He smelled the tar and oakum of the deck as he slept and he smelled the smell of Africa that the land breeze brought at morning.
睡梦中他闻到甲板上柏油和麻絮的味道,嗅到早晨陆风带来的非洲气息。 (海明威的老人与海)
请解释一下at morning用法?为什么不是in the morning?