Combining OMI NO2 and MOPITT CO Measurements to Partition Fire Combustion Phase

Aerosol and gaseous emissions from fires pose significant impacts on climate, biogeochemistry, and human health. One of the uncertainties in fire emissions lies in the fire combustion phase (i.e., flaming, smoldering, or a combination of both). This study simultaneously employs NO2 measurements from OMI and CO measurements from MOPITT over the past 15 years, and explores the potential of NO2/CO to represent the fire combustion phase, thereby improving the quantification of fire emissions. First, Singular Value Decomposition (SVD) was used to pinpoint the fire source regions, as well as the times when fires prevail in these regions. Next, the ratio of NO2 to CO was obtained over the source regions during high fire times, and compared with Modified Combustion Efficiency (MCE), a well-established index of fire combustion phase. NO2/CO and MCE agree reasonably well over Amazon, Central Africa, and Northern Australia, where emissions are dominated by fires with little urbanization. They also agree well over the Middle East, where it is mostly flaming fires from gas flares. However, over the regions where both fires and urbanization contribute to a significant amount of emissions, NO2/CO is dominated by urbanization and therefore diverges from MCE. Overall, NO2/CO is capable of indicating the fire combustion phase, but a more careful spatio-temporal separation of fires and urbanization is required.