Hourly PM_2.5 Estimates across California from 2018 to 2023

This study presents a new data set of hourly PM_2.5 concentrations across California from 2018 to 2023 at a three-kilometer resolution. This data set was developed by assimilating observations from PurpleAir and the U.S. EPA Air Quality System monitors into wildfire smoke forecasts from the High-Resolution Rapid Refresh Smoke (HRRR-Smoke) model using the Gridpoint Statistical Interpolation (GSI) three-dimensional variational data assimilation framework. Archived forecasts of modeled wildfire smoke PM_2.5 from HRRR-Smoke create the background field for assimilation, which is then corrected using surface observations of total PM_2.5. The resulting reanalysis from GSI provides an estimate of total PM_2.5 that minimizes error from both the observational and the model data. Validation results indicate strong performance, with monthly R ² values ranging from 0.73 to 0.91 across the six-year data set, comparable to other PM_2.5 data sets. Case studies are presented for three major fire events, the 2018 Camp Fire, 2019 Kincade Fire, and 2020 Lightning Complex Fires to demonstrate the data set's fidelity in resolving plume dynamics and local exposure patterns. Root-mean-squared error averaged over each month scales with average PM_2.5 concentrations, resulting in a low error under typical conditions but higher absolute errors during extreme smoke events. This is the first long-term, hourly PM_2.5 data set of its kind for California and enables the generation of subdaily exposure metrics, such as peak hourly concentrations, exceedance durations, and time-of-day exposure peaks. The novelty and strong validation of this data set make it a compelling resource for future studies on the impact and significance of subdaily PM_2.5 exposure.