All satellites from Table 1 differ significantly in their measurement capabilities, including spectral range and spatial coverage, spatial resolution, revisiting time. To exploit the extended information content of the synergy, measurements from each instrument should be properly harmonized and merged into a spatial–temporal multi-pixel group for simultaneous retrieval. In particular, this includes (i) spectral band selection within the synergy from different instruments; (ii) harmonization of spatial resolution and gridding; (iii) cloud masking merging.

Independent retrieval with the GRASP algorithm has already been applied to each instrument in Table 1, using 9 spectral bands for the OLCI sensor, 10 for TROPOMI, and 6 for the AHI instrument (Chen et al., 2022b, 2024a; Litvinov et al., 2024; Li et al., 2025; Table 2). These bands were also used in the SYREMIS/GRASP approach. In particular, 19 spectral measurements were included in the polar-orbiting LEO + LEO synergetic retrieval, and 24 bands in the LEO + GEO synergy (Table 2).

All instruments from the synergetic satellite constellation have different spatial resolutions (Table 1). To properly apply the multi-pixel constraints in the GRASP algorithm, all measurements were regridded to the same spatial grid. Following the approach used in GRASP/TROPOMI retrieval (Litvinov et al., 2024; Chen et al., 2024a), an equidistant cylindrical projection and WGS84 coordinate system with a spatial pixel resolution of 0.09° was used for all selected spectral bands, employing a bilinear regridding method (Table 2).

Since the measurements in the SYREMIS synergy are not collocated in time, cloud masking was applied independently for each instrument. In particular, cloud screening for TROPOMI in SYREMIS/GRASP processing is based on the S5P NPP-VIIRS cloud product, with approximately 500 m spatial resolution, as was done for GRASP/TROPOMI processing (Litvinov et al., 2024; Chen et al., 2024a). Similar to GRASP/OLCI retrieval (Chen et al., 2022b), IDEPIX cloud masking was applied to OLCI instruments (Wevers et al., 2022). Himawari-8/AHI cloud screening is based on Level 2 cloud products (Letu et al., 2018, 2020).

The GRASP multi-pixel concept is a key methodological approach in realizing multi-platform inversion. It is implemented within the Multi-Term LSM methodology, as described by Dubovik et al. (2011, 2021a). Following this approach, the retrieval is conducted not for each selected observed satellite pixel, but rather for a set of observations collected over different locations at different time moments (spatial–temporal multi-pixel dataset block, Figs. 2 and 3).

Figure 3 illustrates an example of a spatial–temporal multi-pixel dataset block for 5×5 spatial pixels within a single selected day, filled with S3A/OLCI, S3B/OLCI, S5P/TROPOMI, and Himawari-8/AHI measurements. As one can see, the single-day dataset block consists of data from different sensors, performing observations at different times during the same day: S3A/OLCI and S3B/OLCI make observations in the morning and can overlap spatially for some pixels; S5P/TROPOMI is the afternoon satellite; Himawari-8/AHI is in geostationary orbit, providing multiple observations during a day over the same area.

The balanced measurement “weighting” in the spatial–temporal multi-pixel dataset block (Figs. 2 and 3) is crucial for synergetic retrieval. For example, as demonstrated in Litvinov et al. (2024) and Chen et al. (2024a), the S5P/TROPOMI instrument has the highest information content among all satellites from Table 1, allowing considerable enhancement of extended aerosol and surface characterization in comparison with OLCI and AHI instruments (Chen et al., 2022b; Li et al., 2025). This fact is accounted for by “weighting” measurements from different instruments in the SYREMIS/GRASP retrieval.

The GRASP algorithm is based on the statistical optimization concept (e.g., Dubovik et al., 2021a), where the contributions of different input data are balanced using known covariance matrices of each input dataset, including both satellite measurements and a priori data. Since those datasets are statistically independent (C=Iσ2, where C is the covariance matrix, I is the diagonal unity matrix, and σ2 is the variance (σ is the standard deviation) (Appendix A; Dubovik, 2004; Dubovik et al., 2021a)), the weights (importance of the measurements) are driven by known (or assumed) standard deviations in each dataset: the smaller the standard deviation of measurement fitting (the standard deviation of retrieval residual) required in the retrieval, the greater the “weight” of such measurements that can be assigned in the synergy. Within the framework of the Multi-Term LSM concept, the “weights” are quantitatively determined as ratios of σ12 (variance of the first dataset) to σi2 (variance of the ith dataset). A detailed description of the data weighting Multi-Term LSM concept can be found in Dubovik et al. (2004, 2021a), and a brief discussion is also provided in Appendix A.

Table 3 shows the weighting of different instruments used in the multi-instrument synergy inversion. It can be seen that the S5P/TROPOMI has the highest weight among other observations. At the same time, it is important to emphasize that, in general, the weights are defined with respect to both measurements and the a priori datasets used. This aspect is discussed in the next section and Appendix A.

Application of the GRASP retrieval algorithm independently to OLCI, TROPOMI, and AHI single-view instruments, as well as to multi-angular polarimetric PARASOL measurements, showed good performance of the so-called aerosol “models” approach (Chen et al., 2020, 2022b, 2024a; Dubovik et al., 2021a; Hasekamp et al., 2024; Litvinov et al., 2024). In this approach, the total single-scattering characteristics of aerosols are represented as linear combinations of the characteristics of preselected aerosol components, i.e., assuming an external mixture of four different aerosol models: (1) fine absorbing, representing a climatological biomass burning aerosol model; (2) fine slightly absorbing, corresponding to climatological sulfate aerosols with introduced minor absorption; (3) coarse component representing mainly maritime aerosol type; and (4) coarse component for dust aerosol representation (Lopatin et al., 2021; Litvinov et al., 2024). The state vector for such an aerosol model in the GRASP inversion consists of the following characteristics: aerosol scale height, aerosol concentration, and aerosol model fractions (see, for example, the aerosol model parameters discussion in Litvinov et al., 2024).

The surface reflectance in the GRASP retrieval is usually described by the spectrally constrained renormalized Ross-Li BRDF model over land (Litvinov et al., 2010, 2011a, b, 2024) and a modified Cox and Munk model with accounting for water-leaving reflectance (Cox and Munk, 1954; Litvinov et al., 2024). These aerosol and surface reflectance forward approaches were found to be optimal also in the SYREMIS/GRASP synergetic retrieval for the satellite constellation from Table 1.

The strategic advantage of the GRASP inversion implemented via Multi-Term LSM statistically optimized fitting is that this concept allows the use of multiple a priori constraints in the retrieval. Such a priori constraints can be applied to all aerosol and surface characteristics and parameters (Dubovik et al., 2011, 2014, 2021a). In particular, the “single-pixel” a priori smoothness constraints can be used for limiting variability (and avoiding unrealistic oscillations) of, for example, aerosol column concentration, aerosol model fractions, spectral dependencies of surface BRDF parameters (the “in-pixel” smoothness constraints in Eq. A13). In addition to the “single-pixel” constraints, the “inter-pixel” constraints were used within the so-called “multi-pixel” retrieval when a large group of pixels was retrieved simultaneously and smoothness constraints (the “inter-pixels” smoothness constraints in Eq. A13) can be used for limiting temporal or spatial variability of parameters retrieved in different neighboring pixels (Dubovik et al., 2011, 2021a).

The successful application of the GRASP algorithm to different satellites demonstrated the crucial role of the single- and multi-pixels (or in-pixel and inter-pixel) smoothness constraints for stable and accurate characterization of both atmospheric aerosol and surface (Dubovik et al., 2011, 2014; 2021a; Chen et al., 2022b; Litvinov et al., 2024). A brief summary of the formal implementation of the “multi-pixel” retrieval is provided in Appendix A.

In comparison to the single-instrument retrieval, the SYREMIS LEO + LEO and LEO + GEO synergies have much better diurnal temporal resolution of measurements, when the temporal difference between S3A/OLCI, S3B/OLCI, S5P/TROPOMI, and especially between Himawari-8/AHI measurements can vary from several minutes to several hours within a single day. Moreover, the retrieval is performed on data accumulated in spatial–temporal blocks (Figs. 2 and 3) consisting of hundreds of satellite observations and covering about a month in the LEO + LEO and about two weeks in the LEO + GEO synergies (up to 150 and 200 temporal observations in the LEO + LEO and LEO + GEO synergies, respectively, Table 4). The temporal variability of aerosol and surface properties can be different within a few minutes, hours, or from day to day. Moreover, the temporal variability of aerosol properties is usually stronger than the surface ones. Therefore, the importance of using a priori temporal constraints for aerosol and surface parameters within a few minutes to several days and weeks significantly increases in the LEO + LEO and LEO + GEO configurations.

GRASP limits the temporal variability of the retrieved parameters by applying temporal a priori constraints, introduced as a priori estimates of the first derivatives with respect to time, approximated by finite differences (the differences between the parameters divided by the period over which they were observed; Dubovik et al., 2011, 2021a). The estimates are assumed to be normally distributed values with zero means and the standard deviations assigned on the basis of the expected temporal variability of retrieved parameters. The use of limitations applied to the finite differences is a very convenient way to ensure flexibility in temporal constraints. Indeed, such constraints allow much larger variability for parameters corresponding to more distant in time observations compared to the parameters from observations that are very close in time (full description can be found in Dubovik et al., 2011, 2021a).

However, in cases when observations are nearly simultaneous or very close in time, the finite differences may have very large values that can produce an imbalance in practical fitting. To avoid such difficulties, “temporal thresholds” on aerosol and surface characteristics are used in the SYREMIS/GRASP. The temporal threshold allows for avoiding very large values of the finite differences in cases when the combined synergistic observations are close to each other in time. Specifically, the very small time difference in the denominator of the finite difference is replaced by the specified “temporal threshold” value, allowing stronger temporal constraints on the temporal variability of the retrieved parameters within the threshold. This approach was in particular useful for constraining BRDF parameters, defined by the intrinsic properties of the surface (surface type, reflecting properties, topology), which are very stable in time, and, as a rule, do not change considerably during a day. The temporal thresholds applied to BRDF in the SYREMIS/GRASP approach allow using almost the same BRDF parameters for all synergetic measurements during the specified period. Similarly, the temporal thresholds are also useful for constraining the variability of aerosol model parameters (Table 4). Overall, in the SYREMIS/GRASP approach, different temporal thresholds were applied to different aerosol and surface properties, accounting for their different temporal dependence.

When the time interval between measurements in the synergy exceeds the specified temporal threshold, the multi-pixel temporal constraints (inter-pixel smoothness constraints on t-temporal variability in Eq. A13) in the SYREMIS/GRASP account for the actual temporal interval between the measurements, as was done in the single-instrument GRASP retrieval (Dubovik et al., 2011, 2021a; Chen et al., 2022b; Litvinov et al., 2024; Li et al., 2025). In addition, some spatial multi-pixel constraints were also used for aerosol, as described in Appendix A; however, these constraints played a rather minor role, as they were applied only within a 2-by-2-pixel spatial segment in the spatial–temporal multi-pixel block.

A priori knowledge about single-instrument information content and measurement accuracy provides a good starting point for formulating the initial concept for the retrieval setups: instrument weighting, temporal, spatial, and spectral constraints in the synergy. Nevertheless, practice shows that optimizing and tuning this initial synergetic retrieval setup by conducting a series of remote sensing tests is usually desirable and necessary. In SYREMIS/GRASP, synergy optimization included several tests: (i) validation of retrieved aerosol properties over a limited number of AERONET stations; (ii) intercomparison of surface properties derived from synergetic ground-based and satellite measurements (based on the GROSAT/GRASP synergetic approach; Litvinov et al., 2020, 2022, 2024); and (iii) intercomparison of aerosol and surface synergetic retrieval with reference products from space-borne measurements over small regions (∼1000km×1000km).

Test (i) for the LEO + LEO and LEO + GEO synergies was first performed over a few AERONET stations (Holben et al., 1998; Giles et al., 2019) with different aerosol and surface properties (e.g., Mongu, Banizoumbou, Kanpur, Beijing) and then extended to 30 stations worldwide (e.g., Chen et al., 2022b) for the selected limited period (March–May 2019). The retrieval setups that showed the best AOD, AE, and SSA validation results were selected for further consideration. Test (ii) enabled intercomparison of surface BRDF parameters derived from different instruments using the GROSAT/GRASP synergetic approach, similar to Litvinov et al. (2022, 2024) and Chen et al. (2024a). Since these parameters are independent of illumination/observation geometries, they can be intercompared for different space-borne sensors, allowing identification of possible biases and their accounting for each instrument (Sect. 2.2). In Test (iii), MODIS and VIIRS (Levy et al., 2013; Sayer et al., 2018a; Schaaf and Wang, 2015a, b) aerosol/surface products were used for regional intercomparison with the SYREMIS/GRASP results.

The validation results against AERONET in Test (i) were evaluated using the following statistical characteristics: (i) Pearson correlation coefficient (R); (ii) Root Mean Square Error (RMSE); (iii) the number of data points (N) in the validation; and (iv) the number (and percentage) of data points satisfying the GCOS-based AOD criteria relative to AERONET. The GCOS-based criteria for AOD are based on the AOD requirements from Global Climate Observing System (GCOS-245, 2022) and their adaptation within the ESA aerosol-CCI project (Popp et al., 2016) for validation of AOD, derived from satellite measurements, against AERONET (de Leeuw et al., 2015): absolute difference in AOD is less than 0.04 or relative difference is less than 10 % (whichever is bigger).

The synergetic setup exhibiting the best validation statistical characteristics for aerosol and surface properties from Tests (i) and (ii), and which qualitatively agreed with the well-established aerosol/surface products (Test iii), was selected as a baseline approach for global synergetic retrieval.

The remote sensing tests, performed for the LEO + LEO and LEO + GEO synergies, showed that merging spectrally close measurements (for example, OLCI 442.5 nm and TROPOMI 440 nm, or OLCI 665 nm and TROPOMI 670 nm) does not provide the best results in Test (i). This can be due to different noise in the close spectral bands from different instruments, their different spectral response function, etc. Therefore, the final SYREMIS/GRASP synergetic data were prepared using 19 spectral measurements in the LEO + LEO synergy and 24 bands in the LEO + GEO synergy. The optimization test also showed the crucial role of balanced “weighting” of the multi-instrument measurements within the SYREMIS/GRASP synergetic approach. As discussed in Sect. 2.2 and Appendix A, instrument “weighting” in GRASP can be achieved by defining the standard deviation of the expected uncertainties, which determine weighting matrices in the GRASP multi-term LSM inversion scheme and, thus, the weights (or importance) of different data involved in the inversion. Experimenting with different values of the standard deviation, it was found that application of stronger requirements on the measurement fitting for the TROPOMI bands in the synergy resulted in much better validation results against AERONET (Test i) (Fig. 4). This can be explained by TROPOMI's higher information content (better spectral coverage and wider swath) in comparison with other instruments within the synergy. Specifically, whenever comparable values of the standard deviation were applied to all instruments and spectral bands, the validation against AERONET showed reduced performance relative to single-instrument retrieval (for example, in comparison with the GRASP/TROPOMI results; Litvinov et al., 2024; Chen et al., 2024a). Therefore, in the SYREMIS/GRASP synergetic approach, a few weighting groups with different requirements for the standard deviation of measurement fitting were associated with different instruments from the synergy (Table 3).

In addition, it was discovered that exchanging a few measurements between the weighting groups provided better retrieval results against AERONET. In particular, over land, the same standard deviation of 0.001 as for most TROPOMI channels (Table 3, group “a”) was assigned to OLCI bands 490 and 560 nm. Conversely, one TROPOMI band, 494 nm, was allocated to the OLCI bands' weighting group with a required standard deviation of 0.05 (Table 3, group “b”). Such band inter-exchange was found to be useful for improving OLCI retrieval in the LEO + LEO synergy without loss of accuracy in the SYREMIS/TROPOMI retrieval. This allows transferring information content from TROPOMI to OLCI and vice versa. The inter-exchange of selected bands between OLCI and AHI measurements also improved the consistency of retrieval for all instruments in the LEO + GEO synergy (Table 3). The optimal weighting of the different measurements used in the LEO + LEO and LEO + GEO SYREMIS synergies, according to the applied requirements on the standard deviation, is presented in Table 3.

As one can see, the “weighting” of the LEO + GEO is much more complicated than that of the LEO + LEO synergy. In particular, two additional groups of weights were added: group “c” with “0.01”, applied mainly to AHI instruments to account for diurnal aerosol variability from geostationary measurements; and group “d” with “0.05”, to reduce the “weight” of SWIR AHI measurements, that allowed improving the performance of the remote sensing tests.

The values of the “weights” in Table 3 are related to the specific inversion approach of the GRASP algorithm applied to the instruments listed in Table 1. Moreover, due to the ill-posed character of the inversion, the “weighting” from Table 3 is not unique even within the GRASP algorithm since other combinations of its values are possible with quite comparable retrieved results. The values provided in Table 3 demonstrate a general tendency for the considered synergies: the “weight” of S5P/TROPOMI measurements should be higher compared to other instruments.

Besides satellite measurements' “weighting”, GRASP “single-” and “multi-pixel” a priori constraints (Sect. 2.3, Appendix A) also play a key role in the successful realization of the SYREMIS/GRASP approach. Overall, the spectral and spatial constraints used in GRASP single-instrument retrieval (Chen et al., 2022b; Litvinov et al., 2024; Li et al., 2025) showed good performance in the optimization tests and were subsequently used in synergy.

Due to the merging of measurements from different sensors, the synergetic L1 input data blocks have much better temporal resolution compared to any single instrument from the synergy (Figs. 2 and 3). This affects the optimal temporal constraints applied to aerosol and surface parameters within the SYREMIS/GRASP approach. Properly selected temporal thresholds and smoothness constraints substantially increase the number of quasi-multi-angular measurements in the synergy, a factor crucial for BRDF parameter retrieval and the distinguishing of atmosphere and surface signals. The optimized temporal thresholds and smoothness constraints for aerosol and surface parameters in the SYREMIS/GRASP LEO + LEO and LEO + GEO synergies are presented in Table 4. As one can see, they account for known tendencies: surface BRDF temporal variability is generally smaller than the temporal variability of aerosol properties (stronger thresholds and constraints on the surface BRDF parameters than on the aerosol ones in Table 4); the temporal variability of the aerosol concentration is usually larger than that of the aerosol microphysics, which results in stronger constraints on the aerosol model fraction parameter in the SYREMIS/GRASP approach (Table 4).

The temporal thresholds applied to BRDF parameters in the SYREMIS approach considerably limit the variability of the BRDF parameters for all synergetic measurements during the specified period. They can change depending on pixel latitude due to different satellite overpass times at low and high latitudes. For the considered LEO + LEO synergetic satellite constellation, several hours (for example, ±6h) are sufficient to account for a stable surface within the day globally.

LEO + GEO synergetic measurements cover a wider range of observation/illumination geometries (solar and viewing zenith and azimuth angles) than LEO + LEO. It has already been demonstrated that different BRDF models used in remote sensing perform well only for limited geometries (the limitations of different BRDF models in remote sensing are studied, for example, in Litvinov et al., 2011a, b). Due to this known limitation of the BRDF models, the optimized temporal thresholds for the LEO + GEO synergy are relaxed to a few hours, and the constraints on temporal variability rely mainly on temporal smoothness constraints in the GRASP algorithm, allowing the retrieved properties to change smoothly in time.

Similar to Table 3, due to the ill-posed nature of the inversion problem, the values provided in Table 4, strictly speaking, are not unique, since quite similar retrieval performance of aerosol and surface properties can be obtained within a certain range of the constraints. Nevertheless, the chosen parameters are generally within the optimal range in the sense that they adequately reflect the tendencies in temporal dependencies of aerosol and surface properties, adapted to the information content in the LEO + LEO and LEO + GEO synergies.

Figure 4 shows the SYREMIS LEO + LEO retrieval results obtained before (Fig. 4a) and after (Fig. 4b) harmonization, instrument “weighting”, “single-” and “multi-pixel” setup optimization. The results are intercompared with AERONET AOD at 550 nm. In this and other validation figures below for AOD, AE, and SSA, AERONET Level 2.0 datasets from the Version 3 Direct Sun and Inversion Algorithms were used (Holben et al., 1998; Dubovik and King, 2000; Dubovik et al., 2000; Giles et al., 2019; Sinyuk et al., 2020).

In Fig. 4 (as well as in other similar validation plots below), the color of each data point indicates the AOD Probability Density Function (PDF) value (an increasing PDF value corresponds to a color gradient from blue to red). The straight dotted line represents an ideal one-to-one correspondence in AOD. The straight red line is the linear fit to the data distribution, with the slope and intercept marked in the top-left corner legend. The grey envelope surrounding the one-to-one line in the scatter plot indicates the AOD GCOS-based envelope discussed above: an absolute difference in AOD is less than 0.04, or a relative difference is less than 10 % (whichever is bigger) (GCOS-245, 2022; de Leeuw et al., 2015). Below the scatter plots, the PDF (“Probability” in Fig. 4) of the AOD difference (AOD(SYREMIS/GRASP) – AOD(AERONET) in Fig. 4) is provided for four AOD intervals (bins). “Total” (black color in Fig. 4) corresponds to all AOD cases; “<0.2” (red): AOD<0.2; “[0.2,0.7]” (blue): AOD is between [0.2,0.7]; and “>0.7” (green in Fig. 4): AOD>0.7 cases. In Fig. 4, the bias (“Avg”), standard deviation (“StD”), Root Mean Square Error (“RMSE”), the total number of pixels in the validation (N), and the number and percentage of pixels satisfying GCOS-based requirements (“GCOS”) are indicated as a legend at the top of the PDF plots.

Table 5 lists the AOD validation statistical parameters for the SYREMIS LEO + LEO synergy extracted from Fig. 4. In particular, after optimizing the retrieval settings, “GCOS” (Total) improved from 34.2 %–47.9 %; bias (Total) decreased from 0.04 to -0.02; the correlation coefficient improved from 0.89–0.9; and the RMSE decreased from 0.16–0.14. The biggest improvements are for the low AOD cases (AOD<0.2), containing the majority of validation pixels: “GCOS” improved from 38.8 %–61 %, and “bias” decreased from 0.06–0.01. For moderate AOD (0.2<AOD<0.7), “GCOS” improved from 25.2 %–26.3 %, and “bias” changed from 0.04 to -0.04. For high AOD (>0.7) cases, “GCOS” remained flat around 35 %, and “bias” changed from -0.06 to -0.09. One can observe substantial improvement in the validation statistical characteristics, emphasized by the bold fonts in Table 5, relative to AERONET after properly accounting for the information content of the instruments, measurement accuracy, and adjustment of the retrieval approach to the synergetic multi-instrument constellation.

The validation results for the synergetic SYREMIS/GRASP LEO + LEO retrieval against the global AERONET stations for March, April, and May 2019 are presented in Figs. 5–7. Figures 5a–7a show the validation for all instruments in the synergy (SYREMIS LEO + LEO). Figures 5b–7b present the data associated with S5P/TROPOMI measurements (data extracted from the synergy at the time of TROPOMI measurements, indicated as “SYREMIS LEO + LEO: S5P/TROPOMI” in Figs. 5b–7b). Figures 5c–7c contain the data associated with S3/OLCI measurements (data extracted from the synergy at the time of S3A/OLCI and S3B/OLCI measurements, indicated as “SYREMIS LEO + LEO: S3/OLCI” in Figs. 5c–7c).

To remove the outliers from the retrieval and ensure consistency in the presented results, a filtering similar to the GRASP/TROPOMI quality assurance flag (Litvinov et al., 2024) was applied in Figs. 5–7. In particular, over land, the validation was done for pixels satisfying the following conditions: (i) the relative residual of fitting was less than 0.03 (3 %); (ii) the standard deviation (σAOD) of AOD (670 nm) within the 3×3 pixel window was less than 0.05, or the relative standard deviation σAOD (670 nm)/AOD (670 nm) was less than 0.15; (iii) the number of valid pixels within the 3×3 pixel window was greater than or equal to 5. Over the ocean, the filtering conditions were relaxed, taking into account the overall smaller total reflectance and global AOD values compared to pixels over land: (i) the relative residual was less than 0.1; (ii) σAOD (670 nm) < 0.05. For the Angstrom Exponent (AE) and SSA, an additional filter was applied when only pixels with AOD(550 nm) > 0.2 and AOD(550 nm) > 0.3, respectively, were used in the validation.

In general, the synergetic SYREMIS/GRASP retrieval shows good correspondence to AERONET, with a high percentage (∼54.3% over land and ∼65% over ocean) of fulfillment of GCOS-based (GCOS-245, 2022) requirements for AOD. The values of the statistical characteristics (Pearson correlation coefficient, Root Mean Square Error (RMSE), bias, etc.) indicate the high quality of the retrieval, as shown in Figs. 5–7.

The corresponding SYREMIS LEO + LEO AOD (550 nm) validation statistics are summarized in Table 6. As one can see, the SYREMIS all-instruments AOD (550 nm) retrievals (combining S5P/TROPOMI, S3A/OLCI, and S3B/OLCI, as presented in Fig. 5a and d) show good agreement with AERONET observations. Over land (Fig. 5a), the validation yields the correlation coefficient (R) of ∼0.89, RMSE of ∼0.094, and GCOS-based compliance fraction of 54.3 %. Over the ocean (Fig. 5d), the R value is ∼0.88, the RMSE is ∼0.060, and the GCOS-based compliance fraction is 65.0 %.

These AOD statistical characteristics are consistent with the validation results for all individual instruments extracted from the SYREMIS LEO + LEO retrievals, as shown in Fig. 5b and c (for S5P/TROPOMI and S3/OLCI over land) and Fig. 5e and f (for S5P/TROPOMI and S3/OLCI over ocean), respectively. Moreover, all retrieved extended characteristics (AE and SSA) are also consistent with each other across all instruments in the synergy. Specifically, the validation results for AE and SSA from the SYREMIS LEO + LEO retrievals are presented in Figs. 6 (AE) and 7 (SSA), respectively, and summarized in Table 7. AE from the SYREMIS LEO + LEO (Fig. 6a and d), as well as from the TROPOMI (Fig. 6b and e) and OLCI (Fig. 6c and f) data extraction, demonstrates similarly good agreement with AERONET, with correlation coefficients (R) ranging from 0.70–0.80 and RMSE values of 0.45–0.48 over land. Over the ocean, R values range from 0.60–0.75 with RMSEs of 0.40–0.50. For the SYREMIS LEO + LEO (Fig. 7a and d), as well as for TROPOMI (Fig. 7b and e) and OLCI (Fig. 7c and f) data extraction, the SSA (550 nm) validations against AERONET show that the RMSEs are approximately 0.03–0.035 over land and 0.05–0.07 over ocean, indicating, in general, good agreement taking into account the limited available matchups of SSA under moderate and high AOD conditions.

Figures 8–13 demonstrate the added value of the synergetic approach, comparing validation results against AERONET of AOD, AE, and SSA obtained from the SYREMIS/GRASP LEO + LEO retrieval and from the corresponding GRASP single-instrument retrieval. Specifically, Figs. 8a–10a present the validation results from the SYREMIS LEO + LEO synergy associated with S5P/TROPOMI measurements (data extracted from the synergy at the time of TROPOMI measurements: “SYREMIS LEO + LEO: S5P/TROPOMI” in Figs. 8–10). Figures 8b–10b show the validation obtained from the GRASP/TROPOMI single-instrument retrievals (Litvinov et al., 2024; Chen et al., 2024a). Similarly, Figs. 10–13 present the validation for OLCI data extracted from the SYREMIS LEO + LEO synergy (“SYREMIS LEO + LEO: S3A/OLCI” in Figs. 11a–13a) and for the GRASP/OLCI-A single-instrument retrieval in Figs. 11b–13b (Chen et al., 2022). The corresponding intercomparison of validation statistics is summarized in Tables 8–11. In general, one can observe a higher number of retrievals passing the filtering criteria (mainly the residual filter criteria, Sect. 3.1) in SYREMIS LEO + LEO products compared to the GRASP single-instrument retrievals, indicating increased quality of the retrieval in the synergy. This improvement may be attributed to enhanced aerosol–surface signal decoupling in the synergy, resulting in smaller spectral fitting residuals. In addition, the GRASP/TROPOMI single-instrument AOD (550 nm) retrieval exhibits some scatter, with an RMSE of 0.139 and a GCOS compliance fraction of 48.4 % over land, as shown in Fig. 8b. These metrics are improved in the SYREMISLEO + LEO: S5P/TROPOMI retrievals, which achieve an RMSE of 0.090 and a GCOS fraction of 53.9 %, as shown in Fig. 8a. For the extended aerosol characteristics (AE and SSA), the SYREMIS LEO + LEO: S5P/TROPOMI retrievals show validation statistics that are generally comparable to those from the GRASP/TROPOMI single-instrument retrievals, as presented in Figs. 9 and 10, respectively. On the other hand, a clear improvement is observed for the SYREMIS LEO + LEO: S3A/OLCI aerosol data compared to the GRASP/OLCI-A single-instrument retrievals. Notably, the number of available AOD (550 nm) matchups with AERONET nearly doubles both over land and ocean (Fig. 11a and b). Additionally, the GCOS compliance fraction over land increases from 42.5 %–54.5 % (Fig. 11a and b), comparable to the SYREMIS LEO + LEO: S5P/TROPOMI. Although the validation statistics over the ocean show a slight decrease, as seen in Fig. 11, the number of successful inversions increases by a factor of three (162–543), and more high-AOD values pass the filtering criteria with 64.5 % high GCOS fulfillment, indicating enhanced sensitivity and broader retrieval capability in challenging conditions. For the extended aerosol characteristics (AE and SSA), as shown in Figs. 12 and 13, the validation results for the SYREMIS LEO + LEO: S3A/OLCI datasets are considerably improved in comparison with the single instrument GRASP/OLCI-A retrieval and reached a level comparable with those from the GRASP/TROPOMI single-instrument retrievals. These findings further support the conclusion that S5P/TROPOMI remains the dominant contributor to the information content in the SYREMIS LEO + LEO synergistic retrievals.

Summarizing intercomparison results, one can see better performance of the SYREMIS LEO + LEO: S5P/TROPOMI AOD data extract in comparison with the GRASP/TROPOMI, while the extended properties (AE and SSA) are of similar quality in both (synergy and single-instrument retrieval) cases. The biggest improvement is observed for the OLCI instrument, where the performance of all retrieved parameters in SYREMIS LEO + LEO: S3A/OLCI data extract is essentially improved compared to the single-instrument GRASP/OLCI-A retrieval. In particular, all retrieved characteristics from the synergy (AOD, AE, and SSA) are much better than those from the single instrument GRASP/OLCI-A retrieval, and the number of pixels passed through the quality filter considerably increased. In general, SSA from the SYREMIS LEO + LEO: S3/OLCI dataset is of the same quality as from the SYREMIS/LEO + LEO: S5P/TROPOMI, providing aligned retrieval for all instruments from the synergy.

The presented results clearly show that properly combining measurements from the S3A/OLCI, S3B/OLCI, and S5P/TROPOMI according to information content and accuracy, the synergetic SYREMIS/GRASP approach considerably improves the retrieval of the extended aerosol characterization in comparison to the single instrument retrieval. It enables transfer of information content from one instrument to another, which results in consistent aerosol characterization from non-coincident diverse satellite measurements. As a result, the quality of the retrieval for all instruments in the SYREMIS LEO + LEO synergy is comparable to or better than the quality from the GRASP/TROPOMI single-instrument retrieval and much better than from the GRASP/OLCI single-instrument retrieval. This can be explained by the fact that S5P/TROPOMI measurements have a wider spectral range and swath (consequently, better temporal resolution) than S3/OLCI. They are also known for their high radiometric accuracy (Ludewig et al., 2020; Tilstra et al., 2020). All these are crucial for atmosphere and surface signals differentiation and enhancement of the retrieval as demonstrated in Litvinov et al., 2024 and Chen et al., 2024a, and emphasized in the presented SYREMIS/GRASP synergetic results.

Validation of AOD, AE, and SSA from the SYREMIS LEO + GEO synergy over all AERONET stations within Himawari-8/AHI full scan is presented in Figs. 14–16 and summarized in Tables 12 and 13. Similar to the LEO + LEO synergy, the results are presented for all instruments involved in the SYREMIS/GRASP synergy, as well as for the data extracted for each specific instrument (SYREMIS LEO + GEO: S5P/TROPOMI, SYREMIS LEO + GEO: S3/OLCI (OLCI-A and OLCI-B), and SYREMIS LEO + GEO: Himawari-8/AHI). The retrievals of the AOD, AE, and SSA are highly comparable across all instruments in the synergy. For example, the AOD (550 nm) validation with AERONET shows the correlation coefficients around 0.92, the RMSE values near 0.15, and the GCOS-based fraction of approximately 40 %. While the RMSE and GCOS-based fraction are slightly worse than the global SYREMIS LEO + LEO AOD (550 nm) validation results presented in Fig. 5, this difference can be attributed to the Himawari-8/AHI full scan region, which includes a higher proportion of high-AOD cases and thus poses more challenging retrieval conditions. Moreover, for all instruments, AE (with RMSE ∼0.3) and SSA (with RMSE<0.05) are of similar quality to AE and SSA from GRASP/TROPOMI (Litvinov et al., 2024) and the SYREMIS LEO + LEO synergy (Figs. 6 and 7).

In general, the validation and inter-comparison results show that, similar to the LEO + LEO synergy, richer information content from the S5P/TROPOMI propagates to other instruments of the SYREMIS LEO + GEO synergy (S3A/OLCI, S3B/OLCI, and Himawari-8/AHI), improving extended aerosol characteristics such as AE and spectral SSA. At the same time, AOD is improved for all instruments in the SYREMIS LEO + GEO synergy, including S5P/TROPOMI, due to the additional spatial, temporal, and spectral information content in the synergetic measurements. Moreover, one of the crucial advantages of the LEO + GEO synergy is the high temporal resolution of the extended aerosol characterization (AOD, AE, SSA, etc.). In particular, the considered SYREMIS LEO + GEO synergy provides diurnal variability of aerosol with ∼1h temporal resolution, which allows for monitoring aerosol transport, air quality, and can be used in atmospheric dynamics studies. The observed unique advantages in capturing the diurnal variability of aerosol optical-microphysical properties from the LEO + GEO SYREMIS product will be reported and discussed in a separate study.

In this section, we present a preliminary intercomparison of aerosol and surface products, obtained from the SYREMIS/GRASP synergetic retrieval, with GRASP/TROPOMI, VIIRS, and MODIS aerosol and surface products (Hsu et al., 2013; 2019; Sayer et al., 2018a; Schaaf and Wang, 2015a, b; Chen et al., 2022b; Litvinov et al., 2024). A more detailed analysis of the global SYREMIS product will be performed in separate studies.

Figure 17 shows the global maps of the March 2019 monthly mean AOD at 550 nm for the SYREMIS LEO + LEO: S5P/TROPOMI (TROPOMI data extraction from the synergy, indicated as SYREMIS/TROPOMI in Fig. 17), GRASP/TROPOMI (single-instrument GRASP retrieval from TROPOMI), VIIRS/DB, and their differences. The SNPP/VIIRS AERDB_L2_VIIRS_SNPP aerosol product (Hsu et al., 2013, 2019) was used in the intercomparison. All products were regridded to the same 0.2° global equi-rectangular grid for pixel-to-pixel intercomparison.

Globally, SYREMIS/TROPOMI AOD values show good qualitative agreement with single-instrument GRASP/TROPOMI and VIIRS products. Nevertheless, the quantitative intercomparison (see the maps of the differences “VIIRS/DB-SYREMIS/TROPOMI” and “TROPOMI/GRASP-TROPOMI/SYREMIS” in Fig. 17) shows that the synergetic SYREMIS/TROPOMI AOD is lower than the GRASP/TROPOMI and VIIRS products over: (i) bright surfaces (e.g., over the Sahara and the Arabian Peninsula); (ii) the North Indian Subcontinent and the East coast of China, where AOD is quite large for all products; and (iii) the South polar regions, where AOD is very small for all products. In general, AOD products from the single instruments (GRASP/TROPOMI and VIIRS/DB) correspond to each other better than they correspond to the SYREMIS/GRASP AOD (see maps of the differences: “VIIRS/DB-TROPOMI/GRASP”, “VIIRS/DB-SYREMIS/TROPOMI”, and “TROPOMI/GRASP – TROPOMI/SYREMIS” in Fig. 17). A more detailed analysis of the global aerosol characterization with the synergetic approach will be the subject of separate studies.

Figure 18 shows a qualitatively similar retrieval of the first (isotropic) BRDF parameter from the SYREMIS/TROPOMI data extraction compared to the MODIS product (the MODIS MCD43C1 Terra + Aqua BRDF Model Parameters Daily L3 Global 0.05Deg CMG product suite (Schaaf and Wang, 2015b) was used), regridded to the same 0.2° global equirectangular grid for pixel-to-pixel intercomparison. Depending on the region, SYREMIS/GRASP can provide a brighter or darker surface compared to MODIS. In the blue spectral band, SYREMIS/TROPOMI BRDF1 (the isotropic Ross-Li BRDF parameter) values are generally lower than MODIS BRDF1 values in the Saharan desert, the Middle East, and Tibet, and higher than MODIS BRDF1 in other regions. In the red spectral band, the difference between SYREMIS/TROPOMI and MODIS BRDF1 shows an opposite spatial distribution. In Central Asia and Tibet, MODIS BRDF1 is higher than SYREMIS/TROPOMI BRDF1 in both the 'blue' and 'red' spectral bands. SYREMIS and MODIS BRDF maps differ in spatial completeness over the Amazon and high-latitude regions due to differences in their cloud/snow masks.

The largest difference in the surface retrieval emerging from the synergetic approach can be found in the second (volumetric) and third (geometric) Ross-Li BRDF parameters, which describe the angular profile of the surface reflectance. As can be seen from Fig. 19, the values of these parameters derived by SYREMIS/GRASP over bright surfaces (e.g., over the Sahara) are much higher than those from the MODIS BRDF product. This can be clearly seen from the pixel-to-pixel correlation in Fig. 20. The stronger variability of the second and third BRDF parameters can be explained by the quasi-multi-angular measurements in the synergetic retrieval. In particular, the SYREMIS LEO + LEO accounts for S5P/TROPOMI, S3A/OLCI, and S3B/OLCI measurements within about one month, using temporal thresholds and “multi-pixel” temporal constraints of the GRASP algorithm (Table 4). This allowed for the accumulation of up to hundreds of synergetic measurements obtained at different observation/illumination geometries (zenith and azimuthal angles of the sun and satellites), which are accounted for in the GRASP multi-pixel LSM method (see Sect. 2). As a result, the synergetic retrieval of the angular features of BRDF (provided by the volumetric and geometric Ross-Li BRDF parameters) is expected to be much more comprehensive than any single-instrument retrieval, including a multi-day MODIS Terra+Aqua combination. Similar to the global aerosol product, synergetic surface BRDF retrieval on a global scale will be the subject of subsequent studies.