Quantifying burning efficiency in Megacities using NO2/CO ratio from the Tropospheric Monitoring Instrument (TROPOMI)

Economic development and rapid urbanization have increased the consumption of fossil fuel in megacities degrading the local air quality. Burning efficiency is a major factor determining the impact of fuel burning on the environment. It varies with environmental conditions and influences the ratio at which pollutants are emitted, as expressed by the emission factor. Emission factors are an important source of uncertainty in global emission inventories.

To improve the quantification of burning efficiency and emission factors, this study investigates co-located NO₂ and CO satellite retrievals from TROPOMI over the megacities of Tehran, Mexico City, Cairo, Riyadh, Lahore and Los Angeles. The TROPOMI instrument was successfully launched by the European Space Agency on 13 October, 2017. It measures atmospheric trace gases with daily coverage and a spatial resolution of 7x7 km². At this resolution, TROPOMI detects hotspots of CO and NO₂ pollution over megacities in single satellite overpasses. The Upwind Background and Plume rotation methods are applied to quantify and evaluate TROPOMI derived ∆NO₂/∆CO ratios. TROPOMI derived ∆NO₂/∆CO ratios show a strong correlation (r = 0.85 and 0.7) with emission ratios from the Emission Database for Global Atmospheric Research (EDGAR v4.3.2) and Monitoring Atmospheric Chemistry and Climate and CityZen (MACCity) 2018, with the highest ratio for Riyadh and lowest for Lahore. Inventory-derived emission ratios are larger than TROPOMI-derived total column ratios by 60 to 80%. As we will show, this can largely be explained by the limited lifetime of NO₂ and the different vertical sensitivity of the TROPOMI NO₂ and CO column retrievals. Taking this into account, TROPOMI retrieved emission ratios are generally within 10 to 25% of MACCity. However, larger differences, up to 80%, are found with EDGAR. For Los Angeles, both inventories overestimate NO2/CO ratios compared with TROPOMI. Validation using the air quality monitoring network of Los Angeles supports the lower ∆NO₂/∆CO ratios inferred from TROPOMI, indicating that burning efficiencies in Los Angeles are indeed poorer than indicated by the inventories.