  We present the optical transmission spectrum of the highly inflated
Saturn-mass exoplanet WASP-21b, using three transits obtained with the ACAM
instrument on the William Herschel Telescope through the LRG-BEASTS survey (Low
Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission
Spectroscopy). Our transmission spectrum covers a wavelength range of 4635-9000
Angstrom, achieving an average transit depth precision of 197ppm compared to
one atmospheric scale height at 246ppm. We detect Na I absorption in a bin
width of 30 Angstrom, at >4$\sigma$ confidence, which extends over 100
Angstrom. We see no evidence of absorption from K I. Atmospheric retrieval
analysis of the scattering slope indicates it is too steep for Rayleigh
scattering from H$_2$, but is very similar to that of HD 189733b. The features
observed in our transmission spectrum cannot be caused by stellar activity
alone, with photometric monitoring of WASP-21 showing it to be an inactive
star. We therefore conclude that aerosols in the atmosphere of WASP-21b are
giving rise to the steep slope that we observe, and that WASP-21b is an
excellent target for infra-red observations to constrain its atmospheric
metallicity.
