Why are dtran_a and dtran_b used for both solar and satellite transmittance (tsol, tsen) in l2gen/aerosol.c?

[fengqiaoed]

In l2gen/aerosol.c, the calculation of solar and satellite transmittance (tsol, tsen) uses the same LUT coefficients (dtran_a and dtran_b) for both angles:

```c
read_lut_variable(file, nc_id, "dtran_a", aertab->model[im]->dtran_a[0]);
read_lut_variable(file, nc_id, "dtran_b", aertab->model[im]->dtran_b[0]);
```

These coefficients are applied in model_transmittance() for both solar and satellite angles:

```c
model_transmittance(modmin, wave, nwave, geom->solz, gmult, tauamin, tsolmin, dtmin); // Solar
model_transmittance(modmax, wave, nwave, geom->senz, gmult, tauamax, tsenmax, dtmax); // Satellite
```

However, the solar transmittance (tsol) should logically use dtran_a0 and dtran_b0 (sun-to-ground coefficients), while satellite transmittance (tsen) should use dtran_a and dtran_b (ground-to-sensor coefficients). When I modified the code to use dtran_a0/dtran_b0 for tsol, the results differed by ~0.6% compared to the current implementation.

Questions:

1. What is the rationale behind using dtran_a/dtran_b for both solar and satellite transmittance?
2. Is this a deliberate simplification, or is there an underlying assumption (e.g., symmetry in diffuse transmittance)?
Any insights would be appreciated!