Abstract. This study investigates the use of co-located nitrogen
dioxide (NO2)
and carbon monoxide (CO) retrievals from the TROPOMI satellite to improve the quantification
of burning efficiency and emission factors (EFs) over the megacities of Tehran,
Mexico City, Cairo, Riyadh, Lahore, and Los Angeles. Efficient combustion is
characterized by high NOx (NO+NO2) and low CO emissions, making the
NO2∕CO ratio a useful proxy for combustion efficiency (CE). The local
enhancement of CO and NO2 above megacities is well captured by TROPOMI
at short averaging times compared with previous satellite missions. In this
study, the upwind background and plume rotation methods are used to
investigate the accuracy of satellite-derived ΔNO2∕ΔCO
ratios. The column enhancement ratios derived using these two methods vary
by 5 % to 20 % across the selected megacities. TROPOMI-derived column
enhancement 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 Atmospheric
Chemistry 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 for
Riyadh and lowest emission ratio for Lahore. However, inventory-derived emission ratios are 60 % to 85 %
higher than TROPOMI column enhancement ratios across
the six megacities. The short lifetime of NO2 and the different vertical
sensitivity of TROPOMI NO2 and CO explain most of this difference. We
present a method to translate TROPOMI-retrieved column enhancement ratios
into corresponding emission ratios, thereby accounting for these influences. Except
for 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 higher
than TROPOMI respectively. The air quality monitoring networks in Los Angeles and
Mexico City are used to validate the use of TROPOMI. For Mexico City and Los
Angeles, these measurements are consistent with TROPOMI-derived emission
ratios, demonstrating the potential of TROPOMI with respect to monitoring burning
efficiency.