Modeling attainment in Fairbanks, Alaska, for the wintertime PM2.5 24-hour non-attainment area using the CMAQ (community multi-scale air quality) model.

Abstract

Fairbanks Alaska has some of the highest recorded levels of fine particulate matter (PM_2.5) in the United States (U.S.), exceeding health-based standards since 2009. The National Ambient Air Quality Standard (NAAQS) in the U.S. for 24 h PM_2.5 is 35 μg m^-3 with a 24 h averaging time and takes the form of the 98th percentile averaged over three years; the three-year average is called a design value. Monitored PM_2.5 level design values have been as high as 135 μg m^-3 or almost 4 times higher than the health-based standard. The current monitored PM_2.5 value for 2021-2023 is 56 μg m^-3. Fairbanks winters have strong temperature inversions, trapping pollutants near the ground and leading to elevated concentrations of PM_2.5 and its precursor gases. The two largest species component contributors to PM_2.5 in Fairbanks are organic carbon and sulfate. Control strategies have focused on reducing organic carbon through wood-stove measures and SO₂ through fuel sulfur reductions. State Implementation Plans (SIPs) are mandatory plans that demonstrate the most expeditious path to reaching the health-based standard. In previous SIPs, the Alaska Department of Environmental Conservation (ADEC) based attainment demonstrations on an outdated modeling platform, emissions inventory, meteorological data, and episodes. Recent updates include upgrading to the CMAQ (Community Multi-Scale Air Quality) model version 5.3.3+ and updated Weather Research and Forecast (WRF) meteorology resulting from a collaboration with the United States Environmental Protection Agency Office of Research and Development (EPA-ORD) and recent Alaska Layered Pollution and Chemical Analysis (ALPACA) studies. In addition, there have been updates to the emissions inventory (survey, census, parcel and home-heating energy demand model) for space heating and other pre-processing models. The changes have resulted in improved model performance in representing stable boundary layers in meteorology and Model Performance Evaluation (MPE) of secondary sulfate. Modeled secondary sulfate went from underpredicting 88% of the observed sulfate values using the previous modeling platform, to improved sulfate predictions with only a 2.5% Normalized Mean Bias (NMB) and 40% Normalized Mean Error (NME). Using the sulfur tracking method, CMAQ modeling suggests that in Fairbanks, 60% of the sulfate is primary, and 40% is secondary on average for our wintertime modeling period. The modeled primary and secondary fractions of sulfate are corroborated by Moon et al. 2024 (ACS ES&T Air, 2024, 1, 139-149), showing 62% of the ambient measured sulfate particles were primary and 38% were secondary in Fairbanks, during the ALPACA field campaign. The combination of these updates to emissions, meteorology and the modeling platform have allowed ADEC to accurately represent modeling of control strategies that will bring the area into attainment for the 24 h PM_2.5 standard in the year 2027. All control measures come at a cost to the community. Whether limiting the use of wood stoves at -0 C or mandating costly controls to the electric utilities/point sources for SO₂, the financial hardships are felt by the residents. This modeling informs policy at the state and federal level, to select the control strategies that will result in the fastest path to clean air while avoiding economic harm to the community, which for Fairbanks means focusing on residential wood smoke. In order to focus the costs on wood stoves, a sensitivity model run was conducted with zero SO₂ emissions from the point sources/electric utilities and the resulting secondary sulfate contribution to PM_2.5 was 0.6 μg m^-3 with a concentration of 64 μg m^-3 during the wintertime modeling period. The total PM_2.5 contribution from the electric utilities/point sources is estimated at 2.2 μg m^-3 of PM_2.5 from the modeling results. This contribution of point sources is in corroboration with the modeling work of Brett et al. 2024 (Estimating power plant contributions to surface pollution in a wintertime Arctic environment, 2024, in process) on point-source contribution during the ALPACA campaign in Fairbanks.