# Retrieval of Aerosol and Surface Properties at High Spatial Resolution: Hybrid Approach and Demonstration Using Sentinel-5p/TROPOMI and PRISMA

Cheng Chen, Pavel Litvinov, Oleg Dubovik, David Fuertes, Christian Matar, Franco Miglietta, Monica Pepe, Lorenzo Genesio, Lorenzo Busetto, Lukas Bindreiter, et al.

► To cite this version:

Cheng Chen, Pavel Litvinov, Oleg Dubovik, David Fuertes, Christian Matar, et al. Retrieval of Aerosol and Surface Properties at High Spatial Resolution: Hybrid Approach and Demonstration Using Sentinel-5p/TROPOMI and PRISMA. Journal of Geophysical Research: Atmospheres, 2024, 129 (15), 10.1029/2024JD041041. http://dx.doi.org/10.1029/2024JD041041

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Distributed under a Creative Commons Attribution 4.0 International License
JGR Atmospheres

RESEARCH ARTICLE

10.1029/2024JD041041

**Key Points:**

* Generalized Retrieval of Atmosphere and Surface Properties (GRASP)/ Hybrid approach is developed and demonstrated using Sentinel-5p/ TROPOMI and PRISMA measurements

* PRISMA 100 m aerosol and surface properties are retrieved based on GRASP/Hybrid approach and evaluated using reference data sets

* We demonstrate the big potential of hybrid approach based on the combination of instruments for climate and environment studies

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

P. Litvinov and C. Chen,
pavel.litvinov@grasp-sas.com;
cheng.chen@aiofm.ac.cn

**Citation:**

Chen, C., Litvinov, P., Dubovik, O., Fuertes, D., Matar, C., Miglietta, F., et al. (2024). Retrieval of aerosol and surface properties at high spatial resolution: Hybrid approach and demonstration using sentinel-5p/TROPOMI and PRISMA.
*Journal of Geophysical Research: Atmospheres*, 129, e2024JD041041.
https://doi.org/10.1029/2024JD041041

Received 28 FEB 2024
Accepted 16 JUL 2024

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# Retrieval of Aerosol and Surface Properties at High Spatial Resolution: Hybrid Approach and Demonstration Using Sentinel-5p/TROPOMI and PRISMA

Cheng Chen<sup>1,2</sup> <sup>1</sup>ID, Pavel Litvinov<sup>1</sup> <sup>1</sup>ID, Oleg Dubovik<sup>3</sup> <sup>1</sup>ID, David Fuertes<sup>1</sup> <sup>1</sup>ID, Christian Matar<sup>1</sup>, Franco Miglietta<sup>4</sup>, Monica Pepe<sup>5</sup>, Lorenzo Genesio<sup>4</sup>, Lorenzo Busetto<sup>5</sup>, Lukas Bindreiter<sup>6</sup>, Verena Lanzinger<sup>6</sup>, Martin de Graaf<sup>7</sup> <sup>1</sup>ID, Gijsbert Tilstra<sup>7</sup>, Piet Stammes<sup>7</sup>, and Christian Retscher<sup>8</sup>

<sup>1</sup>GRASP-SAS, Remote Sensing Developments, Lille, France, <sup>2</sup>Now at: Anhui Institute of Optics and Fine Mechanics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, China, <sup>3</sup>Univ. Lille, CNRS, UMR 8518 - LOA - Laboratoire d'Optique Atmosphérique, Lille, France, <sup>4</sup>Institute of Bioeconomy, CNR, Sesto Fiorentino, Italy, <sup>5</sup>Institute for Electromagnetic Sensing of the Environment, CNR, Milano, Italy, <sup>6</sup>Cloudflight Austria GmbH, Linz, Austria, <sup>7</sup>KNMI, De Bilt, The Netherlands, <sup>8</sup>ESA, ESRIN, Frascati, Italy

**Abstract** Satellite remote sensing of aerosol is largely conducted at moderate or coarse spatial resolution around 1–10 km. Nevertheless, at urban areas with high human activity, aerosol can originate from complex emission sources and may also vary strongly in space. Therefore, aerosol characterization at fine spatial resolution is essential for air quality study and assessment of anthropogenic pollution as well as climate effects. However, space-borne instruments with high spatial resolution are usually limited in swath width or spectral coverage which result in lowering information content required for aerosol and surface retrieval. Based on the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm, we propose a hybrid approach by combining fine and coarse spatial resolution measurements to retrieve aerosol and surface properties simulataneously at fine spatial resolution. The instruments with coarse spatial resolution and high revisting time can provide advanced aerosol characterization. At the same time, the instruments with fine spatial resolution are sensitive to spatial variability of aerosol nearby sources. In this study, the GRASP/Hybrid approach is demonstrated and tested based on the European Space Agency Sentinel-5p/TROPOMI together with the Italian Space Agency PRISMA satellite data. Specifically, the detailed aerosol microphysical properties from Sentinel-5p/TROPOMI 10 km retrievals are used as a priori information for PRISMA to derive aerosol loading and surface properties at 100 meter (m) spatial resolution. The PRISMA 100 m aerosol and surface retrieval based on the developed GRASP/Hybrid approach are evaluated using available ground-based and satellite measurements, including AERONET, VIIRS/DB aerosol and PRISMA Level 2 surface reflectance products.

**Plain Language Summary** Both aerosol and surface characterization at high spatial resolution are always highly demanded especially over densely populated urban areas. However, the high spatial resolution space-borne instruments typically have small swaths that, in general, reduce the possibility of aerosol and surface decoupling. In this paper, we develop a hybrid approach based on GRASP algorithm (GRASP/Hybrid) to retrieve aerosol and surface properties at fine spatial granularity by combining fine and coarse spatial resolution satellite data. The GRASP/Hybrid approach is demonstrated using European Space Agency (ESA) Sentinel-5p/TROPOMI and Italian Space Agency (ASI) PRISMA (Hyperspectral Precursor of the Application Mission) measurements. Specifically, the detailed aerosol microphysical properties from Sentinel-5p/ TROPOMI 10 km retrievals are used as a priori information to derive aerosol loading and surface properties exploiting PRISMA at 100 m spatial resolution. Overall, the results obtained in this study demonstrate the big potential of the approach based on the combination of instruments for a number of climate and environment monitoring studies.

# 1. Introduction

A crucial step in the characterization of aerosol and surface using satellite remote sensing measured Top-Of- Atmosphere (TOA) radiance is the decoupling of the atmosphere-surface signal (Dubovik, Schuster, et al., 2021; King et al., 1999; Kokhanovsky et al., 2007; Li et al., 2009; Litvinov, Hasekamp, & Cairns, 2011). Optical and microphysical characteristics of atmospheric aerosol presents, as a rule, are temporally

<footnote>© 2024. The Author(s).
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
inhomogeneous but change spatially slowly and quite continuously (Chen et al., 2021; Dubovik et al., 2002; Redemann et al., 2006; Smirnov et al., 2002). Surface properties are, on the contrary, spatially inhomogeneous and vary temporally slower than aerosol characteristics (Gao et al., 2005; Schaaf et al., 2008). By exploiting this temporal-spatial features, aerosol and surface properties retrieval can be essentially improved. Because of these temporal-spatial differences, aerosol products are largely provided at moderate or coarse spatial resolution around 1–10 km (Chen et al., 2020; Garay et al., 2020; Hsu et al., 2019; Levy et al., 2013; Lyapustin et al., 2018; Remer et al., 2013; Sogacheva et al., 2020), while the surface characterization is more preferable at finer spatial resolutions (e.g., <1 km) (Claverie et al., 2018; Liang et al., 2021; Lyapustin et al., 2011; Roger et al., 2022; Schaaf et al., 2002, 2008; Strahler et al., 1999; Vermote et al., 2016, 2018). Nevertheless, in densely populated urban areas, aerosol can originate from a variety of complicated emission sources and can also fluctuate greatly in space (Li et al., 2013; Singh et al., 2004; Zhang et al., 2012). Therefore, for the purpose of studying air quality and evaluating anthropogenic pollution, aerosol characterization at fine spatial resolution is crucial (Bao et al., 2016; Jin et al., 2019; Li et al., 2022; Wei et al., 2019; Wong et al., 2011).

The Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm is designed to retrieve the complete set of aerosol and full surface Bidirectional Reflectance Distribution Function (BRDF) properties simultaneously (Dubovik, Fuertes, et al., 2021; Dubovik et al., 2011, 2014). The algorithm utilizes the “multi-pixel” concept by simultaneous inversion of a large group of pixels within one or several overpasses to accomplish reliable retrieval of aerosol and surface properties. Such a multi-pixel scheme takes advantage of known intrinsicality on spatial and temporal variability in both aerosol and surface properties. Specifically, the variations of the retrieved parameters horizontally from pixel-to-pixel and/or temporally from day-to-day are enforced to be smooth by an additional, appropriately set of a priori constraints. This concept provides satellite retrieval of higher consistency. The GRASP algorithm has been demonstrated to work on simulated data as well as on real data from the POLDER/PARASOL, MERIS/ENVISAT, OLCI/Sentinel-3 and TROPOMI/Sentinel-5p missions as well as ground-based measurements (Chen et al., 2020, 2022, 2024; Li et al., 2019; Litvinov et al., 2022, 2024; Lopatin et al., 2013, 2021). Considering both computational cost and the pixel revisit frequency to meet the requirement for aerosol and surface contributions decoupling, the above-mentioned satellite aerosol retrievals are conducted at moderate/coarse spatial resolutions about 6–10 km.

Unlike moderate/coarse spatial resolution space-borne instruments with multi-spectral, multi-angular, multi-polarization measurements, the high spatial resolution instruments typically have small swaths that, in general, reduce the possibility of aerosol/surface decoupling. This is especially crucial for aerosol characterization since even small errors in the surface reflectance estimation can yield large errors in aerosol characterization. To achieve the high spatial resolution retrieval, aerosol or surface properties should be essentially constrained. This can be done, for example, with application of some regional specific aerosol microphysical models or a priori information about aerosol and surface properties (Lyapustin et al., 2012, 2018; Vermote & Vermeulen, 1999).

In this paper, we propose a GRASP/Hybrid approach to retrieve aerosol and surface properties simultaneously at fine spatial resolution by combining fine and coarse spatial resolution satellite measurements. The approach is demonstrated and tested based on the European Space Agency (ESA) Sentinel-5p/TROPOMI (Veeftkind et al., 2012) and Italian Space Agency (ASI) PRISMA (Hyperspectral Precursor of the Application Mission) measurements (Cogliati et al., 2021). Specifically, the detailed aerosol microphysical properties from Sentinel-5p/TROPOMI ~10 km retrievals are used as a priori information for PRISMA to derive aerosol loading and surface properties at 100 meter (m) spatial resolution. Section 2 describes the Sentinel-5p/TROPOMI and PRISMA instruments, followed by the description of the GRASP/Hybrid approach that was developed to process Sentinel-5p/TROPOMI and PRISMA data in Section 3. The results and discussion, including the validation with reference measurements, as well as intercomparisons with other independent products are presented in Section 4. Section 5 summarizes and concludes the study.

## 2. Satellite Measurements and Data Sets

### 2.1. Sentinel-5p/TROPOMI Instrument

The Tropospheric Monitoring Instrument (TROPOMI) aboard the Copernicus Sentinel-5p satellite provides hyperspectral measurements which can be beneficial for aerosol and surface characterization. In particular, narrow spectral bands outside of strong absorption bands can be selected which essentially simplify and enhance characterization of aerosol and surface reflectance. TROPOMI observes the ultraviolet-visible (UV-VIS, 267–
<caption>**Table 1**</caption>

TROPOMI and PRISMA Spectral Channels for Aerosol and Surface Characterization Used in This Study

<table><thead><tr><th>Spectrum</th><th>UV</th><th>VIS</th><th>NIR</th><th>SWIR</th></tr></thead><tbody><tr><td>TROPOMI Central Wavelength (nm)</td><td>340</td><td>416</td><td>740</td><td>2,313</td></tr><tr><td>PRISMA Central Wavelength (nm)</td><td>n/a</td><td>416</td><td>670</td><td>2,313</td></tr><tr><td>Band Width</td><td></td><td>440</td><td>494</td><td>1 nm</td></tr></tbody></table>

499 nm), near-infrared (NIR, 661–786 nm), and shortwave infrared (SWIR, 2,300–2,389 nm) wavelength ranges (Veefkind et al., 2012). With a typical pixel spatial resolution about 5.5 km by 3.5 km at nadir (except for the UV1 5.5 km by 28 km and SWIR 5.5 km by 7 km) and a swath width of about 2,600 km, TROPOMI provides almost daily coverage worldwide and can be used for global environment monitoring (Borsdorff et al., 2018; De Smedt et al., 2018; Hu et al., 2016; van Geffen et al., 2020).

## 2.2. PRISMA Instrument

PRISMA (PRecursore IperSpettrale della Missione Applicativa), namely Hyperspectral Precursor of the Application Mission, is an Italian Space Agency (Agenzia Spaziale Italiana, ASI) hyperspectral mission, mainly addressed at scientific use. The hyperspectral mission takes advantage of the combination of two hyperspectral sensors in the spectral range 400–2,500 nm (around 240 channels, overlapping between 920 and 1,010 nm), with a ground sampling distance (GSD) of 30 m, and average spectral resolution of less than 10 nm (Lopinto & Annasso, 2013). The standard radiance generator algorithm, for the L1 products (TOA radiances) is composed by several steps—including Absolute Radiometric Gain (ITF), Defective Pixel, Spectral Coregistration and Geometrical Coregistration—in order to provide quality indicators. VNIR-to-SWIR spatial co-registration analysis conducted independently during the commissioning phase (Cogliati et al., 2021) reported that spatial-shift absolute values are usually below 8 m, with larger shifts in the *x* direction as compared to the *y* direction. The estimated signal-to-noise ratio (SNR) values at different wavelengths nearly agree with characterization measurements performed pre-launch (Coppo et al., 2020). The absolute calibration has been performed on board during commissioning phase, during operational phase also a vicarious calibration using RadCalNet PICs has been run. The BOA reflectance accuracies estimated during the validation project (namely PRISCAV) resulted in an overall Mean Absolute Error, as compared to FRM measurements, is below 5% for most of the bands (Pignatti et al., 2022).

The PRISMA measurements have been successfully used for land and environment monitoring (Bohn et al., 2022; Lima et al., 2023; Mzid et al., 2022; Pepe et al., 2020; Tagliabue et al., 2022). Table 1 summarizes TROPOMI and PRISMA spectral channels used in this study for aerosol and surface characterization. Generally, the selected PRISMA spectral channels (The spectral channel configuration used in this study is the one of Level 1 processor version 3.6.) in VIS, NIR and SWIR are overlapped with TROPOMI that simplify the transmission of TROPOMI aerosol and surface spectral dependent properties to PRISMA retrievals.

## 2.3. TROPOMI/GRASP Products and PRISMA Scenes

The GRASP algorithm has been adapted to retrieve aerosol and surface properties from Sentinel-5p/TROPOMI measurements within the framework of ESA S5p + Innovation Aerosol Optical Depth (AOD)/BRDF project (Chen et al., 2024; Litvinov et al., 2022, 2024). TROPOMI/GRASP product provides aerosol optical properties (spectral aerosol optical depth—AOD, fine mode AOD—AODF, coarse mode AOD—AODC, single scattering albedo—SSA etc.) and surface spectral BRDF, Normalized Difference Vegetation Index, DHR (black sky albedo) and BHRiso (white sky albedo) at 10 km spatial resolution. Chen et al. (2024) comprehensively validated and intercompared TROPOMI/GRASP aerosol and surface product with reference data sets as well as other independent satellite products. Generally, good agreement is observed with aerosol and surface reference data sets. Both TROPOMI/GRASP aerosol and surface products are provided at 10 UV-VIS-NIR-SWIR wavelengths, and it covers the spectral range of PRISMA measurements (Table 1).

In this study, we demonstrate the approach over northern Italy region (Figure 1). The TROPOMI/GRASP 10 km monthly AOD at 440 nm over this region is presented for January 2020 in Figure 1a. Figure 1b shows the selected
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Study area for aerosol and surface characterization at coarse resolution from Sentinel-5p TROPOMI and fine resolution of PRISMA. (a) TROPOMI/Generalized Retrieval of Atmosphere and Surface Properties 10 km monthly Aerosol Optical Depth (440 nm) over Italy for January 2020; (b) PRISMA 30 km × 30 km scenes with 30 m pixel spatial resolution over northern Italy used in this study.</figcaption></figure>

PRISMA 30 km × 30 km scenes with 30 m pixel spatial resolution over northern Italy region. Table 2 summarizes the detailed information of each PRISMA scene processed in this study where the PRISMA L1B 30 m pixels are re-gridded to L1C 100 m × 100 m pixels (grid) using Equidistant Cylindrical projection and then input for GRASP retrievals. Therefore, the spatial resolution of PRISMA/GRASP processing is 100 m. Meanwhile, in order to evaluate the processing results, the PRISMA/GRASP aerosol and surface product are intercompared with AERONET, TROPOMI/GRASP and Visible Infrared Imaging Radiometer Suite (VIIRS)/DB aerosol data sets, as well as PRISMA L2D surface reflectance. PRISMA Level 2D surface reflectance was derived from LUT inversion for a rural aerosol type using RT atmospheric correction approach under Lambertian surface assumption (Cogliati et al., 2021; Mzid et al., 2022; Pepe et al., 2020). Figure 2 shows the flowchart of the PRISMA and TROPOMI hybrid approach and the evaluation of PRISMA/GRASP 100 m aerosol and surface product.

## 3. Methodology

### 3.1. GRASP/Hybrid Approach Demonstration Using TROPOMI and PRISMA

In this section, we demonstrate GRASP/Hybrid approach using Sentinel-5p/TROPOMI and PRISMA measurements. Specifically, TROPOMI/GRASP products with coarse spatial resolution are combined with fine spatial resolution of PRISMA measurements. The results of the GRASP retrieval from TROPOMI measurements allows obtaining at coarse resolution (about 10 km) such aerosol characteristics as AOD and fractions of four different aerosol models: fine absorbing ($c_1$), fine non-absorbing ($c_2$), oceanic ($c_3$) and dust models ($c_4$). AOD characteristic is directly related to total aerosol volume concentration ($c_v$), while the fraction of different aerosol models and aerosol scale layer height ($h$) can be used to derive such advanced aerosol properties as SSA, aerosol

<table><caption>**Table 2**
List of PRISMA Scenes Processed in This Study</caption><thead><tr><th>SceneName</th><th>Center coordinates (latitude, Longitude)</th><th>Size_YX pixels</th><th>Datetimes YYYY/MM/DD</th></tr></thead><tbody><tr><td>Venice</td><td>45.4659 N, 12.1885 E</td><td>600, 600</td><td>2019/08/30, 2020/04/12, 2020/0612, 2020/07/08, 2020/07/14, 2020/08/12, 2020/09/04</td></tr><tr><td>Ispra</td><td>45.7624 N, 8.5703 E</td><td>700, 600</td><td>2020/04/25, 2020/05/24, 2020/07/03, 2020/08/19</td></tr><tr><td>Jolanda di Savoia/ Ferrara</td><td>44.8601 N, 12.0078 E</td><td>400, 500</td><td>2019/10/21, 2020/04/07, 2020/05/11, 2020/05/17, 2020/05/23, 2020/06/26, 2020/07/14, 2020/07/31, 2020/09/16</td></tr><tr><td>Ostend/North_EU_A</td><td>51.3372 N, 3.0722 E</td><td>500, 500</td><td>2019/07/23, 2020/05/14, 2020/07/11, 2020/07/28, 2020/08/09</td></tr><tr><td>Avon/North_EU_B</td><td>48.4003 N, 2.6971 E</td><td>400, 400</td><td>2020/04/09, 2020/06/23, 2020/08/09, 2020/09/12, 2020/09/18</td></tr><tr><td>Demmin/North_EU_C</td><td>53.8439 N, 13.0528 E</td><td>400, 400</td><td>2020/06/15, 2020/06/16, 2020/06/27, 2020/08/07, 2020/08/13, 2020/09/22</td></tr><tr><td>Prague/North_EU_D</td><td>50.1038 N, 14.2572 E</td><td>400, 400</td><td>2020/04/18, 2020/07/14</td></tr><tr><td>Dresden/North_EU_E</td><td>51.0009 N, 13.4622 E</td><td>400, 600</td><td>2020/02/26, 2020/03/26, 2020/04/07, 2020/07/31, 2020/08/06</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Flow chart of the PRISMA + TROPOMI hybrid approach and evaluation of the derived PRISMA 100 m aerosol and surface product.</figcaption></figure>

size, complex refractive index, nonsphericity, etc. Moreover, as it was shown in Litvinov et al. (2022, 2024), together with aerosol extended properties, TROPOMI measurements in the combination with GRASP multi-pixel approach can be used to retrieve directional surface reflectance properties as parameters of BRDF model. Specifically, the Ross-Li BRDF model (Li & Strahler, 1992; Ross, 1981; Wanner et al., 1995) is used for TROPOMI/GRASP retrieval with modified constraints on BRDF model parameter spectral variability (Litvinov et al., 2010; Litvinov, Hasekamp, & Cairns, 2011; Litvinov, Hasekamp, Cairns, & Mishchenko, 2011) where the spectral dependent isotropic term ($a_{iso}(\lambda)$) is retrieved together with spectral independent volumetric and geometric terms ($a_{vol}$ and $a_{geom}$). The Equation 1 gives the state vector of each land pixel for TROPOMI/GRASP retrieval.

$$a_{\text{land}} = [c_v, c_1, c_2, c_3, c_4, h, a_{\text{iso}}(\lambda), a_{\text{vol}}, a_{\text{geom}}]^T \quad (1)$$

PRISMA revisting time is 29 days at-nadir, but also off-nadir acquisitions are possible and frequently made to increase frequency over sites of intertest, while PRISMA tasking is on demand. The overpasses over the same ground areas are more than once per month in the best cases (Figure 1 and Table 2). This aperiodicity of PRISMA measurements limits essentially the possibility of operational extended aerosol and surface characterization at fine spatial resolution. Therefore, in these studies, performing GRASP retrieval from PRISMA measurements, such characteristics as aerosol type and surface directional properties were taken as a priori values from Sentinel-5p/TROPOMI retrieval. Beneficial from the consistent forward modeling of aerosol and surface in the GRASP algorithm, the TROPOMI/GRASP climatology can be directly used for PRISMA retrieval. In addition, the four aerosol model fractions ($c_1, c_2, c_3, c_4$) that define the aerosol type and the surface directional properties ($a_{vol}, a_{geom}$) are spectral independent. Therefore, the designed approach is easily applicable to different combination of fine/coarse resolution measurements.

In the GRASP/Hybrid approach, the PRISMA retrieval provides total aerosol concentration and surface spectral reflectance at fine spatial resolution. Moreover, to stabilize further PRISMA retrieval and separate accurately aerosol and surface signals, a group of 5x5 spatial PRISMA pixels were inverted simultaneously using GRASP spatial multi-pixel approach (Dubovik, Fuertes, et al., 2021). In this approach, the strong spatial constraints on aerosol concentration were applied while the surface reflectance retrieval was considered as independent in a 5x5 spatial segment. This allowed essentially improving the separation of atmosphere and surface signals.

Three different PRISMA GRASP/Hybrid configurations are proposed and will be tested for adoption aerosol and surface climatology:
<table><caption>**Table 3**
Comparison of PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (v1 wo clim, v2 w clim, v3 w clim)
Aerosol Optical Depth (AOD) and AERONET AOD at 550 nm Over AERONET Site Ispra</caption><thead><tr><th rowspan="2">YYYY/MM/DD</th><th rowspan="2">AERONET AOD 550 nm</th><th colspan="3">PRISMA/GRASP AOD 550 nm</th></tr><tr><th>v1 wo clim</th><th>v2 w clim</th><th>v3 w clim</th></tr></thead><tbody><tr><td>2020/04/25</td><td>0.127 ± 0.01</td><td>0.102 ± 0.08</td><td>0.154 ± 0.03</td><td>0.191 ± 0.04</td></tr><tr><td>2020/05/24</td><td>0.080 ± 0.00</td><td>0.028 ± 0.03</td><td>0.090 ± 0.05</td><td>0.100 ± 0.04</td></tr><tr><td>2020/07/03</td><td>0.071 ± 0.00</td><td>0.080 ± 0.08</td><td>0.153 ± 0.09</td><td>0.156 ± 0.09</td></tr><tr><td>2020/08/19</td><td>n/a</td><td>0.040 ± 0.04</td><td>0.068 ± 0.03</td><td>0.095 ± 0.04</td></tr></tbody></table>

1. v1 ("v1 wo clim"): the Sentinel-5p TROPOMI/GRASP aerosol type climatology (i.e., 60% urban - fine non-absorbing; 10% oceanic; 30% dust) is used universally for all PRISMA scenes and the surface is treated as isotropic.

2. v2 ("v2 w clim"): the monthly 2-degree spatial resolution Sentinel-5p TROPOMI/GRASP aerosol type climatology is used and the surface is treated as isotropic.

3. v3 ("v3 w clim"): both the monthly 2-degree resolution Sentinel-5p TROPOMI/GRASP aerosol type climatology and the climatology for the volumetric and geometric surface "BRDF2" and "BRDF3" parameters from TROPOMI/GRASP are used.

The "v1 wo clim" configuration simply fixes the aerosol type climatology for the entire northern Italy region, where the TROPOMI/GRASP aerosol type climatology indicates about 60% urban - fine non-absorbing ($c_2$), 10% oceanic ($c_3$) and 30% dust ($c_4$). The aerosol model fractions are fixed for all processed pixels. In addition, the surface is assumed to be isotropic and the volumetric and geometric terms ($a_{vol}$ and $a_{geom}$) of BRDF are not accounted for and only the $c_v$ and $a_{iso}(\lambda)$ are retrieved.

In the second configuration (v2 w clim), the aerosol type is adopted from the monthly TROPOMI/GRASP 2-degree climatology instead of fixing it in v1 wo clim. The surface treatment in "v2 w clim" is considered as isotropic similarly to "v1 wo clim".

The third configuration (v3 w clim) follows the "v2 w clim" and the monthly 2-degree spatial resolution aerosol type climatology is used. In addition, the "v3 w clim" utilizes the TROPOMI/GRASP surface BRDF volumetric and geometric terms ($a_{vol}$ and $a_{geom}$) climatology which are neglected in the "v1 wo clim" and "v2 w clim" configurations. For example, S5p/TROPOMI GRASP climatology indicate 4 aerosol models fractions: $c_1$ 14.5%, $c_2$ 50.9%, $c_3$ 26.3%, and $c_4$ 8.3%, and angular surface BRDF volumetric and geometric terms: $a_{vol}$ 0.77 and $a_{geom}$ 0.14 over Venice scene in August 2019.

# 4. Results and Validations

## 4.1. Evaluation of PRISMA/GRASP 100 m AOD With AERONET and Intercomparison With Coarse Resolution Satellite AOD Products

For the retrieval of aerosol properties, the AERONET data set is the obvious choice for validation (Giles et al., 2019; Holben et al., 1998). AERONET provides accurate direct sun AOD products with high accuracy 0.01–0.02 (Eck et al., 1999). Strict protocols for the calibration and maintenance assure homogeneity among all its instruments. Due to its high data quality, the AERONET AOD products are widely used as in situ reference data to evaluate satellite remote sensing aerosol products. To validate with reference AERONET point AOD measurements (Version 3 Level direct-Sun, last access: 2021-06-15), we average the nearest 10 x 10 100 m PRISMA/GRASP pixels and compare with AERONET AOD within+/-30 min of the PRISMA overpass. There is only one scene (Ispra) with AERONET colocation and having matchup AOD measurements.

Table 3 summarizes the comparison of PRISMA/GRASP AOD using all 3 configurations (v1 wo clim, v2 w clim, v3 w clim) and AERONET AOD at 550 nm over AERONET site in Ispra (45.803 N, 8.627 E). The standard deviation of AOD within+/-30 min for AERONET and within the 10x10 window for PRISMA are also presented in Table 3. Only 3IRRups are found, and all three GRASP configurations (v1 wo clim, v2 w clim, v3 w clim) obtained PRISMA AOD at 550 nm fit the Global Climate Observing System (GCOS, 2016) requirements (max 0.04 or 10% AOD), except for the v1 wo clim retrieval on 2020/05/24. Overall, the v3 w clim
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Spatial distribution of Aerosol Optical Depth 550 nm on 2020/08/12 over Venice scene from 100 m spatial resolution PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (GRASP), 10 km spatial resolution TROPOMI/GRASP and 6 km spatial resolution Visible Infrared Imaging Radiometer Suite (VIIRS)/DB. (a) PRISMA/GRASP v1 wo; (b) PRISMA/GRASP v2 w_clim; (c) PRISMA/GRASP v3 w_clim; (d) TROPOMI/GRASP; (e) VIIRS/DB.</figcaption></figure>

reports higher AOD than v1 wo_clim and v2 w_clim, however, it is difficult to derive which one is of better
performance from such a small amount of data co-located with AERONET moreover in low aerosol loading
conditions (AERONET AOD 550 nm < 0.15). We would like to emphasis that the PRISMA 100 m AOD is only
evaluated with limited data (1 site 3 cases with AOD 0.05–0.15). With this, it's not enough to conclude that we
manage to capture real AOD spatial distribution at 100 m resolution. While, this high spatial resolution AOD
variability is of paticular intertest for urban environment and climate. In future, some ground-based measurements
can be conducted to verify the AOD variability at high spatial resolution.

Beside the validation against AERONET, PRISMA 100 m resolution AOD was compared with the AOD products
from coarse resolution instruments such as TROPOMI/GRASP of 10 km spatial resolution and the Suomi Na-
tional Polar-orbiting Partnership (Suomi-NPP) SNPP VIIRS deep blue (DB) 6 km Level 2 aerosol product
(AERDB_L2_VIIRS_SNPP) (Hsu et al., 2019; Sayer, Hsu, Lee, Bettenhausen, et al., 2018; Sayer, Hsu, Lee, Kim,
et al., 2018). Figure 3 shows the spatial distribution of AOD 550 nm on 2020/08/12 over the Venice scene.
Figures 3a–3c show the PRISMA/GRASP 100 m spatial resolution AOD using the 3 retrieval configurations
(v1 wo_clim, v2 w_clim, v3 w_clim), and Figures 3d and 3e show TROPOMI/GRASP 10 km and VIIRS/DB
6 km AOD products. Generally, all three PRISMA processing approaches present a similar pattern of AOD spatial
distribution. The “v3 w_clim” tends to provide higher AOD than “v1 wo_clim” and “v2 w_clim”, which is
consistent with the results of AERONET comparisons. Both PRISMA fine resolution and TROPOMI and VIIRS
coarse resolution products indicate a moderate aerosol loading for this case with AOD 550 nm around 0.2–0.5.
While the fine spatial resolution aerosol characterization shows much more spatial patterns with relatively high
AOD are observed near the east coastal regions, which are likely connected to aerosol emission sources located in
Venice seaport. It is necessary to note that stronger NO₂ concentrations over Venice seaport were observed also
from a ground-based air pollution monitoring network. In particular, Figure S1 in Supporting Information S1
shows the mean NO₂ concentration for 6 locations in Venice areas in 2017. The increasing of NO₂ concentration
is clearly observed toward industrial Mestre (via Tagliaamento)—Marghera (via Beccaria) area. In addition, the
high NO₂ emission region is well coincident with the higher AOD values observed from PRISMA/GRASP re-
trievals. Past airborne measurements of organics in the industrial area of Venice also confirm that volatile organic
compounds emissions are not negligible there (Toscano et al., 2011).
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Intercomparison of PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (GRASP) (v1 wo clim, v2 w clim, v3 w clim) 100 m Aerosol Optical Depth (AOD) with (a) Sentinel-5p TROPOMI/GRASP 10 km and (b) SNPP Visible Infrared Imaging Radiometer Suite/DB 6 km AOD at 550 nm over Venice scene (2020/08/12).</figcaption></figure>

We further re-grid PRISMA, TROPOMI and VIIRS AOD products into a common 0.1-degree grid and inter-compare. Figure 4 shows the intercomparison of 3 versions PRISMA/GRASP AOD (v1 wo clim, v2 w clim, v3 w clim) with Sentinel-5p TROPOMI/GRASP (Figure 4a) and SNPP VIIRS/DB (Figure 4b) over the Venice scene on 2020/08/12. The y-axis error bar indicates one standard deviation of PRISMA 100 m spatial resolution AOD in 0.1-degree grid boxes. The root mean square error (difference) (RMSE) and the fraction of pixels within the GCOS requirements max (0.04 or 10% AOD) are also calculated and presented in the figures. Generally, the coarse resolution TROPOMI/GRASP and VIIRS/DB AOD tend to be slightly higher than PRISMA/GRASP fine resolution AOD, while some small area high AOD cases observed at fine resolution are not obvious detectable in coarse spatial resolution data sets. Meanwhile, significant AOD spatial variability (y-axis error bar) is observed inside each 0.1-degree grid boxes, which further emphasizes the importance to derive AOD at fine spatial res-olution especially over urban regions. With respect to 3 versions PRISMA/GRASP AOD, they are spatially consistent especially for v2 and v3. For intercomparison, v2 tends to show slightly better agreement with coarse resolution TROPOMI and VIIRS products than v3 and v1, even though v2 and v3 are statistically close. Overall, the agreement of independent AOD products is reasonable with RMSE 0.1–0.15.

Figure 5 shows another case of spatial distribution PRISMA/GRASP 100 m spatial resolution AOD (550 nm) using 3 the retrieval configurations (v1 wo clim, v2 w clim, v3 w clim) and TROPOMI/GRASP 10 km and VIIRS/DB 6 km AOD (550 nm) over Ispra scene on 2020/05/24. The intercomparison scatter plot of PRISMA, TROPOMI and VIIRS AOD is present in Figure 6. Over urban region (north east part), the PRISMA/GRASP v2 and v3 show higher AOD than v1, which is possibly from the aerosol type differences between v1 (60% urban, 10% oceanic and 30% dust) and v2 and v3 (12.2% smoke, 66.7% urban, 14.6% oceanic and 6.5% dust). According to AERONET validation for this case (Table 3), v1 wo clim underestimates AOD, while v2 w clim and v3 w clim slightly overestimate but the v2 w clim and v3 w clim AOD are within GCOS requirement against AERONET. VIIRS/DB also show relatively higher AOD over this north east urban area, even though it is only covered partially by 1–2 pixels with 6 km spatial distribution. Based on the intercomparison, both PRISMA fine resolution and TROPOMI, VIIRS coarse resolution AOD products indicate relatively low AOD (~0.1–0.15) for this case over entire region, and the RMSE between PRISMA and TROPOMI, VIIRS AOD (550 nm) is generally smaller than 0.09. Again, the details from the fine resolution product are crucial to identify the sources particularly in urban areas. The intercomparison of AOD (550 nm) between PRISMA and TROPOMI, VIIRS for all processed scenes are present in Figure S2 in Supporting Information S1 (Venice) and Figure S3 in Supporting Informa-tion S1 (Ispra). Overall, the results are consistent with the two cases presented in this section. In general, though there is no direct evidence yet to prove this, all indirect facts, based on the results of this study, show the existent relationship between the fine spatial resolution AOD from GRASP/PRISMA and possible emission sources (in particular, in the urban and industrial regions).
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Spatial distribution of Aerosol Optical Depth 550 nm on 2020/05/24 over Ispra scene from 100 m spatial resolution PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (GRASP), 10 km spatial resolution TROPOMI/GRASP and 6 km spatial resolution Visible Infrared Imaging Radiometer Suite (VIIRS)/DB. (a) PRISMA/GRASP v1 wo_clim; (b) PRISMA/GRASP v2 w_clim; (c) PRISMA/GRASP v3 w_clim; (d) TROPOMI/GRASP; (e) VIIRS/DB.</figcaption></figure>

## 4.2. Evaluation of PRISMA/GRASP 100 m Surface Product and Atmospheric Correction

The quality of aerosol retrieval from spaceborne instruments is strongly related to the quality of surface properties retrieval. Therefore, as part of evaluation of PRISMA/GRASP retrieval, we also evaluate the surface properties retrieval from PRISMA and TROPOMI using GRASP/Hybrid approach with available PRISMA standard Level 2D surface reflectance product (processor version 2.2.0). Note that PRISMA/GRASP "v1 wo_clim" and "v2 w_clim" approaches assume isotropic surface reflectance, while "v3 w_clim" derives the full BRDF by fixing the spectrally independent volumetric and geometric terms (Litvinov et al., 2010) from TROPOMI/GRASP surface climatology. The PRISMA Level 2D standard product is geocoded bottom of atmosphere surface reflectance and it is derived by atmospheric correction based on the same assumption of isotropic surface as "v1 wo_clim" and "v2 w_clim".

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Intercomparison of PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (GRASP) (v1 wo_clim, v2 w_clim, v3 w_clim) 100 m Aerosol Optical Depth (AOD) with (a) Sentinel-5p TROPOMI/GRASP 10 km and (b) SNPP Visible Infrared Imaging Radiometer Suite/DB 6 km AOD at 550 nm over Ispra scene (2020/05/24).</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Spatial distribution of PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (GRASP) (v1 wo clim, v2 w clim, v3 w clim) and PRISMA L2D surface reflectance at 440, 670 and 865 nm (blue, red and NIR channels) over Venice scene on 2020/08/12, and the difference between GRASP derive surface reflectance and PRISMA L2D product. Column from left: PRISMA/GRASP V1, V2, V3, PRISMA L2D, V1-L2D, V2-L2D, V3-L2D. Row from upper: 440, 670 and 870 nm.</figcaption></figure>

To perform the intercomparison, the PRISMA Level 2D surface reflectance is re-gridded to the same GRASP processing 100 m spatial distribution using the same method as the GRASP input measurements preparation. Meanwhile, PRISMA/GRASP (v1 wo clim, v2 w clim, v3 w clim) surface reflectance are calculated based on derived surface BRDF together with observational geometry. Figure 7 shows the intercomparison map of PRISMA/GRASP (v1 wo clim, v2 w clim, v3 w clim) with PRISMA L2D surface reflectance at 440, 670 and 865 nm (blue, red and NIR channels) over the Venice scene on 2020/08/12. The corresponding pixel to pixel scatter plot is presented in Figure 8. The linear fit, and related metrics—Person's correlation coefficient (R), root mean square error/difference (RMSE), number of points (N), bias or difference between PRISMA/GRASP and PRISMA L2D (BIAS), fractions of pixels within the surface Optimal (+/-0.01) and Target (+/-0.02) requirements, are also present in the Figure 8. The “Optimal” and “Target” surface requirements are formulated in the Sentinel-5p TROPOMI/GRASP development (Litvinov et al., 2022). Figures 9 and 10 show the same spatial distribution and pixel-to-pixel intercomparison but for the Ispra scene acquired on 2020/05/24.

As seen from the spatial distribution maps and statistics on Figures 7–10, PRISMA/GRASP v2 w clim is slightly higher than PRISMA L2D product around 0.01 at 670 nm and 0.02 at 440 and 865 nm, at least 40%–60% pixels fit the target requirement at 440 and 870 nm, and more than 99% pixels fit the target requirement at 670 nm. The RMSEs are smaller than 0.02 at all wavelengths. The statistical metrics indicate good agreement between PRISMA/GRASP v2 w clim and PRISMA L2 surface reflectance. Generally, PRISMA/GRASP v1 wo clim and v3 w clim shows similar statistics with v2 w clim. While, the PRISMA/GRASP processing with v3 w clim has a slight higher value of surface reflectance than other GRASP/PRISMA and PRISMA L2D reflectance product especially at 865 nm around 0.021–0.023. This fact can be explained by the different approaches in the surface reflectance modeling. In particular, v1 wo clim and v2 w clim as well as PRISMA L2D official reflectance product are based on the atmospheric correction approach for isotropic surface while v3 w clim takes into account full BRDF with directional surface parameters from TROPOMI/GRASP climatology. Overall, although the differences in the surface assumptions in v1 wo clim, v2 w clim, v3 w clim, the derived PRISMA/GRASP surface reflectance show quite good consistency.

Despite of all discussed uncertainties and differences in the intercomparison, overall, both PRISMA L2D and different PRISMA/GRASP surface products are in good agreement as it is demonstrated in the pixel-to-pixel comparison of PRISMA/GRASP (v1 wo clim, v2 w clim, v3 w clim) and PRISMA official L2D surface reflectance quantitatively. The pixel-to-pixel intercomparison of PRISMA/GRASP and PRISMA L2D surface reflectance at 440, 670 and 865 nm over Venice and Ispra scenes for all available dates are present in Figures S4 and S5 in Supporting Information S1.

All PRISMA/GRASP three configurations retrieve aerosol and surface properties simultaneously. Therefore, the atmospheric correction is realized since the spectral surface reflectance is derived using retrieved BRDF parameters together with observational geometry. We therefore use 3 channels (R: 670 nm; G: 545 nm; B: 440 nm)
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Pixel to pixel comparison of PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (GRASP) (v1_wo_clim, v2_w_clim, v3_w_clim) derived surface reflectance with PRISMA L2D surface reflectance at 440, 670 and 865 nm over Venice scene on 2020/08/12. Column from left: 440, 670 and 870 nm. Row from upper: PRISMA/GRASP V1, V2, V3 versus PRISMA L2D.</figcaption></figure>

to compose the RGB images. Figure 11 shows the RGB image for Venice scene on 2020/08/12. The top-of-atmosphere (TOA) RGB (Figure 11a) is composed using PRISMA measurements (before atmospheric correction), and the surface RGB images (Figures 11b–11d) are generated using PRISMA/GRASP retrieved surface reflectance (atmospheric correction) based on 3 different configurations (v1_wo_clim, v2_w_clim, v3_w_clim). In order to illustrate the TOA measured spectral reflectance and retrieved surface reflectance under different aerosol loading and surface types (Hou et al., 2016, 2020), Figure 11e shows TOA and v1_wo_clim, v2_w_clim

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Spatial distribution of PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (GRASP) (v1_wo_clim, v2_w_clim, v3_w_clim) and PRISMA L2D surface reflectance at 440, 670 and 865 nm (blue, red and NIR channels) over Ispra scene on 2020/05/24, and the difference between GRASP derive surface reflectance and PRISMA L2D product. Column from left: PRISMA/GRASP V1, V2, V3, PRISMA L2D, V1-L2D, V2-L2D, V3-L2D. Row from upper: 440, 670 and 870 nm.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Pixel to pixel comparison of PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (GRASP) (v1_wo_clim, v2_w_clim, v3_w_clim) derived surface reflectance with PRISMA L2D surface reflectance at 440, 670 and 865 nm over Ispra scene on 2020/05/24. Column from left: 440, 670 and 870 nm. Row from upper: PRISMA/GRASP V1, V2, V3 versus PRISMA L2D.</figcaption></figure>

and v3_w_clim retrieved spectral surface reflectance for 3 selected pixels. Figure 12 shows the RGB images for Ispra scene on 2,020/05/24. Qualitatively, all 3 PRISMA/GRASP processing eliminate the turbidity brought by the atmosphere to the images. Although we observe deviations of retrieved AOD, the retrieved spectral surface reflectance is quite consistent for all 3 configurations. This is due to the fact that the TOA signal is dominant by surface, and aerosol is secondary and relatively weak contribution, consequently the moderate aerosol differences result in minor difference of derived surface reflectance. Therefore, the constructed RGB images based on the 3 configurations are basically consistent.

**5. Summary and Conclusions**

In this study, a Hybrid approach (GRASP/Hybrid) was developed to retrieve aerosol and surface properties simultaneously at high spatial resolution (100 m) by combining coarse and high spatial resolution measurements. The instruments with coarse spatial resolution and frequent overpassing time can provide advanced aerosol characterization. At the same time, the instruments with high spatial resolution are sensitive to variability of aerosol near emission sources. The method was demonstrated using Sentinel-5p/TROPOMI and PRISMA measurements. The retrieval from Sentinel-5p/TROPOMI with coarse resolution (~10 km) and global coverage can provide information about aerosol type as well as a priori information of surface properties, while the retrieval from PRISMA can use this information to get AOD and surface reflectance at high spatial resolution. Particularly, aerosol type from TROPOMI/GRASP retrieval was a crucial piece of information for PRISMA/GRASP retrieval. It was shown that the combination of PRISMA and Sentinel-5p/TROPOMI allowed transferring TROPOMI advantages of extended aerosol characterization to the high spatial resolution of PRISMA measurements. As a result, the accurate retrieval of AOD and surface reflectance was obtained at high spatial resolution.
<figure><img src="image_13.png" /><figcaption>**Figure 11.** RGB images over Venice scene on 2,020/08/12. (a) RGB image from top-of-atmosphere PRISMA measurements; (b–d) RGB images after atmospheric correction from PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (v1 wo clim, v2 w clim, v3 w clim) retrieved surface reflectance; (e) TOA and retrieved spectral surface reflectance for 3 selected pixels. PRISMA product L1/L2D 2020-08-12©Italian Space Agency (ASI) (2020). All rights reserved.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 12.** RGB images over Ispra scene on 2020/05/24. (a) RGB image from top-of-atmosphere PRISMA measurements; (b–d) RGB images after atmospheric correction from PRISMA/Generalized Retrieval of Atmosphere and Surface Properties (v1 wo clim, v2 w clim, v3 w clim) retrieved surface reflectance; (e) TOA and retrieved spectral surface reflectance for 3 selected pixels. PRISMA product L1/L2D 2020-05-24©Italian Space Agency (ASI) (2020). All rights reserved.</figcaption></figure>
The 100 m aerosol and surface properties retrieval from PRISMA based on the GRASP/Hybrid approach were evaluated using the available ground-based and satellite measurements, including AERONET, VIIRS/DB aerosol products and PRISMA official L2D surface reflectance product.

* Aerosol

The PRISMA/GRASP 100 m spatial resolution AOD is validated against AERONET. Despite the fact that there is only one site (i.e., Ispra) having three matchups of PRISMA and AERONET measurements, 8 of 9 (88.9%) PRISMA AOD from three different approaches fit reasonably the GCOS requirements relative to AERONET. Note that the PRISMA 100 m AOD spatial variability is lack of verification and need to be comprehensively evaluated in future.

Besides AERONET validation, PRISMA/GRASP 100 m AOD products were intercompared with AOD products from the coarse resolution instruments, in particular, from TROPOMI/GRASP (~10 km) and VIIRS/DB (~6 km). The intercomparisons of PRISMA AOD with TROPOMI/GRASP and VIIRS/DB at common 0.1-degree grid show reasonable agreement with RMSE ~0.1. The high resolution AOD retrieval from PRISMA/GRASP evidently provides more details of spatial variability, especially over urban regions where anthropogenic emission is the dominant aerosol source and the coarse resolution products tend to underestimate AOD. The retrieval was demonstrated to be stable for all three tested approaches. In combination with good correspondence of all three approaches with available AERONET measurements as well as with AOD products from coarse resolution instruments, the tested PRISMA/GRASP retrieval approach at high spatial resolution shows good performance.

* Surface

The problems of aerosol and surface properties retrieval are strongly related to each other. Insufficient accuracy of aerosol retrieval yield errors in the surface characterization and vice versa. Therefore, the PRISMA/GRASP surface evaluation was an essential part. The pixel-to-pixel intercomparison between PRISMA/GRASP surface reflectance and PRISMA official L2D surface reflectance show a very good consistency. Identified biases (0.01–0.02 at visible channels and slightly higher at NIR 0.02–0.025) and dispersion (RMSE around 0.01–0.025) are mainly due to existing uncertainties and differences in the atmospheric correction approaches used in PRISMA/GRASP retrieval and PRISMA L2D surface reflectance product. Basically, both PRISMA L2D and different PRISMA/GRASP surface products are in good agreement with each other.

Overall, the results obtained in this study demonstrate the big potential of the approach based on the combination of instruments with coarse and high spatial resolutions for a number of climate and environment monitoring problems. In particular, PRISMA/GRASP 100 m retrieval on the basis of TROPOMI/GRASP (10 km) climatology provides new possibilities for aerosol and surface characterization at high spatial resolution, identification of local aerosol sources and monitoring aerosol emission/pollution. The newly developed GRASP/Hybrid approach can potentially incorporate other satellites to provide global coverage and overpasses over the same area a few times per week with high and moderate resolution. This multi-mission approach opens new possibilities for aerosol sources identifications and aerosol emission/pollution monitoring.

Data Availability Statement

The Sentinel-5P/TROPOMI GRASP aerosol and surface products are publicly available at GRASP-OPEN website (https://www.grasp-open.com/products/tropomi-data-release/). The PRISMA processing results over selected scenes are registered at ZENODO (https://doi.org/10.5281/zenodo.10722723) (Chen, 2024).

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Acknowledgments

The authors would like to gratefully acknowledge the European Space Agency (ESA) for PRISMA + TROPOMI demonstration project (https://eo4society.esa.int/projects/prisma/) under the contract 4000127614/19/I-NS and AIRSENSE (Aerosol and aerosol-cloud Interaction from Remote SENSing Enhancement) project through the contract 4000142902/23/I-NS. The data procurement in this study was supported by the Italian Space Agency (ASI) in the framework of PRISCAV project (grant nr. 2019-5-HH.0). We would also like to thank the AERONET and VIIRS teams for producing the data and making their data available for the community.
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