Each spring a layer of small particles forms between 20 and 30 km altitude in the polar regions. We present the first self‐consistent explanation of the observed “CN layer” from a 2D microphysical model of sulfate aerosol. Our theory relies on photolysis of H₂SO₄ and SO₃, consistent with recent laboratory measurements, to produce SO₂ in the upper stratosphere and mesosphere. An additional source of SO₂ may be required. Nucleation throughout the polar winter extends the top of the aerosol layer to higher altitudes, despite strong downward transport of ambient air. This may affect heterogeneous chemistry at the top of the aerosol layer in polar winter and spring.