We have quantified absorption cross sections of surface-adsorbed HNO3 in the 335-365 nm region at 295 K by using Brewster angle cavity ring-down spectroscopy. Absorption cross sections for HNO3 adsorbed on fused silica surfaces ranged between (3.1 +/- 0.5) x 10 (19) and (0.44 +/- 0.04) x 10 (19) cm(2)/molecule in the wavelength region studied. The HNO3 surface cross section values are at least two orders of magnitude larger than HNO3 cross section values of about < 3 x 10 (22) cm(2)/molecule in the gas phase over the 335-350 nm range. Our results support the field-observed large differences between nitric acid photolysis rates on the surface and that of the gas phase. (C) 2011 Elsevier B. V. All rights reserved.