  Investigating the health impacts of wildfire smoke requires data on people's
exposure to fine particulate matter (PM$_{2.5}$) across space and time. In
recent years, it has become common to use machine learning models to fill gaps
in monitoring data. However, it remains unclear how well these models are able
to capture spikes in PM$_{2.5}$ during and across wildfire events. Here, we
evaluate the accuracy of two sets of high-coverage and high-resolution machine
learning-derived PM$_{2.5}$ data sets created by Di et al. (2021) and Reid et
al. (2021). In general, the Reid estimates are more accurate than the Di
estimates when compared to independent validation data from mobile smoke
monitors deployed by the US Forest Service. However, both models tend to
severely under-predict PM$_{2.5}$ on high-pollution days. Our findings
complement other recent studies calling for increased air pollution monitoring
in the western US and support the inclusion of wildfire-specific monitoring
observations and predictor variables in model-based estimates of PM$_{2.5}$.
Lastly, we call for more rigorous error quantification of machine-learning
derived exposure data sets, with special attention to extreme events.
