In which layer does the atmosphere merge with space?

The atmosphere gradually merges with space in the exosphere, which is the outermost layer of the Earth's atmosphere. This layer extends from about 600 kilometers (370 miles) above the Earth's surface to around 10,000 kilometers (6,200 miles). In the exosphere, the air is extremely thin, and particles are so sparse that they can travel hundreds of kilometers without colliding with one another, making it a transitional zone between the Earth's atmosphere and outer space.