Left: CLOUDY photo-ionization model used to derive the pre-burst physical conditions and metallicity in the GRB 080310 host-galaxy ISM, assuming the extragalactic UV background radiation field at the GRB host-galaxy redshift. In the main panel, column density is plotted against ionization parameter U for a range of metal lines. The observations are shown as large data points, colour–coded by ion. The best-fit log U (and hence the best-fit density) and [Fe/H] are derived by matching the observed Fe iii/Fe ii ratio, as shown in the bottom panel. [C/H], [O/H], [Si/H], and [Cr/H] are determined by matching C ii, O i, Si ii, and Cr ii at the best-fit log U. Right: same as in the left panel but assuming the MW radiation field.