Reconciling Assumptions in Bottom-up and Top-down Approaches for
Estimating Aerosol Emission Rates from Wildland Fires using Observations
from FIREX-AQ
Abstract
Accurate fire emissions inventories are crucial to predict the impacts
of wildland fires on air quality and atmospheric composition. Two
traditional approaches are widely used to calculate fire emissions: a
satellite-based top-down approach and a fuels-based bottom-up approach.
However, these methods often considerably disagree on the amount of
particulate mass emitted from fires. Previously available observational
datasets tended to be sparse, and lacked the statistics needed to
resolve these methodological discrepancies. Here, we leverage the
extensive and comprehensive airborne in situ and remote sensing
measurements of smoke plumes from the recent Fire Influence on Regional
to Global Environments and Air Quality (FIREX-AQ) campaign to
statistically assess the skill of the two traditional approaches. We use
detailed campaign observations to calculate and compare emission rates
at an exceptionally high resolution using three separate approaches:
top-down, bottom-up, and a novel approach based entirely on integrated
airborne in situ measurements. We then compute the daily average of
these high-resolution estimates and compare with estimates from lower
resolution, global top-down and bottom-up inventories. We uncover
strong, linear relationships between all of the high-resolution emission
rate estimates in aggregate, however no single approach is capable of
capturing the emission characteristics of every fire. Global inventory
emission rate estimates exhibited weaker correlations with the
high-resolution approaches and displayed evidence of systematic bias.
The disparity between the low resolution global inventories and the high
resolution approaches is likely caused by high levels of uncertainty in
essential variables used in bottom-up inventories and imperfect
assumptions in top-down inventories.