differences of SSA at 532 nm was -0.02 +- 0.04. The coefficients of determination for the AF comparison were small, and the CALIOP-MODIS retrievals were slightly underestimated. The mean +- standard deviation of the absolute differences of AF at 532 nm was -0.01 +- 0.04. The coefficient of determination for the fine median radius of the CALIOP-MODIS retrieval was small, 0.015, but the mean +- standard deviation of the absolute difference was small, 0.01 +- 0.04 mm. The coarse median radius comparison showed a small coefficient of determination, 0.054, but the mean +- standard deviation of the absolute difference was small, 0.13 +- 0.60 mm. In summary, the AODs at 532 and 1064 nm of the CALIOP-MODIS retrievals showed good agreement with those of the AERONET products, while SSA, AF, and volume size distribution properties were not retrieved well but were not far from those of the AERONET products.

Accepted Answer

The assumed particle model plays a crucial role in aerosol retrievals. In a study, the spheroid particle model for Dust Scattering (DS) was compared with the Voronoi particle model. The results showed that the AOD of DS was greater when using the spheroid model compared to the Voronoi model (Fig. 14). The LDR of the spheroid model was smaller, requiring a larger amount of DS to fit the DR measurements. This led to a decrease in the AOD of WS and LA to compensate for the increase in DS AOD. The DMR of DS was also decreased by about 0.6 mm in heavy dust regions. The scattering intensity ratio for spheroid particles changed from 0.8 to 3.0 in the median radius range of 1.0 to 5.0 mm, while for Voronoi particles, it changed from 0.8 to 2.6. The retrieved DMR of DS was smaller with the spheroid model due to the larger scattering intensity ratio. The AODs of WS and LA were not significantly influenced by the particle model used for DS. The study also explored the impact of BC volume fraction on retrievals, showing that a lower BC fraction resulted in slightly decreased AODs of WS and LA. Different mixture models, such as CGS, CS, and MG, affected the retrievals of WS and LA, with the MG model slightly increasing the AOD of LA due to a larger LR. Overall, the choice of particle model and BC volume fraction can significantly impact aerosol retrievals.

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Most frequently asked questions

1. What are the methods used to retrieve aerosol composition and investigate global distributions?

2. How are CALIOP-MODIS retrievals compared?

3. What technique is used for inversion in the retrieval procedure?

4. How was the surface reflection over the ocean calculated?

5. How were uncertainties evaluated in CALIOP-MODIS retrieval products?

6. How do CALIOP-MODIS retrievals compare with AERONET products in terms of AOD, SSA, AF, and volume size distribution properties?

7. How do different particle models influence aerosol retrievals?

References

AERONET-a federated instrument network and data archive for aerosol Characterization

The Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2)

Discrete-Dipole Approximation For Scattering Calculations

Variability of Absorption and Optical Properties of Key Aerosol Types Observed in Worldwide Locations

Optical Properties of Aerosols and Clouds: The Software Package OPAC

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