The 2020 California fire season yielded the largest fires in California history. TROPOMI nitrogen dioxide (NO2) and carbon monoxide (CO) ratios (i.e. mole density ratios; MDR) have been previously used to distinguish fire combustion among different vegetation types around the world. Here, we analyze the MDR from TROPOMI overpasses to identify fire characteristics over two regions of California where two wildfires were reported. We show that MDR calculated with TROPOMI NO2 and CO columns can be used to assess fire characteristics at the scale of counties through identifying the smoldering fires of the northern region and flaming fire of the southern region with a shift in the combustion at the end of September resulting from a change in fuel types. Improvement in the land cover maps and use of hourly data could help to better attribute MDR to specific fuel source categories.

We evaluate terrestrial net ecosystem-atmosphere exchange (NEE) of CO2 from nine global inversion systems that inferred fluxes from four CO2 observational sources. We use 98 flights in the central and eastern U.S. from the ACT-America aircraft mission to conduct this sub-continental, seasonal-scale evaluation. We use Lagrangian particle dispersion modeling (FLEXPARTv10.4-ERA-Interim) to compare observed and simulated regional biogenic CO2 mole fractions. We find a positive bias (modeled CO2 > observed) in the summer and negative bias (modeled CO2 < observed) in dormant seasons across most flux products, suggesting that the seasonal strength of CO2 NEE is underestimated in these inverse models. Fluxes inferred from OCO-2 v9 satellite land nadir/glint observations yield an error level that is similar to fluxes inferred from in-situ data. Large bias errors are observed in the croplands and eastern forests. Future experiments are needed to determine if these seasonal biases are associated with biases in net annual flux estimates.