An update of emission factors for nitric oxide emissions from croplands and grasslands

Nitric Oxide (NO), an intermediate product in the soil nitrogen cycle, is also an air pollutant causing adverse health effects, including contribution towards the formation of fine particulate matter and tropospheric ozone. Accurate quantification of NO emissions is necessary to mitigate these impacts, but current emission factors used in national greenhouse gas and air pollutant inventories are associated with substantial uncertainty, necessitating an updated assessment based on recent field measurements. In this study, we compiled a comprehensive dataset of 692 field observations of NO measurements from 128 studies conducted globally. Using Bayesian generalized linear mixed-effects models, we derived new emission factors that account for synthetic and organic fertiliser inputs, climate zones and crop groups. The new global emission factor (expressed as mass fraction of nitrogen) for synthetic nitrogen inputs to common crops and grasslands was 0.0042 kg NO-N (kg N) -1 (95% credible interval: 0.0035 – 0.0049), while organic nitrogen inputs exhibited a negligible fertiliser induced emission factor of 0.0001 kg NO-N (kg N) -1 (95% CI: -0.0003 – 0.0005). The emission factors varied markedly upon stratification by climate zones and crop groups, with higher emission factors from synthetic fertilisers in the tropical rainforest regions, while paddy rice cultivation exhibited negligible fertiliser-induced NO emissions and tea plantations showed exceptionally high emissions with a factor of 0.0155 kg NO-N (kg N) -1 (95% CI: 0.0101 – 0.0225). The new emission factors are considerably lower than the Tier 1 value of 0.0133 (0.0015 – 0.0317) kg NO-N (kg N) -1 ), currently recommended in the EMEP/EEA guidelines. Application to nitrogen fertiliser input data from EU member states' 2025 Informative Inventory Reports indicates that, adopting the new emission factors would result in reported emissions from synthetic fertilisers being lower by approximately 65%. These findings highlight the need for refined inventory methodologies, replacing the default emission factor with climate and crop specific values, to better inform strategies for nitrogen management and air quality monitoring. • Compiled and analysed 692 field measurements of nitric oxide emissions from 128 global studies using Bayesian mixed-effects models. • Derived global emission factors for synthetic and organic nitrogen inputs were 0.0042 kg NO–N (kg N) -1 and 0.0001 kg NO–N (kg N) -1 respectively. • Climate and crop stratification showed higher emissions in tropical rainforest zones, negligible fertiliser induced emissions from paddy rice cultivation, and exceptionally high emissions from tea plantations. • Applying the new emission factors to EU nitrogen input data suggests ∼65% lower reported emissions from inorganic fertilisers in national inventories.