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. This study investigates the use of co-located nitrogendioxide (NO2)and carbon monoxide (CO) retrievals from the TROPOMI satellite to improve the quantificationof burning efficiency and emission factors (EFs) over the megacities of Tehran,Mexico City, Cairo, Riyadh, Lahore, and Los Angeles. Efficient combustion ischaracterized by high NOx (NO+NO2) and low CO emissions, making theNO2∕CO ratio a useful proxy for combustion efficiency (CE). The localenhancement of CO and NO2 above megacities is well captured by TROPOMIat short averaging times compared with previous satellite missions. In thisstudy, the upwind background and plume rotation methods are used toinvestigate the accuracy of satellite-derived ΔNO2∕ΔCOratios. The column enhancement ratios derived using these two methods varyby 5 % to 20 % across the selected megacities. TROPOMI-derived columnenhancement ratios are compared with emission ratios from the EDGAR v4.3.2 (Emission Database for Global Atmospheric Research v4.3.2)and the MACCity (Monitoring AtmosphericChemistry and Climate and CityZen) 2018 emission inventories. TROPOMI correlates strongly (r=0.85 and 0.7) with EDGAR and MACCity, showing the highest emission ratio forRiyadh and lowest emission ratio for Lahore. However, inventory-derived emission ratios are 60 % to 85 %higher than TROPOMI column enhancement ratios acrossthe six megacities. The short lifetime of NO2 and the different verticalsensitivity of TROPOMI NO2 and CO explain most of this difference. Wepresent a method to translate TROPOMI-retrieved column enhancement ratiosinto corresponding emission ratios, thereby accounting for these influences. Exceptfor Los Angeles and Lahore, TROPOMI-derived emission ratios are close(within 10 % to 25 %) to MACCity values. For EDGAR, however, emission ratios are ∼65 %higher for Cairo and 35 % higher for Riyadh. For Los Angeles, EDGAR and MACCity are a factor of 2 and 3 higherthan TROPOMI respectively. The air quality monitoring networks in Los Angeles andMexico City are used to validate the use of TROPOMI. For Mexico City and LosAngeles, these measurements are consistent with TROPOMI-derived emissionratios, demonstrating the potential of TROPOMI with respect to monitoring burningefficiency.

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

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