Pollution trapping by strong temperature inversions in Fairbanks, Alaska

Abstract

In cold climates during winter, surface-based temperature inversions reduce vertical dispersion within the atmospheric boundary layer. Reduced vertical dispersion coupled with stagnant horizontal winds causes pollution emitted near the ground level to accumulate to unhealthy amounts. To study pollution trapping, we measured vertical differences in fine particulate matter and ozone across a shallow 20-meter vertical scale during surface-based inversions in Fairbanks, Alaska and showed that pollution trapping occurs on this extremely shallow scale. In winter 2022, trace gases were measured by long-path differential absorption spectroscopy to probe vertical dispersion on a taller scale, up to ~200 m above the surface. We added horizontal dispersion to a one-dimensional Eulerian chemical transport model and used the model with both vertical and horizontal dispersion, but without chemistry, to simulate the vertically resolved measurements of SO₂. The model achieved excellent results with correlation to the observations having a coefficient of R = 0.88. Steady-state transport residence times calculated from the model were on the order of hours, indicating limited time for chemical processing during winter in Fairbanks. Within the model, only ground-based emissions sources were included and there was no interaction between air above and below the boundary layer height, suggesting that ground-based sources dominate pollution measured at ground level. With the knowledge that ground level sources have a large impact on pollution, we carried out an analysis of trace gases and fine particulate matter measured near ground level that was used to better understand the response to pollution control strategies in Fairbanks. This analysis shows that the total amount of pollution in Fairbanks has been trending down over the past nine years. Following a September 2022 legal change that mandated lower sulfur content in heating oil, the amount of sulfur dioxide gas dropped significantly during winter 2022-2023 as compared to the prior three-year average.