Observational Constraints on the Aerosol Optical Depth-Surface PM2.5 Relationship during Alaskan Wildfire Seasons

Tianlang Zhao; Jingqiu Mao; Pawan Gupta; Huanxin Zhang; Jun Wang

doi:10.1021/acsestair.4c00120

Observational Constraints on the Aerosol Optical Depth-Surface PM_2.5 Relationship during Alaskan Wildfire Seasons

ACS EST Air. 2024 Aug 26;1(9):1164-1176. doi: 10.1021/acsestair.4c00120. eCollection 2024 Sep 13.

Authors

Tianlang Zhao¹, Jingqiu Mao¹, Pawan Gupta², Huanxin Zhang³, Jun Wang³

Affiliations

¹ Geophysical Institute and Department of Chemistry and Biochemistry, University of Alaska Fairbanks, Fairbanks, Alaska 99775, United States.
² Goddard Space Flight Center, NASA, Greenbelt, Maryland 20771, United States.
³ Department of Chemical and Biochemical Engineering, Iowa Technology Institute, Center for Global and Regional Environmental Research, The University of Iowa, Iowa City, Iowa 52242, United States.

PMID: 39295742
PMCID: PMC11407303 (available on 2025-08-26)
DOI: 10.1021/acsestair.4c00120

Abstract

Wildfire is one of the main sources of PM_2.5 (particulate matter with aerodynamic diameter < 2.5 μm) in the Alaskan summer. The complexity in wildfire smokes, as well as limited coverage of ground measurements, poses a big challenge to estimate surface PM_2.5 during wildfire season in Alaska. Here we aim at proposing a quick and direct method to estimate surface PM_2.5 over Alaska, especially in places exposed to strong wildfire events with limited measurements. We compare the AOD-surface PM_2.5 conversion factor (η = PM_2.5/AOD; AOD, aerosol optical depth) from the chemical transport model GEOS-Chem (η_GC) and from observations (η_obs). We show that η_GC is biased high compared to η_obs under smoky conditions, largely because GEOS-Chem assigns the majority of AOD (67%) within the planetary boundary layer (PBL) when AOD > 1, inconsistent with satellite retrievals from CALIOP. The overestimation in η_GC can be to some extent improved by increasing the injection height of wildfire emissions. We constructed a piecewise function for η_obs across different AOD ranges based on VIIRS-SNPP AOD and PurpleAir surface PM_2.5 measurements over Alaska in the 2019 summer and then applied it on VIIRS AOD to derive daily surface PM_2.5 over continental Alaska in the 2021 and 2022 summers. The derived satellite PM_2.5 shows a good agreement with corrected PurpleAir PM_2.5 in Alaska during the 2021 and 2022 summers, suggesting that aerosol vertical distribution likely represents the largest uncertainty in converting AOD to surface PM_2.5 concentrations. This piecewise function, η'_obs, shows the capability of providing an observation-based, quick and direct estimation of daily surface PM_2.5 over the whole of Alaska during wildfires, without running a 3-D model in real time.