Low-cost electrochemical gas sensing of vertical differences in wintertime air composition (CO, NO, NO2, O3) in Fairbanks, Alaska.

Abstract

Wintertime Fairbanks, Alaska, experiences episodes of severely poor air quality, when local emissions (e.g., home-heating, vehicular) are enhanced by cold conditions and are trapped by temperature inversions. Monitoring of atmospheric composition, and in particular vertical gradients in composition, is challenging under cold Arctic conditions. This study demonstrates that multiple sets of low-cost electrochemical sensors can provide accurate measurement of CO, NO, NO₂, and O₃ air composition across wide-ranging cold Arctic temperatures (0 °C to -30 °C). The sensors quantify vertical gradients in downtown Fairbanks' atmospheric composition during winter 2021. Low-cost electrochemical sensors (with temperature co-measured) were characterised by cross-comparison to a regulatory air-quality monitoring site. We demonstrate excellent agreement of the electrochemical sensors with the reference monitors (R² > 0.86-0.98), with mean absolute errors <5 ppbv (NO, NO₂, O₃) and <50 ppbv (CO) over gas-ranges of 10-100's, and 3000 ppbv, respectively, sufficient for using the low-cost electrochemical sensors to quantitatively investigate NO-NO₂-O₃ atmospheric chemistry. During four weeks in February-March 2021, sensors placed on the rooftop (20 m) and base (3 m) of a building in downtown Fairbanks identified strong gradients in atmospheric composition over a very short <20 m vertical scale at times when near-surface temperature inversions were present. At night, CO and NO_x were more concentrated at the surface than aloft, and surface ozone was depleted whilst sometimes being present aloft. During the daytime, when solar radiation heated the surface, inversions were disrupted by efficient vertical mixing that mixed in ozone-rich air from above. The low-cost sensor observations demonstrate that near-surface pollutant trapping was correlated with thermal inversions and trace O₃-NO_x atmospheric chemistry, and quantify a local O_x source from direct "primary" NO₂ emissions, with a directly emitted NO₂ : NO_x ratio of 0.13 mol mol^-1. The sensors also characterise NO_x emissions, finding a NO_x : CO of 0.15 mol mol^-1. When well-characterised, low-cost electrochemical sensors can provide valuable measurements of local emissions and vertically-resolved atmospheric composition, with sufficient accuracy to trace atmospheric chemistry in cold and stable wintertime urban environments.