Nitrous acid, HONO, is a source of OH radicals in the polluted atmosphere. Although the atmospheric chemistry of HONO is qualitatively understood, not much quantitative information exists. The magnitude of the OH production by HONO photolysis depends on the spectrum of its absorption cross sections; therefore the knowledge of &sgr;HONO(λ) is essential. The spectrum of the differential cross sections &sgr;HONO'(λ) is needed to detect HONO in the atmosphere by differential optical absorption spectroscopy (DOAS). Here we present measurements of the HONO UV-visible absorption cross sections with a spectral resolution better than 0.1 nm and a high signal-to-noise ratio. The maximum value of the absorption cross sections is &sgr;HONO(354 nm)=(5.19¿0.26)¿10-19 cm2 and agrees well with literature data. Nevertheless, calculations based on data from this work and on literature data reveal that an uncertainty of ~15% remains for the HONO photolysis rates. The new &sgr;HONO(λ) has been employed in DOAS measurements in Milan, Italy. ¿ 2000 American Geophysical Union <