Abstract: We evaluated the United Kingdom Chemistry and Aerosol (UKCA) model's performance in simulating tropospheric nitrogen dioxide (NO2) and ozone (O3) levels against satellite data from NASA's Aura Ozone Monitoring Instrument (OMI), in-service Aircraft for a Global Observing System (IAGOS) measurements, and Ozonesonde network data across South and East Asia. The UKCA model, an integral part of the UK Earth System Model (UKESM), was nudged with ERA-Interim reanalysis meteorology to produce hourly NO2 distributions and analyzed for seasonal patterns, long-term trends, and diurnal variations from 2005 to 2015. While UKCA captures the shape of seasonal NO2 variations, it tends to overestimate concentrations near sources and underestimate in remote areas, with discrepancies attributed to boundary layer height differences and underestimated peroxyacetyl nitrate (PAN) production affecting long-range transport. For ozone, UKCA v8.4, with its comprehensive chemistry-climate modeling including tropospheric and stratospheric chemistry, showed good agreement with observational data from IAGOS over major airports and Ozonesonde data in cities like Delhi and Trivandrum, particularly capturing lower troposphere ozone levels and seasonal variations influenced by the monsoon. The analysis highlights UKCA's capability in simulating atmospheric chemistry dynamics, by pinpointing areas for model improvement. NO2 and O3 in UKCA affects many key chemical and physical processes in UKESM, including oxidants, aerosols, and methane simulation.

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