Critical evaluation of smoke age inferred from different methods during FIREX-AQ

The age of smoke, meaning the time elapsed between its emission at the fire source and its sampling, is a critical parameter for understanding the chemical aging of fire emissions. In this work, we compare and evaluate methods of estimating smoke age at the time of sampling by aircraft during the NASA-NOAA FIREX-AQ field experiment. The first method relies on measurements of mean horizontal wind speed, as observed by the sampling aircraft. Using this wind speed, the smoke age is inferred from the distance between the fire and the aircraft at the time of measurement. The second method uses air parcel trajectories computed in the HYSPLIT trajectory model with multiple high-resolution meteorological datasets. Additional age adjustments account for buoyant plume rise, mismatch between observed and archived winds, and trajectory spatial error in missing the known fire source. Uncertainties in the smoke ages are assessed from the range of results from different methods. While the first method benefits from accurate wind measurements, it assumes that winds are homogeneous in space and time during the plume transport, which can systematically bias the age estimates. In FIREX-AQ these systematic biases are most commonly due to wind changes during the day-to-night transition. The second method accounts for spatial and temporal variations in winds, but is susceptible to errors in the meteorological model. With careful comparison of the simulated trajectories to smoke transport observed from geostationary satellites, however, we can filter out many meteorological errors and improve the smoke age estimates. In favorable conditions, we find that smoke age can be determined with about 20% accuracy (median absolute error) over time scales of several hours and the ensemble statistics identify periods when the smoke age uncertainties are greater.