solid angle, Ω, ω in atmospheric chemistry

https://dx.doi.org/10.1351/goldbook.S05732
Of a cone, the ratio of the area cut out on a spherical surface (with its centre at the apex of that cone) to the square of the radius of the sphere. It is a quantity of dimension one (dimensionless quantity) with unit steradian (sr = 1).
Source:
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 992