I'm comparing the seasonality of different satellite and ground-based AOD products in the high-ish (60N and above) latitudes, and find substantial differences between data products generated from LiDAR versus passive sensors.
I'm wondering whether the AODs calculated from MODIS (Deep Blue Dark Target), for example, are normalized to a particular solar zenith angle, or if they instead show the optical depth that is observed at whatever solar zenith angle is apparent at that time of the year. In other words, with a higher zenith angle (i.e. in the winter months) I would expect the optical depth to increase even if aerosol burdens remained the same, because sunlight would travel a greater distance through any given aerosol layer. Alternatively, if the MODIS data product is instead normalized to a particular zenith angle, do you have any papers/documents that detail how that normalization is calculated?