Surface distributions and vertical profiles of trace gases (CO, O₃, NO, NO₂) in the Arctic wintertime boundary layer using low-cost sensors during ALPACA-2022

Electrochemical gas sensors (EGSs) have been used to measure the surface distributions and vertical profiles of trace gases in the wintertime Arctic boundary layer during the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field experiment in Fairbanks, Alaska, in January-February 2022. The MICRO sensors for MEasurements of GASes (MICROMEGAS) instrument set up with CO, NO, NO2, and O3 EGSs was operated on the ground at an outdoor reference site in downtown Fairbanks for calibration, while on board a vehicle moving through the city and its surroundings and on board a tethered balloon, the helikite, at a site at the edge of the city. To calibrate the measurements, a set of machine learning (ML) calibration methods were tested. For each method, learning and prediction were performed with coincident MICROMEGAS and reference analyser measurements at the downtown site. For CO, the calibration parameters provided by the manufacturer led to the best agreement between the EGS and the reference analyser, and no ML method was needed for calibration. The Pearson correlation coefficient R is 0.82, and the slope of the linear regression between MICROMEGAS and reference data is 1.12. The mean bias is not significant, but the root mean square error (290 ppbv, parts per billion by volume) is rather large because of CO concentrations reaching several ppmv (parts per million by volume) in downtown Fairbanks. For NO, NO2, and O3, the best agreements for the prediction datasets were obtained with an artificial neural network, the multi-layer perceptron. For these three gases, the correlation coefficients are higher than 0.95, and the slopes of linear regressions with the reference data are in the range 0.93-1.04. The mean biases, which are 1 +/- 3, 0 +/- 4, and 3 +/- 12 ppbv for NO2, O3, and NO, respectively, are not significant. Measurements from the car round of 21 January are presented to highlight the ability of MICROMEGAS to quantify the surface variability in the target trace gases in Fairbanks and the surrounding hills. MICROMEGAS flew 11 times from the ground up to a maximum of 350 m above ground level (a.g.l.) on board the helikite at the site at the edge of the city. The statistics performed over the helikite MICROMEGAS dataset show that the median vertical gas profiles are characterized by almost constant mixing ratios. The median values over the vertical are 140, 8, 4, and 32 ppbv for CO, NO, NO2, and O3. Extreme values are detected with low-O3 and high-NO2 and NO concentrations between 100 and 150 m a.g.l. O3 minimum levels (5th percentile) of 5 ppbv are coincident with NO2 maximum levels (95th percentile) of 40 ppbv, which occur around 200 m a.g.l. The peaks aloft are linked to pollution plumes originating from Fairbanks power plants such as those documented during the flight on 20 February.