Evaluating tropospheric nitrogen dioxide in UKCA using OMI satellite retrievals over South and East Asia

Abstract. We compare tropospheric column nitrogen dioxide (NO₂) in the United Kingdom Chemistry and Aerosol (UKCA) model version 11.0 with satellite measurements from NASA’s Earth Observing System (EOS) Aura satellite Ozone Monitoring Instrument (OMI) to investigate the seasonality and trends of tropospheric NO₂ over South and East Asia (S/E Asia). UKCA is the atmospheric composition component of the UK Earth System Model (UKESM). UKCA was run with nudged meteorology, producing hourly output over S/E Asia for 2005–2015. OMI averaging kernels have been applied to the model hourly data sampled at Aura’s local overpass time of 13:45±15 to allow consistent model-data comparison. Background UKCA and OMI tropospheric column NO₂ typically ranges between 0–2 × 10¹⁵ molecules/cm². Diurnal cycles and vertical profiles of the tropospheric NO₂ column in UKCA show that the daily minimum tropospheric column NO₂ occurs around the satellite overpass time. UKCA captures the seasonality but overestimates NO₂, by a factor of ~2.5, especially during winter over Eastern China and North India, at times and locations with high aerosol loadings. Heterogeneous chemistry is represented in the version of UKCA used here as uptake of N₂O₅ on internally generated sulfate aerosol. However, aerosol surface area may be underestimated in polluted locations, contributing to overestimation of NO₂. In addition, the model may underestimate emissions of volatile organic compounds and associated peroxy acetyl (PAN) formation, leading to insufficient long-range transport of oxidised nitrogen, also contributing to overestimation of NO₂ over polluted regions and underestimation over remote regions. Quantifying and understanding discrepancies in modelled NO₂ warrant further investigation as they propagate into modelling of multiple environmental issues.