A prominent source of hydroxyl radicals (·OH), nitrous acid (HONO) plays a key role in tropospheric chemistry. Apart from direct emission, HONO (or its conjugate base nitrite, Φ(NO₂⁻) can be formed secondarily in the atmosphere. Yet, how secondary HONO forms requires elucidation, especially for heterogeneous processes involving complex atmospheric aerosols. We investigated nitrite production from aqueous photolysis of nitrate for a range of conditions (pH, organic compound, nitrate concentration, and cations). Using small oxygenates such as ethanol, n-butanol, or formate as ·OH scavengers, the intrinsic quantum yield of nitrite, Φ(NO₂⁻), averaged at 0.75 ± 0.15%. With near-UV-light-absorbing vanillic acid (VA), however, the effective Φ(NO₂⁻) was strongly pH dependent, reaching 8.0 ± 2.1% for 200 mM nitrate at a pH of 8 and 1.5 ± 0.39% at the more atmospherically relevant pH of 5. Our results suggest that brown carbon (BrC) chromophores may greatly enhance the nitrite production from the aqueous photolysis of nitrate through photosensitizing reactions. The enhanced nitrite formation could not be explained by mere conversion of the coproduced NO₂ to nitrite. Instead, we propose that the triplet excited state of the photosensitizer may generate solvated electrons, which reduce nitrate to NO₂ for further conversion to nitrite. The effective Φ(NO₂⁻) was higher for high concentrations of nitrate at the same VA-to-nitrate ratio, indicating more efficient nitrite formation in concentrated aerosol droplets than in dilute cloud/fog droplets.