What is this?

I'm not a mechanic, although I try to do enough DIY basics. Haven't had this E46 very long.Question is: What is this thing right beside the washer fluid
reservoir. Its round with a half circle cap on the top. Removing the top unveils what looks like a sponge filter.
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I believe it's the filter for the air pump that runs when you first start the car. Extra air to help burn the extra unburned HC in the exhaust until the catalytic converter lights off.
FloydR
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