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The comparison of satellite and model aerosol optical depth (AOD)  fields provides useful information on the strengths and weaknesses of  both. However, the sampling of satellite and models is very different  and some subjective decisions about data selection and aggregation  must be made in order to perform such comparisons. This work examines  some implications of these decisions, using GlobAerosol AOD retrievals  at 550 nm from Advanced Along-Track Scanning Radiometer  (AATSR) measurements, and aerosol fields from the GEOS-Chem chemistry  transport model. It is recommended to sample the model only where the  satellite flies over on a particular day; neglecting this can cause  regional differences in model AOD of up to 0.1 on monthly and annual  timescales. The comparison is observed to depend strongly upon  thresholds for sparsity of satellite retrievals in the model grid  cells. Requiring at least 25% coverage of the model grid cell by  satellite data decreases the observed difference between the two by  approximately half over land. The impact over ocean is smaller. In  both model and satellite datasets, there is an anticorrelation between  the proportion <i>p</i> of a model grid cell covered by satellite  retrievals and the AOD. This is attributed to small <i>p</i> typically  occuring due to high cloud cover and lower AODs being found in large  clear-sky regions. Daily median AATSR AODs were found to be closer to  GEOS-Chem AODs than daily means (with the root mean squared difference  being approximately 0.05 smaller). This is due to the decreased  sensitivity of medians to outliers such as cloud-contaminated  retrievals, or aerosol point sources not included in the model

Some implications of sampling choices on comparisons between satellite and model aerosol optical depth fields