Abstract. Measurements of HONO were carried out at an urban background site near central London as part of the Clean air for London (ClearfLo) project in summer 2012. Data were collected from 22 July to 18 August 2014, with peak values of up to 1.8 ppbV at night and non-zero values of between 0.2 and 0.6 ppbV seen during the day. A wide range of other gas phase, aerosol, radiation, and meteorological measurements were made concurrently at the same site, allowing a detailed analysis of the chemistry to be carried out. The peak HONO/NO_x ratio of 0.04 is seen at  ∼  02:00 UTC, with the presence of a second, daytime, peak in HONO/NO_x of similar magnitude to the night-time peak, suggesting a significant secondary daytime HONO source. A photostationary state calculation of HONO involving formation from the reaction of OH and NO and loss from photolysis, reaction with OH, and dry deposition shows a significant underestimation during the day, with calculated values being close to 0, compared to the measurement average of 0.4 ppbV at midday. The addition of further HONO sources from the literature, including dark conversion of NO₂ on surfaces, direct emission, photolysis of ortho-substituted nitrophenols, the postulated formation from the reaction of HO₂ ×  H₂O with NO₂, photolysis of adsorbed HNO₃ on ground and aerosols, and HONO produced by photosensitized conversion of NO₂ on the surface increases the daytime modelled HONO to 0.1 ppbV, still leaving a significant missing daytime source. The missing HONO is plotted against a series of parameters including NO₂ and OH reactivity (used as a proxy for organic material), with little correlation seen. Much better correlation is observed with the product of these species with j(NO₂), in particular NO₂ and the product of NO₂ with OH reactivity. This suggests the missing HONO source is in some way related to NO₂ and also requires sunlight. Increasing the photosensitized surface conversion rate of NO₂ by a factor of 10 to a mean daytime first-order loss of  ∼ 6 × 10⁻⁵ s⁻¹ (but which varies as a function of j(NO₂)) closes the daytime HONO budget at all times (apart from the late afternoon), suggesting that urban surfaces may enhance this photosensitized source. The effect of the missing HONO to OH radical production is also investigated and it is shown that the model needs to be constrained to measured HONO in order to accurately reproduce the OH radical measurements.