-Atomic aggregation used to inform the neighbor that the originated router aggregated routes
-Aggregator attribute specify the IP & ASN of the router that does
For more information chack:
Atomic aggregate is a Well-known discretionary attribute; it must be recognized by all BGP implementations and does not have to exist in all BGP updates.
The purpose of the attribute is to alert BGP speakers along the path that some information have been lost due to the route aggregation process and that the aggregate path might not be the best path to the destination.
When some routes are aggregated by an aggregator, the aggregator does attache its Router-ID to the aggregated route into the AGGREGATOR_ID attribute and it sets the ATOMIC_AGGREGATE attribute or not; based on whether the AS_PATH information of the aggregated routes were preserved or not.