Blues and purples are created by lighter gases such as hydrogen and helium, while low-level nitrogen can add red fringes to the bottoms of green auroral curtains.

蓝色和紫色是来自较轻的气体比如氢和氦，而更低点的氮气能在绿色极光的底部添上红色的边缘。

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Since the usual test for protein in animal feed just measures the level of nitrogen, these chemicals can be added to far more expensive feed ingredients without anybody noticing.

因为一般的测定动物食物的测试只测量其中氮的含量，这些化学物质可以被加入昂贵的食物成分中而不为人所知。

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Companies using the milk for further production (e.g. of powdered infant formula) normally check the protein level through a test measuring nitrogen content.

而使用牛奶进行下一步加工(比如婴幼儿配方奶粉)的公司，一般通过测量含氮量来检测蛋白质含量。

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But the traffic increase has slowed progress in reducing emissions further, and people in many areas still suffer from nitrogen-related air pollution, including small particulates that get sucked deep into the lungs, and ground-level ozone - a strongly irritant gas formed by the action of sunlight on reactive nitrogen.

BBC: Nitrogen pollution 'costs EU up to ?280bn a year'