An emission of something such as gas or radiation is the release of it into the atmosphere.
双语例句
例： The emission of gases such as carbon dioxide should be stabilized at their present level.
二氧化碳之类气体的排放应该被控制在目前的水平上。
例： Sulphur emissions from steel mills become acid rain.
炼钢厂排放的硫成为酸雨。