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. from 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, 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 co-measured temperature) 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 > 0.77-0.98), with mean absolute errors <5 ppbv (NO, NO2, O3) and <50 ppbv (CO) over gas-ranges of 10’s-100’s, and 3000 ppbv, respectively, sufficient for 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 (20m) and base (3m) of a building in downtown Fairbanks identify 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 NOx were more concentrated at the surface than aloft, and surface ozone was depleted whilst sometimes being present aloft. During 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 near-surface pollutant trapping was correlated to thermal inversions, trace O3-NOx atmospheric chemistry, and quantify a local Ox source from direct “primary” NO2 emissions, with a directly emitted NO2:NOx ratio of 0.13 mol/mol. The sensors also characterise NOx emissions finding a NOx:CO of 0.18 mol/mol. 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.