Processes governing emissions of HONO at a grassland site

Studies have shown that gas-phase nitrous acid (HONO) is a major precursor of hydroxyl (OH) radicals in the boundary layer, which plays an important role in the formation of pollutants such as ozone and secondary aerosol. Despite the many studies undertaken and the development of new techniques to measure HONO, the processes governing formation are not completely understood. As such, current atmospheric models are unable to reproduce daytime HONO concentrations observed by in-situ measurements, suggesting the existence of unknown sources of HONO. Soil has been suggested as a major source of atmospheric HONO under specific conditions. In this study, HONO flux measurements were made on a Scottish grassland using the aerodynamic gradient technique and two Long Path Absorption Spectrometers (LOPAPs) to determine the processes governing the surface-atmosphere exchange of HONO. Fluxes showed a bi-directional behaviour with values between -1.4 ± 0.2 ng N m^-2 s^-1 and 3.2 ± 0.4 ng N m^-2 s^-1. Deposition of HONO were observed at night while HONO emissions were observed during the day. In particular, the average flux diurnal pattern showed a distinctive peak around 9 am, suggesting a recurrent release or formation process in the morning.

Three potential processes were investigated to explain the observed emissions i) soil microbial activity, ii) photolysis of NO₃^- and HNO₃, iii) absorption/desorption of HONO from water films on vegetation. A simple model was developed to replicate the photolysis of NO₃^- and HNO₃, alongside the absorption/desorption of HONO from water films on vegetation. Whereas lab experiments were undertaken to determine whether microbial activity in soil was a possible source. It was discovered that it was unlikely that microbial activity played a major role in the observed emissions. Instead, a combination of photolysis of HNO₃/NO­₃^- and absorption/desorption of water films at the surface were more likely the processes contributing to the observed emissions. The inclusion of both processes into atmospheric chemistry models may help to improve the agreement between measurements and models in the future.