Soils are a non-negligible source of NO in a UK suburban greenspace and SE Australian Eucalyptus forest

Nitrogen oxides, particularly NO and NO₂ (NO_x), are primary air pollutants that also play an essential role in the atmospheric oxidation of volatile organic compounds, resulting in ozone and secondary organic aerosol formation. It is therefore critical to fully characterise NO_x sources and sinks to understand tropospheric photochemistry and hence local- to regional-scale air quality. Human activities such as transport and power plants are well-known NO_x emission sources in urban areas, whereas natural sources such as soils have been considered to contribute more substantially in rural and remote areas. However, soil NO emissions are poorly characterised and therefore underrepresented in models. To improve our understanding of soil as a source of NO, we measured diurnal patterns in soil NO concentrations at a suburban site in the UK and a remote field site in Australia to determine whether soils contribute to local atmospheric NO, and to identify the potential drivers of soil NO emissions. Mean soil NO concentrations in both UK campaigns 1.76 ± 0.92 ppb in summer and 0.91 ± 0.37 ppb in winter) were higher than those measured in Australia (0.73 ± 0.73 ppb). The diel patterns of NO concentrations (both sites) and emissions rates (Australia) showed a clear peak corresponding to local emission sources, but variation in NO was also related to either vapour pressure deficit (R² = 0.88 in UK summer, R² = 0.51 in Australia, both p < 0.05) or solar radiation (R² = 0.06 with p > 0.4 in UK summer, R² = 0.71 with p < 0.05 in Australia) during the daylight hours, indicating biogenic origin of soil NO. Our work demonstrates that biogenic soil emissions of NO are non-negligible, estimated at around 1.32 % of total NO emissions at the remote site, and 0.22 % at the urban site, and must be accounted for in global and regional atmospheric chemistry-climate modelling and NO_x reduction strategies.