Nitrous acid formation in a snow-free wintertime polluted rural area

Abstract. Nitrous acid (HONO) photolysis is an important source of hydroxyl radicals (OH) in the lower atmosphere, in particular in winter when other OH sources are less efficient. The nighttime formation of HONO and its photolysis in the early morning have long been recognized as an important contributor to the OH budget in polluted environments. Over the past few decades it has become clear that the formation of HONO during the day is an even larger contributor to the OH budget and additionally provides a pathway to recycle NO_x. Despite the recognition of this unidentified HONO daytime source, the precise chemical mechanism remains elusive. A number of mechanisms have been proposed, including gas-phase, aerosol, and ground surface processes, to explain the elevated levels of daytime HONO. To identify the likely HONO formation mechanisms in a wintertime polluted rural environment we present LP-DOAS observations of HONO, NO₂, and O₃ on three absorption paths that cover altitude intervals from 2 to 31, 45, and 68 m above ground level (a.g.l.) during the UBWOS 2012 experiment in the Uintah Basin, Utah, USA. Daytime HONO mixing ratios in the 2–31 m height interval were, on average, 78 ppt, which is lower than HONO levels measured in most polluted urban environments with similar NO₂ mixing ratios of 1–2 ppb. HONO surface fluxes at 19 m a.g.l., calculated using the HONO gradients from the LP-DOAS and measured eddy diffusivity coefficient, show clear upward fluxes. The hourly average vertical HONO flux during sunny days followed solar irradiance, with a maximum of (4.9 ± 0.2)  ×  10¹⁰ molec. cm⁻² s⁻¹ at noontime. A photostationary state analysis of the HONO budget shows that the surface flux closes the HONO budget, accounting for 63 ± 32 % of the unidentified HONO daytime source throughout the day and 90 ± 30 % near noontime. This is also supported by 1-D chemistry and transport model calculations that include the measured surface flux, thus clearly identifying chemistry at the ground as the missing daytime HONO source in this environment. Comparison between HONO surface flux, solar radiation, NO₂ and HNO₃ mixing ratios, and results from 1-D model runs suggest that, under high NO_x conditions, HONO formation mechanisms related to solar radiation and NO₂ mixing ratios, such as photo-enhanced conversion of NO₂ on the ground, are most likely the source of daytime HONO. Under moderate to low NO₂ conditions, photolysis of HNO₃ on the ground seems to be the main source of HONO.